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 anesthetized fasted rats (seven hepatic-occluded and one nonoccluded) were intravenously infused with [3-³H]glucose at a rate of 1.7 μCi-min^-1 for 20 min. At the end of infusion, hindlimb muscles were excised and rapidly frozen in liquid nitrogen. Proglycogen was extracted by precipitation in 10% TCA; and macroglycogen as a part of total glycogen by precipitation in 20% KOH-65% ethanol. Along with the tracer, the occluded 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 rate of 10 mU·min^-1 plus glucose at rates of 10.2 and 20.4 μmol·min^-1, respectively (groups 6 and 7). The infusion regimens resulted in up to 30-fold difference in whole-body glucose utilization among the rats. In the rats infused with saline and insulin at a rate of 5 mU·min^-1, [³H]glucose was found to be exclusively incorporated into proglycogen. Incorporation into macroglycogen was found in the rats infused with insulin at rates > 10 mU·min^-1. Supplementary glucose infusion increased the synthesis of [³H]proglycogen (four- to sixfold), and equilibrated the two extractable forms of glycogen in the insulin-infused rats. In the saline-infused nonoccluded rats, only proglycogen was found to be labeled. In conclusion, our data indicate that in the intact and hepatic- occluded rats, proglycogen in the skeletal muscles may undergo synthesis and degradation of its own more readily than exchange between itself and depot macroglycogen.