Effects of internal and external pH on amiloride-blockable Na+ transport across toad urinary bladder vesicles

Abstract

We have examined the effect of internal and external pH on Na+ transport across toad bladder membrane vesicles. Vesicles prepared and assayed with a recently modified procedure (Garty & Asher, 1985) exhibit large, rheogenic, amiloride-sensitive fluxes. Of the total 22Na uptake measured 0.5-2.0 min after introducing tracer, 80 +/- 4% (mean +/- SE, n = 9) is blocked by the diuretic with a KI of 2 X 10(-8) M. Thus, this amiloride-sensitive flux is mediated by the apical sodium-selective channels. Varying the internal (cytosolic) pH over the physiologic range 7.0-8.0 had no effect on sodium transport; this result suggests that variation of intracellular pH in vivo has no direct apical effect on modulating sodium uptake. On the other hand, 22Na was directly and monotonically dependent on external pH. External acidification also reduced the amiloride-sensitive efflux across the walls of the vesicles. This inhibition of 22Na efflux was noted at external Na+ concentrations of both 0.2 microM and 53 mM. These results are different from those reported with whole toad bladder. A number of possible bases for these differences are considered and discussed. We suggest that the natriferic response induced by mucosal acidification of whole toad urinary bladder appears to operate indirectly through one or more factors, presumably cytosolic, present in whole cells and absent from the vesicles.