Calcium pools utilized for contraction in smooth muscle

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Abstract
Isolated intestinal muscles were incubated in a high Ca medium and then transferred to a Ca-free medium where they developed a transient increase in muscle tone. As the period of incubation in the high-Ca medium was lengthened, the fibers accumulated increasingly more Ca and exhibited increasingly larger contractile responses in the Ca-free medium until constant levels were reached. If, in addition to removing Ca from the bathing solution, acetylcholine was introduced, the contractile response was enhanced. When acetylcholine was added to the bathing solution without removing the Ca, the transient response was converted to a sustained one. Both extracellular and cellular Ca may be mobilized to activate contractile elements. Experiments were so designed that equal responses were induced by Ca ions which, in one case, came primarily from an extracellular pool, and, in another case, from an intracellular pool. In the former case the Ca content of the muscle was comparatively smaller and the response was less well sustained in a Ca-free environment. Extracellular Ca ions enter the myoplasm of the smooth muscle fibers by at least 2 parallel pathways.