Independent and combined effects of calorie restriction and AICAR on glucose uptake and insulin signaling in skeletal muscles from 24-month-old female and male rats

Abstract

We assessed the effects of two levels of calorie restriction (CR; eating either 15% or 35% less than ad libitum, AL, food intake for 8 weeks) by 24-month-old female and male rats on glucose uptake (GU) and phosphorylation of key signaling proteins (Akt; AMP-activated protein kinase, AMPK; Akt substrate of 160 kDa, AS160) measured in isolated skeletal muscles that underwent four incubation conditions (without either insulin or AICAR, an AMPK activator; with AICAR alone; with insulin alone; or with insulin and AICAR). Regardless of sex: 1) neither CR group versus the AL group had greater GU by insulin-stimulated muscles; 2) phosphorylation of Akt in insulin-stimulated muscles was increased in 35%-CR versus AL rats; 3) prior AICAR treatment of muscle resulted in greater GU by insulin-stimulated muscles, regardless of diet; and 4) AICAR caused elevated phosphorylation of acetyl CoA carboxylase, an indicator of AMPK activation, in all diet groups. There was a sexually dimorphic diet-effect on AS160 phosphorylation, with 35%-CR exceeding AL for insulin-stimulated muscles in male rats, but not in female rats. Our working hypothesis is that the lack of a CR-effect on GU by insulin-stimulated muscles was related to the extended duration of the ex vivo incubation period (290 minutes compared to 40-50 minutes that was previously reported to be effective). The observed efficacy of prior treatment of muscles with AICAR to improve glucose uptake in insulin stimulated muscles supports the strategy of targeting AMPK with the goal of improving insulin sensitivity in older females and males.