THE CAPILLARY TRANSPORT-SYSTEM FOR FREE FATTY-ACIDS IN THE HEART

The nature of the process by which free fatty acids, which are tightly bound to albumin, traverse the endothelium of cardiac capillaries to reach the cardiac muscle cells, so that they are extracted to a net extent of approximate to 40%, needs clarification. Previous studies have indicated that a membrane fatty acid-binding protein provides for carrier-mediated uptake of free fatty acids by isolated hepatocytes, cardiomyocytes, and jejunal mucosal cells. A monoclonal monospecific antibody was prepared against purified membrane fatty acid-binding protein from rat liver. Multiple-indicator dilution experiments were carried out in the isolated rat heart with labeled albumin, sucrose, and palmitate in the presence of control perfusate or perfusate containing either specific antibody or comparable nonspecific myeloma cell supernatant (each of the latter containing additional albumin, in identical concentrations). Analysis of the labeled-sucrose curves provided a permeability-surface area product for sucrose to which that for palmitate could be compared. In comparison with control supernatants, myeloma supernatant produced a minor inhibition of palmitate uptake, as a result of the increase in albumin concentration. The specific antibody, which contained identical albumin concentrations, produced a major inhibition of palmitate uptake, significantly greater than with the myeloma supernatant. The data indicate that the membrane fatty acid-binding protein mediates the transfer of free fatty acid across the endothelial cells of cardiac capillaries for presentation to heart muscle. Passive intramembrane lateral diffusion of palmitate could not provide an explanation for the findings.