AJP - Regulatory, Integrative and Comparative Physiology, Vol 258, Issue 4 903-R911, Copyright © 1990 by American Physiological Society

ARTICLES

Glycogenesis from lactate in rabbit skeletal muscle fiber types

M. J. Pagliassotti and C. M. Donovan
Department of Exercise Science, University of Southern California, Los Angeles 90089.

The path of glycogen synthesis from three-carbon precursors was studied via single-pass perfusions in three distinct rabbit skeletal muscle preparations, i.e., glycolytic (greater than 99% type IIb), oxidative (greater than 97% type I), and mixed (type I, IIa, and IIb). The extent of interaction between the Krebs cycle and glycogenesis was assessed utilizing [1-14C]- or [2-14C]lactate at basal (1.1 +/- 0.1 mM) and elevated (8.1 +/- 0.3 mM) lactate concentrations (protocols 1 and 2). Under conditions in which the net balance of glucose and lactate, [14C]lactate removal, and venous lactate-specific activity were similar, the yields of 14CO2 and [14C]glycogen were not significantly influenced by position of the label. Additional perfusions were performed with lactate (8.0 +/- 0.1 mM) and acetate (1.0 +/- 0.1 mM) as sole substrates and either [U-14C]lactate or [2-14C]acetate as the tracer. Under conditions of net glycogen synthesis, the incorporation of [14C]lactate into glycogen [in disintegrations/min (dpm).g-1.2 h-1] was 40,940 +/- 3,320, 1,540 +/- 320, and 32,600 +/- 4,100 in the glycolytic, oxidative, and mixed preparations, respectively. However, no incorporation of [2-14C]acetate into glycogen was observed in any preparation, despite a significant yield of 14CO2. Mercaptopicolinic acid, a potent inhibitor of phosphoenolpyruvate carboxykinase (PEPCK), demonstrated no significant effect on net substrate balance, tracer uptake, net glycogen synthesis, incorporation of [14C]lactate and [3H]-glucose into glycogen, or 14CO2 yield. Current results suggest an extramitochondrial route for net glycogen synthesis from three-carbon precursors, exclusive of PEPCK, that is consistent across all mammalian skeletal muscle fiber types.

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