Altered hemodynamic responses to acute hypoxemia in spontaneously hypertensive rats GERALD M. WALSH, MASAYUKI TSUCHIYA, A. CHADWICK COX, ALFONSO J. TOBIA, AND EDWARD D. FROHLICH Alton Ochsner Medical Foundation, New Orleans, Louisiana 70121; and Department University c of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73190 TSUCHIYA, A. CHADWICK WALSH, GERALD M., MA~AYUKI Cox, ALFONSO J. TOBIA, AND EDWARD D. FROHLICH. Altered hen&ynamic responses to acute Izypoxemia in spontaneoudy hypertensive ruts. Am. J. Physiol. 234(3): H275-H279, 1978 or Am. J. Physiol.: Heart Circ. Physiol. 3(3): H275-H279, 1978. Conscious spontaneously . hypertensive rats (SHR), 5-7 wk old, were studied hemodynamically by the direct Fick procedure to determine whether high total peripheral resistance 0’PR) coexisted with increased oxygen consumption (Qo:) at an early stage of hypertension development. Since under resting conditions cardiac output in SHR was not significantly different from normotensive controls, the elevated arterial pressure and $0.) were associated with increased TPR. Arterial hypoxemia was induced to reduce oxygen availability and to assess whether increased TPR in SHR could be reversed by this procedure. During hypoxemia, normotensive controls (WKY) responded with increased cardiac output and decreased arterial pressure and TPR. In contrast, arterial pressure and cardiac output fell in SHR; and the increased TPR persisted. 40~ fell in hypoxemic SHR demonstrating that the relationship between total body oxygen consumption and cardiac output was abnormal in young SHR, and that increased TPR in SHR was not dependent on resting levels of Qo, or oxygen availability. Although Qo, was elevated in SHR compared to age-matched WKY, this condition was not essential for maintained elevated vascular resistance.

vascular resistance; oxygen consumption; direct Fick method; hemodynamics of hypertension; cardiac output; Wistar-Kyoto rat strain; arterial pressure; systemic hypertension; 2,3-DPG; hemoglobin oxygen dissociation -. STUDIES in the spontaneously hypertensive rat (SHR) of Okamoto and Aoki have demonstrated normal cardiac output and elevated total peripheral resistance in the young and adult, studied by openchest flowmetry techniques as well as with the direct Fick method (7, 17, 21-23, 28). The latter method, using unanesthetized rats, however, demonstrated an increased oxygen consumption in SHR associated with normal cardiac output and increased oxygen extraction (21). These findings suggested that oxygen availability may be important in SHR for the maintenance of elevated vascular resistance. To test this possibility, young SHR were studied under resting conditions and during hypoxemia to determine whether high total peripheral resistance (TPR) and increased oxygen consumption coexisted early in the onset of hypertension, PREVIOUS

0363-6135/78/0000-OOoo$o1.25

Copyright

c 1978 the American

Physiological

of Medicine,

and whether increased TPR was dependent upon oxygen consumption and availability. METHODS

Male spontaneously hypertensive rats (SHR) of the strain and their normotensive WistarKyoto (WKY) controls employed in this study were obtained from our own colony, originally developed from breeders provided by the National Heart, Lung, and Blood Institute. All mating was conducted by brother-to-sister inbreeding to insure maintenance of 100% prevalence of hypertension. In preliminary studies, total body oxygen consumption was measured in conscious, untreated, and unrestrained rats in a closedcircuit system, using a Warren-Collins spirometer to determine at which age levels elevated oxygen consumption was greatest in SHR. Rats were selected randomly at various ages ranging from 3 to 70 wk. Each age group contained 9-12 rats. Hemodynamic studies. Cardiac output was measured in young, conscious, unrestrained male rats by the direct Fick procedure (30). The WKY were 44 t 1.5 (SE) days old and weighed 125 t 7 g (n = 14); SHR were 43 2 1.4 days old and weighed 111 2 7 g (n = 14). The difference in body weight between SHR and WKY was not statistically significant. In brief, the procedure involved prior insertion of cannulas into the right ventricle and the right carotid artery under light ether anesthesia with exteriorization of the tubing from the posterior neck. Cardiac output was measured 3 h after anesthesia. We have shown in preliminary studies that increased cardiac output and reduced arterial pressure which occur during ether anesthesia return to normal and stable values 1, 2, 3, and 4 h after termination of ether anesthesia. Each rat was placed in a small chamber connected in series with the closed-circuit system used for measurement of oxygen consumption. The cannulas were brought out of the chamber through rubber stoppers. After a 35min stabilization period, oxygen consumption was measured over 1%min intervals during which arterial pressure was recorded continuously on a multichannel direct-writing polygraph. At the end of this period, arterial and venous blood samples were taken (0.2 ml total) and analyzed for oxygen content (30). Afier obtaining these control (base-line) values, the chamber was filled with a 10% O*, c and 90% Okamoto-Aoki

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Hz75

H276

WALSH

N, gas mixture in order to reduce the arterial blood oxygen content. Ambient chamber oxygen was monitored with a Beckman oxygen analyzer and polarographic sensor to insure that oxygen saturation in the chamber remained at 9-10%. Blood pH, 2,3-DPG, and standard bicarbonate. In six SHR and six WKY (5 wk old), arterial blood pH, 2,3-diphosphoglycerate (2,3-DPG), and standard bicarbonate contents were measured under conditions identical to those described for the hemodynamic studies. Blood samples were collected for measurement while rats were conscious, when breathing room air and during hypoxia. Blood pH was measured by an Instru1 1’ . T 1 TT mentatlon Laboratory micro-pn analyzer rrom samples collected and sealed in polyethylene tubes. 2,3DPG was analyzed by a calorimetric (Sigma Chemical Co., kit 665) procedure (12). Standard bicarbonate content was determined with an automatic titrator (Radiometer) (27) from blood samples collected under mineral oil. Base excess was calculated from the following equation (20): 1

(1 - O.l43Hb)(HCO:,-

QO,

where Hb is the blood hemoglobin content and HCO,is the standard bicarbonate concentration. Hemoglobin content was calculated from the content of oxygen in blood samples saturated with 100% 0,. Base excess is a calculation which indicates deviation of base from the norm, so the value 29 was used in the equation because it was the average standard bicarbonate concentration measured in control WKY rats breathing room air. measurement. The oxygen-hemoglobin dissociaL tion curve was calculated for six SHR and six WKY 5-7 wk of age (25). Blood samples (3 ml) were subjected to tonometry at 38°C in 50-ml flasks at various oxygen concentrations (1, 3, 5, 7, and lO%, Matheson) plus 5% CO*, in a balance of N*. The flasks were rotated to facilitate equilibration of the blood with gas mixtures. Statistical analyses were performed using paired or unpaired t-tests where appropriate. RESULTS

The age groups sampled for oxygen consumption were 3-4, 6-8, 17-19, 25-34, and 50-70 wk IFig. 1). The body weights t SE for WKY vs. SHR at these respective ages were 59 * 4 vs. 37 t 2 g, P < 0.01; i82 t 14 vs. 160 2 8, P < 0.05; 372 2 8 vs. 335 t 14, P < 0.05; 432 k 10 vs. 365 t 5, P < 0.01; and 501 t 14 vs. 380 t 7, P < 0.01. Total body oxygen consumption per unit of body weight was elevated in SHR at all ages (P c 0.01, Fig. I), and the greatest absolute diff&ence between SHR and WKY occurred at the youngest ages. Because body weight was reduced in all SHR, absolute oxygen consumption was not elevated above WKY values. The level of hypoxemia induced by exposure to 10% oxygen was similar for WKY and SHR. Arterial oxygen content fell by 40 t 2 and 34 2 3%, respectively, in the WKY and SHR groups (Table 1). Venous oxygen content also fell by the same amount in each group. Venous oxygen content of SHR at both room air and hypoxic conditions was significantly less than corresponding values in WKY.

AL.

mt/min/kg

70

l =WKY o=SHR

50 30

IO

T

T

3-4

T

6-8

(13)

I,

- 29)

ET

I

1799

25-34

(9) (91 Age (wledts>

00)

1

50-70 (91

FIG. I. Total body oxygen consumption (Qo,) in groups ranging from 3 to 70 wk. Values are expressed SE. Number of rats in each age group is in parentheses.

TABLE 1. Blood oxygen conte nt during condition s and during acute hypoxia

__--------~ Arterial

WKY SHR P Values are means values reduced below statistically significant.

18.46

50.21 18.13 20.39 NS

Control

control 10% Oxygen*

-Venous

10.33 kO.38 9.06 20.50 ==0.05

*SE expressed respective control

various age as means IT

Art&al

11.41

20.35 12.26 kO.5 NS as vol %. * All values, P == 0.001;

-Venous

4.77 kO.27 3.79 kO.39 co.05 hypoxia NS, not

Hemodynamic studies. Under resting conditions mean arterial pressure, heart rate, oxygen consumption, and total peripheral resistance (TPR) were elevated significantly in SHR, although cardiac output, arteriovenous oxygen difference (a-v 0, difference), and hematocrit did not differ from WKY values (Fig. 2). Cardiac index was also not significantly different between the two groups (447 t 14 vs. 481 * 21 ml/min.kg; WKY vs. SHR, respectively). The reduction in arterial oxygen content produced a greater fall in arterial pressure in SHR (-12 t 3 vs. -19 2 2 mmHg) although the difference was not statistically significant. However, all other SHR hemodynamic changes during hypoxia were significantly different from those in WKY (Fig. 2): cardiac output increased 7.2 mllmin in WKY but fell 9.0 mllmin in SHR (P < 0.001); TPR fell 0,375 unit in WKY but did not change in SHR (P < 0.025); oxygen consumption decreased 3.7 units in WKY and 8.5 units in SHR (P < 0.01); and a-v 0, difference fell 1.45 ~01% in WKY, but did not change in SHR (P < 0.001). Although heart rate changes during hypoxia (+17 beats/min for WKY and -9 beats min for SHR) were not significant, the differential response between WKY and SHR Was significant, P < 0.05. Hematocrit was not altered in either group. Under these conditions of hypoxemia, cardiac output was lower, and TPR and a-v 0, difference were greater in SHR than in WKY, whereas mean arterial pressure, oxygen consumption, and heart rate were similar for the two groups (Fig. 2). Cardiac index was also lower in hypoxemic SHR than in hypoxemic WKY (397 k 21 vs. 494 t 20 mffmin per kg, P < 0.005). Blood pH, 2,3-DPG, and standard bicarbonate con-

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HYPOXEMIA

AND

HEMODYNAMICS

IN

H277

SHR

centrations. Under base-line conditions, SHR demonstrated normal arterial blood pH, 2,3-DPG content, hemoglobin concentration, and hematocrit; but standard bicarbonate and base excess were lower in SHR compared to WKY (Table 2). During hypoxia, changes in these indices were similar for SHR and WKY; and the differences between SHR and WKY were not eliminated. For both groups arterial pH increased slightly, but significantly, during hypoxia; standard bicarbonate and base excess decreased; and 2,3-DPG concentrations remained unchanged. The fall in mean arterial pressures was similar to that observed in the hemodynamic study. P,,, measurement. Partial pressure of oxygen at which arterial blood was 50% saturated with oxygen at 38”C, and with PCO~ at approximately 33 mmHg was 37.7 t 2.0 and 39.9 2 1.8 mmHg for WKY and SHR, respectively. These averages were not significantly different between groups. DISCUSSION

Young (5-7 wk old) SHR were mildly hypertensive as a result of increased TPR, confirming findings in barbiturate-anesthetized young SHR (1). The SHR also had elevated rates of oxygen consumption per unit of body weight and had metabolic acidosis relative to WKY. Increased oxygen consumption in SHR could not co

ml /min

Hypemia HR b/min

A -VO26 Vol 24

ml /min /kg 50

IO 1

tt I

6

Hypoxemla

Hypoxemia

Hypoxemia

cmtrol

2. Hemodynamic values (mean arterial pressure, MAP; cardiac output, C0; total peripheral resistance, TPR; and heart rate, HR) and metabolic values (oxygen consumption, Qo,, and arteriovenous blood oxygen difference, A-VO,A) in WKY (s+Zed bars> and SHR (halu/wtl bars) before and during hypoxia. Symbols: + and $$, P < 0.05 and 0.01, respectively, SHR vs. WKY; * and **, P < 0.05 and 0.001, respectively, hypoxia vs. room air. FIG.

TABLE

--_

2. Acid-base values under control (C) conditions and during acute hypoxia (10% OJ -P-P--. ------- - ..-----Arterial

pH

2,3-DPG,

----

SHR P -Values

pInoUR

Standard

Hb

1M

__-----. 7.42 ?0.006 7.4ti -co.024 NS are means

-SE.

P

0:

7.48 f 0.008 7 49 z 0.02u NS

-