Cardiac muscle mechanics from Doca- and aging spontaneously hypertensive LOIS >ANE HELLER Department of Physiology Duluth, Minnesota 55812
HELLER,
LOIS
JANE.
of Minrtesota
University
mX*h~?lk*~
fkm
Dma-
trrd trg-irg qxm tmwwsly hyxv*terwiw rnts. Am. J. Physiol. 235W: H13ZH86, 1978 or Am. J. Physiol.: Heart Circ. Physiol. 44 1): HWH86, 1978. -Contractile properties of isolated papillary muscles from three age groups of spontaneously hypertensive rats (SHRs) were compared to those from age-matched Wistar-Kyoto rats (WKYs) to assess whether- there were differences between the &rains preceding and during the course of the hypertension. In alI three age groups (7, 12, and 50 wk), the mechanical refractory periods IMRPI were longer and aftercontractions more prominent after paired pulse stimulation in preparations from SHRs than from age-matched WKYs. Other isometric twitch properties of SHR papillary muscles at L,,,,,, were not different from WKYs, with the exception of a shorter half-relaxation time in the youngest SHR group. Although hypertension and cardiac hypertrophy increased in SHRs with age, aging had similar influences on most cardiac contractile properties in both strains. None of the isometric properties of papillary muscles from rats made hypertensive by Doca treatment were different from those in normotensive control preparations. This suggests that differences seen between SHRs and WKYs probably represent genetic differences between these strains and are not directly caused by the hypertension, myocardium; hypertension; mechanical refractory periods; tercontractions; isometric twitch; paired pulse stimulation
af-
--. --IN CARDIAC FUNCTION have been implicated in developmental phases of both clinical and experimental hypertension. Most reports indicate that cardiac indices are initially elevated and then later return to normal levels, while a progressive increase in total peripheral resistance occurs to sustain the elevated arterial pressure (4, 6-8, 14, 17, 20, 21, 26). A number of factors that could contribute to the early augmentation of cardiac function have been suggested: 1) alterations in autonomic neural influences on the heart (6, 14, 21, 26), 2) variations in cardiac filling (4, 6, 10, 26), and 3) changes in the cardiac muscle itself (1, 11, 18). The spontaneously hypertensive rat (SHR) has been accepted as a model of genetically linked hypertension (19). The he modynamic characteristics of SHRs have been investigated by several groups who disagree as to the presence and/or time course of a transient increased cardiac output during the developmental stages of hypertension (2, 5, 10, 20-22, 25, 27). In a recent study from this laboratory it was found that certain intrinsic ALTERATIONS
I-I82
School
rats
of Medicine,
mechanical properties of isolated papillary muscles from young SHRs are different from those measured in preparations taken from age-matched normotensive rats of the parent genetic strain (>WKY) (11). The mechanical refractory period (minimal interval between two electrical stimuli for which the second stimulus elicits a contraction) is longer for SHR than WKY In addition, spontaneous aftercontracpreparations* tions (small, slow contractions not dependent on propagated action potentials (3, 23)) occur more frequently and with greater amplitude following paired pulse stimulations in SHRs than WKYs. These findings raise two additional questions: first, are the abnormal mechanical properties found in young adult SHRs temporary in nature as is the initial period of altered cardiac output?; second, are these findings entirely due to the hypertension or might they also represent inherent differences in the SHR and WKY strains? To determine whether these differences exist only transiently during the early stages of hypertension, isometric properties of isolated cardiac muscle from three age-matched groups of SHRs and WKYs (7-, 1%, and 50-wk-old rats) were examined. To determine whether the previously observed differences in responses of SHR and WKY papillary muscles (11) were caused by the hypertension or were rather expressions of the genetic makeup of these two strains of animals, similar experiments were conducted on another strain of rats made hypertensive by unilateral nephrectomy, salt feeding, and treatment with deoxycorticosterone acetate (Doca) (9). Data from these preparations were compared to sham-treated littermate normotensive contrek METHODS
Anirnal~
e
Male, albino rats inbred from the hypertensive strain (SHR) developed by Okamoto and Aoki (19) and agematched normotensive Wistar-Kyoto (WKY) control animals were bred and raised in identical envirunmentcontrolled conditions for use in the first part of this study. The Doca-hypertensive rat model was used in the second part of this study to assessthe cardiac effects of chronic elevated cardiac afterload in animals of the same strain. Male albino rats obtained from SpragueDawley were used for this part of the study. These
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HEART
MUSCLE
MECHANICS
AND
animals were anesthetized with sodium pentobarbital (37 mg/kg ip) and either unilaterally nephrectomized or sham operated. Following recovery, nephrectomized animals were treated with biweekly subcutaneous injections of 10 mg of Doca (Sigma Chemical Co.) dissolved in sesame oil and were given 17~ NaCl as their drinking water. Sham-operated rats received subcutaneous sesame oil alone. Treatment time was 53 k 3 days for Doca-treated and 48 t 3 days for Doca-sham animals. Pclyillcrry . Muxle
H83
HYPERTENSION
Prepuratirv~s
Animals were anesthetized with sodium pentobarbital (37 mg/kg ip), and the carotid artery was cannulated for direct blood pressure measurement using a Statham P23Db transducer connected to a Brush 440 recording system. The heart was rapidly removed and placed in oxygenated physiological salt solution at 27°C containing (mM): NaCl, 115; KCl, 6.0; MgCL, 1.2; CaCl,, 2.5; glucose, 11.5; Tris Cl, 35.0; pH 7.4. The posterior papillary muscle from the left ventricle was removed and mounted in a muscle bath. One end of the muscle was rigidly mounted to the bottom of the bath and the other end attached via a small clamp and silver chain to a Grass FTO3C force transducer. Suprathreshold field stimulation was produced through stainless steel plate electrodes positioned about 1 mm on either side of the muscle by a Grass S88 stimulator. Details concerning the recording and stimulating system have been previously described (11).
percent of the amplitude of the response to the first stimulus of the pair. At the end of each experiment the muscle was removed from the bath and weighed. A tissue density of 1.0 was assumed and the cross-sectional area was estimated by dividing the muscle weight by I+,~ls, Data Analysis Where appropriate, variables were normalized for cross-sectional area, averaged, and reported as means t standard error. The statistical significance of differences between group means was assessed by applying Student’s t test. RESULTS
SHR and WKY Preparations Table 1 includes general information about the animals used for the experiments. The heart weight/body weight ratio decreased with age in WKY. In SHR, however, heart weight/body weight decreased between 7 and 12 wk of age, and then rose between 12 and 50 wk TENSION
(8)
a STlMlJLlJS
-20
250
225mssc
Pr~otocol During 30-60 min of stimulating the muscle in the bath at 0.2 Hz, active tension was permitted to reach a steady state. During this time resting tension, originally set at 1.5 g, decreased to between 0.5 and 1.0 g. Tension-time characteristics at L1ll,l.lb,The muscle was stretched by 0.5mm increments to determine the length at which isometric tension was maximal (L,,,,,,). At this length, resting and active tension, time to peak tension development (TPT), maximum rate of tension development (dT/dt,,,.,, ), and one-half relaxation time (RT, J were measur& Muscle response to paired pulse stimulation. The mechanical response of a muscle to paired pulse stimulation reveals characteristics of the time-dependent recovery processes associated with a contraction. Paired stimuli were given to the muscle such that four single stimuli alternated with four paired stimuli, with a basic frequency of 0.2 Hz (Fig. 1). The interval between the paired stimuli was progressively increased from 60 to 2,500 ms in steps ranging from 25 to 500 ms. The MRP was taken as the pairing interval at which response to the second stimulus of the pair first appeared as a deflection in the relaxation phase of the twitch elicited by the first stimulus. As the pairing interval was further increased, the amplitude of the response to the second stimulus increased and approached the amplitude of the response to the first stimulus. Aftercontractions were noted in many preparations following paired pulses with pairing intervals of 250-350 ms.When present, their amplitude was measured and expressed as
seconds----
275msec
msac
of presentation of paired stimuli to an isolated papillary muscle preparation. P, paired stimuli; S, single stimulus. Intervals between paired stimuli are indicated below stimulus artifact. Arrow identifies one of aftercontractions induced by paired pulse stimulation. FIG.
1. Pattern
1. Animals
TABLE
---. -
SHR
(7 wk)
used in study __.._~_~___ __.._-__---- _------. --- -___ -. ----.-
n
Body Wt, g
Heart
Wt,
13
111 26
0.40 20.02
c:
Heart Wti Body Wt, wck
Cross-Sectional Area Papillary Muscle, mm’
.- _Arterial Pressure, m m Hg
WKY
(7 wk)
16
165 215
0.49 +0.04
3.6 kO.1 t 3.0 50.1
0.65 50.05 *
109 k8
0.83 *o-o7
107 +3
SHR
(12 wk)
13
281 *78
0.93 *O.O3Q
3.3 *O.l$
0.94 *0.108
189 kS$
13
290 Y20§
0.82 kO.075
2.8 -+O.l
0.83 +0,09
120 k5
11
367 k-210
1.34 20.085 I
3.7 &O.lO
1.12 +0.15
t
WKY
SHR
(12 wk)
150 wk)
15
46; L 105
1.12 kO.029
2.4 kO.14
0.94 kO.14
208 &4$ t 90 -+9Ei
Doca-treated (18 wk)
12
394 214
1.11 ~Oo.05
0.89 *0.07
179 26
Doca-sham (18 wk)
14
417 212
0.89 20.03
2.8 20.1 t 2.1 kO.1
0.75 kO.05
134 --?7
.L
WKY
60 wk)
t
t
Values are means +, SE. * P < 0.05, t P < 0.01 (SHR vs. agematched WKY or Doca-treated vs. Doca-sham). $ P < 0.05, 5 P < 0.01 (as compared to the preceding age group of SHRs or WKYs).
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 10, 2019.
H84
L. J. HELLER
(P < 0.01). All three age groups of SHRs had significant cardiac hypertrophy when compared to age-matched WKYs. The cross-sectional area of the SHR muscles tested tended to increase with age, but was not different from the cross-sectional area of the age-matched WKY preparations at 12 and 50 wk. Papillary muscle twitch characteristics at L,,,,,. The relationship between muscle length and resting tension is a curve which, with repeated stretches and releases of the muscle, tends to demonstrate some hysteresis and to shift to the right. Since LlaX was determined during the initial stretching sequence, the resting tensions reported in this study are high. Subsequent release of resting tension to 1.0 g by minor shortening of the muscle did not appreciably affect active tension. Table 2 summarizes the tension-time data for single twitches obtained at Lax. The only isometric twitch property found to be different between SHR and WKY preparations was the RT,,, at 7 wk. Papillary muscles from young WKYs appear to relax more slowly than those from age-matched SHRs. Age-dependent changes that occurred in both SHR and WKY strains include 1) an increase in resting tension at Lax between 7 and 12 wk, 2) an increase in between 7 and 12 wk, 3) a decrease in TPT dmknax between 7 and 12 wk followed by an increase between 12 and 50 wk, and 4) a decrease in RT1/*) between 7 and 12 wk. The decrease in dT/&,,, between 12 and 50 wk for WKYs was the only age-dependent change that did not also appear in SHRs. Papillary muscle response to paired pulse stimulation. Characteristics of papillary muscle responses to paired pulse stimulation are shown in Table 3. MRPs were significantly longer in the SHR preparations than those from age-matched WKYs in all three age groups. In addition, there was a significant shortening of the MRP between 12 and 50 wk in WKYs that was not apparent in SHR preparations. -Aftercontractions appeared in many of the preparaof 250-350 tions when stimuli were paired at intervals ms. These aftercontractions were larger in the SHR preparations than in the age-matched WKYs at 7 and 12 wk of age. Aftercontractions in 7-wk SHRs were TABLE 2. Isometric muscles at L,,,,, Resting Tension, g/mm2
SHR
(7 wk)
WKY
(7wk)
1.3*0.1
properties Active Tension, g/mm2
of papizlary
dT/d.t, ghm2 * s
Time to Peak Tension, ms
Half-Relaxation Time, ms
4.2-b0.4
27.322.7
18856
1.2 + 0.1
4.320.3
23.022.3
190+4
226115 * 2805 14
SHR (12 wk) WKY (12 wk)
2.6+0.3§ 2.620.49
4.950.5 5.4-+0.5
34.7-+3.9$ 35.5+3.9$
166*3$ 167+2§
176+7$ 185269
SHR (50 wk) WKY (50 wk)
2.520.7 2.4~0.6
5.0-+0.7 3.9kO.6
33.324.7 25.0t3.4$
183569 183+6$
193+7 207+12
Doca-treated Doca-sham
1.6kO.2 1.720.1
6.2t0.5 6.4t0.6
38.8+4.1 42.8k4.0
204+5 2131r7
273+ 12 282k9
* P < 0.05, t P -C 0.01 (SHR vs. ageValues are means + SE. matched WKY or Doca-treated vs. Doca-sham). I P < 0.05, 0 P < 0.01 (as compared to the preceding age group of SHRs or WKYs).
TABLE 3. Papillary pulse stimulation
muscle response
Mechanical Refractory Period, ms
SHR
(7 wk)
to paired Aftercontractions
Incidence
227 t 4 *
8of9
Amplitude
8.4 + 1.5 t 1.7 + 0.5
WKY
(7 wk)
216 -t 4
9 of 16
SHR
(12 wk)
246 + 4 *
13 of 13
229 -+ 6
10 of 13
4.2 + 0.5s I2.6 + 0.5
230 + 9
9 of 10
4.1 ?I 0.9
183 t 7s
13 of 14
3.1 t 0.4
209 t 5 193 t 3
8 of 12 8 of 14
2.9 _+ 0.8 2.7 + 1.3
WKY SHR WKY
(12 wk) (50 wk) (50 wk)
Doca-treated Doca-sham
Values are means + SE. Mechanical refractory period is minimum interval between paired pulses that will evoke a second detectable mechanical response. Amplitude is expressed as percentage of the amplitude of the response to the first stimulus of the pair * P < 0.05, t‘ P < which evokes the largest aftercontractions. 0.01 (SHR vs. age-matched WKY or Doca-treated vs. Doca-sham). $ P < 0.05, $ P < 0.01 (as compared to the preceding age group of SHRs or WKYs).
larger than in either of the two older groups of SHRs. In the WKY strain, aftercontractions were largest in the oldest group. Doca-Treated
and Doca-Sham
Preparations
General information about the animals made hypertensive with Doca treatment and the sham-treated control rats is given in Table 1. When compared to agematched sham-treated animals at the end of the treatment, Doca-treated animals had higher mean arterial pressure and their hearts were hypertrophied, as judged either by absolute heart size or the heart weight/body weight ratio. There was no difference in the dry weight/ wet weight ratio of hearts from Doca-treated (23.7 t 0.1%) versus hearts from the Doca-sham animals (23.6 t 0.3%). Papillary muscle twitch characteristics at L,,,,,. There were no detectable differences in any of the isometric properties measured at Lax between papillary muscles from Doca-treated and Doca-sham rats. These data are shown in Table 2. There were some basic differences between the twitch characteristics of papillary muscles from the adult (12 and 50 wk) WKY and the Doca-sham rats. The resting tension at Lax of the Doca-sham rats was significantly less than that of WKYs from both adult’groups (P < 0.01) and the Doca-sham’s active tension was higher (P < 0.01). When compared to the 50-wk WKYs, Docasham rats had a higher dT/&,,, (P < 0.05), longer TPT (P < O.Ol), and longer RT,,, (P < 0.01). Papillary muscle response to paired pulse stimulation. As shown in Table 3, the MRPs of Doca-treated and Doca-sham preparations were not found to be different. This is in contrast to the discrepancy found between SHRs and WKYs in the first portion of this study. Aftercontractions occurred in many of the Doca-
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MEiiRT
MUSCLE
MECHANICS
H85
AND HYPERTENSION
treated and Doca-sham preparations, but there was no detectable difference in the amplitude of these events between these groups (Table 3). Although the incidence of aftercontractions was somewhat lower in the Docasham preparations than in either 12- or 50-wk-old WKYs, the amplitude of these events was similar in both strains of normotensive rats tested. DISCUSSION
The answer to the initial question of whether the previously reported differences in mechanical properties of cardiac muscle from SHRs and WKYs (11) represent a transient occurrence during the early stages of hypertension appears to be a qualified no. That is, a prolonged MRP and an accentuation of aftercontractions is evident in all three age groups of SHRs tested when compared to WKYs. However, the differences in the incidence and amplitude of aftercontractions are must pronounced in the youngest group, whereas the difference in MRP is largest in the oldest group of animals tested. This indicates that the processes responsible for a prolonged MRP may not be directly related to those favoring aftercontractions. Other cardiac muscle mechanical properties measured at Lax appear to be similar when SHRs are compared with age-matched WKYs. The single exception may be the p,T,,, in the 7-wk-old animals. The reasons for the faster relaxation rate in the young SHRs as well as the consistent prolongation of the MRP and exaggeration of aftercontractions in SHRs when compared to age-matched WKYs cannot be precisely determined from this study. In addition, there seems to be no clear correlation between the development of altered met.hanical properties and altered cardiac dynamics or hypertension. At 7 wk, SHRs have been reported to have elevated cardiac function (2, 10) but little or no evidence of hypertension (11, 24). At 12 wk, SHRs have established hypertension, but ventricular performance levels are in dispute. Initial studies by Pfeffer and Frohlich (20) imply that cardiac indices are elevated but later studies indicate that, when taking into consideration the cardiac hypertrophy that has developed, cardiac output per gram of myocardium is normal (22, 25). Reports by Cutilletta et al. (5) and Walsh and Tobia (27) suggest that by this age, cardiac index is now somewhat depressed. At 50 wk, SHRs have chronic hypertension, which is associated with a depressed cardiac function (10, 20). SHR and WKY strains demonstrated similar ageassociated changes in most of the isometric properties of the papillary muscles; the exceptions being a reduction
of dT/dt,,, and MRP with age in the adult WKYs but not SHRs. The present study substantiates previous reports by others who find that in adult rats there are no differences in resting tension or active tension, but a tendency toward prolonged contraction duration associated with aging (12, 15, 16, 28). The youngest group of animals of both SHRs and WKYs had lower resting tensions as well as longer contraction and relaxation times than did the adult animals of these strains. This is in contrast to Hopkins et al, (13) who have reported that after 20 days of age, resting as well as active tension of rat ventricular preparations are not different from adult. The reasons for these differences in findings are not apparent. The second question that initiated this study was whether the differences between mechanical properties of cardiac muscle from SHRs and WKYs are caused by the hypertension or whether they reflect differences between the SHR and WKY strains. The Doca-hypertensive rat, model was chosen to answer this question since the normotensive control preparations are from littermates of the hypertensive preparations, thus eliminating the possibility of genetic differences. Since the Doca-treated hypertensive rats did not demonstrate prolonged mechanical refractory periods or exaggerated aftercontractions when compared to normotensive controls, it is not likely that hypertension per se is responsible for differences between SHRs and WKYs. In this study, a comparison of adult normotensive WKYs and Doca-sham normotensive rats reveals significant differences in isometric twitches measured at Lax, emphasizing the observation that strain-related differences in these properties do exist. Because the preparations from normotensive control animals used for comparison with the two hypertensive preparations are significantly different in so many respects, a comparison of isometric characteristics of cardiac muscle from SHRs and Docatreated animals would be meaningless. The presence of ventricular hypertrophy in SHRs, both before the appearance of hypertension in SHRs, as shown in this study and others (2, 24), as well as when hypertension was prevented by nerve growth factor antiserum (5) or hydralazine treatment (24)) substantiates the suggestion that differences in cardiac properties exist between these strains that are not directly caused by the chronically elevated cardiac afterload. Whether these differences contribute to the development of the hypertension is yet to be established. This work was supported Association and the Minnesota Received
22 August
by grants Medical
1977; accepted
from the Foundation.
in final
form
Minnesota 1 March
Heart 1978.
REFERENCES 1. AOKI, K., N. IKEDA, K. YAMASHITA, AND K. HOTTA. ATPase activity and Ca+i interaction of myofibrils and sarcoplasmic reticulum isolated from the hearts of spontaneously hypertensive rats. Japan. Heart J. 15: 475-484, 1974. BERTE, L., A. CUTILLETTA, P. SODT, AND R. ARCILLA. Myocardial hypertrophy and hemodynamics in hypertensive rats (Abstract). Cirdation 52, Suppl. 2: II-44, 1975. BLINKS, J. R., AND J. KOCH-WESER. Physical factors in the analysis of the actions of drugs on myocardial contractility.
Pharmacd. Reu. 15: 531-599, 1963. 4. COLEMAN, T. G., AND A. C. GUYTON. Hypertension caused by salt loading in the dog. III. Onset transients of cardiac output and other circulatory variables. Circdation Res. 25: 153-160, 1969. 5. CUTLLLETTA, A. F., L. ERINOFF, A. HELLER, J. Low, AND S. OPAREL. Development of left ventricular hypertrophy in young spontaneously hypertensive rats after peripheral sympathectomy. Circdation Res. 40: 428-434, 1977.
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H86 6. ELTJS, C. N., AND S. JULIUS. Role of central blood volume in hyperkinetic borderline hypertension. HI-it. Huwt J. 35: 450-455, 1973. 7. FERRARIO, C. M., T. H. PAGE, AND J. W. MCCUBBIN. Increased cardiac output as ti contributory factor in experimental renal hypertension in dogs. Circdcrfirm Ros. 27: 799-810, 1970, 8. FROHLICH, E. D., V. J. Kozu~, R. C. TARAZI, AND H. P. DUSTAN. Physiological comparison of labile and essential hypertension. (I'iwrrltt tiun R PS. 26 and 27, Suppl. 1: 1-55-I-63, 1970. 9. CAVRAS, H., H. R. BKUNNER, J. I-I. LARAGH, E. D. VAUGHAN, JR., M. Koss, L. J. COTE, AND I. GAVRAS. Malignant hypertension resulting from Doca and salt excess. Cimuidion Res. 36: 300-309, 1975. ’ 10. HALLBACH, M., 0. ISAKSSON, AND E. NORESSEN. Consequences of myocardial structural adaptation on left ventricular compliance ;jnd the Frank-Starling relationship in spontaneously hypertensive ruts. A& Phvsic)f . Stwn(-i. 94: 259-270, 1975. 11. HELI~ER, L. J. Mechanical properties of cardiac muscle from spontaneously hypertensive rats: accentuated after-contractions. Prw. SW. Esptl. Hid. Md 154: 479-482, 1977. 12. HELLER, L. J., AND W. V. WHITEHORN. Age-associated alterations in myocardial contractile properties. Am. J. Physiof. 222: 1613-1619, 1972. 13. HOPKINS, S. F., E. P. MCCUTCHEON, AND D. R. WEKSTEIN. Postnatal changes in rat ventricuIar function. Cimddorz Rcs. 32: 685-691, 1973. 14. JULIUS, S., 0. S. RANDALL, M. D. ESLER, T. KASHIMA, C. ELLIS, AND J. BENNETT. Altered cardiac responsiveness and regulation in the normal cardiac output type of borderline hypertension. C’iwrrkdion Rm 36: I-199-1-207, 1975. 15. LAKATTA, E. G., G. GERSTENBLITH, C.S. ANGELL, N. W. SHOCK, AND M. L. WEISFELDT. Diminished inotropic response of aged myocardium to catecholam ines. Ciwrrlcl tim Km a 36: 262-269, 1975. 16. LAKATTA, E. G., cf. GERSTENBLITH, C. S. ANGELL, N. W. SHOCK, AND M. L. WEISFELDT. Prolonged contraction duration in aged myocardium. J. Clir~ . Inwst. 55: 61-68, 1975.
L. J.
HELLER
17. LEDINGHAM, J. M., AND D. PELLING. Cardiac output and peripheral resistance in experimental renal hypertension. Cirwlutian Res. 20 and 21, Suppl. 2: II-187-11-199, 1967. 18. LIMAS, C. J., AND J. N. COHN. Defective calcium transport by cardiac sarcoplasmic reticulum in spontaneously hypertensive rats. Cirwkrtion Res . 40: I-62-1-69, 1977. 19. OKAMOTO, K., AND K. AOKI. Development of a strain of spontaneously hypertensive rats. Jcrpu 12. Circ~ulution J. 27: 282-293, 1963. 20. PFEFFER, M. A., AND E. D. FROHLICH. Hemodynamics and myocardial function in young and old normotensive and spontaneously hypertensive rats. Cirwlatitm Rm. 32: 1-28-I-38, 1973. 21. PFEFFER, M. A., E. D. FROHLICH, J. M. PFEFFER, AND A. K. WEISS. Pathophysiological implications of the increased cardiac output of young spontaneously hypertensive rats. Cirwlation Res. 34: 1-235-I-242, 1974. 22. PFEFFER, M. A., J. M. PFEFFER, AND E. D. FROI~LICH. Pumping ability of the hypertrophying left ventricle of the spontaneously hypertensive rat. Circdrttion Res. 38: 423-429, 1976. 23. REITER, M. Drugs and heart muscle. Ann. Rw Phur-rtzt~w/. 12: 111-124, 1972. 24. SEN, S., R. C. TARAZI, P. A. KHAIRALLAH, AND F. M. BUMPUS. Cardiac hypertrophy in spontaneously hypertensive rats. Cir-wkrtion Km. 35: 775-781, 1974. 25. TADEPALLI, A. S., G. M. WALSH, AND A. J. TOBIA. Normal cardiac output in the conscious young spontaneously hypertensive rat: evidence for higher oxygen utilization. I,& Sk 15: 1113-1114, 1974. 26. TARAZI, R. C., M. M. IBRAHIM, H. P. DUSTAN, AND C. M. FERRARIO. Cardiac factors in hypertension. Ciwdotitm Rus. 34: 1-213-I-221, 1974. 27. WALSH, G. M., AND A. J. TOFHA. Intrinsic left ventricular performance in the young spontaneously hypertensive rat. Rw Cortztmrn. Chun. Puthot. Phcrmmmt. 8: 623-633, 1974. 28. WEISFELDT, M. L., W. A. LOEVEN, AND N. W. SHOCK. Resting and active mechanical properties of trabeculae carneae from aged male rats. Am. J. Physid. 220: 1921-1927, 1971.
Downloaded from www.physiology.org/journal/ajpheart by ${individualUser.givenNames} ${individualUser.surname} (129.186.138.035) on January 10, 2019.
HELLER,
LOIS
JANE.
of Minrtesota
University
mX*h~?lk*~
fkm
Dma-
trrd trg-irg qxm tmwwsly hyxv*terwiw rnts. Am. J. Physiol. 235W: H13ZH86, 1978 or Am. J. Physiol.: Heart Circ. Physiol. 44 1): HWH86, 1978. -Contractile properties of isolated papillary muscles from three age groups of spontaneously hypertensive rats (SHRs) were compared to those from age-matched Wistar-Kyoto rats (WKYs) to assess whether- there were differences between the &rains preceding and during the course of the hypertension. In alI three age groups (7, 12, and 50 wk), the mechanical refractory periods IMRPI were longer and aftercontractions more prominent after paired pulse stimulation in preparations from SHRs than from age-matched WKYs. Other isometric twitch properties of SHR papillary muscles at L,,,,,, were not different from WKYs, with the exception of a shorter half-relaxation time in the youngest SHR group. Although hypertension and cardiac hypertrophy increased in SHRs with age, aging had similar influences on most cardiac contractile properties in both strains. None of the isometric properties of papillary muscles from rats made hypertensive by Doca treatment were different from those in normotensive control preparations. This suggests that differences seen between SHRs and WKYs probably represent genetic differences between these strains and are not directly caused by the hypertension, myocardium; hypertension; mechanical refractory periods; tercontractions; isometric twitch; paired pulse stimulation
af-
--. --IN CARDIAC FUNCTION have been implicated in developmental phases of both clinical and experimental hypertension. Most reports indicate that cardiac indices are initially elevated and then later return to normal levels, while a progressive increase in total peripheral resistance occurs to sustain the elevated arterial pressure (4, 6-8, 14, 17, 20, 21, 26). A number of factors that could contribute to the early augmentation of cardiac function have been suggested: 1) alterations in autonomic neural influences on the heart (6, 14, 21, 26), 2) variations in cardiac filling (4, 6, 10, 26), and 3) changes in the cardiac muscle itself (1, 11, 18). The spontaneously hypertensive rat (SHR) has been accepted as a model of genetically linked hypertension (19). The he modynamic characteristics of SHRs have been investigated by several groups who disagree as to the presence and/or time course of a transient increased cardiac output during the developmental stages of hypertension (2, 5, 10, 20-22, 25, 27). In a recent study from this laboratory it was found that certain intrinsic ALTERATIONS
I-I82
School
rats
of Medicine,
mechanical properties of isolated papillary muscles from young SHRs are different from those measured in preparations taken from age-matched normotensive rats of the parent genetic strain (>WKY) (11). The mechanical refractory period (minimal interval between two electrical stimuli for which the second stimulus elicits a contraction) is longer for SHR than WKY In addition, spontaneous aftercontracpreparations* tions (small, slow contractions not dependent on propagated action potentials (3, 23)) occur more frequently and with greater amplitude following paired pulse stimulations in SHRs than WKYs. These findings raise two additional questions: first, are the abnormal mechanical properties found in young adult SHRs temporary in nature as is the initial period of altered cardiac output?; second, are these findings entirely due to the hypertension or might they also represent inherent differences in the SHR and WKY strains? To determine whether these differences exist only transiently during the early stages of hypertension, isometric properties of isolated cardiac muscle from three age-matched groups of SHRs and WKYs (7-, 1%, and 50-wk-old rats) were examined. To determine whether the previously observed differences in responses of SHR and WKY papillary muscles (11) were caused by the hypertension or were rather expressions of the genetic makeup of these two strains of animals, similar experiments were conducted on another strain of rats made hypertensive by unilateral nephrectomy, salt feeding, and treatment with deoxycorticosterone acetate (Doca) (9). Data from these preparations were compared to sham-treated littermate normotensive contrek METHODS
Anirnal~
e
Male, albino rats inbred from the hypertensive strain (SHR) developed by Okamoto and Aoki (19) and agematched normotensive Wistar-Kyoto (WKY) control animals were bred and raised in identical envirunmentcontrolled conditions for use in the first part of this study. The Doca-hypertensive rat model was used in the second part of this study to assessthe cardiac effects of chronic elevated cardiac afterload in animals of the same strain. Male albino rats obtained from SpragueDawley were used for this part of the study. These
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HEART
MUSCLE
MECHANICS
AND
animals were anesthetized with sodium pentobarbital (37 mg/kg ip) and either unilaterally nephrectomized or sham operated. Following recovery, nephrectomized animals were treated with biweekly subcutaneous injections of 10 mg of Doca (Sigma Chemical Co.) dissolved in sesame oil and were given 17~ NaCl as their drinking water. Sham-operated rats received subcutaneous sesame oil alone. Treatment time was 53 k 3 days for Doca-treated and 48 t 3 days for Doca-sham animals. Pclyillcrry . Muxle
H83
HYPERTENSION
Prepuratirv~s
Animals were anesthetized with sodium pentobarbital (37 mg/kg ip), and the carotid artery was cannulated for direct blood pressure measurement using a Statham P23Db transducer connected to a Brush 440 recording system. The heart was rapidly removed and placed in oxygenated physiological salt solution at 27°C containing (mM): NaCl, 115; KCl, 6.0; MgCL, 1.2; CaCl,, 2.5; glucose, 11.5; Tris Cl, 35.0; pH 7.4. The posterior papillary muscle from the left ventricle was removed and mounted in a muscle bath. One end of the muscle was rigidly mounted to the bottom of the bath and the other end attached via a small clamp and silver chain to a Grass FTO3C force transducer. Suprathreshold field stimulation was produced through stainless steel plate electrodes positioned about 1 mm on either side of the muscle by a Grass S88 stimulator. Details concerning the recording and stimulating system have been previously described (11).
percent of the amplitude of the response to the first stimulus of the pair. At the end of each experiment the muscle was removed from the bath and weighed. A tissue density of 1.0 was assumed and the cross-sectional area was estimated by dividing the muscle weight by I+,~ls, Data Analysis Where appropriate, variables were normalized for cross-sectional area, averaged, and reported as means t standard error. The statistical significance of differences between group means was assessed by applying Student’s t test. RESULTS
SHR and WKY Preparations Table 1 includes general information about the animals used for the experiments. The heart weight/body weight ratio decreased with age in WKY. In SHR, however, heart weight/body weight decreased between 7 and 12 wk of age, and then rose between 12 and 50 wk TENSION
(8)
a STlMlJLlJS
-20
250
225mssc
Pr~otocol During 30-60 min of stimulating the muscle in the bath at 0.2 Hz, active tension was permitted to reach a steady state. During this time resting tension, originally set at 1.5 g, decreased to between 0.5 and 1.0 g. Tension-time characteristics at L1ll,l.lb,The muscle was stretched by 0.5mm increments to determine the length at which isometric tension was maximal (L,,,,,,). At this length, resting and active tension, time to peak tension development (TPT), maximum rate of tension development (dT/dt,,,.,, ), and one-half relaxation time (RT, J were measur& Muscle response to paired pulse stimulation. The mechanical response of a muscle to paired pulse stimulation reveals characteristics of the time-dependent recovery processes associated with a contraction. Paired stimuli were given to the muscle such that four single stimuli alternated with four paired stimuli, with a basic frequency of 0.2 Hz (Fig. 1). The interval between the paired stimuli was progressively increased from 60 to 2,500 ms in steps ranging from 25 to 500 ms. The MRP was taken as the pairing interval at which response to the second stimulus of the pair first appeared as a deflection in the relaxation phase of the twitch elicited by the first stimulus. As the pairing interval was further increased, the amplitude of the response to the second stimulus increased and approached the amplitude of the response to the first stimulus. Aftercontractions were noted in many preparations following paired pulses with pairing intervals of 250-350 ms.When present, their amplitude was measured and expressed as
seconds----
275msec
msac
of presentation of paired stimuli to an isolated papillary muscle preparation. P, paired stimuli; S, single stimulus. Intervals between paired stimuli are indicated below stimulus artifact. Arrow identifies one of aftercontractions induced by paired pulse stimulation. FIG.
1. Pattern
1. Animals
TABLE
---. -
SHR
(7 wk)
used in study __.._~_~___ __.._-__---- _------. --- -___ -. ----.-
n
Body Wt, g
Heart
Wt,
13
111 26
0.40 20.02
c:
Heart Wti Body Wt, wck
Cross-Sectional Area Papillary Muscle, mm’
.- _Arterial Pressure, m m Hg
WKY
(7 wk)
16
165 215
0.49 +0.04
3.6 kO.1 t 3.0 50.1
0.65 50.05 *
109 k8
0.83 *o-o7
107 +3
SHR
(12 wk)
13
281 *78
0.93 *O.O3Q
3.3 *O.l$
0.94 *0.108
189 kS$
13
290 Y20§
0.82 kO.075
2.8 -+O.l
0.83 +0,09
120 k5
11
367 k-210
1.34 20.085 I
3.7 &O.lO
1.12 +0.15
t
WKY
SHR
(12 wk)
150 wk)
15
46; L 105
1.12 kO.029
2.4 kO.14
0.94 kO.14
208 &4$ t 90 -+9Ei
Doca-treated (18 wk)
12
394 214
1.11 ~Oo.05
0.89 *0.07
179 26
Doca-sham (18 wk)
14
417 212
0.89 20.03
2.8 20.1 t 2.1 kO.1
0.75 kO.05
134 --?7
.L
WKY
60 wk)
t
t
Values are means +, SE. * P < 0.05, t P < 0.01 (SHR vs. agematched WKY or Doca-treated vs. Doca-sham). $ P < 0.05, 5 P < 0.01 (as compared to the preceding age group of SHRs or WKYs).
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H84
L. J. HELLER
(P < 0.01). All three age groups of SHRs had significant cardiac hypertrophy when compared to age-matched WKYs. The cross-sectional area of the SHR muscles tested tended to increase with age, but was not different from the cross-sectional area of the age-matched WKY preparations at 12 and 50 wk. Papillary muscle twitch characteristics at L,,,,,. The relationship between muscle length and resting tension is a curve which, with repeated stretches and releases of the muscle, tends to demonstrate some hysteresis and to shift to the right. Since LlaX was determined during the initial stretching sequence, the resting tensions reported in this study are high. Subsequent release of resting tension to 1.0 g by minor shortening of the muscle did not appreciably affect active tension. Table 2 summarizes the tension-time data for single twitches obtained at Lax. The only isometric twitch property found to be different between SHR and WKY preparations was the RT,,, at 7 wk. Papillary muscles from young WKYs appear to relax more slowly than those from age-matched SHRs. Age-dependent changes that occurred in both SHR and WKY strains include 1) an increase in resting tension at Lax between 7 and 12 wk, 2) an increase in between 7 and 12 wk, 3) a decrease in TPT dmknax between 7 and 12 wk followed by an increase between 12 and 50 wk, and 4) a decrease in RT1/*) between 7 and 12 wk. The decrease in dT/&,,, between 12 and 50 wk for WKYs was the only age-dependent change that did not also appear in SHRs. Papillary muscle response to paired pulse stimulation. Characteristics of papillary muscle responses to paired pulse stimulation are shown in Table 3. MRPs were significantly longer in the SHR preparations than those from age-matched WKYs in all three age groups. In addition, there was a significant shortening of the MRP between 12 and 50 wk in WKYs that was not apparent in SHR preparations. -Aftercontractions appeared in many of the preparaof 250-350 tions when stimuli were paired at intervals ms. These aftercontractions were larger in the SHR preparations than in the age-matched WKYs at 7 and 12 wk of age. Aftercontractions in 7-wk SHRs were TABLE 2. Isometric muscles at L,,,,, Resting Tension, g/mm2
SHR
(7 wk)
WKY
(7wk)
1.3*0.1
properties Active Tension, g/mm2
of papizlary
dT/d.t, ghm2 * s
Time to Peak Tension, ms
Half-Relaxation Time, ms
4.2-b0.4
27.322.7
18856
1.2 + 0.1
4.320.3
23.022.3
190+4
226115 * 2805 14
SHR (12 wk) WKY (12 wk)
2.6+0.3§ 2.620.49
4.950.5 5.4-+0.5
34.7-+3.9$ 35.5+3.9$
166*3$ 167+2§
176+7$ 185269
SHR (50 wk) WKY (50 wk)
2.520.7 2.4~0.6
5.0-+0.7 3.9kO.6
33.324.7 25.0t3.4$
183569 183+6$
193+7 207+12
Doca-treated Doca-sham
1.6kO.2 1.720.1
6.2t0.5 6.4t0.6
38.8+4.1 42.8k4.0
204+5 2131r7
273+ 12 282k9
* P < 0.05, t P -C 0.01 (SHR vs. ageValues are means + SE. matched WKY or Doca-treated vs. Doca-sham). I P < 0.05, 0 P < 0.01 (as compared to the preceding age group of SHRs or WKYs).
TABLE 3. Papillary pulse stimulation
muscle response
Mechanical Refractory Period, ms
SHR
(7 wk)
to paired Aftercontractions
Incidence
227 t 4 *
8of9
Amplitude
8.4 + 1.5 t 1.7 + 0.5
WKY
(7 wk)
216 -t 4
9 of 16
SHR
(12 wk)
246 + 4 *
13 of 13
229 -+ 6
10 of 13
4.2 + 0.5s I2.6 + 0.5
230 + 9
9 of 10
4.1 ?I 0.9
183 t 7s
13 of 14
3.1 t 0.4
209 t 5 193 t 3
8 of 12 8 of 14
2.9 _+ 0.8 2.7 + 1.3
WKY SHR WKY
(12 wk) (50 wk) (50 wk)
Doca-treated Doca-sham
Values are means + SE. Mechanical refractory period is minimum interval between paired pulses that will evoke a second detectable mechanical response. Amplitude is expressed as percentage of the amplitude of the response to the first stimulus of the pair * P < 0.05, t‘ P < which evokes the largest aftercontractions. 0.01 (SHR vs. age-matched WKY or Doca-treated vs. Doca-sham). $ P < 0.05, $ P < 0.01 (as compared to the preceding age group of SHRs or WKYs).
larger than in either of the two older groups of SHRs. In the WKY strain, aftercontractions were largest in the oldest group. Doca-Treated
and Doca-Sham
Preparations
General information about the animals made hypertensive with Doca treatment and the sham-treated control rats is given in Table 1. When compared to agematched sham-treated animals at the end of the treatment, Doca-treated animals had higher mean arterial pressure and their hearts were hypertrophied, as judged either by absolute heart size or the heart weight/body weight ratio. There was no difference in the dry weight/ wet weight ratio of hearts from Doca-treated (23.7 t 0.1%) versus hearts from the Doca-sham animals (23.6 t 0.3%). Papillary muscle twitch characteristics at L,,,,,. There were no detectable differences in any of the isometric properties measured at Lax between papillary muscles from Doca-treated and Doca-sham rats. These data are shown in Table 2. There were some basic differences between the twitch characteristics of papillary muscles from the adult (12 and 50 wk) WKY and the Doca-sham rats. The resting tension at Lax of the Doca-sham rats was significantly less than that of WKYs from both adult’groups (P < 0.01) and the Doca-sham’s active tension was higher (P < 0.01). When compared to the 50-wk WKYs, Docasham rats had a higher dT/&,,, (P < 0.05), longer TPT (P < O.Ol), and longer RT,,, (P < 0.01). Papillary muscle response to paired pulse stimulation. As shown in Table 3, the MRPs of Doca-treated and Doca-sham preparations were not found to be different. This is in contrast to the discrepancy found between SHRs and WKYs in the first portion of this study. Aftercontractions occurred in many of the Doca-
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MEiiRT
MUSCLE
MECHANICS
H85
AND HYPERTENSION
treated and Doca-sham preparations, but there was no detectable difference in the amplitude of these events between these groups (Table 3). Although the incidence of aftercontractions was somewhat lower in the Docasham preparations than in either 12- or 50-wk-old WKYs, the amplitude of these events was similar in both strains of normotensive rats tested. DISCUSSION
The answer to the initial question of whether the previously reported differences in mechanical properties of cardiac muscle from SHRs and WKYs (11) represent a transient occurrence during the early stages of hypertension appears to be a qualified no. That is, a prolonged MRP and an accentuation of aftercontractions is evident in all three age groups of SHRs tested when compared to WKYs. However, the differences in the incidence and amplitude of aftercontractions are must pronounced in the youngest group, whereas the difference in MRP is largest in the oldest group of animals tested. This indicates that the processes responsible for a prolonged MRP may not be directly related to those favoring aftercontractions. Other cardiac muscle mechanical properties measured at Lax appear to be similar when SHRs are compared with age-matched WKYs. The single exception may be the p,T,,, in the 7-wk-old animals. The reasons for the faster relaxation rate in the young SHRs as well as the consistent prolongation of the MRP and exaggeration of aftercontractions in SHRs when compared to age-matched WKYs cannot be precisely determined from this study. In addition, there seems to be no clear correlation between the development of altered met.hanical properties and altered cardiac dynamics or hypertension. At 7 wk, SHRs have been reported to have elevated cardiac function (2, 10) but little or no evidence of hypertension (11, 24). At 12 wk, SHRs have established hypertension, but ventricular performance levels are in dispute. Initial studies by Pfeffer and Frohlich (20) imply that cardiac indices are elevated but later studies indicate that, when taking into consideration the cardiac hypertrophy that has developed, cardiac output per gram of myocardium is normal (22, 25). Reports by Cutilletta et al. (5) and Walsh and Tobia (27) suggest that by this age, cardiac index is now somewhat depressed. At 50 wk, SHRs have chronic hypertension, which is associated with a depressed cardiac function (10, 20). SHR and WKY strains demonstrated similar ageassociated changes in most of the isometric properties of the papillary muscles; the exceptions being a reduction
of dT/dt,,, and MRP with age in the adult WKYs but not SHRs. The present study substantiates previous reports by others who find that in adult rats there are no differences in resting tension or active tension, but a tendency toward prolonged contraction duration associated with aging (12, 15, 16, 28). The youngest group of animals of both SHRs and WKYs had lower resting tensions as well as longer contraction and relaxation times than did the adult animals of these strains. This is in contrast to Hopkins et al, (13) who have reported that after 20 days of age, resting as well as active tension of rat ventricular preparations are not different from adult. The reasons for these differences in findings are not apparent. The second question that initiated this study was whether the differences between mechanical properties of cardiac muscle from SHRs and WKYs are caused by the hypertension or whether they reflect differences between the SHR and WKY strains. The Doca-hypertensive rat, model was chosen to answer this question since the normotensive control preparations are from littermates of the hypertensive preparations, thus eliminating the possibility of genetic differences. Since the Doca-treated hypertensive rats did not demonstrate prolonged mechanical refractory periods or exaggerated aftercontractions when compared to normotensive controls, it is not likely that hypertension per se is responsible for differences between SHRs and WKYs. In this study, a comparison of adult normotensive WKYs and Doca-sham normotensive rats reveals significant differences in isometric twitches measured at Lax, emphasizing the observation that strain-related differences in these properties do exist. Because the preparations from normotensive control animals used for comparison with the two hypertensive preparations are significantly different in so many respects, a comparison of isometric characteristics of cardiac muscle from SHRs and Docatreated animals would be meaningless. The presence of ventricular hypertrophy in SHRs, both before the appearance of hypertension in SHRs, as shown in this study and others (2, 24), as well as when hypertension was prevented by nerve growth factor antiserum (5) or hydralazine treatment (24)) substantiates the suggestion that differences in cardiac properties exist between these strains that are not directly caused by the chronically elevated cardiac afterload. Whether these differences contribute to the development of the hypertension is yet to be established. This work was supported Association and the Minnesota Received
22 August
by grants Medical
1977; accepted
from the Foundation.
in final
form
Minnesota 1 March
Heart 1978.
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H86 6. ELTJS, C. N., AND S. JULIUS. Role of central blood volume in hyperkinetic borderline hypertension. HI-it. Huwt J. 35: 450-455, 1973. 7. FERRARIO, C. M., T. H. PAGE, AND J. W. MCCUBBIN. Increased cardiac output as ti contributory factor in experimental renal hypertension in dogs. Circdcrfirm Ros. 27: 799-810, 1970, 8. FROHLICH, E. D., V. J. Kozu~, R. C. TARAZI, AND H. P. DUSTAN. Physiological comparison of labile and essential hypertension. (I'iwrrltt tiun R PS. 26 and 27, Suppl. 1: 1-55-I-63, 1970. 9. CAVRAS, H., H. R. BKUNNER, J. I-I. LARAGH, E. D. VAUGHAN, JR., M. Koss, L. J. COTE, AND I. GAVRAS. Malignant hypertension resulting from Doca and salt excess. Cimuidion Res. 36: 300-309, 1975. ’ 10. HALLBACH, M., 0. ISAKSSON, AND E. NORESSEN. Consequences of myocardial structural adaptation on left ventricular compliance ;jnd the Frank-Starling relationship in spontaneously hypertensive ruts. A& Phvsic)f . Stwn(-i. 94: 259-270, 1975. 11. HELI~ER, L. J. Mechanical properties of cardiac muscle from spontaneously hypertensive rats: accentuated after-contractions. Prw. SW. Esptl. Hid. Md 154: 479-482, 1977. 12. HELLER, L. J., AND W. V. WHITEHORN. Age-associated alterations in myocardial contractile properties. Am. J. Physiof. 222: 1613-1619, 1972. 13. HOPKINS, S. F., E. P. MCCUTCHEON, AND D. R. WEKSTEIN. Postnatal changes in rat ventricuIar function. Cimddorz Rcs. 32: 685-691, 1973. 14. JULIUS, S., 0. S. RANDALL, M. D. ESLER, T. KASHIMA, C. ELLIS, AND J. BENNETT. Altered cardiac responsiveness and regulation in the normal cardiac output type of borderline hypertension. C’iwrrkdion Rm 36: I-199-1-207, 1975. 15. LAKATTA, E. G., G. GERSTENBLITH, C.S. ANGELL, N. W. SHOCK, AND M. L. WEISFELDT. Diminished inotropic response of aged myocardium to catecholam ines. Ciwrrlcl tim Km a 36: 262-269, 1975. 16. LAKATTA, E. G., cf. GERSTENBLITH, C. S. ANGELL, N. W. SHOCK, AND M. L. WEISFELDT. Prolonged contraction duration in aged myocardium. J. Clir~ . Inwst. 55: 61-68, 1975.
L. J.
HELLER
17. LEDINGHAM, J. M., AND D. PELLING. Cardiac output and peripheral resistance in experimental renal hypertension. Cirwlutian Res. 20 and 21, Suppl. 2: II-187-11-199, 1967. 18. LIMAS, C. J., AND J. N. COHN. Defective calcium transport by cardiac sarcoplasmic reticulum in spontaneously hypertensive rats. Cirwkrtion Res . 40: I-62-1-69, 1977. 19. OKAMOTO, K., AND K. AOKI. Development of a strain of spontaneously hypertensive rats. Jcrpu 12. Circ~ulution J. 27: 282-293, 1963. 20. PFEFFER, M. A., AND E. D. FROHLICH. Hemodynamics and myocardial function in young and old normotensive and spontaneously hypertensive rats. Cirwlatitm Rm. 32: 1-28-I-38, 1973. 21. PFEFFER, M. A., E. D. FROHLICH, J. M. PFEFFER, AND A. K. WEISS. Pathophysiological implications of the increased cardiac output of young spontaneously hypertensive rats. Cirwlation Res. 34: 1-235-I-242, 1974. 22. PFEFFER, M. A., J. M. PFEFFER, AND E. D. FROI~LICH. Pumping ability of the hypertrophying left ventricle of the spontaneously hypertensive rat. Circdrttion Res. 38: 423-429, 1976. 23. REITER, M. Drugs and heart muscle. Ann. Rw Phur-rtzt~w/. 12: 111-124, 1972. 24. SEN, S., R. C. TARAZI, P. A. KHAIRALLAH, AND F. M. BUMPUS. Cardiac hypertrophy in spontaneously hypertensive rats. Cir-wkrtion Km. 35: 775-781, 1974. 25. TADEPALLI, A. S., G. M. WALSH, AND A. J. TOBIA. Normal cardiac output in the conscious young spontaneously hypertensive rat: evidence for higher oxygen utilization. I,& Sk 15: 1113-1114, 1974. 26. TARAZI, R. C., M. M. IBRAHIM, H. P. DUSTAN, AND C. M. FERRARIO. Cardiac factors in hypertension. Ciwdotitm Rus. 34: 1-213-I-221, 1974. 27. WALSH, G. M., AND A. J. TOFHA. Intrinsic left ventricular performance in the young spontaneously hypertensive rat. Rw Cortztmrn. Chun. Puthot. Phcrmmmt. 8: 623-633, 1974. 28. WEISFELDT, M. L., W. A. LOEVEN, AND N. W. SHOCK. Resting and active mechanical properties of trabeculae carneae from aged male rats. Am. J. Physid. 220: 1921-1927, 1971.
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