EFFECTS OF ISOPROTERENOL INFUSION ON THE MYOCARDIAL UPTAKE OF FATTY-ACIDS AND OTHER SUBSTRATES IN LAMBS WITH AN AORTOPULMONARY SHUNT

Isoproterenol, used in the management of infants with left-to-right sh unts and circulatory congestion, increases myocardial work load and ox ygen consumption. In addition, it may selectively enhance myocardial f atty acid utilization, The less efficient oxidation of FFA could induc e an oxygen wasting effect and thus further increase myocardial oxygen consumption. The combination of such an oxygen wasting effect and the chrono-inotropic and inotropic effects of isoproterenol could induce an imbalance between myocardial oxygen supply and demand in hearts of which resting oxygen consumption is already elevated. We studied myoca rdial substrate uptake (FFA, triglycerides, glucose, lactate, pyruvate , beta-OH-butyrate, and acetoacetate) in 10 7-wk-old lambs with an aor topulmonary left-to-right shunt (57 +/- 4% of left ventricular output, mean +/- SEM) and 9 control lambs during isoproterenol infusion (0.1 mu mol . min(-1). kg(-1)). Myocar dial blood flow and oxygen consumpti on increased in both groups but less in shunt than in control lambs be cause of the smaller rise in heart rate in the shunt lambs. The arteri al FFA concentration increased 3-fold in both groups and was not diffe rent between the two groups. The FFA arteriocoronary sinus difference, however, was not affected by the isoproterenol infusion. The myocardi al FFA uptake thus followed the changes in myocardial blood flow and d id not increase more in shunt than in control lambs. Isoproterenol inf usion does in spite of a 3-fold increase in arterial FFA concentration , not induce a shift toward a greater percentage uptake of fatty acids compared with other substrates in lambs with aortopulmonary left-to-r ight shunt, so that the possibility of an oxygen wasting effect can be ruled out as an unwanted side effect.