Role of nitric oxide in regulation of renal sympathetic nerve activity during hemorrhage in conscious rats

The effect of inhibition of nitric oxide (NO) synthesis on the responses of blood pressure (BP), heart rate (HR), and renal sympathetic nerve activity (RSNA) during hemorrhaging was examined with the use of an NO synthase inhibitor, N-G-nitro-L-arginine methyl ester (L-NAME), in conscious rats. In the 0.9% saline group, hemorrhage (10 ml/kg body wt) did not alter BP but significantly increased HR and RSNA by 88 +/- 12 beats/min and 67 +/- 12%, respectively. Intravenous infusion of L-NAME (50 mu g.kg(-1).min(-1)) significantly attenuated these tachycardic and sympathoexcitatory responses to hemorrhage (14 +/- 7 beats/min and 26 +/- 12%, respectively). Pretreatment of L-arginine (87 mg/kg) recovered the attenuation of HR and RSNA responses induced by L-NAME (92 +/- 6 beats/min and 64 +/- 10%, respectively). L-NAME by itself did not alter the baroreceptor reflex control of HR and RSNA. Hemorrhage increased the plasma vasopressin concentration, and its increment in the L-NAME-treated group was significantly higher than that in the 0.9% saline group. Pretreatment with the vascular arginine vasopressin V-1-receptor antagonist OPC-21268 (5 mg/kg) recovered the attenuation of RSNA response induced by L-NAME (54 +/- 7%). These results indicate that NO modulated HR and RSNA responses to hemorrhage but did not directly affect the baroreceptor reflex arch. It can be assumed that NO modulated the baroreflex function by altering the secretion of vasopressin induced by hemorrhage.