American Joixnal of Physiology

HURWITZ, LEON, PAUL D. JOINER, AND STANLEY VON HAGEN. Calcium pooh utili<ed for contraction in smooth muscle. Am. J. Physiol. 213(5): 1299-1304. 1967.-Isolated intestinal muscles were incubated in a high-calcium medium and then transferred to a calcium-free medium where they developed a transient increase in muscle tone. As the period of incubation in the high-calcium medium was lengthened, the fibers accumulated increasingly more calcium and exhibited increasingly larger contractile responses in the calcium-free medium until constant levels were reached. If, in addition to removing calcium from the bathing solution, acetylcholine was introduced, the contractile response was enhanced. When acetylcholine was added to the bathing solution without removing the calcium, the transient response was converted to a sustained one. These observations suggest that both extracellular and cellular calcium may be mobilized to activate contractile elements. Experiments were so designed that equal responses were induced by calcium ions which, in one case, came primarily from an extracellular pool, and, in another case, from an intracellular pool. In the former case the calcium content of the muscle was found to be comparatively smaller and the response was less well sustained in a calcium-free environment. These results suggest that extracellular calcium ions enter the myoplasm of the smooth muscle fibers by at least two parallel pathways.