VOLTAGE-INDUCED SLOW ACTIVATION AND DEACTIVATION OF MECHANOSENSITIVE CHANNELS IN XENOPUS OOCYTES

1. The relationship between stretch and voltage activation of mechanos ensitive (MS) channels from Xenopus oocytes was studied in excised pat ches of membrane using the patch clamp technique. 2. As is characteris tic of MS channels in oocytes, stretching the membrane by applying neg ative pressure to the patch pipette at -50 mV activated the MS channel s rapidly. The channels then deactivated rapidly when the stretch was removed. The stretch-activated MS channels entered a main conductance level (45 pS) and one or more subconductance levels in the range of ab out 75-90 % of the main conductance level. 3. In the absence of stretc h, a depolarizing step from -50 to +50 mV activated apparent MS channe ls after long delays of typically 1-20 s (range, 100 ms to 6 min). Upo n repolarization, the channels deactivated slowly with a single expone ntial (mean time constant of 4 s) or double exponential (mean time con stants of 0.8 and 3 s) time course. 4. Delayed activation with depolar ization and slow deactivation upon repolarization were also observed f or apparent MS channels in on-cell patches. 5. The voltage-activated c hannels were cation selective and had the same selectivity and conduct ance levels as the stretch-activated MS channels. Applying stretch dur ing voltage induced channel activity did not activate any additional c hannels, and the same maximal number of channels were typically activa ted by either stretch or by voltage. These observations suggest that v oltage activates the same MS channels that are activated by stretch. 6 . The opening of MS channels following steps to +50 mV occurred in an apparently co-operative manner in 70 % of the excised patches containi ng multiple MS channels. 7. In the absence of stretch, the opening fre quency and open probability of MS channels increased with depolarizati on in the examined voltage range of -60 to -20 mV. 8. Applying a brief stretch during the delay to activation at +50 mV activated the MS cha nnels rapidly, which then remained active when the stretch was removed . In contrast, applying a brief stretch during the slow deactivation i nduced by stepping from +50 to -50 mV abruptly terminated the voltage- induced channel activity upon release of the stretch and inhibited sub sequent depolarization-induced activity. 9. Depolarizing steps from -5 0 to +50 mV inhibited any spontaneous channel activity that was presen t before the depolarizing step. If the potential was stepped back to - 50 mV before the channels activated at +50 mV, a delayed activation co uld occur at -50 mV, followed by normal deactivation, indicating that the depolarizing step initiated activation processes that were initial ly masked by inhibition. 10. These observations suggest that voltage a nd stretch can induce different functional gating configurations of MS channels with associated structures, and that these different gating configurations can interconvert.