Journal of Applied Physiology, Vol 67, Issue 2 671-676, Copyright © 1989 by American Physiological Society
Aminophylline enhances contractility of frog skeletal muscle: an effect dependent on extracellular calcium
J. W. Ridings, S. R. Barry and J. A. Faulkner
Department of Physiology, University of Michigan Medical School, Ann Arbor 48109.
The effects of aminophylline (10-500 microM) on isometric twitch and
tetanic forces were studied in vitro on frog semitendinosus muscle. Two
hypotheses were tested: 1) that micromolar concentrations of aminophylline
enhanced contractility of isolated skeletal muscle and 2) that the
potentiating effect of aminophylline was dependent on the presence of
extracellular calcium ions. Muscles were removed, placed in aerated Ringer
solution at 20 degrees C, attached to a force transducer, and stimulated
directly. Muscles in normal Ringer and aminophylline Ringer were compared
throughout the frequency-force relationship from twitches to maximum
tetanic force. Aminophylline increased twitch force significantly at
concentrations as low as 25 microM. Over a range of stimulation
frequencies, but especially at 10 and 20 Hz, aminophylline increased
tetanic force. The potentiating effect of aminophylline (100 microM) was
reduced or eliminated in calcium-free Ringer containing 10 mM magnesium. We
conclude that aminophylline, at therapeutic concentrations, enhances muscle
contractility, and the enhancement is dependent on the presence of
extracellular calcium. These findings support the concept that
aminophylline is effective in improving respiration in humans with airway
obstruction by enhancing diaphragmatic contractility.
