Pagliassotti M J, Donovan C M
Department of Exercise Science, University of Southern California, Los Angeles 90089.
Am J Physiol. 1990 Apr;258(4 Pt 2):R903-11. doi: 10.1152/ajpregu.1990.258.4.R903.
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.
通过在三种不同的兔骨骼肌制剂中进行单程灌注，研究了由三碳前体合成糖原的途径，即糖酵解型（超过99%为IIb型）、氧化型（超过97%为I型）和混合型（I型、IIa型和IIb型）。利用[1-14C]-或[2-14C]乳酸盐，在基础（1.1±0.1 mM）和升高（8.1±0.3 mM）的乳酸盐浓度下（方案1和2），评估了三羧酸循环与糖原生成之间的相互作用程度。在葡萄糖和乳酸盐的净平衡、[14C]乳酸盐清除率和静脉乳酸盐比活性相似的条件下，14CO2和[14C]糖原的产量不受标记位置的显著影响。用乳酸盐（8.0±0.1 mM）和乙酸盐（1.0±0.1 mM）作为唯一底物，并用[U-14C]乳酸盐或[2-14C]乙酸盐作为示踪剂进行了额外的灌注。在净糖原合成的条件下，在糖酵解型、氧化型和混合型制剂中，[14C]乳酸盐掺入糖原的量[以每分钟衰变数（dpm）·g-1·2 h-1计]分别为40940±3320、1540±320和32600±4100。然而，尽管14CO2有显著产量，但在任何制剂中均未观察到[2-14C]乙酸盐掺入糖原。巯基吡啶酸是磷酸烯醇式丙酮酸羧激酶（PEPCK）的有效抑制剂，对净底物平衡、示踪剂摄取、净糖原合成、[14C]乳酸盐和[3H]-葡萄糖掺入糖原或14CO2产量均无显著影响。目前的结果表明，存在一条不依赖PEPCK的、由三碳前体合成净糖原的线粒体外途径，该途径在所有哺乳动物骨骼肌纤维类型中都是一致的。
