Activation of mechanosensitive currents in traumatized membrane

Mechanosensitive (MS) channels, ones whose open probability varies with mem brane tension in patch recordings, are diverse and ubiquitous, yet many are remarkably insensitive to mechanical stimuli in situ. Failure to elicit me chanocurrents from cells with abundant MS channels suggests that, in situ, the channels are protected from mechanical stimuli. To establish what condi tions affect MS channel gating, we monitored Lymnaea neuron stretch-activat ed K (SAK) channels in cell-attached patches after diverse treatments. Mech anosensitivity was gauged by rapidity of onset and extent of channel activa tion during a step pressure applied to a "naive" patch. The following treat ments enhanced mechanosensitivity: actin depolymerization (cytochalasin B), N-ethylmaleimide, an inhibitor of ATPases including myosin, elevated Ca (u sing A-23187), and osmotic swelling (acutely and after 24 h). Osmotic shrin king decreased mechanosensitivity. A unifying interpretation is that trauma tized cortical cytoskeleton cannot prevent transmission of mechanical stimu li to plasma membrane channels. Mechano-protection and capricious mechanose nsitivity are impediments to cloning efforts with MS channels. We demonstra te a potpourri of endogenous MS currents from L-M(TK-) fibroblasts; others had reported these cells to be MS current null and hence to be suitable for expressing putative MS channels.