Rise in cytosolic Ca2+ concentration induced by P-2-purinoceptor activation in isolated myocytes from the rat gastrointestinal tract

1 The changes in the free cytosolic Ca2+ concentration ([Ca2+](i)) in response to agonists of P-2-purinoceptors were studied in myocytes isolated from the longitudinal muscle layer of different regions of the rat gastrointestinal tract (stomach, jejunum, ileum, caecum and colon). [Ca2+](i) was estimated by emission from the fluorescent dye, indo-1. 2 ATP and the P-2Y-purinoceptor agonist, 2-methylthio-ATP (2-MeSATP), transiently increased [Ca2+](i) in single myocytes from all segments of the gastrointestinal tract, whereas alpha,beta-methylene-ATP, a P-2X-purinoceptor agonist, had no effect. 3 The rise in [Ca2+](i) induced by ATP and 2-MeSATP was maintained in Ca2+-free solution but was abolished by depletion of the intracellular store with thapsigargin (1 mu M). 4 Single myocytes from stomach, caecum and colon also responded to UTP by a transient increase in [Ca2+](i). 5 Individual myocytes responded to ATP, 2-MeSATP and UTP in a nearly all-or-nothing manner. The increasing of agonist concentration enhanced the number of responding cells but did not increase the amplitude of the [Ca2+](i) rise. 6 These results suggest that myocytes from the longitudinal layer of gastrointestinal muscle do not possess functional P-2X-purinoceptors and that agonists of P-2Y and P-2U-purinoceptors induced a rise in [Ca2+](i), probably via an all-or-nothing mobilization of Ca2+ from intracellular stores.