Effect of Dihydropyridines on Visceral Smooth Muscle
The primary role of calcium ions as an intermediary in the regulation of excitation-contraction coupling is still accepted. Our present views on the cytoplasmic calcium, the intracellular stores of this ion and its regulation are based on indirect experimental evidence. The dimensions of smooth muscle cells and the complex structure of tissues composed of such cells have only allowed in a few instances to visualize cellular Ca compartments . Recently the changes of cytoplasmic calcium concentration have also been investigated using the bioluminiscent calcium indicator aequorin [2, 3]. The various experimental results obtained in studying smooth muscle cells suggest that these cells have an intracellular store from which calcium can be released and that a maintained contraction depends on a continuous supply of calcium from the external medium [4, 5]. Also the amplitude of these force developments is determined by the external calcium concentration. The cells could be considered as a one-way system with calcium flowing into the cells and being extruded out of the cell. The recycling of calcium between the cytoplasm and the cellular calcium store, as observed in skeletal muscle fibres is probably only poorly developed in smooth muscle. In Ca-free medium a single supramaximal stimulus with an agonist induces only a transient contraction and a second application of the stimulus during a maintained exposure to Ca-free medium is neither accompanied by a second phasic contraction nor by a release of Ca from the tissue.
KeywordsHistamine Prostaglandin Noradrenaline Progesterone Catecholamine
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