A Shift of Cellular Calcium to a More Slowly Exchangeable Fraction During Contraction in Guinea Pig Taenia Coli

It is now generally agreed that Ca ion is essential for the contraction of smooth muscle. On the role of Ca ion in the contraction cycle of smooth muscle, many electrophysiological studies have suggested that the action potential of the muscle cell is associated with an inward movement of Ca ions from anionic sites of cell membrane or from extracellular space and this Ca flow may initiate contractile response (1, 2). It has also been suggested from radioisotope studies (3–6) that contraction in smooth muscle may be triggered and maintained by release of Cellular Ca and/or by influx of extracelluar Ca.