Bicarbonate and CO₂ transport across the skin of the leopard frog, Rana pipiens: Effect of PGF(2α)

The amphibian skin represents an important organ for osmoregulation and, like the mammalian kidney, maintains acid-base balance by secreting protons or base. However, the lack of a reliable and accurate method to measure the contribution of unidirectional fluxes of HCO₃ ions to this mechanism has been an obstacle for the determination of the role of bicarbonate in epithelial acid-base homeostasis. Recently, one of us developed a method that allows for the reliable determination of transepithelial fluxes of bicarbonate, and this method was applied to determine unidirectional fluxes of ¹⁴CO₂ and H¹⁴CO₃/^- under a variety of conditions. We report that the combined CO₂ and HCO₃/^- mucosal-to-serosal flux under 5% CO₂ was 40% larger than the opposing flux, giving a net flux in the mucosal-to-serosal direction. This net flux was inhibited by acetazolamide. In CO₂-free conditions, there was no detectable net flux; however, acetazolamide and PGF(2α) attenuated the mucosal-to-serosal flux and established an apparent secretion of HCO₃/^-. A model is presented that depicts twelve vectors or components to the CO₂ plus HCO₃/^- fluxes in the frog skin. This model can accurately reproduce the experimental values measured from unidirectional fluxes of CO₂ and HCO₃ under a variety of conditions and can explain the effects of PGF(2α) on unidirectional ¹⁴C-labeled fluxes as a consequence of inhibition of H⁺ secretion to the apical bath, similar to what was previously suggested by our laboratory using a different methodological approach. The present method, utilizing radiolabeled HCO₃/^-, may be useful as a means to evaluate the mechanism of action of hormones and drugs that may regulate acid-base homeostasis by altering proton and bicarbonate transport processes.