Reduction of PTG protein levels leads to reduced glycogen stores due to decreased basal and insulin-stimulated glycogen synthesis. (a) PTG+/– mice display reduced glycogen stores. Glycogen levels were analyzed from tissues isolated from nonfasted 1- to 2-month-old male animals (n = 6 per group: adipose and heart; n = 7–8 per group: liver, epitrochlearis, and quadriceps; n = 3–6 per group: gastrocnemius) or from fasted 18-month-old male animals (n = 4–10 per group, liver; n = 4–12 per group, heart). Results are reported as mean ± SEM. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.005. (b) Glycogen synthase activity ratio is decreased in insulin-responsive tissues of PTG+/– mice. Glycogen synthase activity was assayed in tissue homogenates prepared from 1- to 2-month-old male animals in the nonfasting state (n = 9 per group, adipose; n = 7–8 per group, liver). Results are reported as mean ± SEM (*P ≤ 0.002; **P ≤ 0.001). G6P, glucose-6-phosphate. (c) Basal and insulin-stimulated glycogen synthase activity ratio is decreased in skeletal muscle of PTG+/– mice. Glycogen synthase activity was assayed in homogenates of forelimb epitrochlearis muscle in the basal state and after stimulation with 2 mU/g body weight human recombinant insulin. Animals were 1- to 2-month-old males in the nonfasting state (n = 4–7 per group). Results are reported as mean ± SEM (*P ≤ 0.05; **P ≤ 0.002). (d) The levels of total cellular glycogen synthase (GS) and PP1 protein are unchanged in adipose, liver, and epitrochlearis muscle of PTG+/– mice. Total cellular levels of glycogen synthase or PP1 protein were analyzed by Western blotting of tissue homogenates prepared from 1- to 2-month-old male animals in the nonfasting state. It should be noted that the anti–skeletal muscle glycogen synthase antibody used cannot detect the liver isoform of glycogen synthase.