Extracellular calcium participates in responses to acetylcholine in Xenopus oocytes.
We tested the contribution of extracellular calcium (Ca2+) 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 Ca2+o 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 Ca2+o-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 of inositol 1,4,5-trisphosphate (IP3) resulted in a two-component depolarizing current. IP3 injection promoted the appearance of Ca2+o-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.
Lupu-Meiri
M
M
Division of Physiology and Pharmacology, Sackler Faculty of Medicine, Tel Aviv University, Ramat Aviv, Israel.
Shapira
H
H
Oron
Y
Y
D000109:Acetylcholine / Q000494:pharmacology*; D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism; D005071:Evoked Potentials; D005260:Female; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000187:drug effects* / Q000378:metabolism; D014981:Xenopus
D015220:Calcium Channels; D002712:Chlorides; D011956:Receptors, Cell Surface; D000109:Acetylcholine; D002118:Calcium