l-Carnitine uptake in differentiating human cultured muscle

We studied carnitine uptake in human skeletal muscle growing in culture for up to 30 days, and correlated it to the degree of muscle differentiation revealed by myotube formation and muscle-specific creatine-kinase isozyme accumulation. In our study carnitine uptake was a saturable specific process with two distinct components: a high affinity uptake at carnitine concentration between 0.5 and 10 μM and a low affinity uptake at carnitine concentration between 25 and 200 μM. High affinity uptake (K_m 4.17-5.50 μM, V_max 11.78-19.6 pmol/h per mg protein) did not change during muscle maturation in culture. Low affinity uptake showed significant changes in K_m and V_max in the various stages of muscle differentiation. Our studies suggest the existence of a muscle-specific system, operating at physiological carnitine concentration, which gradually developes during muscle maturation in culture. We hypothesize that a defect of the low affinity muscle-specific uptake might be the cause of the primary muscle carnitine deficiency syndrome.