ROLE OF NITRIC-OXIDE AND ANGIOTENSIN-II IN THE REGULATION OF SYMPATHETIC-NERVE ACTIVITY IN SPONTANEOUSLY HYPERTENSIVE RATS

This study evaluated the actions of nitric oxide on the blood pressure and renal sympathetic nerve activity responses produced by angiotensin II (Ang II) blockade in conscious spontaneously hypertensive rats. Two days after implantation of electrodes, we measured mean arterial pressure, heart rate, and renal sympathetic nerve activity. Baroreceptor reflex function was assessed with a logistic function curve; the maximum slope of the curve estimated the baroreceptor reflex gain. Data were obtained in rats given acute intravenous administration of either vehicle, the Ang II type I receptor antagonist losartan, the type 2 antagonist CGP 42112A, or the converting enzyme inhibitor lisinopril. In comparison with vehicle (-1.1+/-0.2%/mm Hg), both losartan (-1.8+/-0.3%/mm Hg) and lisinopril (-2.4+/-0.2%/mm Hg) significantly increased the maximum gain of the baroreceptor reflex control of nerve activity (p<0.05). In contrast, the type 2 receptor antagonist did not alter baroreceptor reflex function. Similar studies were performed in rats that received an intravenous injection of N(G)-monomethyl L-arginine (10 mg/kg). The nitric oxide synthase inhibitor increased baseline blood pressure and decreased renal sympathetic nerve activity. Subsequent administration of losartan or lisinopril returned blood pressure to initial hypertensive level, whereas sympathetic nerve activity was increased to a level above the initial control value. The maximum gain of the baroreceptor reflex control of renal nerve activity was increased after the nitric oxide inhibition. The present study demonstrates that blunted baroreceptor reflex function in conscious spontaneously hypertensive rats is mediated by an Ang II type 1 receptor. Moreover, nitric oxide may play a dual role in the antihypertensive effect of Ang II blockade. First, nitric oxide contributes to the antihypertensive effects of losartan and lisinopril. Second, the enhancement of the baroreceptor reflex after inhibition of the actions of Ang II may be related to the fall in arterial pressure.