L-carnitine Uptake Research Articles

Transport of L-carnitine induced by prednisolone in an established cell line (CCL 27) : A possible explanation of the therapeutic effect of glucocorticoids in muscular carnitine deficiency syndrome

Prednisolone (10 −8-10 −5 mol/l) in the growth medium for 24 h increased the rate of uptake of L-[ 3H]carnitine in an established cell line (CCL 27) to 164 ± 6% (mean ±S.E.) of the rate observed in untreated cells. At the same time the intracellular content of free L-carnitine increased about 20%. The simultaneous addition of prednisolone (10 −6 mol.l for 24 h) and L-carnitine (10 −4 mol/l for 96 h) to the growth medium increased the rate of uptake to 225 ± 8% (mean ±S.E.) of that in untreated cells. The increase seemed to be mediated through an increase in number of carriers, as judged by the increase in V of the transport process with unchanged K m. Phosphodiesterase I, an enzyme mainly localized in the plasma membrane, increased its activity about 3.5 times when cells were stimulated with prednisolone. Thus, it seems that the increase in the rate of uptake of L-carnitine mediated by glucocorticoids, is part of a more general effect on the plasma membrane. The observations offer an explanation to the observed clinical improvement in patients with muscular carnitine deficiency treated with glucocorticoids and/or L-carnitine.

Carnitine-induced uptake of l-carnitine into cells from an established cell line from human heart (CCL 27)

l-Carnitine is actively transported into Girardi human heart cells, an established cell line from human heart. The present study was undertaken to investigate the effect of different concentrations of l-carnitine in the growth medium on the rate of uptake of l-[ 3H]carnitine. Increasing the concentration of l-carnitine from 2 to 100 μmol/1 in the growth medium of the cells, increased the rate of uptake of l-[ 3H]carnitine by about 50%. The maximal effect was reached after approx. 72 h incubation. The increase in rate seemed to be caused by synthesis of increased number of carriers, as judged by the increase in V with unchanged apparent K m for the transport process. This effect of l-carnitine could be inhibited by cycloheximide, indicating the dependence on intact protein synthesis. The morphology of the cells was studied by electron microscopy. No myofilaments were found, thus the cells are dedifferentiated and no longer typical muscular cells.

Carnitine movement across muscle cell membranes. Studies in isolated rat muscle

l-Carnitine uptake and exodus was studied in rat extensor digitorum longus muscle in vitro. A saturable transport process was observed, which had an apparent K m of 60 μM and V of 22 nmol/h per g tissue. Transport was inhibited by 2,4-dinitrophenol, sodium azide, anaerobiosis, ouabain, and sodium ion depletion. Analogs of l-carnitine containing a quarternary ammonium group were found to inhibit uptake ( d-carnitine, K i = 400 μ M γ-butyrobetaine, K i = 60 μ M , choline chloride, K i = 14 mM ), while those not containing this functional group (γ-aminobutyrate, d,l-β-hydroxybutyrate) had no significant effect at concentrations 100 times the apparent K m of l-carnitine. Carnitine exodus from rat extensor digitorum longus muscle consisted of two phases. The rapid initial phase was attributed to leakage of l-carnitine from damaged muscle fibers, as it proceeded at nearly the same rate at 0° and 37°C, and leveled off to a rate of near zero after 1 h of incubation in vitro. The quantitatively more important phase of exodus showed a latency of 1–2 h and then proceeded at a linear rate of 40–45 nmol/h per g tissue. The results of this study support the contention that l-carnitine is taken up by a carrier-mediated, active transport system in rat extensor digitorum longus muscle. Functionally, the transport system for uptake is distinct from the process by which carnitine is lost from this muscle.