Comparative effects of aminophylline on diaphragm and cardiac contractility

The mechanisms by which aminophylline increases inspiratory muscle contractility are unclear. The present study compared the effects of aminophylline on cardiac as well as on diaphragm contractility and examined the interaction of aminophylline with verapamil, a calcium channel-blocking agent, on both types of muscle. Experiments were performed in mongrel dogs anesthetized with pentobarbitone. Diaphragm contractility was assessed from transdiaphragmatic pressure (Pdi) developed during supramaximal electrical stimulation of the cervical phrenic rootlets, and cardiac contractility was assessed from peak left ventricular (LV) pressure and its rate of rise (dPv/dt). Measurements were made before and after each of 3 sequential IV infusions of aminophylline (6 mg/kg) and subsequent IV infusions of verapamil (0.1 mg/kg bolus and 0.02 mg/kg/min for 5 to 7 min.). Transient decreases in Pdi were frequently observed immediately after aminophylline infusion in association with decreases in mean arterial blood pressure. With recovery of mean blood pressure, Pdi increased above baseline values. Aminophylline increased Pdi in a dose-dependent fashion over the entire frequency range studied (1 to 40 Hz). Aminophylline increased the rate of rise of Pdi (dPd/dt) without affecting the period over which pressure was developed or dissipated during single twitches. Aminophylline increased both peak LV pressure and dPv/dt. The magnitude of the cardiac response was greater than the diaphragmatic response. Subsequent verapamil infusion completely reversed the effects of aminophylline on LV contractility but had only a small effect on diaphragm contractility.(ABSTRACT TRUNCATED AT 250 WORDS)