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Contractile activity in smooth muscle is usually too variable to permit a meaningful analysis of the time course of the active state. A regular activity of uniform contractions was obtained in the present experiments on isolated rat portal veins by simultaneous increases in the external potassium and calcium concentrations. The contractions were associated with electrical discharges which lasted between 0.8 and 1.5 seconds. The active state during these uniform contractions was determined from the shortening velocities after isotonic releases and from the redevelopment of tension after controlled releases. Results obtained with the two methods agreed satisfactorily. The active state increased over the first 0.4-0.6 seconds. Suggestive evidence of a plateau phase was found in some experiments. The active state clearly began to fall about 1 second after the start of the contraction, and this fall coincided with the fall in the first time derivative of isometric force (dP/dt). The maximal intensity of the active state was, on the average, 173% of the maximal isometric force. During isometric contractions, the series elastic element was stretched by as much as 15% of the total muscle length. Active-state values were therefore corrected for changes in contractile element length. The curve then obtained might represent the time course of the level of activation, reflect variations in internal calcium concentration, and relate to the electrical response. Concepts developed for the mechanics of striated muscle can apparently be applied to propagating vascular smooth muscle, and this applicability might help to clarify electromechanical relations and the actions of vasoactive substances in blood vessels.

Active State in the Smooth Muscle of the Rat Portal Vein in Relation to Electrical Activity and Isometric Force