Uptake and esterification of circulating carnitine by aorta and heart in rabbits in vivo

Disappearance of intravenously injected DL-[methyl-14C]carnitine from the bloodstream and its uptake and esterification by heart and aorta were studied in rabbits fed atherogenic or non-atherogenic control diets. The disappearance rate of [14C]carnitine from the bloodstream was approximately 2-fold greater in animals fed the control diet than in those fed the atherogenic diets. No evidence was found for carnitine esterification in the blood. Circulating [14C]carnitine was taken up and esterified in both the heart and aorta of all animals regardless of diet; however, on comparing dpm/mg lipid-free dry weight, uptake of [14C]carnitine and accumulation of [14C]carnitine esters by the heart was greater (6-fold and 12-fold, respectively) than by the aorta. Analysis of defined arterial segments indicated that aortas in animals fed the atherogenic diet contained greater [14C]carnitine activity (4- to 8-fold) and greater acetyl-[14C]carnitine activity (4-fold) when compared to aortas of control animals; uptake of plasma [14C]carnitine and accumulation of acyl-[14C]carnitine compounds by the heart was independent of diet. Butyryl-[14C]carnitine, although not detected in aortas from animals fed the non-atherogenic or atherogenic diet for only 7 weeks, was detected in aortas from animals fed the atherogenic diet 17 weeks. Butyryl-[14C]carnitine was detected in heart tissue regardless of the animals' dietary regime. The increased uptake of circulating [14C]carnitine and its accumulation as both free and esterfied carnitine in atherosclerotic aortas occurred before the development of extensive macroscopic atherosclerotic lesions; this response of the aorta to atherogenic stimuli was not a general tissue response in that the heart did not respond similarly. Since blood carnitine is found predominantly in the plasma fraction, it is likely that these results refect the uptake and metabolism of plasma carnitine in vivo.