Electrogenic bicarbonate secretion in the turtle bladder: Apical membrane conductance characteristics

Summary We have recently shown that stimulation of electrogenic HCO₃^– secretion is accompanied by a simultaneous increase in short-circuit current (I_sc, equivalent to HCO₃^– secretion rate under these conditions), apical membrane capacitance (C_a, proportional to membrane area), and apical membrane conductance (G_a, proportional to membrane ionic permeability). The current experiments were undertaken to explore the ionic basis for the increase inG_a and the possibility that the rate of electrogenic HCO₃^– secretion is regulated by changes inG_a. 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 inG_a includes a Cl^–-conductive component; (iii)G_a 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 reducesG_a andI_sc appear cAMP-stimulated hemibladders; and (vi) alterations inI_sc appear to be mediated by changes inG_a.