The role of phosphatidylinositol (PI) turnover in excitation-contraction coupling was investigated in canine antral smooth muscle. Acetylcholine (ACh; 0.1-1 microM) transiently increased tissue levels of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and increased the amplitudes of the plateau phase of slow waves and associated Ca2+ transients and phasic contractions. ACh also increased basal concentrations of cytosolic Ca2+ ([Ca2+]c), but these changes were not associated with an increase in resting tension. ATP (0.3 mM) had similar effects on Ins(1,4,5)P3 levels, basal [Ca2+]c, and resting tension. However, in contrast to the effects of ACh, ATP transiently reduced the amplitude of the plateau phase of slow waves and reduced the amplitudes of associated Ca2+ transients and phasic contractions. We investigated the possibility that two products of PI turnover, diacylglycerol (DAG) and Ins(1,4,5)P3, might provide negative feedback to regulate Ca2+ entry during slow waves. 1) DAG is known to activate protein kinase C (PKC). Activation of PKC by phorbol 12,13-dibutyrate (PDBu, 0.5 microM) reduced the amplitude of the plateau phase of slow waves and corresponding Ca2+ transients and phasic contractions. Assay of PKC showed that ACh, ATP, and PDBu stimulated enzyme activity. 2) Ins(1,4,5)P3 is known to increase [Ca2+]c by release of Ca2+ from internal stores. Basal [Ca2+]c was also increased by elevated external K+, ionomycin, thapsigargin, or caffeine. Each of these compounds reduced the amplitude and duration of slow waves. Results suggest that products of PI turnover may provide negative-feedback control of Ca2+ influx during slow waves, tending to reduce the amplitude of phasic contractile activity in gastric muscles. Differences in responses to ACh and ATP can be explained by a G protein-dependent mechanism in which ACh suppresses the voltage dependence of Ca(2+)-activated K+ channels.
Excitation-contraction coupling in smooth muscle cells of the guinea-pig mesenteric artery
