Extracellular calcium participates in responses to acetylcholine in Xenopus oocytes

We tested the contribution of extracellular calcium (Ca_o²⁺) to membrane electrical responses to acetylcholine (ACh) in native Xenopus oocytes. Removal of Ca_o 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 Ca_o²⁺ 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 Ca_o²⁺-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 Ca_o²⁺-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.