THE EXCHANGE BETWEEN PROGLYCOGEN AND MACROGLYCOGEN AND THE METABOLIC ROLE OF THE PROTEIN-RICH GLYCOGEN IN RAT SKELETAL-MUSCLE

The aim of this study is to determine if proglycogen and macroglycogen are kinetically related in rat skeletal muscle. Eight groups of anest hetized fasted rats (seven hepatic-occluded and one nonoccluded) were intravenously infused with [3-H-3]glucose at a rate of 1.7 mu Ci-min-( 1) for 20 min. At the end of infusion, hindlimb muscles were excised a nd rapidly frozen in liquid nitrogen. Proglycogen was extracted by pre cipitation in 10% TCA; and macroglycogen as a part of total glycogen b y precipitation in 20% KOH-65% ethanol. Along with the tracer, the occ luded rats were also infused with: saline (group 1); insulin at rates ranging from 5 to 50 mU . min(-1) (groups 2 to 5); and insulin at a ra te of 10 mU . min(-1) plus glucose at rates of 10.2 and 20.4 mu mol . min(-1), respectively (groups 6 and 7). The infusion regimens resulted in up to 30-fold difference in whole-body glucose utilization among t he rats. In the rats infused with saline and insulin at a rate of 5 mU . min(-1), [H-3]glucose was found to be exclusively incorporated into proglycogen. Incorporation into macroglycogen was found in the rats i nfused with insulin at rates > 10 mU . min(-1). Supplementary glucose infusion increased the synthesis of [H-3]proglycogen (four- to sixfold ), and equilibrated the two extractable forms of glycogen in the insul in-infused rats. In the saline-infused nonoccluded rats, only proglyco gen was found to be labeled. In conclusion, our data indicate that in the intact and hepatic-occluded rats, proglycogen in the skeletal musc les may undergo synthesis and degradation of its own more readily than exchange between itself and depot macroglycogen.