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Isolated muscles from the rabbit forelimb synthesised protein at a lower rate than that in the same muscles in vivo; rates of protein degradation were changed only slightly. Insulin stimulated protein synthesis and prostaglandin F2 release in isolated muscles incubated either under a constant tension or with mechanical stretching, having a greater effect on muscles from fasted rabbits. Inhibitors of prostaglandin synthesis, indomethacin and meclofenamate, abolished these effects but had no effect on the controls (-insulin). When added two hours after insulin, indomethacin did not reduce the hormonally induced increase in protein synthesis whilst still reducing PG release, suggesting that prostaglandins are involved in the action of insulin on protein synthesis only at an early stage in the response. In muscles from fully fed rabbits dexamethasone reduced protein synthesis and PFG2 release, after a transient increase (during the first hour); in muscles from fasted animals the glucocorticoid reduced PG release only, suggesting that dexamethasone-induced reductions in protein synthesis and PG release may not be causally related. PGE2 release and the rate of protein degradation were generally affected similarly by the two hormones but to a lesser extent than PGF2 release and protein synthesis. The combined effect of the two hormones differed depending on the nutritional status of the donors. Together insulin and dexamethasone had antagonistic effects on muscles from fed animals, the stimulatory effect of insulin on protein synthesis being diminished by the glucocorticoid. In muscles from fasted rabbits dexamethasone did not affect the response to insulin. Phospholipase A2 inhibitory proteins were extracted from incubated muscles. The anti-phospholipase activity of these proteins was increased by prior incubation with dexamethasone and was reduced, to control values or below, by incubation with dexamethasone + insulin. Some of the antagonistic effects of insulin and glucocorticoids may involve their opposing actions on phospholipase A2. |
