Constraints on the uptake of labeled palmitate by the heart. The barriers at the capillary and sarcolemmal surfaces and the control of intracellular sequestration.

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Abstract
While long chain free fatty acids (FFA) are known to be a major fuel of the mammalian myocardium, little is known about the processes which limit their uptake in the intact heart. Conventional biochemical techniques necessitate the destruction of membrane barriers which could be significant in vivo. Steady state extraction techniques yield information only on net extraction. The necessary information for the demonstration of rate-limiting steps can be obtained by a technique that preserves the structure of the organ, yet is sensitive to processes with rate constants on the order of tenths of a second. The multiple indicator dilution technique is ideal for this type of investigation. Using a perfused in situ dog heart, simultaneous injections of 125I-albumin (intravascular reference), 14C-sucrose (capillary surface and interstitial space reference) and 3H-palmitate (a representative of the long chain free fatty acids) were made into the left coronary artery and samples were collected rapidly from the coronary sinus. The activity of each isotope in the venous samples was divided by the activity per milliliter in the injection to give the outflow fraction per milliliter and plotted vs. time to give the normalized coronary sinus concentration-time curve for each tracer. The curves were analyzed by a 2-barrier model which includes the constraints at the capillary and muscle cell surfaces and an irreversible sequestration reaction inside the myocyte for palmitate. The capillary permeability of palmitate was only 2- to 3-fold greater than that for sucrose; the palmitate entered the myocyte: the sucrose did not. The intracellular sequestration of palmitate was found to take place at a rate inversely proportional to the arterial lactate concentration. Normally some palmitate returns to the interstitial space and capillary after entering the cell. Toxic doses of flavaspidic acid or dinitrophenol changed this. They caused virtually complete sequestration of all the palmitate that entered the cell. The capillary endothelium is apparently a major barrier to the extraction of FFA. The activity of the reaction catalyzed by long chain fatty acyl thiokinase may be controlled by the blood lactate concentration. Flavaspidic acid and the uncoupler dinitrophenol in some way apparently accelerated FFA activation.

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