Voltage-induced slow activation and deactivation of mechanosensitive channels in Xenopus oocytes.

Abstract

1. The relationship between stretch and voltage activation of mechanosensitive (MS) channels from Xenopus oocytes was studied in excised patches of membrane using the patch clamp technique. 2. As is characteristic of MS channels to oocytes, stretching the membrane by applying negative pressure to the patch pipette at -50 mV activated the MS channels 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 about 75-90% of the main conductance level. 3. In the absence of stretch, a depolarizing step from -50 to +50 mV activated apparent MS channels after long delays of typically 1-20 s (range, 100 ms to 6 min). Upon repolarization, the channels deactivated slowly with a single exponential (mean time constant of 4 s) or double exponential (mean time constants of 0.8 and 3 s) time course. 4. Delayed activation with depolarization and slow deactivation upon repolarization were also observed for apparent MS channels in on-cell patches. 5. The voltage-activated channels were cation selective and had the same selectivity and conductance levels as the stretch activated MS channels. Applying stretch during voltage-induced channel activity did not activate any additional channels, and the same maximal number of channels were typically activated by either stretch or by voltage. These observations suggest that voltage 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 containing multiple MS channels. 7. In the absence of stretch, the opening frequency and open probability of MS channels increased with depolarization 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 channels rapidly, which then remained active when the stretch was removed. In contrast, applying a brief stretch during the slow deactivation induced by stepping from +50 to -50 mV abruptly terminated the voltage induced channel activity upon release of the stretch and inhibited subsequent depolarization-induced activity. 9. Depolarizing steps from -50 to +50 mV inhibited any spontaneous channel activity that was present before the depolarizing step. If the potential was stepped back to -50 mV before the channels activated at +50 mV, a delayed activation could occur at -50 mV, followed by normal deactivation, indicating that the depolarizing step initiated activation processes that were initially masked by inhibition. 10. These observations suggest that voltage and stretch can induce different functional gating configurations of MS channels with associated structures, and that these different gating configurations can interconvert.