Regulation and possible physiological role of the Ca2-dependent K+ channel of cortical collecting ducts of the rat

In the luminal membrane of rat cortical collecting duct (CCD) a big Ca²⁺-dependent and a small Ca²⁺-independent K⁺ channel have been described. Whereas the latter most likely is responsible for the K⁺ secretion in this nephron segment, the function of the large-conductance K⁺ channel is unknown. The regulation of this channel and its possible physiological role were examined with the conventional cell-free and the cell-attached nystatin patch-clamp techniques. Patch-clamp recordings were obtained from the luminal membrane of isolated perfused CCD segments and from freshly isolated CCD cells. Intracellular calcium was measured using the calcium-sensitive dye fura-2. The large-conductance K⁺ channel was strongly voltage- and calcium-dependent. At 3 mol/l cytosolic Ca²⁺ activity it was half-maximally activated. At 1 mmol/l it was neither regulated by cytosolic pH nor by ATP. At 1 mol/l Ca²⁺ activity the open probability (P_o) of this channel was pH-dependent. At pH 7.0 P_o was decreased to 4±2% (n=9) and at pH 8.5 it was increased to 425±52% (n=9) of the control. At this low Ca²⁺ activity the P_o of the channel was reduced by 1 mmol/l ATP to 8±4% (n=6). Cell swelling activated the large-conductance K⁺ channel (n=14) and hyperpolarized the membrane potential of the cells by 9±1 mV (n=23). Intracellular Ca²⁺ activity increased after hypotonic stress. This increase depended on the extracellular Ca²⁺ activity. A possible physiological function of the large-conductance K⁺ channel in rat CCD cells may be the reduction of the intracellular K⁺ concentration after cell swelling. Once this channel is activated by increases in the cytosolic Ca²⁺ activity it can be regulated by changes in cellular pH and ATP.Supported by DFG Schl 277/2-3