A calcium-permeable stretch-activated cation channel in renal proximal tubule

Isolated Necturus proximal tubules were split to expose the apical membrane surface for patch clamping. When both pipette and bath solutions contained only Ca, N-methyl-D-glucamine, and methanesulfonate, inwardly directed Ca currents were observed through a stretch-activated (SA) cation channel with conductance of 18 +/- 1 pS (n = 19). The SA cation channel exhibited little discrimination among Na, K, and Ca but was at least nine times more selective for cations than anions. The channel was not significantly gated by either membrane potential or cytosolic Ca. However, application of 15 cmH2O suction to patch pipette significantly increased the mean number of open channels by a factor of 6.5, from 0.04 +/- 0.02 to 0.26 +/- 0.08 (n = 11). Ca currents through the SA cation channel were reversibly blocked by 10 microM gadolinium, which was applied to outside surface of excised patches. This is similar to gadolinium block of stretch-activated channels in Xenopus oocytes (X.-C. Yang and F. Sachs. Science Wash. DC 243: 1068-1071, 1989). A Ca-dependent, maxi-K channel (92 +/- 9 pS, n = 5) was also found at the apical membrane of the same proximal tubules. In some cases this maxi-K channel appeared to be indirectly activated by pipette suction, raising the possibility that Ca influx through the SA cation channel may regulate K efflux via the maxi-K channel. Such a process could mediate cell volume regulation and maintain electrolyte homeostasis during normal variations in Na-substrate cotransport.