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 (~1. channel μm^-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 antisense oligonucleotides. 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 (P(K/)P(Cl) approximate 1) membrane conductance that was not blocked by 100 μM 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.