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

L-Carnitine (L-C) is involved in the transport of acyl groups into mit ochondria for beta-oxidation, although its role in the adult brain is still uncertain. We have shown before that the uptake of L-carnitine i nto cultured rat cortical neurones was dependent on temperature as wel l as the Na gradient and is inhibited by compounds resembling its stru cture, like gamma-aminobutyric acid (GABA), but most potently by speci fic GABA uptake blockers. In this study we have characterised this upt ake 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. T he L-C uptake was inhibited by substituent anions in the order glucona te (83%) >isethionate (32%), with propionate being ineffective, wherea s GABA uptake was inhibited most potently by propionate substitution ( 79%) and equally by isethionate and gluconate (67%), This L-C uptake p rocess was not affected by the amino acids, glutamine or lysine, up to 1 mM concentration, although beta-alanine at 500 mu M caused a 38% in hibition. The uptake of L-C was also significantly inhibited by struct urally-related compounds, with a carbon chain length of three to six a toms, possessing an amine group and/or a carboxyl group. At a concentr ation of 500 mu M, 3-aminopropane sulphonic acid (53%), gamma-butyrobe taine (31%), gamma-hydroxybutyric acid (34%) and 4 methylaminobutyric acid (33%). Other compounds were effective only at the lower concentra tion of 10 mu M, such as butyric acid (25%), nicotinic acid (26%), iso nicotinic acid (26%), hexanoic acid (23%) and at 100 mu M, like 6-amin ocapric acid (22%). Drugs suggested to affect membrane properties, suc h as chlorpromazine, was without effect at 1 or 10 mu M, whereas fluna rizine (FLU) at 1 mu M inhibited 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 1 mu M by 22 and 21% respectively, although higher concentrations were toxi c (>100 mu M). Pretreatment of the cells with neuraminidase (50 U ml(- 1), 10 min) reduced the subsequent uptake of both L-C (18%) and GABA ( 42%). Hypoxia (3 h) also significantly attenuated L-C uptake (42%), ho wever 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 sim ilar, to that of GABA or may even in part occur via the GABA transport er. (C) 1996 The Italian Pharmacological Society