Inhibition of Na/H exchange in avian intestine by atrial natriuretic factor.

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Abstract
Effects of 8-bromo-cGMP (8-Br-cGMP) and synthetic rat atriopeptin III (APIII) on sodium absorption by isolated chicken villus enterocytes and intact chicken ileal mucosa were determined. In isolated cells, both agents significantly decreased initial rates of influx of 22Na and caused a persistent decrease in intracellular pH (pHi); effects that are not additive to those caused by amiloride (10(-3) M). The ED50 for APIII was 0.3 nM. In intact mucosa, both 8-Br-cGMP (10(-4) M) and 5-(N-methyl-N-isobutyl)amiloride (MIBA) (10(-5) M) reduced JNams and JNa.net, their effects were not additive. APIII (10(-7) M) significantly increased cellular cGMP but not cAMP. Both 8-Br-cGMP (10(-4) M) and APIII (10(-7) M) stimulated a persistent increase in cytosolic calcium (Cai), which could be prevented by pretreating the cells with the cytosolic calcium buffering agent MAPTAM or with H-8, an inhibitor of cyclic nucleotide-dependent protein kinases. Furthermore, pretreatment of cells with H-8 or the calmodulin inhibitor, calmidazolium (CM), prevented the effects of 8-Br-cGMP and APIII on pHi. However, the pHi response to subsequent addition of the calcium-ionophore ionomycin was blocked only by CM and not by H-8. These data suggest that APIII and 8-Br-cGMP inhibit amiloride-sensitive Na/H exchange by increasing Cai, an event requiring activation of cGMP-dependent protein kinase.