Calcium-, voltage- and osmotic stress-sensitive currents in Xenopus oocytes and their relationship to single mechanically gated channels.

1. Patch recordings from Xenopus oocytes indicated that mechanically gated (MG) channels are expressed at a uniform surface density ( approximately 1 channel microm-2) with an estimated > 3 x 106 MG channels per oocyte that could generate microamps of current at +/-50 mV. 2. Removal of external Ca2+ induced a membrane conductance that differed from MG channels in ion selectivity, pharmacology and sensitivity to connexion-38. 3. Depolarization to +50 mV activated a Na+-selective, a Cl--selective and a non-selective conductance. Hyperpolarization to -150 mV activated a non-selective conductance. None of these conductances appeared to be mediated by MG channels. 4. Hypotonicity (25 %) failed to evoke any change in membrane conductance in the majority of defolliculated oocytes. Hypertonicity (200 %) evoked a large non-selective (PK /PCl approximately 1) membrane conductance that was not blocked by 100 microM Gd3+. 5. Although the above stimuli could activate a variety of whole-oocyte conductances, including three novel conductances, they did not involve MG channel activation. Possible mechanisms underlying the discrepancy between observed conductances and those anticipated from patch-clamp studies are discussed.