FATTY-ACID TRANSFER ACROSS THE MYOCARDIAL CAPILLARY WALL - NO EVIDENCE OF A SUBSTANTIAL ROLE FOR CYTOPLASMIC FATTY-ACID-BINDING PROTEIN

It has recently been hypothesized that fatty acid (FA) transfer across the myocardial capillary wall is mediated by cytoplasmic fatty acid-b inding protein (FABP). Therefore, we studied the type and content of F ABP in endothelial cells from rat heart, using molecular biological, i mmunochemical, and FA-binding assays. Studies were performed on short term cultured endothelial cells, two established endothelial cell line s and ultrathin cryosections from adult rat heart. Northern blotting a nalysis of endothelial cell RNA failed to detect either heart-type (H- ) FABP or liver-type (L-) FABP mRNA, but the reversed transcription-po lymerase chain reaction revealed both H- and L-FABP mRNAs, indicating the presence of minor amounts of these mRNAs. Highly sensitive immunoc hemical assays (sandwich ELISAs) using specific antibodies raised agai nst rat H- or L-FABP showed the contents of these FABP-types in endoth elial cells to be 1-5 ng/mg cytosolic protein, which is more than thre e orders of magnitude lower than the contents of H-FABP in heart or L- FABP in liver. Immuno-electron microscopy also showed that the concent ration of H-FABP in endothelial cells is at least two orders of magnit ude lower than that in cardiomyocytes. Finally, cytosolic protein samp les from endothelial cells revealed no significant FA-binding activity in the 15-kDa region. We conclude that rat heart endothelial cells co ntain only minor quantities of cytoplasmic FABP and that, therefore, F A transport over the endothelium is mediated by FABP only to a minor e xtent. It is postulated that aqueous diffusion of FA through the endot helial cytoplasm most likely accounts for the experimentally observed rates of cardiac FA utilization.