Roles of stored calcium on the mechanical response evoked in smooth muscle cells of the porcine coronary artery.

Effects of acetylcholine (ACh), caffeine and procaine on the membrane and mechanical properties of the intact and skinned muscle cells of the porcine coronary artery were investigated using the micro-electrode and isometric tension recording methods.1. ACh (10(-8)-10(-5)m) had no effect on the membrane potential and membrane resistance, as assessed from the current-voltage relationship.2. Caffeine possessed dual actions on the membrane property, i.e. a low concentration (0.3-1 mm) hyperpolarized and a high concentration (over 1 mm) depolarized the membrane, while caffeine (over 0.3 mm) consistently increased the ionic conductance of the membrane. Procaine (over 1 mm) depolarized the membrane and decreased the ionic conductance of the membrane.3. The spike and contraction evoked by outward current pulses in the presence of 10 mm-TEA were suppressed by treatment with caffeine (5 mm) or procaine (3 mm), but the spike was slightly and the electrically induced contraction was significantly suppressed by ACh (10(-6)m) due to the marked increase in resting tension.4. In Ca-free 2 mm-EGTA containing solution, the K-induced contraction (118 mm) rapidly ceased, but not the contraction evoked by ACh (10(-5)m) or caffeine (5 mm). The ACh-induced contraction rapidly ceased in the presence of 1 mm-procaine; however, over 5 mm-procaine was required to abolish the caffeine-induced contraction.5. The pCa-tension relationship was measured in saponin-treated skinned muscles. The minimum concentration of free Ca required to produce contraction was 2 x 10(-7)m, while maximum contraction was observed at 10(-5)m-free Ca. The maximum amplitude of Ca-induced contraction observed in skinned muscles was larger or the same as that evoked by ACh in the intact muscle. ACh (10(-5)m), caffeine (5 mm) and procaine (5 mm) had no effect on the pCa-tension relationship.6. After Ca has been loaded in skinned muscles, caffeine and replacement of K with choline (116 mm) in the Ca-free EGTA containing solution produced the contraction. However, the application of ACh did not result in a release of the stored Ca, and procaine suppressed the release of Ca activated by caffeine.7. The present results indicate that ACh activates the muscarinic receptor distributed on the plasma membrane, thus releasing the stored Ca. However, this mechanism may not be a prerequisite for direct action of ACh on the Ca releasing site. The possible mechanism of ACh action on the Ca releasing site, mainly sarcoplasmic reticulum, was discussed in relation to actions of caffeine and procaine, particularly with regard to the functional relations between plasma membrane and sarcoplasmic reticulum.