The whole cell recording mode of the patch-clamp technique was used to study the effect of hypotonic NaCl or isotonic high-KCl solution on membrane currents in a human osteoblast-like cell line, C1. Both hypotonic NaCl or isotonic high-KCl solution activated Cl- channels expressed in these cells as described previously. The reversal potential of the induced Cl- current is more negative when activated through hypotonic NaCl solution (-47 +/- 5 mV; n = 6) compared with activation through isotonic high-KCl solution (-35 +/- 3 mV; n = 8). This difference can be explained by an increase in intracellular [Cl-] through the activity of a K-Cl cotransporter. Potassium aspartate was unable to activate the current, and furosemide or DIOA suppressed the increase in Cl- current induced by isotonic high-KCl solution. In addition, we used the polymerase chain reaction to demonstrate the presence of KCC1-KCC4 mRNA in the osteoblast-like cell line. From these results, we conclude that human osteoblasts express functional K-Cl cotransporters in their cell membrane that seem to be able to induce the indirect activation of volume-sensitive Cl- channels by KCl through an increase in the intracellular ion concentration followed by water influx and cell swelling.