Mechanisms for Calorie Restriction Effects on Insulin-stimulated Glucose Uptake by Rat Skeletal Muscle.

Abstract

Hallmarks of calorie restriction (CR; consuming ~60% of ad libitum, AL, intake) include greater whole body insulin sensitivity and insulin-stimulated glucose uptake (ISGU) by skeletal muscle. This thesis aimed to extend knowledge on mechanisms for CR-effects on ISGU by elucidating: 1) CR-effects on insulin signaling and muscle ISGU in old age, 2) mechanisms for CR-enhanced Akt phosphorylation (pAkt), and 3) in vivo signaling and ISGU in multiple muscles of varying fiber types. The results demonstrated that 24-month-old CR versus AL rats had greater ISGU in isolated epitrochlearis and soleus muscles with greater pAkt and phosphorylation of Akt-substrate filamin C, but not greater phosphorylation of Akt-substrates AS160 or TBC1D1 or insulin receptor tyrosine phosphorylation. Greater pAkt in isolated soleus from 9-month-old CR versus AL rats was not attributable to greater activation of signaling steps proximal to Akt (insulin receptor phosphorylation, insulin receptor substrate-1 tyrosine phosphorylation or greater insulin receptor substrate-1-phosphatidylinositol-3-kinase activity at 5, 15 or 50 minutes insulin exposure). Analysis of six muscles from 9-month-old rats with in vivo insulin exposure revealed that CR versus AL rats had greater pAkt in each of four predominantly Type II muscles (epitrochlearis, gastrocnemius, tibialis anterior and plantaris) and one of two predominantly Type I muscles (soleus, but not adductor longus) with greater ISGU for three of four predominantly Type II muscles (not plantaris) and neither predominantly Type I muscle and no diet-effect on insulin receptor tyrosine phosphorylation for any

muscle. In conclusion: 1) the CR-related increase in insulin-stimulated pAkt regardless of age or experimental model (in vivo or ex vivo) in multiple muscles suggests that Akt may be necessary, but not sufficient for a CR-related increase in muscle ISGU; 2) greater pAkt for CR versus AL rats was not attributable to detectable diet-related differences in insulin signaling steps proximal to Akt suggesting CR-effects on pAkt may rely on other mechanisms such as modulation by regulatory proteins that bind to Akt; and 3) the similarity for CR-related effects on ex vivo and in vivo ISGU for the epitrochlearis, but not the soleus, suggested a muscle-specific difference in susceptibility to the influence of systemic factors.