The action of procaine on pharmaco-mechanical coupling activated by application of acetylcholine (ACh) was investigated using collagenase-treated dispersed intact and skinned smooth muscle cells and intact muscle tissues of the porcine coronary artery. ACh reduced stored45Ca2+, and this action was prevented by procaine in intact dispersed cells. The maximum reduction in the level of stored45Ca induced by caffeine (25 mM) or inositol 1,4,5-trisphosphate (InsP,; 3 μM) was also prevented by procaine in the skinned muscle cells in the presence or absence of ATP. However, inhibitions of the latter required higher concentrations of procaine than the former. Release by 10 μM ACh of Ca2+from its store site in the presence or absence of extracellular Ca2+was also inhibited by procaine and was detected using the quin2 fluorescence method. In these smooth muscle tissues, ACh (above 10 nM) reduced the amount of phosphatidylinositol 4,5-bisphosphate (PI-P2) and dose dependently increased the amount of phosphatidic acid. Procaine inhibited the hydrolysis of PI-P2activated by ACh, thus reducing the amount of InsP3and the release of Ca2+from the store site. It is concluded that procaine has multiple actions on the porcine coronary artery, and one of the actions related with pharmaco-mechanical coupling appears through inhibition of hydrolysis of PI-P2induced by ACh.