Electrogenic bicarbonate secretion in the turtle bladder: apical membrane conductance characteristics.

We have recently shown that stimulation of electrogenic HCO3- secretion is accompanied by a simultaneous increase in short-circuit current (Isc, equivalent to HCO3- secretion rate under these conditions), apical membrane capacitance (Ca, proportional to membrane area), and apical membrane conductance (Ga, proportional to membrane ionic permeability). The current experiments were undertaken to explore the ionic basis for the increase in Ga and the possibility that the rate of electrogenic HCO3- secretion is regulated by changes in Ga. Membrane electrical parameters were measured using impedance-analysis techniques before and after stimulation of electrogenic HCO3- secretion with cAMP in three solutions which contained different chloride concentrations. In another series of experiments, the effects of an anion channel blocker, anthracene-9-carboxylic acid (9-AA), were measured after stimulation of electrogenic HCO3- secretion with cAMP. The major conclusions are: (i) a measurable apical Cl- conductance exists in control hemibladders; (ii) the transport-associated increase in Ga includes a Cl(-)-conductive component; (iii) Ga also appears to reflect a HCO3- conductance; (iv) the relative magnitudes of the apical membrane conductances to Cl- and HCO3- are similar; (v) 9-AA reduces Ga and Isc in cAMP-stimulated hemibladders; and (vi) alterations in Isc appear to be mediated by changes in Ga.