STRUCTURAL, METABOLIC AND IONIC REQUIREMENTS FOR THE UPTAKE OF1-CARNITINE BY PRIMARY RAT CORTICAL CELLS

Abstract

L-Carnitine (L-C) is involved in the transport of acyl groups into mitochondria for β-oxidation, although its role in the adult brain is still uncertain. We have shown before that the uptake of L-carnitine into cultured rat cortical neurones was dependent on temperature as well as the Na gradient and is inhibited by compounds resembling its structure, like ‰-aminobutyric acid (GABA), but most potently by specific GABA uptake blockers. In this study we have characterised this uptake process further. We have shown that the uptake of L-carnitine may be dependent on Cl ions, in addition to Na ions, but non on Ca ions. The L-C uptake was inhibited by substituent anions in the order gluconate (83%) >isethionate (32%), with propionate being ineffective, whereas GABA uptake was inhibited most potently by propionate substitution (79%) and equally by isethionate and gluconate (67%). This L-C uptake process was not affected by the amino acids, glutamine or lysine, up to 1mM concentration, although β-alanine at 500μ Mcaused a 38% inhibition. The uptake of L-C was also significantly inhibited by structurally-related compounds, with a carbon chain length of three to six atoms, possessing an amine group and/or a carboxyl group. At a concentration of 500μ M, 3-aminopropane sulphonic acid (53%), ‰-butyrobetaine (31%), ‰-hydroxybutyric acid (34%) and 4 methylaminobutyric acid (33%). Other compounds were effective only at the lower concentration of 10μ M, such as butyric acid (25%), nicotinic acid (26%), isonicotinic acid (26%), hexanoic acid (23%) and at 100μ M, like 6-aminocapric acid (22%). Drugs suggested to affect membrane properties, such as chlorpromazine, was without effect at 1 or 10μ M, whereas flunarizine (FLU) at 1μ Minhibited both L-C (24%) and GABA uptake (17%). Other drugs like the cholinesterase inhibitors, tacrine and eserine, also had a small inhibitory effect on L-C uptake, reducing it at 1μ Mby 22 and 21% respectively, although higher concentrations were toxic (>100μ M). Pretreatment of the cells with neuraminidase (50Uml −1, 10min) reduced the subsequent uptake of both L-C (18%) and GABA (42%). Hypoxia (3h) also significantly attenuated L-C uptake (42%), however part of these effects were related to the loss of cell viability. In summary, L-C uptake occurs by a complex mechanism which at least in part may occur by a Na/Cl cotransport mechanism, which could be similar, to that of GABA or may even in part occur via the GABA transporter.