The effect of aminophylline on the contraction threshold of rat diaphragm fibers and its modification by 9-aminoacridine.

We studied the effect of aminophylline (1mM) and 9-aminoacridine (100 microM) on the contraction threshold (CT) of rat diaphragm fibers (25 degrees C). The CT was measured by direct visualization (200 X) of the fiber under current-clamp conditions. The main findings are the following: 1) Aminophylline lowers the CT toward more negative values of the resting membrane potential (Vm). 2) 9-aminoacridine, a drug that diminishes Ca2+ release from the sarcoplasmic reticulum (SR), shifts the CT toward more positive values: 3) this effect is overcome by aminophylline. 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 Ca2+ release from the SR.