Theophylline produces an adverse effect on myocardial lactate metabolism at a therapeutic serum concentration: an effect blocked by verapamil.

To assess the actions of theophylline on coronary blood flow and myocardial energetics, 1 mg/kg/min of this methylxanthine was infused i.v. in the dog for 15 min, producing an average concentration of 18 +/- 3 micrograms/ml. Heart rate increased by 20 +/- 7 min-1, P less than .05, but left ventricular (LV) systolic and end-diastolic pressures, cardiac output and LV peak dp/dt did not change. Although coronary vascular resistance decreased by 0.26 +/- 0.08 mm Hg/ml/min, P less than .05, coronary flow and myocardial oxygen consumption and extraction did not change. Myocardial lactate extraction decreased, from 22 +/- 2 to 1 +/- 5%, P less than .05. The decrement in lactate extraction was not related to heart rate, but to the change in LV peak dp/dt, r = 0.74, P less than .05. Furthermore, with verapamil, 0.2 mg/kg, pretreatment, and 0.2 mg/kg, during the theophylline infusion, reduction in lactate extraction was blocked and LV peak dp/dt increased by 843 +/- 311 mm Hg/sec, P less than .05. Thus, at therapeutic concentrations, theophylline reduces myocardial lactate extraction, an effect that is associated with the absence of the expected inotropic actions of theophylline. However, when verapamil is administered with theophylline, a reduction of myocardial extraction does not occur and myocardial inotropy is enhanced.