A rise in internal Ca2+ is the signal which activates the contractile mechanism in muscle. In striated muscle there is a well developed sarcoplasmic reticulum which stores and releases the bulk of the Ca2+ involved in the contraction cycle1,2, allowing the muscle to operate relatively independently of external calcium levels. In smooth muscle the sarcoplasmic reticulum system is much less extensive3,4 and normal contractile activity is very sensitive to calcium levels in the bathing medium5?7. These findings, together with the small size of smooth muscle cells which minimises diffusion time from the plasma membrane to the contractile apparatus, have suggested to some investigators that the Ca2+ which enters the cell during Ca2+-mediated action potentials is either sufficient by itself to activate contraction or is a necessary trigger for the further release of Ca2+ from internal stores8?11. This notion is given support by the observation that contractile activity rapidly dies out when smooth muscle preparations are exposed to calcium-free solutions. However, electrical activity is eventually lost in these solutions. We report here on spontaneous contractile and electrical activity, and evoked mechanical activity, recorded after prolonged exposure to calcium-free EGTA saline in intestinal smooth muscle of cat.
Contractions of cat small intestinal smooth muscle in calcium-free solution