Effect of length and caffeine on isometric tetanus relaxation of frog sartorius muscles.

In an isometric tetanus of frog sartorius muscle the total relaxation time increased linearly with change in length from 0.7 to 1.4 times rest length. Maximal rate of relaxation, measured from the time derivative (dp/dt) of tension decay, decreased with both decrease and increase from rest length in correlation with the generated tetanus tension. Stretching the muscle did not significantly affect the times to maximal rate, positive and negative inflexion points but greatly increased the time to total relaxation from the negative inflexion point. Caffeine at 2 mM, acting on muscles at rest length, also slowed the relaxation and decreased the maximal rate of tension decay. However, caffeine increased the times to maximal rate, positive and negative inflexion points without significantly affecting time to total relaxation from the negative inflexion point. These results suggest that caffeine slows an earlier step in relaxation, while stretch slows a later step. It is proposed that muscle relaxation is a two step process: an initial step that is regulated by the rate of Ca2+ uptake by sarcoplasmic reticulum, and a later step that is mostly controlled by the speed of dissociation of remaining cross-bridges.