Effect of perfusate Ca2+ on the relation between metabolism and mechanical performance in the rat heart

Langendorf perfused rat hearts (n = 25) were used to study the effects of changes in perfusate Ca2+ concentration ([Ca2+p]) on the relation between metabolism and mechanical performance with either glucose or pyruvate as the exogenous substrate. Increased [Ca2+p] (from 1.3 to 3.9 mM) produced an increase (243 +/- 38%) in left ventricular developed pressure regardless of the substrate. With glucose as the substrate, the NADH fluorescence intensity increased by 11.8 +/- 1.2% (n = 17) relative to control indicating a more reduced state of the respiratory chain. Increasing [Ca2+p] in the pyruvate perfused heart produced the expected NADH oxidation (-6.2 +/- 1.1%; n = 8). Hence the change in NADH fluorescence associated with increased [Ca2+p] is substrate dependent. The data show that, with glucose as the substrate but not with pyruvate, increases in [Ca2+p] increase the availability of reducing equivalents to the respiratory chain above the level necessary to compensate for the increased demand resulting from the greater contractile performance.