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 (similar to 1 channel mu m(-2)) with an estimated >3 x 10(6) 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 to 1) membrane conductance that was not blocked by 100 mu M 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.