Contraction of isolated smooth muscle cells by inophore A23187

Ionophore A23187 causes contraction of smooth muscle cells isolated from the stomach muscularis of Bufo marinus. The peak response is reached within 10 sec of inophore addition and then decays to a lesser, sustained contraction. The initial phase of contraction is maximal at A23187 concentrations greater than 5 X 10(-6) M and half maximal at 5 X 10(-7) M. Chelation of extracellular calcium by ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) does not affect the initial contraction produced by A23187, whereas it inhibits completely the sustained, secondary phase. Addition of calcium in excess of EGTA restores this phase rapidly and fully to control levels of contraction. A23187 produces a maximal, rapid contraction of muscle cells previously depolarized by incubation in high potassium medium. The ionophore also contracts isolated strips of stomach smooth muscle. A23187 appears to produce contraction of the isolated smooth muscle cells by transmembrane transport of calcium into the cytoplasm from both intracellular and extracellular sources.