Effect of Zinc Ions on Caffeine-Induced Contracture in Vascular Smooth Muscle and Skeletal Mu-scle of Rat

Abstract

The effect of zinc ions on caffeine-induced contracture in vascular smooth muscle and skeletal muscle of rat was studied. In aortic strips, caffeine contracture was depressed by Zn²⁺ in a dose-dependent manner. Moreover, the extent of the depression of caffeine contracture in the Zn²⁺ loaded smooth muscle increased with repetitive caffeine exposures. For instance, in the preparations perfused with a medium containing 100 µM [Zn²⁺] for 15 or 30 min, the first caffeine contractures were similarly depressed to about 60% of the control. However, the subsequent caffeine exposure at 15 min interval could not evoke any contracture. In this study this feature is referred to as activation dependence of the Zn²⁺ effect. In small bundles of soleus muscle 2∼100 µM [Zn²⁺] similarly caused a depression of caffeine contracture, and the activation dependence also was evident. However, an evident potentiation of caffeine contracture was seen in the preparations loaded with lower [Zn²⁺] such as 0.5 µM, indicating that the effect of Zn²⁺ on caffeine contracture of skeletal muscle was somewhat different from that seen in smooth muscle. By observing how the depression effect depends on the intervals between caffeine exposures as well as on caffeine concentrations, it is indicated that the activation dependence of the Zn²⁺ effect, at least in skeletal muscle, may not be explained by depletion of intracellular Ca²⁺ stores alone. The possible mechanism for that caffeine contracture of smooth muscle and skeletal muscle was differentially affected by Zn²⁺ ions is discussed.