Substitution of strontium for calcium in mammalian heart muscle delayed the onset of maximum unloaded active velocity to 30-35% of the time to peak force, compared to 16% in calcium solutions.
The substitution of strontium for calcium delays the onset of mechanical activation in mammalian heart muscle, highlighting a close relationship between the force-velocity-length time course and the degree of activation.
The time course of the relationship between force, shortening velocity, and length during shortening was examined in mammalian heart muscle in calcium (Ca)-containing solutions and solutions with strontium (Sr) substituted for Ca by imposing load clamps of various amplitudes at various times during external shortening. The onset of this force-velocity-length relationship was measured by abruptly unloading the muscle at different times to nearly zero external load--by zero load clamping the muscle--under conditions of optimal damping; the subsequent unloaded active muscle shortening velocity was then analyzed with respect to time and length. In Ca- containing solution, the unloaded velocity in the length range around 90% of L max rose within 16% of the time required to reach peak isometric force to a maximum level appropriate to a given contractile state. Subsequently, the force-velocity-length relationship was independent of time at any load over a well-determined portion of external shortening. In Sr-containing solution, unloaded active velocity rose much slower to reach a steady state after 30-35% of the time to peak force, and the time independence of the force-velocity-length relationship was delayed until after this time. In Na-free Sr-containing solution, maximum unloaded shortening velocity and the time-independent portion of the force-velocity-length relationship were achieved again within 20% of the time to peak force. The effects of substitution of Sr for Ca thus resemble those of caffeine on mammalian heart muscle. These findings suggest a close relationship between the time course of the force-velocity-length relationship and the degree of activation.
Brutsaert et al. (Sun,) conducted a other in Mammalian heart muscle mechanics. Strontium (Sr)-containing solutions vs. Calcium (Ca)-containing solutions was evaluated on Onset of the force-velocity-length relationship (unloaded active muscle shortening velocity). Substitution of strontium for calcium in mammalian heart muscle delayed the onset of maximum unloaded active velocity to 30-35% of the time to peak force, compared to 16% in calcium solutions.