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A B S T R A C T Chemically skinned fibers from guinea pig taenia caecum were prepared by saponin treatment to study the smooth muscle contractile system in a state as close to the living state as possible. The skinned fibers showed tension development with an increase of Ca 2+ in the solution, the threshold tension occurring as 5 x 10 -7 M Ca 2 § The maximal tension induced with 10 -4 M Ca 2+ was as large and rapid as the potassium-induced contracture in the intact fibers. The slope of the pCa tension curve was less steep than that of skeletal muscle fibers and shifted in the direction of lower pCa with an increase of MgATP. The presence of >1 mM Mg 2+ was required for Ca2+-induced contraction in the skinned fibers as well as for the activation of ATPase and superprecipitation in smooth muscle myosin B. Mg 2+ above 2 mM caused a slow tension development by itself in the absence of Ca 2 § Such a Mg2 § tension showed a linear relation to concentrations up to 8 mM in the presence of MgATP. Increase of MgATP concentration revealed a monophasic response without inhibition of Ca2+-induced tension development, unlike the biphasic response in striated muscle. When MgATP was removed from the relaxing solution, the tension developed slowly and slightly, even though the Mg ~+ concentrations was fixed at 2 mM. These results suggest a substantial difference in the mode of actin-myosin interaction between smooth and skeletal muscle.

We studied the effects of Ca++ antagonists on intact and skinned muscles of rabbit mesenteric artery. Intact muscle contractions were inhibited by 10(-6) M diltiazem, whereas greater levels were required to abolish contractions in skinned muscle fibers. In contrast, nisoldipine had no effect on skinned muscle contractions, although it inhibited, almost completely, the contraction of intact muscle at concentrations below 10(-6) M. In the presence of EGTA, norepinephrine-induced contractions result from a release of Ca++ from an intracellular store. Diltiazem inhibited these contractions at concentrations between 10(-6) and 10(-4) M. Higher doses were required in studies with skinned muscle preparations. Unlike diltiazem, nisoldipine only partially inhibited the Ca++-free norepinephrine-induced contractions in the range of 10(-7) to 10(-5) M. From these results, we assumed that at low concentrations (below 10(-6) M), diltiazem induced relaxation by blocking Ca++ influx, whereas at relatively high concentrations (above 10(-6) M), an inhibition of Ca++ release from an intracellular store also occurred. A similar conclusion was reached regarding the mechanism whereby nisoldipine inhibits force developments.

A comparison was made between the properties of the norepinephrine- and caffeine-sensitive Ca²⁺ store in both intact and skinned smooth muscle of the rabbit mesenteric artery. After a first application of 10^-5M norepinephrine, reapplication of norepinephrine did not induce a second contraction in Ca²⁺-free medium. However, following this sequence 25 mM caffeine still induced a large contraction. The rates of Ca²⁺ leakage and Ca²⁺ filling of the norepinephrine-sensitive store were much faster than those of the caffeine-sensitive one. The amplitude of the norepinephrine-induced contraction in Ca²⁺-free medium also depended on the amount of Ca²⁺ present in the caffeine-sensitive store. In the saponin-treated skinned muscle caffeine induced a Ca²⁺ release only after loading with Ca²⁺, whereas norepinephrine was unable to induce Ca²⁺ release in the skinned preparation even after loading with Ca²⁺. The release of Ca²⁺ from the caffeine-sensitive store could be activated by Ca²⁺ itself when the skinned muscle was loaded with Ca²⁺ above 10^-6M. These results suggest that the norepinephrine-sensitive Ca²⁺ store is distinct from a large fraction of the caffeine-sensitive one, and that the norepinephrine-sensitive store is close to the cell membrane. In vascular smooth muscle, under physiological conditions, Ca²⁺ released from the norepinephrine-sensitive store by norepinephrine may induce Ca²⁺ release from the caffeine-sensitive Ca²⁺ store which may be comprised of the sarcoplasmic reticulum.

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