Excitation, contraction, and excitation-contraction coupling of frog muscles in hypotonic solutions

Abstract

Twitch, tetanus, K contracture, and caffeine contracture tensions, and extracellularly recorded action potentials were measured in frog toe muscles bathed in hypotonic Ringer solutions. Tetanic tension is potentiated in these solutions down to 0.4 normal tonicity (0.4T), but falls off sharply to about 15% of the 1T value at 0.25T. The extracellularly recorded action potential also falls off at low tonicities but still is at 99% of the 1T value at 0.25T where the tetanus is only 15% of the 1T value. By contrast, the caffeine contracture tension is maintained at all tonicities indicating that hypotonic solutions have little direct effect on contractile proteins. It is shown that the decline of tension in very hypotonic solutions is due both to a loss in excitability and a true uncoupling of excitation and contraction. Raising [Ca] 0 to 3 times normal (5.4mM) allows recording of action potentials with no contraction. The loss of excitability and the decline of tension are not purely due to the decreased [Na] 0 because when NaCl is replaced with an osmotic equivalent of sucrose, tension and compound action potentials did not decline to zero. The loss of excitability may be due to the membrane depolarization observed in microelectrode studies.