VALPROIC ACID IMPAIRS CARNITINE UPTAKE IN CULTURED HUMAN SKIN FIBROBLASTS - AN IN-VITRO MODEL FOR THE PATHOGENESIS OF VALPROIC ACID ASSOCIATED CARNITINE DEFICIENCY

The mechanisms of valproate-associated carnitine deficiency are contro versial. The urinary excretion of valproylcarnitine is insufficient to account for tissue carnitine depletion. To explore this mechanism, we studied the effects of valproic acid (VPA) on carnitine uptake in cul tured human skin fibroblasts by the method of Tein et al. (Pediatr Res 28:247-255, 1990). Fibroblasts were preincubated with varying concent rations (0-2000 muM) of VPA for 1, 3, 5, 7, 10, 14, 21, and 28 d and t hen incubated with fixed carnitine concentrations of 50 muM (normal ph ysiologic concentration), 20 muM (as seen in secondary carnitine defic iency disorders), or 5 muM (as seen in the plasma membrane carnitine t ransport defect). There was an exponential dose-dependent decrease in carnitine uptake with increasing VPA concentrations, and the relative inhibitory effect was the same for all three carnitine concentrations. The mean percentages +/- SD (n - 1) of residual carnitine uptake for all combined preincubation periods (1-28 d) and combined carnitine con centrations (5, 20, and 50 mumol/L) with increasing concentrations of VPA varied from 83.4 +/- 2.6% (10 muM VPA) to 56.7 +/- 0.1% (500 muM) to 19.8 +/-1.3% (2000 muM). The degree of inhibition was directly prop ortional to the time of VPA preincubation and parallel for all three c arnitine concentrations; the longer the preincubation period, the lowe r the toxic dose of VPA (to a minimum of 450 muM), resulting in a 50% suppression of carnitine uptake (TD50). The mean TD50 of the combined carnitine concentrations for increasing preincubation periods of VPA v aried from 1898 +/- 214 muM (1 d) to 447 +/-9 muM (28 d), tapering tow ard an asymptote of 450 muM when the preincubation period exceeded 14 d. This in vitro TD50 value may be comparable to the in vivo therapeut ic range of serum VPA concentrations (350-700 mumol/L) for anticonvuls ant therapy. We conclude that one mechanism by which long-term VPA the rapy induces serum and tissue carnitine depletion is through inhibitio n of plasmalemmal carnitine uptake, including decreased renal reabsorp tion of free carnitine. This effect is directly proportional to the du ration of exposure and concentration of VPA.