Ca2+ responses to acetylcholine and adenosine triphosphate in the otocyst of chick embryo

The action of acetylcholine and adenosine triphosphate (ATP) on cytoplasmic Ca2+ concentration ([Ca2+]i) was studied in the otocyst epithelium of embryonic day 3 chicks with Ca(2+)-sensitive fluorescence measurements. Increases in [Ca2+]i were evoked by the bath application of acetylcholine (1 microM or higher). The rise in [Ca2+]i was due to the release of Ca2+ from intracellular Ca2+ stores, since the Ca2+ response occurred even in a Ca(2+)-free medium. The Ca2+ response to acetylcholine was mediated by muscarinic receptors. Atropine of 1 microM abolished the response to 10 microM acetylcholine; muscarine and carbamylcholine (100 microM each) evoked Ca2+ rises. Increases in [Ca2+]i were also evoked by the bath application of ATP (10 microM or higher). The Ca2+ rise by ATP was evoked even in a Ca(2+)-free medium. Adenosine (500 microM) did not cause any Ca2+ response. Suramin and reactive blue 2 (200 microM each) completely blocked the Ca2+ response to 500 microM ATP. Uridine triphosphate (500 microM) caused comparable Ca2+ responses with those to 500 microM ATP. These results suggested the involvement of P2U purinoceptors. The potentiation of Ca2+ rise was observed when acetylcholine and ATP were co-applied at submaximal concentrations (10 microM and 100 microM, respectively). We conclude that undifferentiated cells in the otocyst epithelium have Ca2+ mobilizing systems activated by acetylcholine and ATP.