Interactions between intrarenal epinephrine receptors and the renal baroreceptor in the control of PRA in conscious dogs.

We investigated the relationship between plasma renin activity (PRA) and renal arterial pressure (RAP) during (1) control conditions, (2) intravenous epinephrine infusion, and (3) intrarenal epinephrine infusion. In five uninephrectomized conscious dogs maintained on a low-salt diet, we controlled the RAP with an inflatable constricting cuff around the renal artery. (1) The control stimulus-response curve of the renal baroreceptor consists of a relatively flat section (slope = −0.03 ng Al/ml/hr per mm Hg) at RAP's close to control pressure and a much steeper section (slope = −0.48 ng Al/ml/hr per mm Hg) at RAP's below a threshold pressure of approximately 75 mm Hg. (2) Intravenous infusion of epinephrine at a dose that yields physiological plasma concentrations shifts the stimulus-response curve to the right by 10–15 mm Hg without altering either slope. (3) Intrarenal epinephrine infusion also displaces the stimulus-response curve to the right; intrarenal and intravenous epinephrine infusions that produce similar renal arterial epinephrine concentrations shift the response curve by the same amount. (4) However, as previously reported, intravenous administration of epinephrine decreases mean aortic pressure (MAP) (93 to 88 mm Hg, P < 0.05) and increases PRA (1.1 to 1.6 ng Al/ml per hr, P < 0.05), while intrarenal epinephrine has no effect on either MAP or PRA. Taken together, these results demonstrate that both infusions act intrarenally, increasing the threshold pressure, but that only intravenous epinephrine has an additional systemic effect, lowering MAP and, therefore, RAP. The combination of an increase in threshold pressure and a decrease in RAP causes the RAP to fall below the threshold pressure, raising PRA. These observations explain why PRA increases during intravenous but not intrarenal infusion.