REGULATION AND POSSIBLE PHYSIOLOGICAL-ROLE OF THE CA-2+ -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 mumol/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 mumol/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.