THE EFFECT OF AMINOPHYLLINE ON THE CONTRACTION THRESHOLD OF RAT DIAPHRAGM FIBERS AND ITS MODIFICATION BY 9-AMINOACRIDINE

We studied the effect of aminophylline (1 mM) and 9-aminoacridine (100 mu M) on the contraction threshold (CT) of rat diaphragm fibers (25 d egrees C). The CT was measured by direct visualization (200 X) of the fiber under current-clamp conditions. The main findings are the follow ing: 1) Aminophylline lowers the CT toward more negative values of the resting membrane potential (V-m). 2) 9-aminoacridine, a drug that dim inishes Ca2+ release from the sarcoplasmic reticulum (SR), shifts the CT toward more positive values; 3) this effect is overcome by aminophy lline. We suggest that the displacement in the CT to more negative V-m plays an important role in the potentiating effect of aminophylline. This could be the result of an enhancement of Ca2+ release from the SR .