Stretch-activated ion channels in smooth muscle: a mechanism for the initiation of stretch-induced contraction.

As in many smooth muscle tissue preparations, single smooth muscle cells freshly dissociated from the stomach of the toad Bufo marinus contract when stretched. Stretch-activated channels have been identified in these cells using patch-clamp techniques. In both cell-attached and excised inside-out patches, the probability of the channel being open (Po) increases when the membrane is stretched by applying negative pressure to the extracellular surface through the patch pipette. The increase in Po is mainly due to a decrease in closed time durations, but an increase in open time duration is also seen. The open-channel current-voltage relationship shows inward rectification and is not appreciably altered when K+ is substituted for Na+ as the charge-carrying cation in Ca2+-free (2 mM EGTA) pipette solutions bathing the extracellular surface of the patch. The inclusion of physiological concentrations of Ca2+ (1.8 mM) in pipette solutions (containing high concentrations of Na+ and low K+) significantly decreases the slope conductance as well as the unitary amplitude. The channel also conducts Ca2+, since inward currents were observed using pipette solutions in which Ca2+ ions were the only inorganic cations. When simulating normal physiological conditions, we find that substantial ionic current is conducted into the cell when the channel is open. These characteristics coupled with the high density of the stretch-activated channels point to a key role for them in the initiation of stretch-induced contraction.