Angiotensin II enhances norepinephrine spillover during sympathetic activation in conscious rabbits

To investigate the potential modulating influence of angiotensin II (ANG II) on sympathetic activity in response to changes in baroreflex activity, renal and total norepinephrine (NE) spillover rates were examined during sodium nitroprusside (SNP) and phenylephrine (PE) infusions in four groups of conscious rabbits: 1) saline (control); 2) subpressor ANG II (ANG II, 2 ng.kg-1.min-1); 3) enalaprilat (MK-422, 200 micrograms/kg and 3.3 micrograms.kg-1.min-1); and 4) MK plus ANG II (MK+ANG II). Upper plateaus of baroreflex-NE spillover curves for renal and total NE spillover were reduced in the MK group (25 and 81 ng/min) compared with control (38 and 125 ng/min) and MK+ANG II (37 and 155 ng/min). To investigate the interaction of ANG II and sympathetic activity during treadmill exercise, hindlimb NE spillover rate was examined in three groups of rabbits: 1) control, 2) MK, and 3) MK+ANG II. Exercise at 6 and 12 m/min produced similar effort-related hemodynamic responses in the three groups. At maximal exercise, hindlimb NE spillover was reduced in the MK group (29 +/- 3 ng/min) compared with control (62 +/- 17 ng/min, P < 0.05) and MK+ANG II group (51 +/- 10 ng/min). It is concluded that endogenous ANG II enhances sympathetic activity during pharmacological (baroreflex) and physiological stimulation.