Membrane stretch: A physiological stimulator of Ca²⁺-activated K⁺ channels in thick ascending limb

The effects of membrane stretch on Ca²⁺-activated (maxi) K⁺ channels were examined in the apical membrane of cultured medullary thick ascending limb (MTAL) cells. Using cell-attached patch-clamp technique, we found that negative pressure (-33 ± 5 cmH₂O) applied to the patch membrane increased fractional open probability (NP(o)) from 0.3 ± 0.2 to 29.9 ± 7.6% (n = 12) in the presence of 1.8 mM Ca²⁺ in the pipette. The activity returned to control on releasing the negative pressure. Reduction of extracellular osmolality from 293.2 ± 1.6 to 219.8 ± 1.1 mosmol/kg also activated K⁺ channels (NP(o) = 43.8 ± 12.2%, n = 8) in cell-attached patches. Removal of Ca²⁺ from both pipette and bathing solution inhibited osmotic activation of K⁺ channels. K⁺ channels were shown to be Ca²⁺-activated K⁺ channels by their conductance (146 ± 7 pS, n = 5) and Ca²⁺ dependence. Our data suggest that membrane stretch caused by swelling or possibly by tubular flow enhances Ca²⁺ entry across the apical cell membrane of MTAL cells activating maxi K⁺ channels.