Glucagon increases cellular uptake and plasma membrane binding of L-carnitine in S49 lymphoma cells

The carnitine carrier was investigated in S49 lymphoma cells, a murine cell type cultured in suspension culture and used widely in signal transduction studies. Carnitine uptake in S49 lymphoma cells was stimulated almost twofold by pretreatment of intact cells by 0.5 microM glucagon for 4 h. Plasma membranes derived from S49 lymphoma cells bound 556 +/- 81 pmol/mg protein whereas pretreatment by 0.5 microM glucagon for 4 h of cells, before cell harvesting and preparation of plasma membranes, increased the number of carnitine binding sites to 1196 +/- 52 pmol/mg protein. The glucagon pretreatment also altered the carnitine binding characteristics from a two site model to a single binding site. S49 lymphoma cells were further shown to contain 50.9 +/- 2.6 fmol glucagon receptors per 10(6) cells. We conclude that glucagon stimulated cellular uptake of carnitine by a mechanism that at least partially operated through increasing the number of available carnitine binding sites in plasma membranes.