Coupling of mitochondrial fatty acid uptake to oxidative flux in the intact heart

The coordination of long chain fatty acid (LCFA) transport: across the mitochondrial membrane (V-PAL) with subsequent oxidation rate through beta-oxidation and the tricarboxylic acid (TCA) cycle (V-toa) has been difficult to characterize in the intact heart. Kinetic analysis of dynamic C-13-NMR distinguished these flux rates in isolated rabbit hearts. Hearts were perfused in a 9.4 T magnet with either 0.5 mM [2,4,6,8,10,12,14,16-C-13(8)] palmitate (n = 4), or 0.5 mM C-13-labeled palmitate plus 0.08 mM unlabeled butyrate (n = 4). Butyrate is a short chain fatty acid (SCFA) that bypasses the LCFA transporters of mitochondria. In hearts oxidizing palmitate alone, the ratio of V-TCA to V-PAL was 8:1. This is consistent with one molecule of palmitate yielding eight molecules of acetyl-CoA for the subsequent oxidation through the TCA cycle. Addition of butyrate elevated this ratio; V-TCA/V-PAL = 12:1 due to an SCFA-induced increase in V-TCA of 43% (p < 0.05). However, SCFA oxidation did not significantly reduce palmitate transport into the mitochondria: V-PAL = 1.0 +/- 0.2 mu mol/min/g dw with palmitate alone versus 0.9 +/- 0.1 with palmitate plus butyrate. Thus, the products of beta-oxidation are preferentially channeled to the TCA cycle, away from mitochondrial efflux via carnitine acetyltransferase.