Mechanical activity of mammalian heart muscle: variable onset, species differences, and the effect of caffeine.

The time course of the ability to shorten during contraction was measured using velocity of shortening of the contractile element corrected for length after quick releases to constant loads, in isolated papillary muscles of the cat, rabbit, dog, and rat. The ratios of time-to-peak shortening ability at preload (TTPA), to time-to-peak active force (TPF), were 0.30, 0.36, 0.32, and 0.70 in the cat, dog, rat and rabbit, respectively. When the pacing rate was increased from 12 to 60/min, peak force was augmented in the cat (36%) and rabbit (108%), while TPF decreased in the cat (by 30%) but not in the rabbit. Various inotropic interventions did not alter the ratio of TTPA/TPF in any species. However, caffeine (10mM) increased this ratio to that normally found in the rabbit, in which it was not altered. Afterloaded force-peak velocity relations of the cat, dog, and rat were curvilinear, but in the rabbit in the control state, and in the cat and dog in the presence of caffeine, these relationships tended to be linear. The differences in the time course of the ability to shorten among these mammalian species may be related to differences in excitation-contraction coupling.