Effects of hypotonic stress on the apical ion conductance and intracellular Ca²⁺ concentration of principal cells in freshly isolated rat kidney cortical collecting duct.

Abstract

The apical membrane of renal cortical collecting duct (CCD) faces the tubular fluid whose osmolarity is relatively low. Using the patch-clamp technique, we investigated the effect of reducing extracellular osmolarity on ion conductance of the apical membrane of principal cells in freshly isolated rat CCD. In cell-attached patches, three distinct types of the ion conductance formed by 30 pS K⁺, 10 pS Cl⁻ and 190 pS K⁺ channels were observed. Although the open probability (NP_o) of the former two channels was relatively high, that of the 190 pS large K⁺ channel was extremely low under the control condition. NP_o of the large K⁺ channel was raised by cytosoic free Ca²⁺ and depolarization. The exposure of the cells to hypotonic solution did not significantly influence NP_o of 30 pS K⁺ and 10 pS Cl⁻ channels, while NP_o of the large K⁺ channel was markedly enhanced within 5 min after the exposure to the hypotonic solution. The hypotonic solution-induced increase of NP_o of the large K⁺ channel was not observed in the absence of extracellular Ca²⁺ . Fluorescence imaging of intracellular Ca²⁺ concentration ([Ca²⁺]_i) with fura-2 showed that the hypotonic solution evoked transient increase in [Ca²⁺]_i, but did not increase [Ca²⁺]_i when the extracellular Ca²⁺ was removed. These results indicate that reducing extracellular osmolarity induces Ca²⁺-dependent activation of the large K⁺ channel due to an increase in [Ca²⁺]_i by a Ca²⁺ entry from the extracellular source in rat CCD. [J Physiol Sci. 2007;57 Suppl:S120]