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To examine the origin of velocity deceleration in the shortening of smooth muscle, guinea pig tenea coli which contracts isometrically in a high-potassium solution was released quickly (1st release), restretched to the original length, kept at this length for 0.06-0.2s and quickly released again (2nd release). The muscle shortened isotonically under the same very-light load after the 1st and 2nd releases. The length change of the muscle during shortening was examined using a video camera. The rapid shortening phase, seen after the 1st release, which followed the instantaneous, elastic recoil and lasted for about 0.3s, disappeared after the 2nd release. This effect was prominent when the stretch duration was short. The time course of the slow phase of shortening subsequent to the rapid one was not affected by stretching. These results suggest that the rapid shortening phase is due largely to adjust of the cross-bridges between the thick and thin filaments to the change from an isometric to an isotonic contraction, while the slow phase reflects a steady turnover in the cross-bridges. The velocity of the slow shortening phase reached a maximum before the isometric force peaked and decreased as the activation level was lowered by reducing the external Ca2+ concentration from 2.5 to 0.5 mM. This suggests that the steady turnover rate of the cross-bridges depends on the phase of contraction and level of activation. The cause of the gradual velocity decrease in the slow phase of shortening was discussed.

Effect of a Brief Stretch on Time Course of Shortening after a Quick Release in Guinea Pig Tenea Coli.