| Abstrakt: |
Insulin has been shown to stimulate collagen, noncollagen protein and nucleic acid synthesis in bone cells in vitro. However, the effects of insulin on intermediary carbohydrate metabolism in osteoblasts, and in particular on the key regulatory enzyme glycogen synthase, have not been directly examined. Accordingly, we developed a microassay for glycogen synthase (GS) to examine insulin and glucose regulation of this enzyme in cultured osteoblastlike cells. In osteoblast-enriched rat calvarial bone explants incubated for 24 hours in 5 mM glucose, insulin (0.1-100 nM) produced a dose-related stimulation of GSa (glucose-6-phosphate independent GS activity). The insulin-stimulated increase in GSa ranged from a 30% increase in the presence of 0.1 nM insulin to a 163% increase produced by 100 nM insulin, both significant at P<0.01. In contrast, GSb (glucose-6-phosphate dependent GS activity) was significantly increased only at a supraphysiologic insulin concentration (100 nM). The GS activity ratio (GSa/GSb) increased with insulin concentration from 0.1-100 nM. Basal values for GSa and GSb activity did not differ between explants incubated in 5mM or 15 mM glucose for 24 hours. However, incubation in the presence of 15 mM glucose blunted the insulin-stimulated increase in GSa activity, with 100 nM insulin producing only a 75% increase in GSa activity. In contrast, maximal insulin-stimulated GSb levels were not affected by high glucose medium. In explants incubated in glucose-free medium, basal GSa activity was significantly greater than in the presence of glucose. In addition, in explants incubated in the absence of glucose for 24 hours, 100 nM insulin produced a significant increase in GSa activity, demonstrating that exogenous glucose is not required for insulin stimulation of GS activity in the osetoblast. Therefore, both glucose and insulin regulate GS activity in osteoblastlike cells, and GS activity appears to be a highly sensitive marker for the effects of insulin on intermediary glucose metabolism in the osteoblast. [ABSTRACT FROM AUTHOR] |
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