Mechanical restitution at different temperatures in papillary muscles from rabbit, rat, and hedgehog

Mechanical restitution curves, i.e., peak isometric force as a function of the duration of the preceding test interval, were investigated in papillary muscles from rabbit, rat, and hedgehog. Peak force of rabbit papillary muscle increased with prolongation of the test interval from about 0.3 s to about 1.0 s and for longer intervals peak force declined (called type I mechanical restitution). On the other hand, in rat and hedgehog, papillary muscles' force reached a maximum value at intervals of 30-120 s (called type II mechanical restitution). When temperature was decreased from 35 to 15 degrees C, maximum force of type I mechanical restitution shifted from 1.0 to 10 s, whereas maximum force of type II restitution did not change significantly. Type II mechanical restitution consisted of two different phases, designated phase A and phase B, respectively. As temperature was decreased from 35 to 0 degree C in the hedgehog preparation, the two phases became even more separated. At 35 degrees C, the rising part of mechanical restitution in the rabbit muscle could not be distinguished from phase A of the hedgehog preparation and was also very similar to phase A of the rat muscle. Phase A is thus present in both type I and type II mechanical restitution, but phase B is a special feature of type II mechanical restitution. Phase A and phase B might be a manifestation of activator calcium originating from two different sources, e.g., the sarcoplasmic reticulum and the sarcolemma.