Enhancement of the pressor response to norepinephrine by angiotensin in the conscious rabbit.

The pressor interactions between angiotensin II and norepinephrine were investigated in conscious New Zealand white rabbits receiving a low sodium diet. Angiotensin II was administered continuously by intraperitoneal osmotic pumps in a subpressor dose so as to avoid the potentially confounding effects of experimentally-induced hypertension. Norepinephrine challenges were given as a series of graded intravenous boluses. During the 3 days of study the baseline blood pressure in the angiotensin-treated rabbits (n=10) did not differ from that in controls (n=10) whose intraperitoneal pumps contained only diluent. After 24 hours the systolic and diastolic blood pressure responses to norepinephrine in the angiotensin-treated group were, on average, 45% and 30% higher than in the controls; after 72 hours, they were 46% and 34% higher. Although the pressor amplitudes were increased by angiotensin II, they were not prolonged. Thus, facilitation by the subpressor angiotensin II of the blood pressure responses to norepinephrine did not seem dependent upon alterations in endogenous sympathetic mechanisms or the uptake of norepinephrine; nor could it be explained by sodium retention. It is possible that angiotensin II exhibits its effect by enhancing contractile responsiveness to norepinephrine at the postreceptor level.