Effects of blockade of brain angiotensin II receptors in conscious, sodium-deprived dogs

Effect of intravertebral angiotensin II on cardiac output and its distribution in conscious dogs

Intravertebral infusion of angiotensin II (Ang II) increases mean arterial pressure (MAP), but the contribution of cardiac output (CO) and total peripheral resistance (TPR) to this increase is unclear. In the present study, the effects of Ang II infusion on CO and regional blood flow was determined by the microsphere technique in eight conscious, chronically catheterized dogs. Ang II was infused into both vertebral arteries at 0.33 and 1.0 ng/kg/min, and intravenously at 0.66, 2.0 and 5.0 ng/kg/min. Intravertebral infusion of Ang II at 0.33 ng/kg/min increased MAP by increasing CO without changing TPR or peripheral plasma Ang II concentration. MAP also was increased with intravertebral infusion of Ang II at 1.0 ng/kg/min, but this resulted from small increases in both CO and TPR. In contrast, intravenous infusion of Ang II at 2.0 and 5.0 ng/kg/min increased MAP by increasing TPR in association with a decrease in CO. The increase in CO with intravertebral infusion of Ang II at 0.33 ng/kg/min was distributed primarily to the muscles, kidneys, heart, and brain. Intravenous infusion of Ang II at 5.0 ng/kg/min and, to a lesser extent, 2.0 ng/kg/min decreased blood flow to the skin, splanchnic region, and kidneys. These data indicate that the increase in MAP produced by a low intravertebral dose of Ang II results from an increase in CO, which is distributed primarily to the muscle, kidney, heart, and brain. In contrast, the increase in MAP produced by a higher intravertebral dose of Ang II results from increases in CO and TPR. This latter action is apparently due to a peripheral action of Ang II to increase resistance in the skin, splanchnic, and renal circulations.

Interaction between angiotensin II and the baroreceptor reflex in the control of adrenocorticotropic hormone secretion and heart rate in conscious dogs

The present studies were designed to examine the effect of angiotensin II on baroreflex control of adrenocorticotropic hormone secretion and heart rate in conscious dogs. Baroreflex function was assessed by examining the relationship between blood pressure and plasma corticosteroid concentration (used as an index of adrenocorticotropic hormone secretion) and between blood pressure and pulse interval (the inverse of heart rate). Blood pressure was varied by intravenous infusion of four doses (0.3, 0.6, 1.5, and 3.0 micrograms/kg per min) of the vasodilator nitroprusside. Nitroprusside infusion produced an increase in plasma corticosteroid concentration and a decrease in pulse interval, both of which were linearly related to the fall in arterial blood pressure. During infusion of angiotensin II (10 ng/kg per min), however, the lines relating blood pressure to plasma corticosteroid concentration or blood pressure to pulse interval were shifted to a higher pressure level, suggesting that angiotensin II resets the baroreceptor reflex. Angiotensin II blockade with saralasin, an angiotensin II antagonist, or with captopril, a converting enzyme inhibitor, in sodium-depleted dogs with elevated plasma angiotensin II levels produced a shift in the opposite direction in these relationships. Because baroreflex function in sodium-depleted dogs before angiotensin II blockade was similar to that in sodium-replete dogs, despite the increased plasma angiotensin II levels, and because treatment of sodium-replete dogs with saralasin did not affect baroreflex function, these results suggest that endogenous angiotensin II is necessary for the maintenance of normal baroreflex control of adrenocorticotropic hormone secretion and heart rate during sodium depletion. Previous studies designed to evaluate the physiological significance of angiotensin II in the control of adrenocorticotropic hormone secretion showed that high, perhaps supraphysiological, levels of exogenous angiotensin II are required to increase adrenocorticotropic hormone secretion. An additional finding of the present study was that exogenous angiotensin II produces a larger increase in adrenocorticotropic hormone secretion when the pressor effect of angiotensin II was eliminated with simultaneous infusion of the vasodilators nitroprusside or hydralazine. This result suggests that experiments that evaluate the physiological role of angiotensin II in the control of adrenocorticotropic hormone secretion with infusions of exogenous AII may underestimate the importance of endogenously produced angiotensin II, which is normally released without an increase in pressure.