"Calcium-induced release of calcium" in rectal smooth muscle of mice

Contractile responses to K, ACh, Ba and exogenous Ca of rectal strips from mice were recorded isotonically. These responses consisted of phasic contraction and subsequent tonic contraction. In Ca-depleted preparations exposed to Ca-free bath medium, the contractions by K and ACh disappeared but the contraction by Ba remained constant. This residual contraction by Ba is presumably due to a direct stimulation of Ba to contractile elements of muscle. Metabolic inhibitors (anoxia and DNP) abolished tonic contraction without affecting phasic contraction by exogenous Ca. In Ca-free bath medium, tonic contraction by exogenous Ca immediately disappeared whereas phasic contraction gradually decreased and about 3 hr later remained constant in parallel with the occurrence of residual contraction by Ba. After pretreatment with the removal of Na from bath medium, which produced Ca-release action, phasic contractions by exogenous Ca in Ca-free bath medium were depressed like those by K, ACh and Ba. These results suggest that the phasic contraction by exogenous Ca is produced mainly by release of Ca and partly by influx of Ca. Thus, it is suggested that the same mechanism of Ca-induced release of Ca from the storage sited as described in skeletal muscle is also operating in rectal smooth muscle of mice.