Shusaku Nagahama

Altered vascular reactivity and baroreflex sensitivity induced by chronic central administration of captopril in the spontaneously hypertensive rat.

Previous studies from our laboratory have shown that chronic intracerebroventricular administration of captopril attenuates the development of hypertension in the spontaneously hypertensive rat of the Okamoto strain (SHR) without altering sodium and water balance, plasma renin, or sympathoadrenal activities. To determine whether the depressor effect of intracerebroventricular captopril was associated with an alteration in peripheral vascular reactivity and/or baroreflex sensitivity, vascular reactivity to phenylephrine and vasopressin was assessed in renal, mesenteric, and hindquarter vascular beds using pulsed Doppler flow probes. Captopril was infused into the jugular vein or lateral ventricle of male SHR and Wistar-Kyoto (WKY) rats for 4 weeks (osmotic mini pump, 1.25 micrograms/0.5 microliter/hr). Control SHR or WKY received intracerebroventricular infusions of vehicle. Four weeks of captopril decreased arterial pressure in both SHR and WKY. In response to phenylephrine and vasopressin, SHR and WKY treated with intracerebroventricular captopril showed significantly attenuated increases in arterial pressure and vascular resistance in comparison to either vehicle-treated rats or rats receiving intravenous captopril. Reflex decreases in heart rate in response to phenylephrine were also greater in SHR and WKY treated with intracerebroventricular captopril than in the other rat groups. Neither vascular reactivity nor baroreflex sensitivity was altered in rats treated with intravenous captopril. We conclude that the depressor effect of captopril involves a blunting of vascular reactivity to vasoconstrictors and a potentiation of the baroreflex.

Attenuation of the Development of Spontaneous Hypertension in Rats by Chronic Central Administration of Captopril

Captopril infused into the lateral ventricle (ICV) of adult spontaneously hypertensive rats (SHR) decreases blood pressure. The current study was designed to explore the effects of brain converting-enzyme inhibition in young animals before the development of established hypertension and to characterize changes induced by captopril in a variety of pressor systems that might be responsible for the development of hypertension in this strain. Captopril (1.25 micrograms/0.5 microliter/hr) was infused into male SHR starting at 7 weeks of age. Four weeks later systolic blood pressure was only 157 +/- 3.3 compared to 181 +/- 3.9 mm Hg in vehicle-infused controls, and the pressor effect of ICV-injected angiotensin I was attenuated by 50%. When the same dose of captopril was infused intravenously, hypertension progressed as in vehicle-treated rats. Serum angiotensin-converting enzyme activity (SACE) and plasma arginine vasopressin (AVP) concentration were significantly higher (p less than 0.001 and 0.05, respectively), in the ICV captopril group than in the ICV vehicle group, while plasma aldosterone concentration and renin activity, fluid intake, urine volume, and urinary sodium excretion were similar in the two groups. Peripheral sympathetic nervous system activity assessed in the resting state was not altered by captopril treatment. In addition, AVP content of the telencephalon, diencephalon, mesencephalon, and pons medulla were not altered by ICV captopril. Renin activity was elevated in the telencephalon of ICV captopril-treated animals but unaltered in the other brain regions examined. These data demonstrate that ICV administration of captopril attenuates the development of hypertension in young SHR by mechanisms apparently independent of altered fluid and sodium balance and the sympathoadrenal system. The effect on blood pressure occurs in the absence of changes in renin activity or AVP content of plasma or those brain regions most often associated with blood pressure control.

Effect of Angiotensin I Converting Enzyme Inhibitor, SQ 14225, on Renin Production and Release

Renin production and release induced by angiotensin I converting enzyme inhibitor (CEI), SQ 14225, were studied in male Wistar rats. Acute studies on the effects of CEI (4.6 mumol/kg, i.m. every 6 h for 48 H) in rats revealed a significant reduction in angiotensin II levels and a marked increase in plasma renin activity (PRA). There were no appreciable changes in renal content (RRC) and the juxtaglomerular cell granulation index (JGI). In rats which had received CEI in their drinking water (0.92 mumol/ml) for 12 days, angiotensin II levels were decreased markedly and urinary excretion of sodium was increased, but the reduction in blood pressure was slight. PRA, JGI and RRC were markedly increased in these animals. The kidneys of CEI-treated rats exhibited conspicuous vascular dilatation. CEI treatment of sodium-depleted rats resulted in a reduction of the increased JGI and RRC and in a further increase in PRA. Our findings suggest that CEI administration may initially effect stimulation of renin release from the juxtaglomerular apparatus and that prolonged CEI administration results in a gradual stimulation of renin production.