EFFECTS OF MATURATION AND AGING ON THE SKELETAL-MUSCLE GLUCOSE-TRANSPORT SYSTEM

Insulin resistance in old, compared with young, humans and animals has been well documented. The resistance is due primarily to defects in skeletal muscle. In the present study, skeletal muscle sarcolemmal membranes were purified from five age groups of female Fischer rats ranging from 2 to 24 mo. Basal specific D-glucose transport was not significantly different among any of the groups. Maximum insulin-stimulated transport was progressively decreased from 96.4 +/- 5.0 pmol.mg-1.15 s-1 in the 2-mo-old animals to 70.8 +/- 8.9 pmol.mg-1.15 s-1 in the 24-mo-old animals. Most of the decrease occurred during maturation, and in fact there was no significant difference in maximum transport among the 8-, 16-, and 24-mo-old rats. The decrease in insulin-stimulated transport in the 24-mo-old animals was due to a reduction in the number of glucose transporters translocated into the sarcolemma membrane (9.8 +/- 0.6 vs. 7.8 +/- 0.6 pmol/mg protein). The intracellular or microsomal pool of glucose transporters was not significantly different between the 2- and 24-mo-old animals (8.8 +/- 0.6 vs. 8.5 +/- 0.9/mg protein). Western blotting revealed no differences in the cellular GLUT-4 contents between the 2- and 24-mo-old rats. The number of insulin receptors (2.3 +/- 0.4 vs. 2.1 +/- 0.5 pmol/mg protein) was not significantly different. Tyrosine kinase activity of the insulin receptor was, however, significantly reduced in the 24-mo-old compared with the 2-mo-old animals. Maximum autophosphorylation was reduced from 623 /- 3 to 566 /- 6 counts.min-1.10-mu-g protein-1, and maximum phosphorylation of the synthetic substrate polyglutamyltyrosine (4:1) [poly(Glu4Tyr1)] was reduced from 2.94 /- 0.07 to 2.59 /- 0.08 fmol P-32/fmol insulin binding. Kinetic studies showed that the maximum velocity for poly(Glu4Tyr1) was reduced from 6.7 to 4.3 fmol P-32.fmol insulin binding-1.30 min-1 while the K(m) was unchanged [approximately 0.28 mg poly(Glu4Tyr1)/ml].