Influence of endogenous angiotensin on the renovascular response to norepinephrine

The purpose of this study was to elucidate the role of endogenous angiotensin II in mediating the renovascular effects of renal adrenergic stimulation. Six conscious dogs instrumented for monitoring of renal blood flow were subjected to step increases every 10 minutes in the rate of norepinephrine infusion into the renal artery. Under control conditions, infusion of norepinephrine (10-40 ng/min per milliliter per minute of control renal blood flow) increased plasma renin activity and decreased renal blood flow progressively by approximately 10-75%. When increments in angiotensin II during norepinephrine infusion were abolished by fixing plasma levels of angiotensin II at either normal or high concentrations by chronic infusion of captopril plus angiotensin II, renal blood flow responses to adrenergic stimulation were greatly attenuated at rates of norepinephrine infusion that decreased renal blood flow up to approximately 40% under control conditions. Thus, acutely generated angiotensin II appeared to contribute to the renovascular effects of norepinephrine. However, when endogenous levels of angiotensin II were suppressed to low levels by chronic infusion of captopril alone, norepinephrine induced severe renal ischemia at much lower rates of infusion than occurred when the renin-angiotensin system was intact. Since this enhanced sensitivity to norepinephrine did not occur during chronic captopril infusion when angiotensin II was given simultaneously at rates that restored mean arterial pressure to normotensive levels or higher, low arterial pressure during chronic captopril administration may predispose the kidneys to excessive renal vasoconstriction during renal adrenergic stimulation.