Changes in contractility of frog muscle due to fatigue and inhibitors

Abstract

Continuous activity in excised and curarized frog sartorius muscles reduced twitch tension, contraction rate, and half-relaxation rate, and increased contraction and half-relaxation times. These changes are essentially the same as those reported by others for single fibers. 0.05 mm 2,4-dinitrophenol (DNP), 1 mm NaN3, and 1 mm KCN reduced twitch and tetanus tension in rested muscles. DNP and KCN shortened contraction and half-relaxation times, but NaN3 induced a late prolongation. DNP and NaN3 reduced contraction and half-relaxation rates, but KCN enhanced them. DNP and NaN3 accentuated fatigue declines in tension output. KCN consistently induced twitch treppe, while tetanus tension declined as in unpoisoned fatigue. All the agents enhanced fatigue prolongations of contraction and half-relaxation times. DNP and NaN3 accelerated the fatigue declines of contraction and half-relaxation rates, but KCN retarded them. All inhibitor effects were at least partially reversible. These results differ in many respects from equivalent results obtained with mouse muscles. Known inhibitor mechanisms do not as yet account adequately for the present findings.