Effects of in-vivo administration of insulin-like growth factor-I on the rate of glucose utilization in the soleus muscle of the rat

Abstract

ABSTRACT This study investigated the effects of insulin-like growth factor-I (IGF-I) administered to rats in vivo on the soleus muscle isolated from these rats. In order to study the interactions between IGF-I and insulin, the soleus muscles were incubated in the presence of various concentrations of insulin. IGF-I (190–200 μg) was given twice daily; the rats were killed 1 h after one injection of IGF-I (acute administration) or after treatment with IGF-I for 10 days (prolonged administration). The level of IGF-I in plasma was increased by ∼ 100% after acute administration and by around 30% after 10 days of treatment with IGF-I. Acute administration of IGF-I to the rats increased the flux of glucose to hexose monophosphate and the rates of lactate formation and glycogen synthesis in the soleus muscles; however, the responsiveness of these muscles to insulin was lost: the increase in the rate of glucose utilization by IGF-I at physiological concentrations of insulin (10 or 100 mU/l) was similar to that observed at maximal concentrations of insulin (1000 mU/l). Similar results were obtained after prolonged treatment of the rats with IGF-I; however, the increase in the rate of glucose utilization was less pronounced than when IGF-I was given acutely and the muscles were still capable of responding to insulin. These results suggest that: (a) acute or chronic increases in the serum level of IGF-I in the rat in vivo increase the basal rate of glucose utilization in skeletal muscle; this increase is independent of insulin; (b) the effects of insulin on the rate of glucose utilization are not additive to those of IGF-I; however, this may depend on the level of IGF-I in serum. Journal of Endocrinology (1992) 133, 37–43