THE T-TUBULE IS A CELL-SURFACE TARGET FOR INSULIN-REGULATED RECYCLINGOF MEMBRANE-PROTEINS IN SKELETAL-MUSCLE

(1) In this study we have determined the distribution of various membr ane proteins involved in insulin-activated glucose transport in T-tubu les and in sarcolemma from rat skeletal muscle. Two independent experi mental approaches were used to determine the presence of membrane prot eins in T-tubules: (i) the purification of T-tubules free from sarcole mmal membranes by lectin agglutination, and (ii) T-tubule vesicle immu noadsorption. These methods confirmed that T-tubules from rat skeletal muscle were enriched with dihydropyridine receptors and tt28 protein and did not contain the sarcolemmal markers dystrophin or beta(1)-inte grin. Both types of experiments revealed an abundant content of GLUT4 glucose carriers, insulin receptors and SCAMPs (secretory carrier memb rane proteins) in T-tubule membranes. (2) Acute administration in vivo of insulin caused an increased abundance of GLUT4 in T-tubules and sa rcolemma. On the contrary, insulin led to a 50% reduction in insulin r eceptors present in T-tubules and in sarcolemma, demonstrating that in sulin-induced insulin receptor internalization affects T-tubules in th e muscle fibre. The alteration in the content of GLUT4 and insulin rec eptors in T-tubules was a consequence of insulin-induced redistributio n of these proteins. SCAMPs also redistributed in muscle membranes in response to insulin. They were recruited by insulin from intracellular high-density fractions to intracellular lighter-density fractions and to the cell surface, showing a pattern of insulin-induced cellular re distribution distinct from those of GLUT4 and the insulin receptor. (3 ) In conclusion, the T-tubule is a cell-surface target for membrane pr oteins involved in recycling such as SCAMPs or for membrane proteins t hat acutely redistribute in response to insulin such as GLUT4 or insul in receptors.