Radiolabeled acetate as a tracer of myocardial tricarboxylic acid cycle flux, Circ

The kinetics of [l-l4C]acetate oxidation in isolated perfused rat hearts have been determined over a range of perfusion conditions. Effluent measurements demonstrated that I4CO2 cleared biexponentially over 50 minutes after bolus injection of [l-l4C]acetate into normoxic hearts perfused with 5 mM glucose and 10 mU/ml insulin. The clearance half-time (tr/2) for the predominant initial clearance phase was 3.1 ±0.5 minutes (n = 4). MVo2 was varied over a fourfold range by hypoxia and phenylephrine stimulation (tw, 7.2 ± 1.2 and 2.2 ±0.2 minutes, respectively) and in the presence of alternate substrates (lactate, 2 mM; DL-3-hydroxybutyrate, 20 mM; and palmitate, 0.1 mM), which did not modify either tricarboxylic acid (TCA) cycle flux or acetate kinetics. A good correlation (r=0.93) was observed between k, the rate constant for the initial phase of 14CO2 clearance, and TCA cycle flux, estimated from oxygen consump-tion. In contrast to results with [l-14C]acetate, lactate (2 mM) increased t w for 14CO2 clearance from a bolus injection of [l-14C]palmitate from 3.0 ±0.4 minutes (n = 3) at control to 4.3 ±0.2 minutes (n = 3, p<0.01). Addition of acetate in nontracer amounts (0.5 or 5 mM) caused significant underestimation of TCA cycle flux when estimated with [l-l4C]acetate. 14CO2 clearance accounted for 88-98 % of total effluent 14C between 10 and 20 minutes afte