ABSTRACT-Sodium nitroprusside, nitroglycerin and carperitide (a-human ANP) all reduced mean blood pressure, but only carperitide increased the hematocrit in rats with bilateral renal artery and ureter-ligation. Nc-Monomethyl L-arginine, a selective inhibitor of nitric oxide synthesis, elevated the mean blood pressure but did not change the hematocrit significantly. These findings suggest that ANP has a physiological role in regulating circulatory blood volume distinct from that of NO, although both increase intracellular cyclic GMP in the vasculature. Nitric oxide (NO) is produced in vascular endothelial cells, and atrial natriuretic peptide (ANP) is produced in and released from atrial cells. Both NO and ANP re lax vascular smooth muscles (1, 2). ANP activates the particulate guanylate cyclase in smooth muscle (3), while NO activates soluble guanylate cyclase (4). Since ANP has a potent diuretic action (2), it has been sug gested that ANP might have a role in regulating blood volume (5). Several investigators have reported that administered atriopeptin (rat ANP) increases the hema tocrit in rats (6, 7). However, the increase in hemato crit has not been totally explained by fluid loss due to the diuretic action of ANP. Almedia et al. (6) have hypothesized that the rat ANP-induced increase in hematocrit may be due to an increase in the fluid efflux from capillaries, which thereby causes a decrease in plasma volume. On the other hand, kidney function has also been reported to be regulated by NO (8), but to our knowledge, the effect of NO on hematocrit is not known yet. To assess whether exogenous NO and ANP can modulate the hematocrit, we have examined the effect of NO-generating drugs and carperitide (a-hu man ANP) on the hematocrit by concomitantly moni toring arterial blood pressure and heart rate in anesthe tized rats.Male Wistar rats weighing 200 290 g were used (Shizuoka Agr. Coop., Hamamatsu, Japan). Each of the drug-treatment groups consisted of 5 rats. Rats were anesthetized with urethane plus a -chloralose (500 mg/kg and 100 mg/kg, i.p., respectively). The bilateral renal arteries, veins and ureters exposed by a dorsal approach were ligated concomitantly. A tracheotomy was performed to keep good ventilation. Polyethylene catheters (PE 50) were inserted into the left juglar vein for infusion of drugs and into the right common carotid artery for measurement of systemic blood pressure and withdrawal of blood sample for determination of hemato crit. Blood pressure (BP) was measured by a pressure transducer (MPU-0.5A, Nihon Kohden, Tokyo), and heart rate (HR) was measured with a heart rate counter (AT-600T, Nihon Kohden) triggered by the BP pulse. Changes in BP and HR were continuously recorded on a polygraph recorder (RM-6200, Nihon Kohden). At first, the effect of physiological saline solution (vehicle) given intravenously in a volume of 80 p1/kg/min for 30 min was studied in all animals used, and then each of the test drugs was cumulatively in fused by increasing the concentration in a stepwise fashion at ...