Pyruvate potentiates inotropic effects of isoproterenol and Ca2+ in rabbit cardiac muscle preparations

Catecholamines and elevated extracellular Ca2+ concentration ([Ca2+](o)) augment contractile force by increased Ca2+ influx and subsequent increased sarcoplasmic reticulum (SR) Ca2+ release. We tested the hypothesis that pyruvate potentiates Ca2+ release and inotropic response to isoproterenol and elevated [Ca2+](o), since this might be of potential importance in a clinical setting to circumvent deleterious effects on energy demand during application of catecholamines. Therefore, we investigated isometrically contracting myocardial preparations from rabbit hearts at 37 degrees C, pH 7.4, and a stimulation frequency of 1 Hz. At a [Ca2+](o) of 1.25 mM, pyruvate (10 mM) alone increased developed force (F-dev) from 1.89 +/- 0.42 to 3.62 +/- 0.62 (SE) mN/mm(2) (n = 8, P < 0.05) and isoproterenol (10(-6) M) alone increased F-dev from 2.06 +/- 0.55 to 25.11 +/- 2.1 mN/mm(2) (P < 0.05), whereas the combination of isoproterenol and pyruvate increased F-dev overproportionally from 1.89 +/- 0.42 to 33.31 +/- 3.18 mN/mm(2) (P < 0.05). In a separate series of experiments, we assessed SR Ca2+ content by means of rapid cooling contractures and observed that, despite no further increase in F-dev by increasing [Ca2+](o) from 8 to 16 mM, 10 mM pyruvate could still increase F-dev from 26.4 +/- 6.8 to 29.7 +/- 7.1 mN/mm(2) (P < 0.05, n = 9) as well as the Ca2+ load of the SR. The results show that the positive inotropic effects of pyruvate potentiate the inotropic effects of isoproterenol or Ca2+, because in the presence of pyruvate, Ca2+ and isoproterenol induced larger increases in inotropy than can be calculated by mere addition of the individual effects.