Human muscle glycogen resynthesis after exercise: insulin-dependent and -independent phases

Abstract

To study the effects of glycogen depletion and insulin concentration on glycogen synthesis, gastrocnemius glycogen was measured with 13C-nuclear magnetic resonance at 4.7 T after exercise. Subjects performed single-leg toe raises to deplete gastrocnemius glycogen to 75, 50, or 25% of resting concentration (protocol I). Insulin dependence of glycogen synthesis was assessed after depletion to 25% with (protocol II) and without (protocol III) infusion of somatostatin to inhibit insulin secretion. After depletion to 75 and 50%, glycogen resynthesis rates were similar (2.4 +/- 0.7 and 2.8 +/- 0.6 mM/h, respectively). When glycogen was depleted to 25% (< 30 mM), the resynthesis rate was significantly higher (P < 0.02) at 33 +/- 7 mM/h, and it declined to 3.5 +/- 0.9 mM/h at > 35 mM glycogen. At < 35 mM glycogen, synthesis was not affected by low insulin (24 +/- 4 mM/h, protocol vs. 19 +/- 3 mM/h, protocol III), whereas at > 35 mM glycogen, synthesis ceased without insulin (-0.07 +/- 0.19 mM/h, protocol II). After depletion to 25% (protocol III), plasma lactate transiently increased (0.81 mM at rest, 1.82 mM 0 h after exercise, and 0.76 mM 2 h after exercise), whereas other plasma constituents did not significantly change. We conclude that after depletion to < 30 mM initial glycogen resynthesis is insulin independent and glycogen dependent, which suggests local control.

Enhancement of whole body glucose uptake during and after human skeletal muscle low-frequency electrical stimulation

“…In contrast, increased insulin sensitivity is undetectable in the early phase of the postexercise period (at 30 and 60 min) and becomes prominent at 180 min after exercise (48a). Consistent with these findings, Price et al (38) have shown in human muscle that postexercise glycogen repletion occurs in an insulin-independent manner for ϳ1 h after exercise, and thereafter insulin-dependent glycogen repletion becomes significant. Thus it appears that the increase in GDR after ES found in this study is likely due, at least in a large part, to the insulin-independent effect of muscle contraction, and the insulin-dependent effect may come into play particularly during the latter part of the poststimulation period.…”

Regulation of Glycogen Synthase by Glucose and Glycogen

“…Glycogen levels are depleted during exhaustive exercise (15). Restoration of these levels is in part independent of insulin action (15) and involves increases in both glucose uptake (33,34) and GS activity (35).…”

“…Restoration of these levels is in part independent of insulin action (15) and involves increases in both glucose uptake (33,34) and GS activity (35). However, to date the molecular mechanisms underpinning these events have been poorly understood.…”

“…GS becomes activated following glycogen depletion (such as occurs during exhaustive exercise) in an insulinindependent manner (15). The molecular mechanism underlying this phenomenon has not been elucidated, although a number of hypotheses have been proposed.…”

Dietary macronutrient recommendations for optimal recovery post-exercise: Part I

“…42,43 The rapid phase only occurs when post-exercise muscle glycogen concentrations are below 30 -35 mmol/kg ww 43 and CHO is provided immediately post-exercise. 28 Hence, of the abovementioned dietary factors, CHO availability remains the key factor affecting both phases of postexercise glycogen synthesis and rate of recovery.…”