Angiotensin II-mediated catecholamine release during the pressor response in rats

Angiotensin II (ANG II)-mediated catecholamine release and its possible contribution to the pressor response was assessed in baroreceptor-denervated rats. Neonatal male Sprague-Dawley rats were injected with the sympatholytic drug, guanethidine monosulphate (50 mg/kg s.c., 6 days/week) for 40 days. Plasma catecholamine concentrations were measured using a 3H-radioenzymatic assay as follows: (a) before and 30 s after the injection of saline or ANG II (79.3 pmol/kg i.v.), at the peak of the pressor response, then 50 s and 80 s thereafter, in guanethidine-treated (GUAN) and saline-injected (SHAM) rats, and (b) before and after adrenalectomy (ADX), following the same time-sequence for ANG II as in (a). Peak pressor responses to graded doses of ANG II (6.6, 26.4, 53.0 and 79.3 pmol/kg i.v.) were measured in GUAN+ADX and ADX rats. Destruction of peripheral sympathetic nerves was confirmed by measurements of plasma noradrenaline (NA), adrenaline (AD) and dopamine (DA) concentrations and by changes in pressor responses and heart rates following i.v. doses of tyramine. ANG II induced significantly (P < 0.05) greater pressor responses in GUAN+ADX rats than in ADX rats, especially after the 53.0 and 79.3 pmol/kg doses. Plasma AD concentrations increased within seconds after the pressor response to ANG II in both GUAN and SHAM rats but there was no change in plasma NA or DA concentrations (P < 0.05). ANG-II-mediated AD release from the adrenal medulla may contribute to the overall pressor action of the peptide. The vasculature became more sensitive to ANG II at a time when NA and DA depletion occurred following sympathectomy and/or adrenalectomy. This heightened sensitivity to ANG II was not due to a decrease in circulating ANG II in sympathectomized rats because even though plasma renin activity fell from 6.54 +/- 0.52 to 3.77 +/- 0.26 ng ANG I/ml per h it remained within the normal range.