POORLY SELECTIVE CATION CHANNELS IN THE APICAL MEMBRANE OF A6 CELLS

This paper describes a Ca2+-blockable, poorly selective cation pathway in the apical membrane of A6 epithelia. This pathway has properties t hat resemble the cation-selective channels in the toad urinary bladder and frog skin. Transepithelial short circuit currents (I-sc) and powe r density spectra (PDS) of the fluctuations in current were recorded. The basolateral surface of the tissues was exposed to Cl- or SO42- sol utions with Na+ as the major cation. Ca2+-blockable inward oriented cu r rents and Lorentzian noise were recorded with isotonic (215 mosmol/k g) mucosal Cl- and hypotonic (144 mosmol/kg) serosal SO42- solution wi th Na+, K+, Rb+ or Cs+ as the major mucosal cation. Experiments with m ucosal K+ demonstrated that the cation-selective channel was markedly activated by serosal hypotonicity. Effects of an increased electrical driving force were excluded on the basis of the results obtained with microelectrode experiments and transepithelial voltage clamping. Cell volume expansion induced by isotonic replacements of serosal sucrose b y glycerol or urea also activated the cation-selective pathway. Furthe rmore, the presence of Cl- in the mucosal solution was a prerequisite for a sustained response to hypotonicity or replacements of the organi c compounds. Moreover, we found that the cation-selective channels are mainly expressed in the cells during the early period of epithelial g rowth.