Beta-adrenergic effects on carbohydrate metabolism in the unweighted rat soleus muscle

Abstract

The effects of insulin on carbohydrate metabolism in atrophied rat soleus muscle are increased after unweighting by tail-cast suspension. This work has been extended by testing the effect of unweighting on the response of carbohydrate metabolism to isoproterenol, a beta-adrenergic agonist. Isoproterenol promoted glycogen degradation more in the unweighted than in the weight-bearing soleus but showed no differences in the extensor digitorum longus, which is unresponsive to hindlimb unweighting. In soleus muscles depleted of glycogen, to avoid varied inhibitory effects of glycogen on glycogen synthesis, isoproterenol inhibited this process more in the unweighted muscle. Isoproterenol did not have a greater inhibitory effect on net uptake of 2-deoxy-D[1,2-3H]glucose by the unweighted muscle. Measurements of intracellular 2-deoxy-[3H]glucose 6-phosphate and 3-O-methyl-D-[1-3H]glucose, which cannot be phosphorylated, showed that isoproterenol inhibited glucose phosphorylation but not transport. This effect could be explained by an increase of glucose 6-phosphate, an inhibitor of hexokinase. At 100 microU insulin/ml but not at a lower amount (10 microU/ml), isoproterenol inhibited hexose phosphorylation more in the control than in the unweighted muscle. This result may be explained by greater insulin antagonism in the unweighted muscle owing to increased insulin sensitivity. However, insulin antagonism of isoproterenol stimulation of glycogenolysis or inhibition of glycogenesis was not altered by unweighting. Therefore, for some aspects of carbohydrate metabolism, the unweighted muscle has an increased response to beta-adrenergic activation, just as this muscle shows increased responses to insulin.