Graded activation in rabbit mesotubarium smooth muscle.

The outer margin of the mesotubarium superius, an accessory ligament from the femal rabbit reproductive system, contains a long and slender bundle of smooth muscle fibers well aligned with the long axis of the tissue. Very little connective tissue is present. Massive alternating-current electrical field stimuli (variable in frequency, amplitude, and duration) applied to isolated mesotubaria from mature, nonpregnant animals produced contractions in which isometric force (P) and its first derivative (dP/dt) could be continuously graded; an optimum existed for stimulus frequency and amplitude, but not for its duration. Twitchlike contractions could not be produced. Isometric contractions in which P was plotted against dP/dt to generate a phase-plane trajectory were used to predict the ultimate (time = infinity) force (Po) in graded contractions; this value agreed with the Po derived from isotonic force-velocity curves fitted to the Hill equation. Quick stretches applied during the rise of contractile force revealed a slow onset of the ability to bear the Po force.