Peculiarities of phospholipase C-dependent release of CA2+ from intracellular stores upon activation of choline and purine receptors in myocytes of the guinea-pig small intestine

Using a two-wave fluorescence probe, Fura-2, we studied changes in the intracellular concentration of calcium ions ([Ca2+]i) resulting from activation of muscarinic and purine receptors in single myocytes of the guinea-pig small intestine. Applications of the respective agonists added to the normal Krebs solution (1.0, 10.0, and 100.0 µM carbachol, CCh, as well as 10.0 and 100.0 µM ATP) induced a rise in the [Ca2+]i. Carbachol evoked an increase in the [Ca2+]i, including two components (a rapid and a plateaulike), while ATP under analogous conditions led only to a short-lasting rise in the [Ca2+]i. Transients induced by CCh or ATP applied in different concentrations, which exceeded a certain level, did not significantly differ from each other in their amplitudes, i.e., they were generated according to an all-or-none principle. In the nominally Ca-and Mg-free solution, CCh and ATP induced only rapid increases in the [Ca2+]i in myocytes. The absence of the slow component in the [Ca2+]i elevation upon the action of CCh under such conditions indicates that the effect of ATP, as compared with that of CCh, is not related to activation of the entry of Ca2+ ions into cells through voltage-operated calcium channels. After the addition of CCh, repeated application of CCh or ATP induced no effect, while application of CCh after the addition of ATP initiated a rise in the [Ca2+]i. These data show that intracellular calcium stores are depleted completely upon the action of CCh, while they are depleted only partially after the action of ATP. An inhibitor of phospholipase C (PLC), U-73122 (5.0 µM), completely blocked rises in the [Ca2+]i induced by both CCh and ATP; therefore, the release of Ca2+ ions from the intracellular calcium stores after application of these agonists is mediated by PLC. We hypothesize that the difference in the release of Ca2+ ions from the intracellular stores observed in our experiments upon activation of choline and purine receptors (partial and complete depletion of the stores upon the action of ATP and CCh, respectively) is responsible for the opposite functional effects of the above-mentioned neurotransmitters on smooth muscles.