Hyposmotic activation of Ca-activated K channels in cultured rabbit kidney proximal tubule cells.

The effects of hypotonicity on K currents were examined in the single cells isolated from cultured rabbit kidney proximal tubule cells. Using the whole cell clamp technique [pipette-filling solution was 145 mM KCl with 0 Ca plus 0.08 mM ethylene glycol-bis(beta-aminoethylether)-N,N,N',N'-tetraacetic acid], we found that the K currents increased from 0.8 to 43.5 +/- 12.5 pA at a holding potential (Vp) = 0 mV in response to reduction of extracellular osmolality from 290 to 218 mosmol/kgH2O (n = 8). Removal of Ca from bath reduced osmotic activation of K currents by 74% (n = 10). Application of 5 mM Ba to the bath completely inhibited the K currents at Vp = 0 mV but not at Vp = 60 mV. Dominant K currents were shown to be Ca-activated (maxi) K channels by their single-channel conductance (175 +/- 8.6 pS) and Ca dependence. Relative permeability ratio (PK/PNa) was 9. In conclusion, hypotonic-induced cell swelling stimulates Ca-activated K channels, which may play a role in cell volume regulation.