Preservation of renal function by angiotensin during chronic adrenergic stimulation

Hypertension induced by chronic renal adrenergic stimulation is angiotensin dependent

We designed these studies to assess the role of the renin-angiotensin system in mediating the hypertensive and renal functional effects of chronic renal adrenergic stimulation. Norepinephrine was infused at 0.1 microgram/kg per minute for 7 days directly into the renal artery of uninephrectomized dogs under control conditions (n = 5) or after plasma angiotensin II (Ang II) concentration was fixed at control levels (n = 5) by chronic intravenous infusion of captopril (14 micrograms/kg per minute) and Ang II (0.58 +/- 0.04 ng/kg per minute). During the first 60 minutes of norepinephrine infusion in control dogs, mean arterial pressure increased 9 +/- 4 mm Hg in association with a twofold to threefold rise in plasma renin activity. Additionally, glomerular filtration rate, renal plasma flow, sodium excretion, and fractional sodium excretion decreased to 70 +/- 5%, 64 +/- 5%, 31 +/- 4%, and 38 +/- 6% of control, respectively, while filtration fraction increased 15 +/- 2%. In contrast to the pronounced short-term effects of norepinephrine on renal function, during chronic norepinephrine infusion, all indexes of renal function returned to control levels. However, elevations in both plasma renin activity and mean arterial pressure were sustained and on day 7 were 2.3 +/- 0.6 ng angiotensin I/mL per hour (control, 0.5 +/- 0.1) and 110 +/- 7 mm Hg (control, 90 +/- 3). In dogs with fixed plasma levels of Ang II, acute and chronic changes in renal function induced by norepinephrine were similar to those in control dogs except that acute reductions in glomerular filtration rate tended to be more severe, and changes in filtration fraction and fractional sodium excretion were either attenuated or abolished. Moreover, in the absence of a rise in plasma Ang II concentration, mean arterial pressure did not change either acutely or chronically during norepinephrine infusion. These findings suggest a critical role for Ang II in mediating the hypertension associated with elevated levels of renal adrenergic stimulation that have little or no long-term effect on renal blood flow.

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.