Contraction of depolarized smooth muscle by electric fields

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Abstract
Smooth muscle strips of the circular layer of cat intestine depolarized with potassium (150 mEq/l.) were subjected to electric fields. The field strengths necessary to generate equal tensions were much higher in depolarized muscle (5–25 v/cm) than in non-depolarized muscle (0.5–2 v/cm). It was found that maximal tension generated varied linearly, within limits, with strength and duration of the field. A longitudinal field, with respect to the long axis of the fibers, was more effective than a transverse field. Tensions generated in sinusoidal a-c fields decreased markedly between 40–1,000 cycles/sec; the low tensions generated at the higher frequencies could be increased by increasing the field strength. With rectangular d-c pulses, tension generated at 10,000 cycles/sec was greater than that at 10 cycles/sec. Treatment with a detergent irreversibly blocked the action potential-dependent contractions but did not prevent the field-induced contractions. A single, brief contracture generally occurred immediately after addition of acetylcholine to the bath (5–50 µg/ml); in addition, the field-induced tension was markedly increased without an effect on the resting tension. Prior atropinization blocked both acetylcholine effects. Acidification of the bathing solution enhanced the field-induced tensions. Sr replaced Ca in the field contractions but not in excitation-contraction coupling. It is suggested that, in depolarized smooth muscle, displacement of charged particles by the application of large voltage gradients leads to the generation of tension; the excitation-contraction coupling process appears to be bypassed.