Radiolabeled acetate as a tracer of myocardial tricarboxylic acid cycle flux

The kinetics of [1-¹⁴C]acetate oxidation in isolated perfused rat hearts have been determined over a range of perfusion conditions. Effluent measurements demonstrated that ¹⁴CO₂ cleared biexponentially over 50 minutes after bolus injection of [1-¹⁴C]acetate into normoxic hearts perfused with 5 mM glucose and 10 mU/ml insulin. The clearance half-time (t 1/2 ) for the predominant initial clearance phase was 3.1 ± 0.5 minutes (n = 4). MV̇O₂ was varied over a fourfold range by hypoxia and phenylephrine stimulation (t 1/2 , 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 consumption. In contrast to results with [1-¹⁴C]acetate, lactate (2 mM) increased t 1/2 for ¹⁴CO₂ clearance from a bolus injection of [1-¹⁴C]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 [1-¹⁴C]acetate. ¹⁴CO₂ clearance accounted for 88-98% of total effluent ¹⁴C between 10 and 20 minutes after [1-¹⁴C]acetate bolus injection; rate constants for clearance of ¹⁴CO₂ and total ¹⁴C clearance were very similar during this period, and these two rate constants did not differ significantly from each other under any conditions tested. It is concluded that radiolabeled acetate kinetics are insensitive to myocardial substrate supply and may thus provide the basis for quantitative estimation of myocardial TCA cycle flux, allowing ¹¹C acetate to be used in vivo to assess directly myocardial oxidation metabolism with positron emission tomography.