ROLE OF TRANSVERSE TUBULES IN INSULIN-STIMULATED MUSCLE GLUCOSE-TRANSPORT

Although the strongest evidence for recruitment of glucose transporter s in response to insulin comes from studies with adipocytes, studies i n muscle seem in general to confirm that glucose transporters are also translocated to the cell membrane in muscle in response to insulin. H owever, the observation that transverse tubule (T-tubule) membranes co ntain approximately five times more glucose transporter than sarcolemm a raised a question as to where glucose transport occurs in muscle. Th e T-tubule membrane system is continuous with the surface sarcolemma a nd is a tubule system in which extracellular fluid is in proximity wit h the interior of the muscle fiber. The purpose of this Prospects arti cle is to evaluate the possibility that the T-tubule membrane may repr esent a major site of glucose transport in skeletal muscle. Using immu nocytochemical techniques we have located GLUT4 glucose transporters o n the T-tubule membrane and in vesicles near T-tubules. Since T-tubule s form channels into the interior of the muscle fiber, glucose could d iffuse or be moved by some peristaltic-like pumping action into the tr ansverse tubules and then be transported across the membrane deep into the interior of the muscle fiber. This mode of transport directly int o the interior of the cell would be advantageous over transport across the sarcolemma and subsequent diffusion around the myofibrils to reac h the interior of the muscle. Thus, in addition to the role of the T-t ubule in ion fluxes and contraction, this unique membrane system can a lso provide a pathway for the delivery of substrates into the center o f the muscle cell where many glycolytic enzymes and glycogen deposits are located.