Abstract
Effects of a Ca2+ channel facilitator, CGP 28,392, on smooth muscle contractions were examined in order to delineate characteristics of Ca2+ channels in rabbit and rat aortae and guinea-pig taenia caeci. Application of increasing concentrations of KCl induced contractile responses in these smooth muscles and CGP 28,392 shifted the concentration-response curve for KCl to the left. The maximum response was also increased in rat aorta and guinea-pig taenia. CGP 28,392 also shifted the concentration-response curves for noradrenaline in rat aorta and for histamine in taenia to the left and increased the maximum response in rat aorta. However, the corresponding curve for noradrenaline in rabbit aorta was not affected by CGP 28,392. The sustained contractions induced by KCl were inhibited by cumulative application of verapamil in these smooth muscles. Pretreatment of the muscle with CGP 28,392 decreased the inhibitory effect of verapamil. The noradrenaline-induced contraction in rat aorta and the histamine-induced contraction in taenia were also inhibited by verapamil, and CGP 28,392 antagonized the effect of verapamil. The noradrenaline-induced contraction in rabbit aorta was only slightly inhibited by verapamil, and CGP 28,392 did not modify the effect of verapamil. In these smooth muscles, cumulative application of Ca2+ to the Ca2+-depleted, KCl-treated muscle induced contraction, and the concentration-response curve for Ca2+ was shifted to the left by CGP 28,392 and to the right by verapamil. The concentration-response curves for Ca2+ in Ca2+-depleted, noradrenaline-treated rabbit and rat aortae and in Ca2+-depleted, histamine-treated taenia were also shifted to the left by CGP 28,392 and to the right by verapamil. In some contractions, CGP 28,392 increased and verapamil decreased the maximum responses. CGP 28,392 antagonized the inhibitory effect of verapamil. 5 These results suggest that the Ca2 channel facilitator, CGP 28,392, has a relatively selective activating effect on voltage-dependent Ca2+ channels in rabbit aorta. However, it also activates receptor-linked Ca2+ channels in rabbit aorta when Ca2+ concentrations are low. In rat aorta and guinea-pig taenia this facilitator activates both types of Ca2+ channels.
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