Intracellular calcium events activated by ATP in murine colonic myocytes

ATP is a candidate enteric inhibitory neurotransmitter in visceral smooth m uscles. ATP hyperpolarizes visceral muscles via activation of small-conduct ance, Ca2+-activated K+ (SK) channels. Coupling between ATP stimulation and SK channels may be mediated by localized Ca2+ release. Isolated myocytes o f the murine colon produced spontaneous, localized Ca2+ release events. The se events corresponded to spontaneous transient outward currents (STOCs) co nsisting of charybdotoxin (ChTX)-sensitive and -insensitive events. ChTX-in sensitive STOCs were inhibited by apamin. Localized Ca2+ transients were no t blocked by ryanodine, but these events were reduced in magnitude and freq uency by xestospongin C (Xe-C), a blocker of inositol 1,4,5-trisphosphate r eceptors. Thus we have termed the localized Ca2+ events in colonic myocytes "Ca2+ puffs." The P-2Y receptor agonist 2-methylthio-ATP (2-MeS-ATP) incre ased the intensity and frequency of Ca2+ puffs. 2-MeS-ATP also increased ST OCs in association with the increase in Ca2+ puffs. Pyridoxal-phospate-6-az ophenyl-2',4'-disculfonic acid tetrasodium, a P-2 receptor inhibitor, block ed responses to 2-MeS-ATP. Spontaneous Ca2+ transients and the effects of 2 -MeS-ATP on Ca2+ puffs and STOCs were blocked by U-73122, an inhibitor of p hospholipase C. Xe-C and ryanodine also blocked responses to 2-MeS-ATP, sug gesting that, in addition to release from IP3 receptor-operated stores, rya nodine receptors may be recruited during agonist stimulation to amplify rel ease of Ca2+. These data suggest that localized Ca2+ release modulates Ca2-dependent ionic conductances in the plasma membrane. Localized Ca2+ releas e may contribute to the electrical responses resulting from purinergic stim ulation.