Time course of mechanical activity in mammalian cardiac muscle: dependence on species, loading, and displacement.

We have found that animal species is of great importance when mechanics of contraction are analyzed in mammalian cardiac muscle. Mechanics of contraction have been studied comparatively in the isolated papillary muscles of the rat, cat, and rabbit under control conditions and following alterations of loading and inotropic interventions. The time course of the shortening ability measured in terms of the velocity of shortening at constant contractile element length and load following quick releases to preload peaked at 20, 30, and 70% of time to peak force (TPF) in the rat, cat, and rabbit, respectively, and declined more slowly in the rabbit. The shortening ability peaked progressively later in contraction at increasing loads in the cat, but not in the rabbit. Shortening ability measured after releases to zero load or after imposing "load clamps" of variable magnitude and duration reached the maximum much later in the rabbit than in the cat. The contractile response to changes of the contraction mode from isometric to isotonic was different in cat and rabbit. Upon increase in the stimulation rate from 6 to 60/min, force increased to a greater extent in the rabbit than in the cat, while TPF did not decrease in the rabbit as seen in the cat. Caffeine (10mM) increased developed force more in the rabbit than in the cat, while even 1 mM caffeine decreased force in the rat. The onset of the shortening ability was greatly delayed by caffeine in the cat and rat but not in the rabbit. Afterloaded force-peak velocity relations were curvilinear in the cat and rat; these relations approached linearity in the rabbit in the control state, and in both cat and rabbit in the presence of caffeine. The differences in the mechanics of contraction, in the contractile response to alterations in loading, and to caffeine in the ventricular muscle of the rat, cat, and rabbit are viewed as indicators of differences in excitation-contraction coupling of ventricular cardiac muscle from these different mammalian species.