Journal of Applied Physiology, Vol 71, Issue 4 1409-1414, Copyright © 1991 by American Physiological Society
Effect of aminophylline on the contraction threshold of rat diaphragm fibers
A. S. Losavio and B. A. Kotsias
Instituto de Investigaciones Medicas A. Lanari, Universidad de Buenos Aires, Argentina.
We studied the effect of aminophylline (0.1-1 mM) on the contraction
threshold (CT) of rat diaphragm fibers (25 degrees C). The CT was measured
by direct visualization (x200) of the fiber under current-clamp conditions.
The main findings are the following: 1) Aminophylline lowers the CT, in a
dose-dependent manner, toward more negative values of the resting membrane
potential (Vm). 2) Dibutyryl adenosine 3',5'-cyclic monophosphate (2 mM)
shifts the CT, although this change is smaller than in the presence of
xanthine. 3) Tetracaine (1 mM), a drug that diminishes Ca release from the
sarcoplasmic reticulum, reduces the shift induced by 1 mM aminophylline;
this is partially overcome by increasing aminophylline concentration to 5
mM. 4) Hyperpolarization of the fibers shifts the CT to more negative Vm.
We suggest that the displacement in the CT to more negative Vm plays an
important role in the potentiating effect of aminophylline. This could be
the result of an enhancement of Ca release from the sarcoplasmic reticulum.
