Blood pressure and plasma renin activity responses to chronic stress in the borderline hypertensive rat

Physiology & Behavior, Vol. 32, pp. 101-105.Copyright ©PergamonPress Ltd., 1984.Printedin the U.S.A.

0031-9384/84$3.00 + .00

Blood Pressure and Plasma Renin Activity Responses to Chronic Stress in the Borderline Hypertensive R a t a J A M E S E. L A W L E R , 2 G R E G O R Y F. B A R K E R , J O H N W. H U B B A R D , R O N A L D H. C O X 3 A N D G A R Y W. R A N D A L L

Psychophysiology Laboratory o f the Department o f Psychology and Department o f Zoology University o f Tennessee, Knoxville, TN 37996 R e c e i v e d 5 July 1983 LAWLER, J. E., G. F. BARKER, J. W. HUBBARD, R. H. COX AND G. W. RANDALL. Blood pressure and plasma renin activity responses to chronic stress in the borderline hypertensive rat. PHYSIOL BEHAV 32(1) 101-105, 1984.--The purpose of this study was to examine the systolic blood pressure and plasma renin activity (PRA) responses to chronic stress in normotensive rats and in rats with one hypertensive parent. Twenty-four male Wistar-Kyoto (WKY) and 24 male Ft offspring of spontaneously hypertensive and WKY rats (BHR) were randomly assigned to 3 groups of 8 each. Experimental (E) animals were subjected to 2 hr daily of shock-shock conflict. Each response produced a 0.2 sec, 0.2-0.4 mA cutaneous electric shock. Failure to respond in 10 sec resulted in a train of 5 shocks (0.2 sec each sec). Yoked animals (Y) received the same shocks as E but had no control over their presentation. Finally, a control group (C) for maturation received no shocks. The E and Y animals were subjected to 14 weeks of conflict and were then monitored an additional 14 weeks in the absence of shock. All animals had their tail cuff blood pressures taken weekly except for 3 times when bloods were obtained for PRA assays. Analysis of blood pressure data revealed that: (1) BHR animals showed more of a blood pressure response to shock than WKY animals; (2) Y animals showed more of a response to conflict than E, especially for the BHR group; and (3) BHR shocked animals remained permanently elevated compared to BHR control animals even in the 14 week post-conflict period during which no shocks were given. Although PRAs for BHR animals were significantly higher than for WKY at the beginning of study, the stress-induced hypertension was associated with either normal or suppressed PRA values, suggesting that the hypertension in these animals is not a high renin hypertension. Borderline hypertensive rat Hypertension Shock-shock conflict Yoked control group

Blood pressure

THE spontaneously hypertensive rat (SHR) has been studied extensively as a potential model for human essential hypertension [17,20]. While many studies utilizing SHRs have sought to elucidate the mechanisms of this hypertension [3,20], relatively few have addressed the role of environmental influences. This reflects one of the drawbacks for examining such influences in this strain: the SHR becomes hypertensive at an early age [18] in the absence of any unusual environmental manipulations. Although environmental influences have been shown to produce a slight aggravation [6] or diminution [10] in the development of hypertension, the SHR is, in either case, hypertensive. An animal which had a genetic history of hypertension, but became hypertensive only following exposure to the appropriate set of environmental inputs, might in the long run be more relevant to human essential hypertension than the SHR. In preliminary studies [12,13], we have demonstrated that the male F1 offspring of SHR and Wistar-Kyoto rats

Plasma renin activity

(WKY) have mildly elevated blood pressure, which does not develop spontaneously into frank hypertension. We have designated this animal model the borderline hypertensive rat (BHR). The BHR has a resting systolic blood pressure (SBP) between 140-160 mm Hg. While studies utilizing environmental stressors have generally failed to produce permanent, frank hypertension in normal animals [4, 7, 8], we have found that such stressors can permanently elevate the blood pressure of the BHR [14]. Given these initial findings, the present study sought, first and foremost, to replicate the hypertensinogenic response of the BHR to an environmental stressor. To establish the reliability of the BHR's response by independent replication of previous results would suggest to us that the BHR has the potential to be a unique animal model of neurogenic hypertension which does not require an artificial intervention (chemical or electrical) into the central nervous system. The second goal of the present study was to compare the

tResearch supported by NIH grant HL-19680 and by an NIH Biomedical Support Grant (7088). Portions of this paper were presented at the 65th Annual Meeting of the Federation of American Societies for Experimental Biology, Atlanta, GA, April 12-17, 1981. 2Research Investigator of the American Heart Association, Tennessee Affiliate. Requests for reprints should be addressed to J. E. Lawler, c/o the Department of Psychology. 3Supported by post-doctoral fellowship HL-06036 from the USPHS.

101

102

I,AWLER. BARKER, HUBBARD, C()X AN[) RAN1)AI.i

SBP response of the BHR and WKY. We sought to determine whether or not the BHR starts out with a higher SBP, shows more of a SBP response to an environmental stressor, and remains elevated longer following the termination of stress compared to WKY. A third goal was to compare the SBP response of two groups differing in the degree of control over, and predictability of, the stressor. In the present study, one group of animals could predict, by their failure to make a response (wheel turn), that multiple, cutaneous electric shocks would occur. They also had some degree of control over shock presentation, since one response produced only one shock and failure to respond produced five. A second group of animals, the "'yoked control" group, could neither predict nor control shock. They merely received the same shocks as experimental animals, independent of their own responses. We have previously suggested that these yoked animals may show more of ~. SBP elevation to this type of shock schedule than experimental animals [12]. We based this hypothesis on studies which have shown the importance of psychological variables such as predictability and controllability in the production of stomach ulceration [22]. The present study allows a direct comparison of the SBP response in experimental and yoked animals. Finally, the fourth goal of this study was to determine if plasma renin activity (PRA) is altered in rats subjected to a long-term environmental stressor. PRA has been shown to be altered in at least some types of experimental [16, 19, 21] and human hypertension [11]. METHOD

Twenty-four male BHR (F, offspring of female SHR and male WKY rats), and 24 WKY males were used in this study. Both parent groups were purchased from Taconic Farms, and tail cuff SBP measurements were used to verify that SHRs and WKYs were indeed hypertensive and normotensive, respectively. All BHR and WKY offspring were born in our Animal Facility within 3 days of each other. Blood Pressure Determinations

Beginning at 11 weeks of age, tail-cuff systolic blood pressures (SBP) were determined once weekly in all 48 animals (except weeks 15, 35, and 48, when bloods were drawn), until the animals were 49 weeks of age. Rats were placed in a heated restraint (baseplate temperature, 37°C) for approximately 10 minutes before the onset of BP determinations (Narco Biosystems). The mean of seven artifact-free determinations served as an index of SBP for the week. Weeks 13-17 served as baseline SBP weeks. Stress Procedure

When the rats were 17 weeks of age, six groups of 8 rats each were formed with random assignment within each strain. There were four groups exposed to psychological stress (BHR-E, BHR-Y, WKY-E, and WKY-Y) and two non-shock maturation control groups (BHR-C and WKY-C). The experimental (E) animals were trained in two phases to rotate a 7.6 cm diameter wheel one quarter of a turn. In the first phase, lasting 3 weeks, a 1 kHz, 75 dB tone sounded each 10 sec for 5 sec. I f a response was not made during this 10 sec interval, the E animal received 5 brief (0.2 sec each sec), low current (0.2-0.4 mA) cutaneous electric shocks. Any response during the 10 sec interval terminated the tone

and postponed its onset by 5 sec. l)uring this phase. ] animals could avoid receiving any shocks by responding duving the 10 sec interval. In the second phase (shock-shock conflict), however. each response made produced one 0.2-0.4 mA cutaneous shock, varying randomly in duration from 0-200 msec. Failure to respond produced 5 pulsed shocks just as in the first phase. All animals learned to avoid multiple shocks during the first week of the second phase. More details about the behavioral responses to this protocol are fot, nd in l,awler ct al. [12]. Yoked (Y) animals received the same intensity, duration and distribution of electric shock as E animals. However. responses of Y animals had no effect on tone or shock presentations. Control (C) animals were not subjected to the shock paradigm. Shocked animals were studied 2 hours daily, 5 days per week for a 3 week training period and for 14 weeks of conflict. There was a subsequent 14 week. post-conflict recovery period during which no animals were exposed to the conflict paradigm. Tail cuff blood pressures were taken once weekly in all animals, at least two hours following conflict in E and Y animals. Tail cuffs in C animals were taken at the same time of day as groups E and Y. to control for circadian rhythm effects. Plasma Assays

Blood was withdrawn at weeks 15, 35, and 48 tbr analysis of PRA. Plasma levels of sodium (Na') and potassium (K ~/ were also determined at week 15. Blood was obtained using a tail clip procedure similar to that of Bagby et al. [2]. Animals were placed in the same heated restraints at the same time of week during which tail cuffs were normally taken. After a 10 minute warm-up period, approximately I cm of tail was rapidly clipped with a scalpel, and approximately 1.5 ml of blood was obtained in a tube containing 2.7 mg EGTA and 1.7 mg glutathione. The sample was usually obtained within 15 sec, but never took longer than 30 sec. All assays were performed in duplicate for each subject's plasma sample. As Bagby et al. have noted, the tail clip procedure is mildly stressful, and values obtained cannot be considered representative of true resting levels. PRA was determined by a modification of the technique of Haber et al. [5] using 100 /zl of plasma, as previously described [9]. The intraassay coefficient of variation was less than 7%. Plasma Na ~ and K ~ concentrations were determined by flame photometry. RESULTS

B a s e l i n e D~fJ~,rences in B H R a n d W K Y

Between 13-15 weeks of age, and before any experimental treatment was initiated, tail cuff blood pressures and hormone levels of the 24 WKYs were compared to those of the 24 BHRs. These data are summarized in Table 1. Statistical analyses (t-tests) revealed that 13-15 week old BHRs had significantly higher SBP, heart rate (based on 15 sec strips obtained from the tail-cuff plethysmograph), and PRAs, and significantly lower plasma sodium, than comparably aged WKYs. However, there were no strain differences in plasma potassium levels. B l o o d Pressure

An analysis of variance was performed on the tail-cuff SBPs in the six groups of animals, using groups and weeks as

103

BP AND PRA RESPONSES TO STRESS 200

TABLE 1 BLOODPRESSURE, HEARTRATE, HORMONEAND ELECTROLYTE DIFFERENCES BETWEENBHR AND WKY RATS AT BASELINE (13-15 WEEKSOF AGE)

CONFLICT 190

BHR-Y

180

SBP

HR

PRA

NA÷

K÷

E

BHR

MN SEM N

WKY MN SEM N t p

..

~'

-

BHR-I:

17o

E

157.0 1.24 24

340.7 3.90 24

19.93 1.76 21

139.6 1.03 23

5.109 0.139 22

a.

16o

OHR-C

~ ~

WKY-Y

I--O9

134.4 0.98 24 14.24 0.0001

318.0 4.98 24 3.58 0.0008

12.40 1.24 22 3.53 0.001

146.5 1.41 23 3.97 0.0003

5.255 0.090 22

~ 14c

WKY-E

~ ~9o

WKY-C

hi

0.88 ns

ilo I 13-14

SBP=systolic blood pressure (mm Hg); PRA=plasma renin activity (ng Al/ml/hr; HR=heart rate (beats/min); NA÷=sodium {mEq/1); K÷=potassium {mEq/l).

! 16-17

I 10-20

I I I 21-22 ~1-24 ~ 1 - ~

I z'r-l~

I 29-~)

I I 31-~. 3 ~ k - ~

WEEKS

FIG. I. Mean (_+SEM) systolic blood pressure values for experimental (E), yoked (Y), and control (C) borderline hypertensive (BHR) and normotensive Wistar-Kyoto rats (WKY) during baseline (PRE), training on avoidance (TRNG) and exposure to conflict. factors. Analysis revealed a significant effects of Groups (F=77.38, p<0.0001), and Weeks (F=29.62, p<0.0001), and a significant interaction of Groups × Weeks (F=4.29, p<0.001). This interaction is depicted in Figs. l and 2 for the conflict and post-conflict weeks, respectively. Inspection of Fig. 1 would suggest the following: (l) yoked animals tend to show more of a blood pressure elevation to conflict than experimental animals; (2) this effect appears to be more prominent in BHR animals; (3) conflict animals tend to have higher pressures than control animals; and (4) this effect appears more prominent in the BHRs. Statistical assessment of these observations utilized Tukey A post-hoc comparisons with a pooled error term, as suggested by Winer [23]. A significance level of p<0.05 was chosen for all post-hoc comparisons. During the PRE and training (TRNG) periods, the only differences observed were that all three BHR groups had higher SBPs than the three WKY groups. The WKY-Y animals had significantly higher pressures than the WKY-C group only during the first two (21-22) and last two (33-34) weeks of conflict. WKY-E animals never differed significantly from WKY-C rats throughout the conflict period. This is in sharp contrast to the BHR data. The BHR-Y group had significantly higher SBP than the BHR-C group throughout the entire period of conflict. During the first 4 weeks of conflict, BHR-Y animals had SBPs of 180 mm Hg compared to 163 mm Hg in BHR-C. During the last 4 weeks of conflict, BHR-Y animals had SBPs of 184 mm Hg, compared to 158 mm Hg in BHR-C. Again in contrast to the WKY animals, the BHR-E rats had significantly higher SBPs than BHR-C from weeks 27-28 throughout the remainder of conflict. During the first 4 weeks of conflict, the difference between the SBP of BHR-E and BHR-C rats was only 8 mm Hg. During the last four weeks of conflict, this difference was 18.5 mm Hg. The significant differences from weeks 27-28 throughout the remainder of conflict appear accountable by both a rise in the pressures of BHR-E and a drop in pressures of BHR-C. Figure 2 depicts the mean SBPs for the six groups in the

190

POST

CONFLICT

19O

~--::-:~--. :~ 17o E E

16o

_~

__z_

~

--~

--~. . . . . . ~ . . . . . . ~- . . . . _Q..."-~( ~ - ,

~____~__~.

"'~ ...~

150

Z

W

"~9~..~

120

w~-c

IIO I 36

I 37-345

I 39-40

I 41-42

I 43-44

I 45-46

I 47-48

WEEKS

FIG. 2. Mean (-+SEM) systolic blood pressure values for same groups depicted in Fig. 1 during a 14 week post-conflict recovery period.

post-conflict period. Visual inspection suggests two conclusions: (I) conflict animals (especially BHR) tend to maintain an elevated SBP compared to their control groups; and (2) all groups show a tendency to reduce SBP across weeks. Statistical analyses of the post-conflict data utilized Tukey A post-hoc comparisons (p<0.05). For the BHR rats, group Y showed the least trend for a decrease in SBP. Comparing all weeks with each other in this group, the only weeks which showed significantly different SBPs were Week 36 vs. Weeks 47-48. For BHR animals, the SBP for Week 36 was significantly higher than for weeks

t04

[.AWLER, BARKER, HUBBARD. COX AND RANDAI.I.

OY Ic WKY

oi g (=

DISCUSSION

BHR

50

o. 40

30

=o

10

I

WKY-Y animals had significantly lower PRA values than WKY-C (t=2.58, p<0.02; t=2.88, p<0.007, respectively). There were no significant differences in BHR animals during recovery. It is also worth noting that the significant differences between BHR and WKY animals during pre-stress were absent throughout the remainder of study. That is, there were no significant differences between WKY-C and BHR-C during either post-stress or recovery.

I

I

Pre-Stress

I

I

1

Post-Stress

1

I

|

Recovery

FIG. 3. Mean (_+SEM) plasma renin activity (PRA) for same groups depicted in Fig. I during baseline, following 14 weeks exposure to conflict, and following a 14 week recovery period.

41-48. Weeks 37-38 were significantly higher than weeks 45-48. For BHR-C animals, Week 36 was significantly higher than Weeks 43--48. Again, the data for WKY groups were less clear-cut. For the E group, the only comparison which was significantly different was between Weeks 37-38 and Weeks 47-48. For the Y group, the SBP for Week 36 was significantly higher than that for Weeks 45-48. In addition, Weeks 41-42 were significantly different from Weeks 47-48. Finally, for the C group, Weeks 36-40 were significantly different from Weeks 45-46. Plasma Renin Activity

Figure 3 depicts the PRA values for E, Y and C animals of the two strains for the three times when blood was drawn: at 15 weeks of age (pre-stress), following 14 weeks of conflict (post-stress), and following 14 additional weeks of recovery from conflict (recovery). During the pre-stress period, t-tests revealed that there were no differences within each strain. This merely reflects the random assignment to groups. The significant difference between strains has already been discussed (Table 1). During the post-stress period, WKY-Y animals had significantly lower PRA values than WKY-C (t=2.42, p<0.03), and marginally lower values than WKY-E (t = 1.69, p <0.10). Although BHR animals showed the same general response as WKYs, none of the comparisons yielded significant differences. Finally, during the recovery period, both WKY-E and

With regard to SBP changes, the present study basically replicated that of Lawler el al. [14]. BHRs subjected to chronic conflict showed significant elevations in tail-cuff SBP compared to control animals. During a recovery period, the conflict animals maintained their elevation relative to the control subjects. In addition, the present study demonstrated that: (1) yoked animals show more of a SBP elevation than experimental animals; (2) WKY animals are not as reactive as BHR; and (3) effects of conflict on the chronic SBP of WKY are not permanent. However, there are some differences in results between Lawler et al. [14] and the current study. In the former study, SBPs at 13-14 weeks of age were about 152 mm Hg, compared to 157 mm Hg in the present study. Many more animals in the present study had pressures between 160-170 mm Hg. In fact, of the 56 male offspring used in this and 2 unrelated studies run concurrently, 22 had pressures between 160-169 mm Hg, and 3 had pressures between 170-175 mm Hg. It is with some misgiving, therefore, that we continue to call these animals BHRs. The slightly higher mean SBPs in the current study meant that several animals had pressures outside the range many consider borderline hypertensive. However, we will maintain the label, since it is both more descriptive than F, and since we will in the future attempt to breed enough animals so that none outside the range of 140-160 mm Hg will be included for study. A second difference between the present study and that of Lawler et al. [14] is that blood pressures did not increase as much in the present study (c. 177 mm Hg in BHR-E vs. c. 187 mm Hg in the previous study). This may be partly due to a maturational factor: animals in the present study were three weeks older at the start of the experiment. There may be a critical period during which environmental influences have their greatest effect, Nevertheless, the elevations in the present study were still quite large and statistically significant. Finally, recovery SBPs of BHR-E animals did not show any tendency to decrease in our previous study, compared to a marked reduction in the present study. While this is certainly of concern, it is important to note that the BHR-Es and Y animals kept the same magnitude difference compared to BHR-C throughout the recovery period. While it is possible that this represents an adaptation to restraint in all groups, it is difficult to believe it would take so many weeks to develop. In addition, both control groups also decreased pressure across weeks, an effect we have not seen in several previous (and subsequent) studies. One possible source ot this general decrease across weeks in all animals is the bulb which senses the pulse distal to the occlusion cuff. We have subsequently noted that old bulbs, which do not appear worn, give us lower pressure readings on the same animal than new bulbs. It is possible that the gradual SBP decrease in all groups across weeks represents

BP A N D PRA R E S P O N S E S TO STRESS the gradual deterioration of the bulb. The same bulbs were used throughout the study. Unfortunately, we no longer have those particular bulbs, so we cannot say for certain whether that is the source of the lower pressure. However, we have no other tentative explanation for the reduction in pressures of all groups. The procedure utilized to obtain blood for PRA assays was the same as that detailed by Bagby et al. [2]. Our values compare favorably with theirs (WKY=24 ng AI/ml/hr; SHR=40 ng/AI/ml/hr; their Fig. 4), and with those of others using mild restraint or shock stress [16,19]. However, these values cannot be construed as a measurement of true basal PRA, as this is often below 10 ng AI/ml/hr ([12], and unpublished observations). What it does yield are values of PRAs obtained under nearly the same conditions as the tail cuff blood pressures. The most interesting aspect of these PRA data are the differences between E and Y groups despite little or no difference in blood pressure. That is, both Y groups had lower PRA values at the post-stress measurement (following 14 weeks of conflict) than the C groups. Thus, it remains possible that elevated blood pressure is produced through different mechanisms in animals with or without control over shock. We are not aware of any more definitive data relating PRA with blood pressure and degree of control over the aversive stimulus, so this possibility is merely speculative at this point. The case would have been stronger if the PRA

105 values had been obtained from indwelling catheters while animals were resting in their home cage. However, this procedure was not utilized since we had no reason to suspect that E and Y groups would differ in PRA. We mention this point only because of the significance it would have if it could be more definitively demonstrated that different stressors produce hypertension through different physiological mechanisms. Indirectly related to this are data which show that elevated blood pressure in animals in a pre-avoidance period is caused by an elevated total peripheral resistance [t], while during avoidance it is cardiac output which causes increased pressure [15]. In conclusion, we have replicated our original finding of conflict-induced hypertension in BHRs, and have additionally shown that yoked animals show more of a blood pressure response than experimental animals. Furthermore, the current study has demonstrated that W K Y animals similarly stressed do not become hypertensive, although the yoked-experimental trends are quite similar to what we saw with BHRs. We have found yoked-experimental differences in PRA, which need further study. Finally, we have no evidence of high PRA in the BHR model of conflict-induced hypertension. ACKNOWLEDGEMENTS We gratefully acknowledge the technical assistance of Vivian H. Bachuss, and the helpful comments of Arthur J. Vander, MD.

REFERENCES 1. Anderson, D. E. and J. G. Tosheff. Cardiac output and total 11. Laragh, J. H., L. Baer, H. R. Brunner, F. R. Buhler, J. E. peripheral resistance changes during pre-avoidance periods in Sealey and E. D. Vaughan. Renin, angiotensin and aldosterone the dog. J Appl Physiol 34: 650-654, 1973. system in pathogenesis and management of hypertensive vascu2. Bagby, S. P., W. J. McDonald and R. D. Mass. Serial reninlar disease. Am J Med 52: 633-652, 1972. angiotensin studies in spontaneously hypertensive and Wistar12. Lawler, J. E., G. F. Barker, J. W. Hubbard and M. T. Allen. Kyoto rats. Transition from normal-to-high-renin status during The effects of conflict on tonic levels of blood pressure in the the established phase of spontaneous hypertension. Hypertengenetically borderline hypertensive rat. Psychophysiology 17: sion 1: 347-354, 1979. 363-~70. 1980. 3. Barnes, K. L., K. B. Brosnihan and C. M. Ferrario. Animal 13. Lawler, J. E., G. F. Barker, J. W. Hubbard and R. G. Schaub. models, hypertension, and central nervous mechanisms. Mayo Pathophysiological changes associated with stress-induced Clin Proc 52: 387-390, 1977. hypertension in the borderline hypertensive rat. Clin Sci 59: 4. Findley, J. D., J. V. Brady, W. W. Robinson and W. J. Gilliam. 307~310s. 1980. Continuous cardiovascular monitoring in the baboon during 14. Lawler, J. E., G. F. Barker, J. W. Hubbard and R. G. Schaub. long-term behavioral performance. Commun Behav Biol 6: Effects of stress on blood pressure and cardiac pathology in rats 49-58, 1971. with borderline hypertension. Hypertension 3: 496-505, 1981. 5. Haber, E., T. Koerner, L. B. Page, B. Kliman and A. Pernode. 15. Lawler, J. E., P. A. Obrist and K. A. Lawler. Cardiovascular Application of a radioimmunoassay for angiotensin I to the function during pre-avoidance, avoidance and post-avoidance in physiologic measurements of plasma renin activity in normal dogs. Psychophysiology 12: 4-11, 1975. human subjects. J Clin Endocrinol Metab 29: 1349-1355, 1969. 16. Leenen, F. H. and A. P. Shapiro. Effect of intermittent electric 6. Hallback, M. and B. Folkow. Cardiovascular responses to acute shock on plasma renin activity in rats. Proc Soc Exp Biol Med mental 'stress' in spontaneously hypertensive rats. Acta Physiol 146: 534-538, 1974. Scand 90: 684-698, 1974. 17. Okamoto, K. and K. Aoki. Development of a strain of spon7. Harris, A. H., W. J. Gilliam, J. D. Findley and J. V. Brady. genesis and Complications. Berlin: Springer-Verlag, 1972. Instrumental conditioning of large magnitude daily, 12-hour 18. Okamoto, K. and K. Aoki. Development of a strain of sponblood pressure elevations in the baboon. Science 182: 175-177, taneously hypertensive rats. Jpn Circ J 27: 282-293, 1963. 1973. 19. Sigg, E. B., K. L. Keim and T. D. Sigg. On the mechanism of 8. Herd, J. A., W. H. Morse, R. T. Kelleher and L. G. Jones. renin release by restraining stress in rats. Pharmacol Biochem Arterial hypertension in the squirrel monkey during behavioral Behav 8: 47-50, 1978. experiments. Am J Physiol 217: 24-29, 1969. 20. Trippodo, N. C. and E. D. Frohlich. Similarities of genetic 9. Hubbard, J. W., R. H. Cox, J. E. Lawler, M. L. Blank and F. (spontaneous) hypertension: Man and rat. Circ"Res 48:309-319, Snyder. Antihypertensive effects of 1-hexadecyl-2-acetyl-sn1981. glycero-3-phosphocholine on plasma renin activity and cate21. Vander, A. J., J. P. Henry, P. M. Stephens, L. L. Kay and D. cholamine responses in spontaneously hypertensive rats. Life R. Mouw. Plasma renin activity in psychological hypertension Sci 32: 221-232, 1983. of CBA mice. Circ Res 42: 496-502, 1978. 10. Lais, L. T., R. A. Bhatnagar and M. J. Brody. Inhibition by 22. Weiss, J. M. Effects of punishing the coping response (conflict) dark adaptation of the progress of hypertension in the spontaneon stress pathology in rats. J Comp Physiol Psychol 77: 14-21, ously hypertensive rat (SHR). Circ Res 34-35: Suppl 1, 1971. 155-160, 1974. 23. Winer, B. J. Statistical Principles in Experimental Design. New York: McGraw-Hill, 1971.