Renal norepinephrine overflow during graded renal nerve stimulation before and after beta-adrenoceptor blockade

These experiments were designed to determine if renal venous norepinephrine (NE) overflow provides a valid index of renal sympathetic nerve activity. In addition, we evaluated the effect of beta-adrenoceptor blockade on renal NE overflow during graded renal nerve stimulation in order to examine the possibility that presynaptic beta-adrenoceptors facilitate neuronal release of NE in the kidney. In 6 pentobarbital-anesthetized dogs, the renal nerves were transected to remove tonic nerve activity and the distal ends were electrically stimulated (8-25 V, 0.5 ms) over the range of 0.3-5.0 Hz for consecutive 4-min periods. NE overflow rate was calculated as the product of the veno-arterial NE concentration difference and renal plasma flow. Control values for NE overflow (-8.7 ± 1.8 ng/min) demonstrated net clearance of NE from the renal circulation. NE overflow rate rose to -3.1 ± 2.2, 1.0 ± 4.0, and 33.2 ± 15.4 ng/min at 0.3, 0.6, and 1.2 Hz stimulation, respectively, with no accompanying change in renal blood flow. At 2.4 and 5.0 Hz, renal blood flow decreased by 21 ± 4% and 37 ± 3%, but there was no further increase in NE overflow rate (38.8 ± 9.4 and 27.8 ± 6.5 ng/min). Propranolol (0.5-1.0 mg · kg^-1 plus 0.4-0.5 mg · kg^-1 · h^-1 i.v., n = 4) did not alter the effect of nerve stimulation on either NE overflow or renal blood flow. Thus we were unable to demonstrate the presence of functional renal presynaptic beta-adrenoceptors. Further, our data indicate that renal norepinephrine overflow rate is not always a reliable index of renal nerve activity, since NE overflow was not proportional to renal nerve stimulation rate at frequencies high enough to cause vasoconstriction.