A stretch-activated (SA) cation channel was investigated in the apical membrane of cultured kidney tubule cells (A6) with the patch-clamp technique. The mean numbers of open channels increased from 0.005 (without suction) to 0.05, 0.11, and 0.30 at pressures of -32, -48, and -64 cmH2O in the pipette, respectively (cell-attached patch). The channel activity increased at hyperpolarized membrane potentials and decreased with increasing Ca2+ in the pipette. It was completely blocked by 10 microM gadolinium in the pipette when suction was between -16 and -48 cmH2O. The channel was permeable to Na+, K+, and Ca2+, but not to Cl-. The single-channel conductances (120 mM NaCl in the pipette) were decreased by adding 2-10 mM Ca2+ in the pipette solution: they were 57.8 +/- 1.6, 36.4 +/- 3.3, 28.2 +/- 3.4, and 25.9 +/- 1.3 pS with 0.5, 2, 4, and 10 mM Ca2+, respectively. When the NaCl in the pipette was replaced with 80 mM CaCl2, the conductance was 25.6 pS. These results indicate that when cells are stretched in a standard solution (Ca(2+)-containing Na(+)-rich solution), Na+ and Ca2+ enter the cell through the SA channel. The SA channel in A6 cells may be responsible for the Na+ and Ca2+ entry pathway which is sensitive to membrane stretch.