Transport of L-carnitine in isolated cerebral cortex neurons.

The accumulation of carnitine was measured in cerebral cortex neurons isolated from adult rat brain. This process was found to be lowered by 40% after preincubation with ouabain and with SH-group reagents (N-ethylmaleimide and mersalyl). The initial velocity of carnitine transport was found to be inhibited by 4-aminobutyrate (GABA) in a competitive way (Ki = 20.9 +/- 2.4 mM). However, of various inhibitors of GABA transporters, only nipecotic acid and very high concentrations of 1-[2-([(diphenylmethylene)amino]oxy)ethyl]-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NO-711) acid decreased carnitine accumulation while betaine, taurine and beta-alanine had no effect. The GABA transporters expressed in Xenopus laevis oocytes did not transport carnitine. Moreover, carnitine was not observed to diminish the accumulation of GABA in cerebral cortex neurons, which further excluded a possible involvement of the GABA transporter GAT1 in the process of carnitine accumulation, despite the expression of this protein in the cells under study. The absence of carnitine transporter OCTN2 in rat cerebral cortex neurons (K. A. Nalecz, D. Dymna, J. E. Mroczkowska, A. Broër, S. Broër, M. J. Nalecz and R. Cecchelli, unpublished results), together with the insensitivity of carnitine accumulation towards betaines, implies that a novel transporting protein is present in these cells.

Transport of carnitine in neuroblastoma NB-2a cells.

Carnitine accumulation was measured in cultured neuroblastoma NB-2a cells. This process was found partially sodium dependent and its kinetics to be a sum of a saturable transport (Km = 123 +/- 13 microM) and diffusion (D = 63 +/- 7 pmol/mg protein/min/mM). On the contrary to previous reports on neural cells, the accumulation of carnitine was found insensitive to gamma-aminobutyric acid (GABA). Measurements of carnitine accumulation in the presence of different compounds resulted in the conclusion that carnitine transport does not occur through the known systems specific toward choline and/or amino acids. For instance, an observed inhibition of carnitine transport by serine and cysteine, without any effect of alanine, excluded a possible role of ASC amino acid transport system. An involvement of a new transporter is thus postulated, specific toward compounds with a polar group in the beta position with respect to the carboxylic group.

Effect of gamma-aminobutyric acid on the carnitine metabolism in neural cells.

Isolated rat cerebral cortex cells were able to accumulate L-carnitine and this process was competitively inhibited by 1 mM gamma-aminobutyric acid (GABA) with a shift of Km from 7.8 +/- 1.9 mM to 14.6 +/- 4.0 mM. Addition of GABA also affected distribution of carnitine derivatives. The decrease of acetylcarnitine level by 1.6 fold was correlated with the inhibition of carnitine acetyltransferase (1.77 times). A postulated involvement of this enzyme in delivering acetyl moieties for acetylcholine synthesis would suggest a negative feedback between GABA and the level of acetylcholine.

Effect of externally added carnitine on the synthesis of acetylcholine in rat cerebral cortex cells.

Acetylcholine synthesis from radiolabelled glucose was monitored in cerebral cortex cells isolated from brains of suckling and adult rats. Acetylcholine synthesis was found much higher in suckling animals, both in the absence and presence of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) inhibitor, paraoxon. Together with choline (20 microM), carnitine was found to stimulate acetylcholine synthesis in a synergistic way in cortex cells from adult rats (18%). Choline, however, was incapable of reversing an inhibitory effect exerted by carnitine on acetylcholine synthesis in cortex cells from suckling animals. Distribution of carnitine derivatives was found significantly different in the cells from young and old animals, the content of acetylcarnitine decreased with age with a corresponding increase of free carnitine. The observed differences in carnitine effect on acetylcholine synthesis suggested that high acetylcarnitine in cells capable of beta-oxidation might be correlated with the lower level of acetylcholine synthesis.

Synergistic effect of choline and carnitine on acetylcholine synthesis in neuroblastoma NB-2a cells.

An influence of carnitine on acetylcholine synthesis from radiolabeled glucose was monitored in neuroblastoma NB-2a cells. Upon addition of carnitine the distribution of its derivatives was found significantly different than the values published for brain, the level of long-chain acyl derivatives being much higher and reaching 60%. Carnitine itself did not change acetylcholine level. Together with choline (20 microM), carnitine was observed to stimulate (by 36%) acetylcholine synthesis in a synergistic way, which indicated that both substrates could be limiting factors of this process in NB-2a cell line of neuroblastoma.