We tested the contribution of extracellular calcium (Cao 2+) to membrane electrical responses to acetylcholine (ACh) in native Xenopus oocytes. Removal of Cao caused a decrease in both the rapid (D1) and the slow (D2) chloride currents that comprise the common depolarizing response to ACh in native oocyte. The effect of Cao 2+ removal on the muscarinic response was mimicked by the addition of 1 mM Mn2+, an effective antagonist of calcium influx, though not by antagonists of voltage-sensitive calcium channels. When oocytes were challenged with ACh in Ca2+-free medium, subsequent addition of 1.8 mM CaCl2 resulted in a rapid, often transient, depolarizing current. Similarly to the Cao 2+-dependent component of membrane electrical responses, the Ca2+ -evoked current was reversibly abolished by Mn2+, though not by antigonists of voltage-sensitive calcium channels. Depletion of cellular calcium potentiated the Ca2+-evoked current, implying negative feedback of calcium channels by calcium. Injection of 10–100 fmol ofinositol 1,4,5-trisphosphate (IP3) resulted in a two-component depolarizing current. IP3 injection promoted the appearance of Cao 2+-evoked current that was significantly potentiated by previous calcium depletion. We suggest that activation of cell-membrane muscarinic receptors causes opening of apparently voltage-insensitive and verapamil or diltiazem-resistant calcium channels. These channels may be activated by IP3 or its metabolites, which increase following the activation of cell membrane receptors coupled to a phospholipase C. The channels may be identical to receptor-operated channels described in other model systems.
