Species differences

in cardiac energetics

D. S. LOISELLE AND C. L. GIBBS Department of Physiology, Monash University,

LOISELLE, D. S., AND C. L. GIBBS. Species differences in cardiac energetics. Am. J. Physiol. 237(l): H90-H98, 1979 or Am. J. Physiol.: Heart Circ. Physiol. 6(l): H90-H98, 1979.The energy flux of rat, guinea pig, and cat papillary muscles was measured myothermically under resting, isometric, and isotonic conditions at 27OC. Resting heat rate was highest in the smallest species and declined with body size. The slope of the isometric heat-stress relationship was constant across species, whereas the stress-independent heat component was least for rat muscles. The shape of the load-enthalpy relationship was similar across species. Maximum mechanical efficiency, work-enthalpy, occurred with lighter loads than for skeletal muscle (-0.2 PO). Rat muscle had the smallest enthalpy per beat and the highest active mechanical efficiency, but this advantage was nullified by the higher basal heat rate. The myothermic data are compared with cardiac oxygen consumption values in the literature and it is concluded, contrary to the deductions of common dimensional arguments, that cardiac energy expenditure across species is not directly proportional to heart rate. Reasons for this discrepancy are considered together with the likely contribution of cardiac metabolism (&) to total body metabolism (EB). It seems likely that smaller species have lower EH/EB. cat; rat; guinea pig papillary muscle; myocardial heat production; myothermal technique; length-stress relation; load-enthalpy relations; cardiac resting metabolism

using tetanized papillary muscles from several species, that species variation in rates of cardiac energy expenditure correlates well with biochemical estimates of cardiac actomyosin ATPase activity (19). However, from the point of view of in vivo cardiac metabolism, it is obviously of more interest to have comparative information about the energy cost of single contractions. It has been shown that there are considerable species differences in cardiac mechanics (3)) biochemistry (9,28), and excitation-contraction coupling mechanisms (22); it would be surprising if there were not some differences in their components of cardiac energy output. Indeed, the claim that animals of small species, by virtue of their reduced dimensions, pay a disproportionate cost for cardiac performance seems well entrenched in the literature (21). The argument in support of this, which we refer to as a dimensional argument (because it relates cardiac and whole-body metabolism to heart and whole-body dimensions according to wellestablished phenomenological relations), is as follows. If it is assumed that stroke volume is proportional to heart weight (21, 23) and that all adult mammalian cardiac systems operate at about the same mean arterial pressure (Z), then it follows that the pressure-volume

WE HAVE

H90

RECENTLY

SHOWN,

Clayton,

Victoria

3168, Australia

work of the heart, expressed per beat and per gram of heart tissue, must be constant across species. It also follows that the resting rate of cardiac work production per gram of heart must be proportional to the resting heart rate (HR). If it is further assumed that mechanical efficiency, defined as stroke work to total energy production, is constant across species, then the total cardiac energy consumption (&n) per gram of heart tissue must be proportional to heart rate: I&/g oc HR. Because resting heart rate varies inversely with body size (5, Zl), it follows that the per gram cardiac energy expenditure is expected to be extreme in small species. With these considerations in mind, we have examined the energy production of papillary muscles from three common laboratory species (cat, rat, and guinea pig) under physiological conditions where mechanical output was close to maximal. The data are compared where possible with existing literature values for other species. The results lead us to question some of the common dimensional arguments concerning cardiac energy expenditure and to appraise the likely contribution of cardiac metabolism to total body metabolism in different species at rest. METHODS

Papillary muscles from adult animals of three species, rat, cat, and guinea pig, were studied. Animals were anesthetized with ether and the heart was removed and perfused with warmed (35°C) modified Krebs-Ringer solution of the following millimolar composition: NaCl 118, KC1 4.75, CaClz 2.54, KHaPO* 1.18, MgS04 1.18, NaHC03 24.8, and glucose 10. Insulin, 10 U/l, was added and the solution was aerated with 95% 02-5s CO2 to pH 7.4. Papillary muscles (characteristics given in Table 1) were removed on a small spring under sufficient force to maintain resting length, from the right ventricle of cat and guinea pig hearts and the left ventricle of rat hearts, and mounted on the thermopile as previously described (17). The muscle thermopile arrangement was lowered into a thermostatically controlled bath (17) at a temperature of 27°C and the muscle left to contract isometrically under a small preload of 0.5-1.5 g. This equilibration period (about l-l.5 h) was deemed complete when the extent of shortening under the preload had stabilized. Mechanical measurements. Force and displacement were measured as previously described (17) and displayed on a six-channel Gould Brush model 260 pen recorder. In isotonic experiments, work per beat was calculated as the product of the average displacement and the sum of afterload and preload. The preloads ranged from 0.5 to

0363-6135/79/oooO-0000$01.25

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0 1979 the American

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MAMMALIAN TABLE

CARDIAC

Cat, n = 9 Guinea

pig, n

H91

1. Average muscle characteristics w

Rat, IL = 17

ENERGETIC23

7.6 HI.3

7.7 HI.3 6.0 kU.4

5.5 M.6 6.4 zku.7

2.8 HI.4

A

HL

0.70 wM36 0.84 S3.08 0.47 z!AkU?

13.4 *a7 13.7 321.1 17.1 k1.6

Stress

l/6

6

l/5

20

1

20

41.0 k4.8 47.4 k3.2

52.4 k6.1

= 8

All values are means zk SE. lo, muscle length, mm; zu, muscle weight, mg; A, cross-sectional area of an equivalent cylinder, mm2; HL, rate of heat loss, % s-‘*, f& stimdus frequency, Hz; n, number of stimuli at fo in a train of isotonic contractions; stress, maximum active force; PO,per cross-sectional area, A, mN/mm*. For each preparation POwas averaged over n so values are modified by the force-staircase phenomena.

1.5 g depending on muscle size and tended to be somewhat heavier in rats. Heat measurements. Two thermopiles, of output 3.45 and 3.64 mV/“C, were used routinely, and on several occasions the calibration procedure was corroborated by use of an integrating thermopile. In each case, the total heat recorded -during calibration was at least 92% of that liberated. Thermopile output was amplified by either an Astrodata model 120.nV amplifier or a Keithley model 1400nV amplifier. In either case, amplifier output was passed through a ftiter network (20.Hz cutoff frequency) and the filtered signal was corrected electrically for heat loss as previously described (17,20) and displayed on the pen recorder. The heat output of the muscle (per gram) per beat was calculated by subtracting the stimulus-heat from the total heat recorded in a train of twitches and dividing by the number of twitches in the train. The stimulus heat was determined for each muscle by reducing the stimulus duration, at constant voltage, well beyond the point where the muscle failed to give a mechanical response. The total heat from a large number of such “stimuli” could then be recorded under high gain and the stimulus heat in the standard stimulus train deduced with accuracy. The relative contribution of the stimulus heat varied widely among muscles and among treatment conditions (ranging from negligible to 28%) and averaged 4% of the total heat liberated in a contraction train at lo and 20% of the stress-independent (activation) heat. Stimdation. Stimulation was achieved by use of a Digitimer model 4030 stimulator. Two electrodes, one in the plane of the thermopile and the other cantilevered from the thermopile frame and resting on the surface of the muscle, were arranged to stimulate the muscle transversely. Stimulus pareuneters were adjusted to be barely suprathreshold to minimize the stimulus heat and catecholamine liberation from sympathetic nerve endings. The stimulus frequencies were similar to those employed by Bodem and Sonnenblick (3) and produce close to maximal stress development for the prevailing conditions in the case of rat and guinea pig muscles .A 1040% stress improvement could be achie ved by employing somewhat higher stimulus rates for cat papillary muscles, but as these muscles had the largest cross-sectional area, we considered it prudent to keep the rate of oxygen consumption, greatly influenced by the stimulus rate, below that calculated to be critical for muscles of this

size. The criteria that must be met have recently been considered in some detail (14), Experimental protocol. During the l- to 1.5-h equilibration period, the muscles contracted isotonically under a preload sufficient to prevent passive shortening (M1.5 g)- During the 1st h the initial rate of resting heat production was determined. When the mechanical performance of the muscle had stabilized, the rate of heat loss from the muscle was determined and the experiment begun. It should be emphasized that all active heat measurements were made on mechanically stable preparations. Three relationships were determined for each muscle: resting heat rate vs. time of day, active heat vs. active stress, and enthalpy vs. load. The heat-stress relation was explored by making use of both the frequency-stress relation and the length-stress relation. The muscle length at which stress was maximal, &, was determined and in the first series of experiments the heat liberated at & in response to a series of isometric contractions of various frequencies was measured. In the second series, a wide range of isometric muscle stress was achieved by altering muscle length while keeping the stimulus frequency, fo, constant. The load-enthalpy relation was determined over two series of experiments. In the first, a sequence of afterloads ranging from 0.1 PO to 1.0 PO (isometric force at ZOand fo) was applied to the muscle in either ascending or descending order and the heat liberated in response to a train of contractions recorded. In the final series the loads were applied to the muscle in mirror-image sequence, The muscle was returned to solution for about 30 min between each series of experiments and the resting heat was measured prior to each series, i.e., at about hourly intervals. This protocol ensured that mechanical performance was stable throughout an experiment; in no case did the decline in peak force at & exceed 10% over the 4- to 5.5h experimental period. At the completion of the experiment the muscle underwent electrocution and calibration was achieved by liberating a known amount of heat (capacitor discharge) into the muscle via the stimulating electrodes. The muscle was removed from the thermopile, its length (lo) and diameter (&J were measured by use of a graduated eyepiece, and its blotted weight (w) was determined to the nearest 0.1 mg. The diameter (&) of an equivalent cylinder was calculated according to: & = (4~/aZ#/~. In no case did the difference between & and &, exceed 0.15 mm (maximum relative error: 12.5%). Because all muscle force values were expressed as stress, i.e., force per crosssectional area of an equivalent cylinder of length & and unit specific gravity, this served as a check that the calculated cross-sectional areas were reliable and allowed confidence in retaining data from muscles with unexpectedly high or low stress values. All data from each muscle studied were included in the analysis. STATISTICAL

TREATMENT

Resting heat rate. Because resting heat rate could be measured only at the beginning of each of the experimental seiies rather than at regular intervals, only two

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H92

D. S. LOISELLE

questions were asked: does the resting heat rate decline during the experiment and is there an interspecies difference in resting heat rate? Thus only the initial measurement (between 0.75 and 1.5 h) and the final measurement (between 3 and 5 h) were used in the Multivariate Analysis of Variance (see below). The remaining data are merely presented graphically. Heat-stress relations. For each muscle the data for heat vs. stress as a function of stimulus frequency and heat vs. stress as a function of length were pooled to determine the heat-stress relationship. The pooled data for each muscle were fitted by quadratic regression equations of the form H = a0 + aIS + a2S2 (16, 17). The resulting regression coefficients (ao, al, and a2) were then subjected to a Multivariate Analysis of Variance (27) and mean interspecies differences examined using the methods of Rodger (34). Differences were adjudged significant if P < 0.05. Load-enthal’py relations. Because the afterloads were in general not identical fractions of PO for each muscle, the enthalpy, E, and work, WV, corresponding to the fractional loads (P/PO) 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 were estimated, when necessary, by linear interpolation. The mechanical efficiency, E, defined as the ratio of work performed to enthalpy liberated (i.e., work + active heat production), was calculated for each fractional load. For each quantity, E, W, and E, a Multivariate Analysis of Variance was performed and interspecies differences examined as above. It should be emphasized that this method of analysis, unlike the conventional Randomized Blocks Analysis of Variance, takes into account the inequality of covariances between the variates in a repeated measures design and is thus a more conservative approach for a given number of degrees of freedom.

AND

C. L. GIBBS

on the other hand, tended to decline considerably for the first 2 or 3 min. For purposes of comparison, the values at 4.5 min were measured, and it is these values that are shown in Fig. 2. As can be seen in Fig. 2, the initial (0.75-1.5 h) resting heat rate of the rat was significantly higher than that of either cat or guinea pig and there was no significant difference between the latter two. It seemslikely that the guinea pig resting heat rate is intermediate between that of rat and cat, but additional experiments would be

.

RAT n=13

n=l2

1

n=12

T

T

4

e; J

n=lO

-+-

n=6

; PIG 1=8 7

CAT n=9 \

RESULTS

Figure 1 shows the interspecies variability in resting heat rate response during the 5-min measurement period. On draining the solution from the muscle-thermopile system at the commencement of the measurement period the heat records for all cat and guinea pig muscles reached a plateau value of heat output within about 2 min and this value was maintained. Rat resting heat rate, 50 4.62mWhf GUINEA -“I PIG

RAT

. -

5.15 mw/g

4 min

I

2

3 TIME (hr)

FIG. 2. Resting heat rate (mW/g tissue) of rat, guinea pig, and. cat papillary muscles vs. time (origin representing removal of heart from animal). Measurements taken at unequal intervals and clustered into bins of to.5 h width. Experiments of variable duration so decreasing numbers of observations with time.

FIG. 1. Resting heat rate (mW/g) of a papillary muscle from each species: raw data uncorrected for rate of heat loss. At extreme left muscle is under solution and sawtooth wave form results from passage of carbogen bubbles through solution past muscle/thermopile. Amplitude of sawtooth is proportional to magnitude of resting heat. When solution is drained, temperature of muscle/thermopile rises immediately. For statistical analysis, deflection of trace was read at 4.5 min after draining. Superimposed on rat resting heat trace is effect of a 6-twitch isotonic contraction train at 0.6 PO. Each trace recorded at 0.75 h after removal of heart. Muscles weighed 2.3, 7.6, and 8.2 mg and had lengths of 5.5, 7.0, and 8.5 mm and rates of heat loss of 21.3,14.3, and 12.3%/ s for guinea pig, cat, and rat, respectively.

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MAMMALIAN

CARDIAC

H93

ENERGETICS

needed to make this statement statistically secure. In addition, the resting heat rates of cat and rat (but not guinea pig) showed a significant decline (to about onehalf their initial values) over a period of 4 or 5 h. There was some difficulty in making precise interspecies comparisons because the total duration of experiments was quite variable. This may account for the nonsignificant decline in the guinea pig values over the total experiment. The decline of resting heat over a number of hours has been previously reported in myothermic studies of rabbit cardiac muscle from this laboratory (4) and accords well with K’-arrested whole-heart studies (see DISCUSSION). This decline in resting heat rate occurs while the mechanical performance of the muscle is either still improving or has stabilized. Heat-stress relation. The maximum twitch stress values were inversely related to muscle cross-sectional area as shown in Fig. 3. The guinea pig muscles tended to be considerably smaller in cross-sectional area (see Table 1) and hence developed somewhat higher stresses. The highest individual stress and heat liberation values were found in a rat muscle. This muscle (0.23 mm2) developed 106 mN/mm2 stress and liberated 33.7 mJ/g per twitch. Actual records of the total heat production in a train of isometric twitches at Zo are shown for each species in Fig. 4. Of interest are the force staircase phenomena, positive for cat and guinea pig, but negative for rat, and the rapid evolution of recovery heat at this temperature-essentially complete in 1 min. To optimize mechanical performance for each species, different rates of stimulation (1, 0.2, and 0.16 Hz) and number of twitches comprising the train (20, 20, and 6 for guinea pig, cat, and rat, respectively) were necessary. The complete data from one rat muscle is shown in Fig. 5. Notice the curvilinear nature of the plot and that the stress-independent heat has been measured and not estimated by extrapolation. The interval-stress relation rarely accounted for more than 40% of the stress range and in the rat tended to be considerably less. In all three species a second-order polynomial regression equation adequately describes the heat-stress relation. In particular a~, the estimate of stress-independent heat, is less subject to underestimation than with linear regression.

The coefficients al and a2 must reflect, in a complex way, the efficiency of the underlying energy transduction process; significantly, neither shows interspecies variation. The zero-order coefficient, on the other hand, reflects the magnitude of the activation heat and is lower in the rat than in either the cat or guinea pig. The average regression equations for each species resulting from a parabolic fit to the heat-stress data for each muscle are given in Fig. 6. A Multivariate Analysis of Variance of the regression coefficients shows no difference among species in the coefficients of the first- and second-order terms. However, for the intercepts (or stress-independent heat) the average value (1.6 mJ/g) for rat was significantly less than that for cat or guinea pig, which did not differ from each other (mean value 2.5 mJ/g). Load-enthalpy relations. The average per beat values of work production, active enthalpy consumption, and resulting active efficiency (work/active enthalpy) at specified fractional loads (P/P,) are given in Table 2 and Fig. 7. Note that any given average efficiency value may not correspond to the quotient of average work and average enthalpy because the ratio was formed for each muscle and then averaged (i.e., an average of ratios need not equal the ratio of averages). As has been shown for rabbit papillary muscle (16, 18), the enthalpy consumption per beat per gram of cardiac tissue is a monotonically increasing function of relative load for all three species examined. In addition, the work curve is bell-shaped and peaks at about 0.4 PO for each species. The mechanical efficiency vs. relative load curve is skewed to the left, yielding peak values in the 0.2-0.4 PO range. Because both the work and efficiency curves peak in the vicinity of 0.4 PO, it is convenient to compare the species at this point. As one of us has argued elsewhere (13), it seems likely that the peak wall stress at the completion of the isovolumic phase of the cardiac contraction cycle corresponds roughly to the stress produced by lifting a 0.4 PO load. At this point of the efficiency vs. relative load relationship (see Fig. 7)) the active mechanical efficiency of rat muscle is maximally divergent from that of cat or guinea pig. For each of work, enthalpy, and efficiency, the Multivariate Analysis of Variance showed no significant differences between cat and guinea pig values, but each were

FIG. 3. Linear regression of maximum active stress (mN/mm”) vs. papillary muscle cross-sectional area (mm2). Regression equations: S = 83 - 65A, S = 63 - 19A, and S = 69 - 40A; correlation coefficients: r = -0.74, -0.48, and -0.50; standard errors of regres. sion: s,., = 12.7, 9.0, and 17.8 for guinea pig, cat, and rat, respectively.

0

I

I

I

0.2

0.4

0.6

CROSS-SECTIONAL

I

0.8 AREA

li0

I

I

1.2

14

(mm2)

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H94

D. S. LOISELLE

HEAT 5OmJlg

[ J

' GUINEA- PIG 20 @ 3~OHZ

C. L. GIBBS

FIG. 4. Total heat (mJ/g muscle) and twitch stress (mN/mm2) in a contraction train of 20, 20, or 6 isometric twitches at & with stimulus frequencies of 1.0, 0.2, and 0.16 Hz for guinea pig (1.2 mg, 6.7 mm), cat (3.9 mg, 6.5 mm), and rat (12.6 mg, 9.5 mm) papillary muscles, respectively .

1 I

AND

I

300 TIME (set) CAT 20 @ 0.2Hz

RAT 6 @ 0.16Hz

HEAT - STRESS

.O 1 Hz a 0

8-

00

-G 6: E

lo

o ‘2OHz 0

2 !e4

0 0

0

0

0

I 10

I 20 STRESS

I 1 30 40 (mN/mm2)

1 50

FIG. 5. Twitch heat (mJ/g) vs. isometric stress (mN/mm’) for a single rat papillary muscle. Fitled cirdes: variation in stimulation frequency from 0.1 to 2.0 Hz (peak stress occurred at 0.16 Hz); open circles: variation in muscle length from Zo (8.5 mm) to Zo - 2.5 mm (length where active stress = 0). Muscle wt = 7.2 mg. Quadratic regression equation: H = 1.3 + 0.046s + 0.0030S2.

significantly different from rat. Thus rat papillary muscles performed less work per beat, consumed less enthalpy per beat, and were more efficient than either cat or guinea pig. It should be stressed that in no case did a test of difference at a particular fractional load level prove significant. Although overall interspecies differences were found, they were not simply related to load. Examination of Fig. 7 or Table 2 shows PO enthalpy values of 12, 11, and 9 mJ/g per beat for guinea pig, cat, and rat, respectively, and approximately 10, 9, and 6.5 mJ/g at the 0.4 POload level. The difference between the cat and guinea pig values is slight and a choice of a higher stimulus rate for cat might have further narrowed the discrepancy. The load-enthalpy relation at 1.0 PO (isometric case) may also be examined by the isometric heat coefficient (IHC), which is a measure of the economy of such a

I I

I

0

10

1

I

I

20 30 40 STRESS (mN/mm2)

1

50

1

60

6. Average quadratic regression equations relating heat or en‘g) per twitch and stress (mN/mm2) (varied by alterin ,g both muscle length and stimulus frequency): H = 2.91 + 0.142s + 0.0009Sz (n = 7), H = 2.16 + 0.133s + 0.0013S2 (n = 9), and H = 1.59 + 0.094s + 0.0018S2 (n = l7), for guinea pig, cat, and rat, respectively. No difference in slopes; stress-independent heat significantly less for rat. FIG.

ergy bJ/

contraction. If POis given in force units, then IHC = P&/ Ho. However, throughout this study POis given in stress (force/area) units so IHC = PO/&. Using POdata from Table 1 and Ho data from Table 2, the respective IHC values (mean t SE) for guinea pig, cat, and rat were 4.44 t 0.38, 4.41 t 0.24, and 4.78 t 0.21. There are no significant differences among these values. DISCUSSION

Basal metabolism. A number of facets of the rate of resting heat production are worthy of comment. The fall in resting heat rate with time is particularly striking and similar effects have been reported in whole-heart experiments. Lochner et al. (25) reported a decline in resting oxygen consumption from 3.3 to 1.5 ml 02/100 gemin from 10 to 90 min after K+ arrest in rat hearts at 24OC. These values correspond to 11 and 5 mW/g and show remarkable agreement with the values for rat presented

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MAMMALIAN TABLE

CARDIAC

2. Average

ENERGETICS

coefficients

H95 and fractional

load val-

ues 12.

Isometric Heat-stress* Guinea pig Cat Rat

2% 2.16 1.59

o.z2 0.133 0.093

o.z90 0.00129 0.00197

10. I

RAT

Is0 tonic P/PO

0.1

0.2

0.4

0.6

0.8

Work-Load? Guinea pig Cat Rat

0.74 0.62 0.55

1.40 1.20 1.04

1.92 1.60 1.47

1.88 1.53 1.29

1.24 0.91 0.64

Enthalpy-load? Guinea pig Cat Rat

6.0 4.9 3.3

7.8 6.5 4.5

10.1 8.7 6.4

11.5 9.7 7.5

12.0 10.4 8.1

Efficiency-load* Guinea pig Cat Rat

11.6 12.4 16.1

17.5 18.2 22.4

18.4 18.1 22.1

15.6 15.4 17.0

9.4 9.0 7.7

1.0

12.4 10.8 8.9

* H = a0 + a$3 + a#; a for rat significantly less than cat or guinea tWork and active enthalpy in mJ/g; rat significantly less than Pk. #Active efficiency % = work/active enthalpy; rat cat or guinea pig. significantly more than cat or guinea pig.

in Fig. 2. Similarly, Penpargkul and Scheuer (33) reported a decline from 0.38 to 0.15 ml 02/g dry wt mminfrom 5 to 60 min after K+ arrest in rat hearts at 37°C. These values correspond to 25 and 10 mW/g, respectively. The initial value of resting heat rate for cats (2 mW/g) is somewhat lower than that reported in oxygen consumption studies at 29OC. Thus Coleman et al. (6) report the equivalent of 2.4 mW/g, but it is not clear how soon these measurements were made and it must be remembered that the earliest heat measurements reported in the present study were at about 1 h. The final values of resting heat rate for cat and guinea pig are similar to those reported for rabbit (1.85 mW/g) at 20-22°C with glucose as substrate (4) and are less than for the cat. It is clear that rat papillary muscle has a high basal metabolism. This accords with the suggestion of Nayler et al. (28), supported by the study of Penpargkul et al. (32), that the rat SR has poor Ca2+-retaining properties and as a consequence Ca2+ continues to cycle between the SR and the sarcoplasm even in the absence of activation. Another unusual aspect of the rat cardiac resting heat production is its decline throughout the 5-min measurement period (Fig. 1). It is difficult to believe that this is related to anoxia as rat papillary muscle seems particularly able to withstand lengthy periods out of solution without a decrement in mechanical performance. On the other hand, the resting heat rate (either the initial or final value) is negatively correlated with muscle crosssectional area (r = -0.6, s~.~= 1.2 mW/g, n = 17). This finding is open to the explanation that muscles of larger diameter have an anoxic core in our experimental arrangement (muscle out of solution during heat measurements). The fact that active muscle stress is likewise negatively correlated with muscle cross-sectional area (see Fig. 3) might be seen as further support for this idea. However, this is true both in vitro, for muscles contract-

0.2

0.4 l.OAD

0.6

0.8

1.0

('/P,)

per beat active enthalpy (active heat + work, mJ/ and active mechanical efficiency (work/active enthalpy, W) vs. relative load for guinea pig (n = 8), cat (n = 9), and rat (n = 17) papillary muscles. Note doubling of work scale. FIG.

7. Average

g), work (mJ/ g ),

ing under solution (36)) and in situ, for muscles being perfused normally (10). Furthermore, the basal heat rate can be doubled by altering substrate (4); so equally plausible explanations of the latter phenomenon are a better mechanical arrangement of muscle fibers inserting into the tendon or a higher contractile protein-to-mitochondria ratio in thin muscles (14). Heat-stress relation. In comparing the stress values with those reported in the literature, it must be remembered that, unlike studies where purely mechanical measurements are made, there are several factors, recently discussed by Gibbs and Loiselle (19), which act to lower the values obtained in myothermic studies. It is therefore encouraging to note the agreement of our mean POvalues (52.4 t 6.1,47.4 t 3.2, and 41.0 -+ 4.8 mN/mm2 for guinea pig, cat, and rat, respectively, Fig. 6) with steady-state values reported by Bodem and Sonnenblick (3) for muscles under solution: 58.9 and 40.7 mN/mm2 for cat and rat papillary muscles, respectively, at 30°C. It is worth noting that these peak stress values are considerably in excess of those measured midwall during ventricular systole. Most estimates of canine and human peak wall stress are in the 15-30 mN/mm2 range; the data have been reviewed recently (14). Although wall stress is the major physiological determinant of the energy output per beat it must be emphasized that there are other important components to the energy balance sheet and that the enthalpy-load, rather than enthalpystress curves, are more valuable in predicting likely in

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D. S. LOISELLE

H96 vivo rates of energy expenditure. It is reassuring, however, that a load of about 0.4 PO yields a value of papillary muscle stress that closely corresponds to that calculated using the formula proposed by Mirsky (26) to approximate the physiological case; e.g., for rats 0.4 PO = 16 mN/ mm2. There is no difference among species in the slopes of the heat-stress relations, suggesting similar underlying chemomechanical transduction efficiencies. However, the stress-independent or activation heat, which we believe to largely reflect the release and sequestration of Ca2’ from the SR, and which is represented by a0 in the heat-stress relation, is significantly less for rats (see Fig. 6 and Table 2). Thus the metabolic cost of a given amount of stress will be less for rat by the difference in activation heats. If the metabolic cost comparison is made at PO, however, and expressed as the IHC, then in spite of the significantly reduced activation heat found in rat papillary muscle, the IHC is not significantly different among the three species. From values for ao (activation heat) and Ho given in Table 2, it may be seen that the mean a&& ratios are 23, 20, and 18% for guinea pig, cat, and rat, respectively. These ratios are likewise not significantly different despite differences in CQ because the activation heat difference is masked by variation in the larger stress-dependent heat fraction. Load-enthalpy relation. As shown in Fig. 7, rat heart has a slightly higher active mechanical efficiency. However, the efficiency difference is not large and would become insignificant in vivo if the higher resting heat value were taken into account in any total efficiency calculation. Before attempting to make predictions about whole-heart oxygen consumption usage, it is reasonable to check the reliability of the myothermic data. The experimental precautions taken are outlined in METHODS, and in Table 3 the myothermic data are compared to in vivo whole-heart oxygen measurements. Unfortunately, there are few cross-species oxygen consumption studies on isolated papillary muscles. The only literature values, obtained under comparable conditions and with unambiguous use of units, have been obtained with cats. The oxygen consumption values have been read at PO and TABLE

3. Cardiovascular 2

1 Species

Rat Guinea Rabbit Cat Dog Man

MB,

pig

and total body metabolic

kg

0.3 0.6 2.0 2.5 15.0 70.0

MH,

3 g

1.1 2.5 4.8 11.5 120 370

HR, min-’

320 280 200 170 120 70

4 Relative HR

4.6 4.0 2.9 2.4 1.7 1.0

5 Basal @H, mW/g

25 12 8 8 8 7

35 47 30 27 23 23

C. L. GIBBS

converted to mJ/g making use of the calorific equivalent of oxygen usage in cardiac muscle: 20 kJ/l 02 (8). Representative literature values are: 8 mJ/g (35), 14 mJ/g (7), and 16 mJ/g (6). Thus our mean isometric value of 11 mJ/g seems reasonable; indeed, in terms of the arguments developed later, higher values would only strengthen our conclusions. Relation to whole-heart studies. If the ordinate of the load-enthalpy relation is resealed in units of oxygen consumption, VOW, then the average per beat Vo2 at 0.4 PO is seen to be 0.32 ~102/g for rats. It is interesting to compare this value with data reported by Gamble et al. (11, 12) for the isolated spontaneously beating (200-360/min) blood-perfused rat heart at 37°C. These studies report a linear relationship between the per beat Vo2 and left ventricular systolic pressure, LVSP, such that at 100 mmHg the rat heart consumes about 0.5 ,ul 02/g (in the 1970 paper the value is 0.47 and in the 1973 paper it is 0.55). This figure includes resting 02 usage, whereas the myothermically derived value (0.32) does not. The resting heat contribution on a per beat basis is not negligible even at rates near 300/min. Selecting the Penpargkul and Scheuer (33) initial (5-min) value of rat resting heat rate (25 mW/g) and expressing it on a per beat basis, using a HR of 320/min, yields a value of 0.23 ~102/g (see column 10, Table 3). Thus the active energy cost would be about 0.27 ,ul 02/g per beat, which is in fair agreement with the myothermic value of 0.32 obtained at the lower temperature. It should be emphasized that for comparable stress development, temperature has little effect on the per beat enthalpy value (16, 20). Agreement is also good with the Neely et al. (29, 30) working heart preparation value of about 2.3 mM 02/g dry wt h. (The value is 2.2 in the 1967 paper, Table 3, and 2.4 in the 1973 paper, Fig. 4). When this value, the equivalent of 0.54 ~1 02/g per beat, is corrected for the resting heat contribution using either their estimated value of 0.7 mM/g dry wt. h or the higher value used above (1.0 mM), the resulting active oxygen consumption per beat (320/min) ranges from 0.30-0.37 ~102/g per beat (column 10, Table 3) . Dimensional analysis of cardiac-energy expenditure. The commonly accepted interspecies dimensional argul

data for different 6 Active E?IH, mWYg

AND

7 @H,

mW/g

60 59 38 35 31 30

species

8 Equivalent EH,

ml

02/

100 g - min

17.6 17.3 11.4 10.3 9.3 8.9

9 Relative, EH

1.98 1.94 1.28 1.16 1.04 1.00

10 Literaturt2,

$02/g

-

11 Relative, EB

0.23-0.37 (0.32) (0.5) 0.25-0.5 (0.45) 0.3-0.78 (0.45) 0.5-0.7 0.8-l. 1

3.9 3.3 2.4 2.3 1.5 1.0

12 tiHI&

3.6 4.7 2.4 4.4 10.4 9.6

1) Body wt values for rat, cat, guinea pig and rabbit are similar to those used experimentally. Dog and man values are estimates. 2) Heart wt estimated from heart wt/body wt (see text). 3) Values are estimates from Biological Handbooks. 5) Basal myocardial metabolism rat, guinea pig, rabbit, and cat values are based on myothermic and oxygen consumption data, &lo = 2, zero time (see text). Dog and man values estimated from whole heart oxygen consumption data (11, 15). 6) Active myocardial metabolism, product HR x 0.4 PO enthalpy value. See text for rat, guinea pig, rabbit, and cat values. Dog and man values are based on determination of per beat stroke work and assumption of 20% active mechanical efficiency. 7) Addition cohmns 5 and 6. 8) Oxygen equivalent of cohmn 7 assuming 1 1 02 liberates 20.3 kJ. 9) Cohmn 8 data normalized with respect to man. 10) Refs. for literature values. Rat and dog, (11); rabbit: (8); cat: (6); man: (15). Nos. in parentheses are equivalent values reported in this study. II) Total body metabolism per g of tissue with respect to man, calculated from Kleiber relation (EB 0~ iW$.75). 12) Myocardial oxygen usage calculated from MS, cohmn 2 and coZumn 8 data assuming LV mass = 0.7 1MH and LV work = 7 x RV work. Total body resting oxygen usage from Kleiber relation: Vo, = 0.057 iUBo-75 (31).

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MAMMALIAN

CARDIAC

ENERGETICS

ment, which concludes that both resting stroke work and resting stroke energy usage (per gram of cardiac tissue) are proportional to heart rate, was outlined in the introduction. This argument rested on three assumptions: a) mean arterial blood pressure is constant across species; b) stroke volume is directly proportional to heart weight; and c) cardiac mechanical efficiency is constant across species. Now the whole-heart studies of Gamble et al. (11, 12) have shown that the rat heart consumes only about 55% more 02 per gram per minute than the dog heart despite a two- to threefold difference in heart rate. This result is in agreement with the present myothermic data when enthalpy values and species heart rates are used to calculate $H as shown in Table 3, columns 7-9. Literature values for other species support the conclusion that the rate of cardiac energy usage is not linearly related to heart rate. Thus, in spite of uncertainty in selecting appropriate values of species body weight, resting heart rates, and basal cardiac energy rates, such calculations make it clear that rates of total cardiac energy consumption are quite comparable across species differing in size by as much as dogs and rats. This result clearly conflicts with the common dimensional argument that hearts from small species must expend energy at very much higher rates than hearts from large species (21) . The narrowing of the oxygen usage range across species must mean that some of the above assumptions are i.ncorrect. On the basis of the present data we would suggest that smaller animals have a lower work and energy output per gram per beat (they may also have slightly higher active mechanical efficiencies). The myothermic data and the literature oxygen consumption data completely rule out the possibility that the basal, i.e., nonbeating, rate of energy usage is high in large species and low in small species. Indeed the reverse is obviously true. Cardiac metabolism as a fraction of whole-body metaboZism. The question now arises as to the interspecies constancy of the fraction of whole-animal basal metabolism provided by cardiac metabolism. Dimensional analysis, as outlined above, predicts that the rate of cardiac energy expenditure, per gram of heart tissue, is proportional to heart rate: &/g 0~ HR. But heart weight (MH) is a constant fraction of body weight (MB) across species: MH 0~ MB’*’ (Kleiber (24) gives the exponent as 1.0, Adolph (1) as 0.95, and Grande and Taylor (21) as 0.9); and heart rate is inversely proportional to body weight: HR oc Mg-o-25 (Clarke (5) gives the exponent as -0.27); so i!& cc Mgo75. Because this is identical to the wellknown Kleiber relation (24) between body weight and basal metabolic rate (Z&), J!& 0~ &O-75, it follows that &&!?~, i.e., the proportion of total body basal metabolism contributed by cardiac metabolism, is expected to be constant across species. In man the heart is assumed to consume about 10% of the resting whole-body oxygen uptake (the literature has been reviewed recently by Gibbs and Chapman) (15). Is this fraction higher in smaller animals? From the Gamble et al. (12) figure of 14.3 ml/100 gamin (see their Fig. 8) or the 17.6 ml/100 gemin estimate (see Table 3) plus corrections for the differing energy output of the left and right ventricle (see footnote to column 12, Table 3, a l.l-

H97 g heart would have an oxygen consumption between 0.11 and 0.14 ml/min. If one estimates the resting whole-body oxygen consumption according to the Kleiber relation as given by Pasquis et al. (31) (Vo2 = 0.057 Mg”*75) and uses the value 4.1 ml/min for a 300-g rat, the cardiac contribution becomes a surprisingly low 3.6% or only about one- ‘third of the accepted value for humans. Indeed, even this value would be more than halved if the measured average basal oxygen consumption of rats reported by Pasquis et al. (31) were used. One of the reasons this result occurs is that rats tend to have a lower heart weight-to-body weight ratio (3.8 g/kg) than man (5.3 g/ kg) (see Table 1, Grande and Taylor (21)) contrary to the general rule that all species have the same relative cardiac mass. Support for this line of argument comes from the comparison made by Gambl .e et al. (11 ) with their data for rats and data for dogs taken from the literature (see their Fig. 10). At 100 mmHg LVSP the mean cardiac oxygen consumption for dogs was 9.1 ml/100 gemin or about 11 ml/min for a 120-g heart. Now dogs have one of the highest heart weight-to-body weight ratios, a value of 8 being realistic (21). From the Kleiber relation, a dog weighing 15 kg would be expected to have a basal total body metabolism of 77 ml Oz/min, of which the cardiac contribution would be about 10% (see column 12, Table 3). It appears, therefore, that although both the cardiac oxygen consumption per gram of cardiac tissue and the basal total body metabolism per gram of body weight are about twice as high in rats as in dogs, the relative contribution of cardiac metabolism to whole-body basal metabolism is about 3 times less in rats even though the relative heart weight ratios differ by only a factor of about 2 (3.8 vs. 8). Now the relative heart weights (i.e., heart weight (g)/ body weight (kg)) of the other species (domestic rabbit, guinea pig, and domestic cat) studied in this laboratory are also low: 2.4, 4.2, and 4.6, respectively (21). But even if rats, cats, rabbits, and guinea pigs had the mammalian average heart weight-to-body weight ratio of 5-6 the argument would not be substantially changed. It thus appears th .at the heart makes a disproportionately small contribution to the relatively high resting whole-body metabolism of small animals. This can be seen more clearly with reference to column 12 of Table 3 where data, both from this study and from the literature, have been used to support the contention that the ratio of myocardial to total body basal metabolism is species (size) dependent. It is seen in Table 3 that across species, from rats to man, the heart rate decreases about fivefold, whereas the minute cardiac oxygen usage per unit weight decreases only by a factor of about 2. This discrepancy seems to reflect 1) a continual fall in basal cardiac metabolism as species weight increases; 2) an approximately threefold increment in the cost per beat of cardiac contractions (6.5-20 mJ h8 across the species range; and 3) the variation in heart weight-to-body weight ratio, the more active species tending to have higher ratios. - With these facts in mind it would be surprising if there were not distinct morphological, biochemical, and mechanical differences in cardiac tissue among species. Such

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H98

D. S. LOISELLE

differences do exist, their myothermic consequences being clearly shown in tetanized cardiac preparations (19) . This

work

was supported

by grants-in-aid

from

the Australian

Re-

search

Grants

Committee

and the National

AND

Heart

C. L. GIBBS

Foundation

of Aus-

tralia* Received

24 April

1978; accepted

in final

form

20 February

1979.

REFERENCES 1. ADOLPH, E. F. Quantitative relations in the physiological constitution of mammals. Science 109: 579-585, 1949. 2. ALTMAN, P. L. AND D. S. DITTMER (editors). Biology Data Book (2nd ed.). Bethesda, MD; FASEB, 1974, vol. III. 3. BODEM, R., AND E. H. SONNENBLICK. Mechanical activity of mammalian heart muscle: variable onset, species differences, and the effect of caffeine. Am. J. PhysioZ. 228: 250-261, 1975. 4. CHAPMAN, J. B., AND C. L. GIBBS. The effect of metabolic substrate on mechanical activity and heat production in papillary muscle.

Cardiovasc. Res. 8: 656-667,1974.

5. CLARKE, A. J. Comparative Physiology of the Heart. New York: Cambridge, 1927. 6. COLEMAN, H. N., E. H. SONNENBLICK, AND E. BRAUNWALD. Mechanism of norepinephrine-induced stimulation of myocardial oxygen consumption. Am. J. Physiol. 221: 778-783, 1971. 7. COOPER, G., IV, R. M. SATAVA, JR., C. E. HARRISON, AND H. N. COLEMAN III. Mechanisms for the abnormal energetics of pressureinduced hypertrophy of cat myocardium. Circ. Res. 33: 213-223, 1973. 8. COULSON, R. L. Energetics of isovolumic contractions of the isolated rabbit heart. J. Physiol. London 260: 45-53, 1976. 9. DELCAYRE, C., AND B. SWYNGHEDAUW. A comparative study of heart myosin ATPase and light subunits from different species.

Pfluegers Arch. 355: 39-47, 1975. 10. FISHER, V. J., AND F. KAVALER. Maximal force development by hypertropied right ventricular papillary muscles remaining in situ. In: Cardiac Hypertrophy, edited by N. R. Alpert. New York: Academic, 1971, p. 371-385. 11. GAMBLE, W. J., P. A. CONN, A. EDALJI KUMAR, R. PLENGE, AND R. G. MONROE. Myocardial oxygen consumption of blood-perfused, isolated, supported rat heart. Am. J. Physiol. 219: 604-612, 1970. 12. GAMBLE, W. J., C. PHORNPHUTKAL, A. EDALJI KUMAR, G. L. SANDERS, F. J. MANASEK, AND R. G. MONROE. Ventricular performance, coronary flow, and Mv02 in aortic coarctation hypertrophy. Am. J. Physiol. 224: 877-883, 1973. 13. GIBBS, C. L. Cardiac energetics. In: The MammaLian Myocardium, edited by G. A. Langer and A. J. Brady. New York: Wiley, 1974, p. 105-133. 14. GIBBS, C. L. Cardiac energetics. Physiol. Rev. 58: 174-254, 1978. 15. GIBBS, C. L., AND J. B. CHAPMAN. Cardiac energetics. In: Handbook of Physiology. CardiovascuZar System. Bethesda, MD: Am. Physiol. Sot., 1979, sect. 2, vol. 1, chapt. 22, p. 775-804. 16. GIBBS, C. L., AND W. R. GIBSON. Effect of ouabain on the energy output of rabbit cardiac muscle. Circ. Res. 24: 951-967, 1969. 17. GIBBS, C. L., AND W. R. GIBSON. Effect of alteration in the stimulus rate upon energy output, tension development and tension-time integral of cardiac muscle in rabbits. Circ. Res. 28: 611-618, 1970. 18. GIBBS, C. L., AND W. R. GIBSON. Isoprenaline, propranolol and the energy output of rabbit cardiac muscle. Cardiovasc. Res. 6: 508515,1972. 19. GIBBS, C. L., AND D. S. LOISELLE. The energy output of tetanized cardiac muscle: species differences. Pfluegers Arch. 373: 31-39,

1978. 20. GIBBS, C. L., AND P. VAUGHAN. The effect of calcium depletion upon the tension-independent component of cardiac heat production. J. Gen. Physiol. 52: 532-549, 1968. 21. GRANDE, F., AND H. L. TAYLOR. Adaptive changes in the heart, vessels, and patterns of control under chronically high loads. In: Handbook of PhysioZogy. CircuZation. Bethesda, MD: Am. Physiol. Sot., 1965, sect. 2, vol. III, chapt. 74, p. 2615-2677. 22. HAJDU, S. Mechanism of the Woodworth phenomenon in heart and skeletal muscle. Am. J. Physiol. 216: 206-214, 1969. 23. HILL, A. V. The dimensions of animals and their muscular dynamics. Proc. R. Inst. GB 34: 450-471, 1950. 24. KLEIBER, M. Body size and metabolic rate. Physiol. Rev. 27: 511541,1947. 25. LOCHNER, W., G. ARNOLD, AND E. R. MULLER-RUCHHOLTZ. Metabolism of the artificially arrested heart and of the gas-perfused heart. Am. J. CardioZ. 22: 299-311, 1968. 26. MIRSKY, I. Review of various theories for the evaluation of left ventricular wall stress. In: Cardiac Mechanics, edited by I. Mirsky, D. Ghista, and H. Sandler. New York: Wiley-Interscience, 1974, p. 381-409. 27. MORRISON, D. F. MuLtivariate StatisticaL Methods (2nd ed.). New York: McGraw, 1976. 28. NAYLER, W. G., J. DUNNETT, AND W. BURIAN. Further observations on species-determined differences in the calcium-accumulating activity of cardiac microsomal fractions. J. Mol. CeZZ.Cardiol. 7: 663-675, 1975. 29. NEELY, J. R., H. LIEBERMEISTER, E. G. BATTERSBY, AND H. E. MORGAN. Effect of pressure development on oxygen consumption by isolated rat heart. Am. J. Physiol. 212: 804-814, 1967. 30. NEELY, J. R., M. J. ROVETTO, J. T. WHITMER, AND H. E. MORGAN. Effects of ischemia on function and metabolism of the isolated working rat heart. Am. J. PhysioZ. 225: 651-658,1973. 31. PASQUIS, P., A. LACAISSE, AND P. DEJOURS. Maximal oxygen uptake in four species of small mammals. Respir. PhysioZ. 9: 298309, 1970. 32. PENPARGKUL, S., D. I. REPKE, A. M. KATZ, AND J. SCHEUER. Effect of physical training on calcium transport by rat cardiac sarcoplasmic reticulum. Circ. Res. 40: 134-137, 1977. 33. PENPARGKUL, S., AND J. SCHEUER. Metabolic comparisons between hearts arrested by calcium deprivation or potassium excess. Am. J. Physiol. 217: 1405-1412, 1969. 34. RODGER, R. S. The number of non-zero, post hoc contrasts from ANOVA and error rate I. Br. J. Math. Stat. Psychol. 28: 71-78, 1975. 35. SKELTON, 6. L., H. N. COLEMAN, K. WILDENTHAL, AND E. BRAUNWALD. Augmentation of myocardial oxygen consumption in hyperthyroid cats. Circ. Res. 27: 301-309, 1970. 36. SNOW, T. R., AND P. B. BRESSLER. Oxygen sufficiency in working rabbit papillary muscle at 25°C. J. MOL. CeZZCardioZ. 9: 595-604, 1977.

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