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Rat liver glycogen metabolism in the perinatal period
Insights
This study tracked liver glycogen metabolism in developing rats. Postnatal hypoglycemia, not direct metabolite effects, likely drives rapid changes in glycogen and enzyme activity after birth.
Area of Science:
- Biochemistry
- Developmental Biology
- Physiology
Background:
- Liver glycogen metabolism is crucial for maintaining blood glucose homeostasis.
- Enzyme regulation of glycogen synthesis and breakdown is complex and dynamic during development.
Purpose of the Study:
- To investigate the correlation between blood glucose, glycogen concentration, and key enzyme activities (glycogen synthase, phosphorylase, kinases, phosphatases) in rat liver from late gestation to early postnatal life.
- To elucidate the regulatory mechanisms of glycogen metabolism during this critical developmental transition.
Main Methods:
- Measurement of blood glucose levels, liver glycogen concentration, and activities of glycogen synthase, glycogen phosphorylase, and their regulatory kinases and phosphatases.
- Analysis of enzyme kinetics, including Km for glucose, and metabolite concentrations (AMP).
Main Results:
- Prenatal development (days 18-21) showed increased glycogen and synthase activity, with rising kinase activities and fluctuating phosphatase activities.
- Postnatal hypoglycemia (after birth) triggered rapid glycogenolysis, decreased synthase activity, and increased phosphorylase activity.
- Metabolite levels (glucose, AMP) did not appear to directly regulate enzyme activity during the prenatal period.
Conclusions:
- Enzyme activities and glycogen concentration undergo significant shifts during late fetal development and immediately after birth.
- Hormonal regulation, triggered by hypoglycemia, is suggested as the primary driver of rapid postnatal metabolic changes, rather than direct metabolite effects on enzymes.
Abstract:
The correlation between blood glucose levels, the concentration of glycogen, the activities of glycogen synthase and phosphorylase and their respective kinases and phosphatases was examined in liver of rat fetuses between day 18 of gestation and one day after birth. Between day 18 and 21 there is a rapid increase in the concentration of glycogen and in the activity of synthase a and a much slower increase in the activity of phosphorylase a. The activity of the respective kinases increased rapidly during this period and reached maximum on day 21. The activity of synthase phosphatase and phosphorylase phosphatase increased after day 18, to reach a maximum on day 19 and 20, respectively, but decreased again towards day 21. The possibility that the changes in glycogen concentration and enzyme activities were related to an effect of glucose or AMP on the respective phosphatases was considered. It was found that the Km of phosphorylase phosphatase for glucose in the prenatal period was 5--7 mM, as in the adult. Since the level of blood glucose during this period was constant (2.8 mM), an effect of glucose on phosphatase activity seems unlikely. AMP concentration increased between day 18 and 21 from 6--15 nmol/g. In view of the low level of phosphorylase a activity during this period, the increase in AMP concentration is not considered to be important in the regulation of glycogen breakdown at this time. Immediately after birth blood glucose levels dropped to 5 mg/dl. This was accompanied by a rapid decrease in glycogen concentration and in the activity of glycogen synthase and a rise in phosphorylase activity. Blood glucose levels returned to the initial level within 1 h after birth, whereas the changes in glycogen concentration and enzyme activities continued for at least 3 h after birth. On day 22 all parameters examined had reached the level found in adult rat liver. It is suggested that the rapid changes observed immediately after birth are due to an effect of gypoglycemia mediated by hormones and cannot be ascribed to direct effects of metabolites on the enzyme systems involved.