Interaction between ANP and amiloride in renal tubular sodium handling in anesthetized rabbits

To examine whether amiloride, an inhibitor of a conductive sodium channel in the distal tubule, modifies a possible tubular action of atrial natriuretic peptide (ANP), alpha-human ANP (0.05 micrograms.kg-1.min-1) was infused intravenously with or without pretreatment of amiloride (5 mg/kg + 0.04 mg.kg-1.min-1) in anesthetized rabbits. No significant changes in arterial pressure and renal blood flow were observed between two groups after ANP infusion. However, increases in filtered sodium load (FNa) and net tubular sodium reabsorption (RNa) induced by ANP were attenuated after the amiloride pretreatment. These attenuations in FNa and RNa could be due to the failure of increase in glomerular filtration rate, although the reason is not clear. Amiloride pretreatment abolished the ANP-induced kaliuresis, whereas ANP-induced increases in urinary sodium and fractional sodium excretion were additive after amiloride pretreatment. Thus, ANP elicited a further natriuretic effect even when the amiloride-sensitive component was blocked. This suggests that there is another mechanism by which ANP induces natriuresis, in addition to any ANP effects mediated by interference with sodium conductive channels.