The effect of glucagon on muscle protein synthesis in the perfused rat hemicorpus [proceedings].

unoperated rats. At this time (16 days) T, rats were divided into three groups, i.e. given tri-iodothyronine and fed ad libitum, given tri-iodothyronine and fed a restricted amount of food (14 g per day, slightly less than that eaten by the T, rats) or left untreated. In this way the effects of tri-iodothyronine replacement could be measured in rats in which muscle growth was unrestricted or restricted. Measurements were made after 7 days of these regimes. The results are shown in the table. At 5 days, although growth was unchanged in the T, rats, protein synthesis and degradation were decreased. No further decrease in the fractional degradation rate was observed in the T, rats, but the fractional synthesis rate did continue to decrease, so that at 16 days growth was markedly slowed compared with unoperated rats and the decreased turnover was maintained throughout the 7 day treatment period. In the tri-iodothyronine-treated groups muscle growth was increased and this involved increases in both the fractional synthesis rate and the fractional degradation rate. In the food-restricted treated group growth was not stimulated (above the slow rate in the untreated T, rats), but nevertheless the fractional synthesis rate and the fractional degradation rate were increased. The degradation rate in the tri-iodothyroninetreated groups was higher than in unoperated controls (16 days) and is certainly as high as in young controls (sham-operated). This may represent a certain elevation above the true normal rate of degradation for these rats. It is apparent that the changes in the fractional synthesis rate involved mainly changes in protein synthetic capacity (i.e. RNA/protein ratios) rather than RNA activity (rates of synthesis per unit of RNA). This would indicate that there is not a requirement for tri-iodothyronine for normal translation. Measurements of circulating insulin concentrations in these rats, a hormone that has been implicated in the regulation of translation in muscle, indicated normal values in all groups.