Effects of 7-nitroindazole on renal sympathetic nerve activity during acute cardiac tamponade in conscious rabbits.

To investigate whether nitric oxide (NO) in the central nervous system is involved in the decrease in renal sympathetic nerve activity (RSNA) during acute cardiac tamponade in conscious rabbits, we examined the effect of 7-nitroindazole (7-NI), a selective inhibitor of neuronal nitric oxide synthase in vivo, on RSNA during acute cardiac tamponade in chronically installed conscious rabbits. Cardiac tamponade was produced by intrapericardial infusion of physiological saline at 2 ml/30 s. Mean arterial pressure (MAP) remained constant initially but RSNA increased to 218+/-24% when we started injection of physiological saline into the pericardial space. Concomitantly after MAP fell to 51+/-1 mm Hg by subsequent injection of the saline into the pericardial space, RSNA decreased to 45+/-6%. If 7-NI (50 mg/kg) was administered intraperitoneally 35 min before the beginning of cardiac tamponade, the decline in RSNA caused by cardiac tamponade was markedly counteracted. Brain nitric oxide synthase (NOS) activity in the cerebral cortex and medulla oblongata, assessed by the conversion of labelled arginine to citrulline, was inhibited by 48% and 44% after the intraperitoneal administration of 7-NI. These results indicate that acute cardiac tamponade elicits a biphasic effect on RSNA, which rises during non-hypotensive period and then falls during hypotension in conscious rabbits. The decrease in RSNA was abolished by treatment with 7-NI, suggesting that the abrupt decrease in RSNA during hypotension induced by acute cardiac tamponade is mediated by NO in the central nervous system.