CHLORIDE CHANNELS OF MITOCHONDRIA-RICH CELLS IN ANURAN SKIN - PHYSIOLOGICAL SIGNIFICANCE AND REGULATION

Osmotic and ionic balance in anuran amphibians depends on regulation o f ion fluxes in heterocellular epithelia in skin, bladder, and kidney collecting duct. ion mechanisms in amphibian skin have been studied at tissue level and at levels of single cells and individual transport m olecules. It has been found that the major active Na+ flux is flowing through the principal cells, and that the chloride fluxes are handled by the mitochondria-rich cells. Both active and passive fluxes of chlo ride have been identified. Evidence is discussed that the switching be tween these two transport modes involves regulation of the activity of single chloride channels in the apical membrane of mitochondria-rich cells. In patch clamp studies it was found that this membrane contains more than one population of Cl- channels of which two types are discu ssed here: (a) Small (7-10 pS) ohmic chloride channels which seem resp onsible for the cyclicAMP activated macroscopic Cl- current. (b) Giant channels (150-550 pS) which are activated by membrane depolarization. This latter population constitutes candidate-channels for controlling the voltage activated inward flux of Cl- through the skin. A low freq uency of occurrence of active channels in apical membrane patches of i solated mitochondria-rich cells has prevented us from comparing, quant itatively, the measured Cl- currents through single channels with macr oscopic transcellular Cl- currents.