ExtractTotal glycogen synthetase and synthetase I (glucose 6-PO, independent) and total phosphorylase and phosphorylase a activities were determined in muscle from fetal, neonatal, infant, and adult rhesus monkeys (Macaca mulatta) and correlated with glycogen concentrations. O n the basis of nitrogen content, glycogen levels in fetuses in the 55-to 65-day series were higher than those in fetuses in the 100-day series, but similar to those in the fetuses near term (table I). Both glycogen synthetase and phosphorylase activities were demonstrated histochemically in the immature muscle fibers from fetuses at 55, 62, and 65 days of gestation. Total synthetase activity (assayed with 10 m M glucose 6-PO,) was demonstrated quantitatively at 78 days of gestation; this activity was highest at term (table 11). No synthetase I activity could be found in the 78-and 90-day series (table 11) ; hence, it is suggested that in fetal muscle, synthetase D is responsible for the high levels of glycogen present early in gestation. No correlation between the glycogen content and the percentage of synthetase I present could be found in fetal muscle ( fig. 1). Total phosphorylase, lowest at 78 days of gestation, increased almost 10-fold by term (table 11). No significant phosphorylase a activity could be demonstrated a t 78 days; however, by 90-100 days, phosphorylase a activity was present and remained a t about the same level (2-7 %) thereafter.I t is concluded that the enzymes for glycogen formation and degradation are present in the immature muscle fibers of the rhesus fetus as early as the first third of gestation (55 days). The ratio of total synthetase to phosphorylase activities favors glycogen deposition in the younger fetus (78-125 days) and glycogen breakdown in the fetus at term.
SpeculationIn adult animals, control of muscle glycogen metabolism depends greatly on hormonal activity; insulin promotes glycogen formation by increasing synthetase D to I conversion, and epinephrine causes glycogenolysis by promoting phosphorylase a formation while depressing synthetase activity. The role of hormonal control of glycogen metabolism may be less critical in fetal muscle than it is in adult muscle. If synthetase D is the active form of the enzyme in glycogen formation in fetal muscle, it would appear that at the cellular level, insulin is a less important regulator of glycogen synthesis than a metabolite control.