Fig. 39.15 ). Glucose tolerance refers to the ability of an individual to minimize the increase in blood glucose concentration after a meal. A primary way by which insulin promotes glucose tolerance is activation of glucose transporters in skeletal muscle. Insulin stimulates translocation of preexisting GLUT4 transporters to the cell membrane (reaction 1D). Insulin also promotes storage of glucose in muscle as glycogen (reaction 2D) and promotes oxidation of glucose through glycolysis (reaction 3D). During the fasting phase, low insulin results in a low number of GLUT4 transporters at the membrane (reaction 1F), so these cells consume less glucose (glucose sparing). Skeletal muscle fibers with mitochondria switch to the use of FFAs from adipocytes and KBs from hepatocytes (reaction 3F). Skeletal myocytes do not express glucagon receptors. Uptake of FFAs and KBs and their oxidation for ATP is largely upregulated by intracellular Ca++ levels and a high AMP:ATP ratio. Exercise also activates these pathways, as does glycogenolysis (reaction 2F), through adrenergic receptor stimulation.
2.
(Fig. 39.16A ). Insulin also stimulates GLUT4-dependent uptake of glucose and subsequent glycolysis in adipose tissue (reactions 1D and 2D). Adipose tissue uses glycolysis for energy needs but also for generating G3P (reaction 3D), which is required for esterification of FFAs into TGs (reaction 4D). During the fasting phase, insulin is low, so GLUT4 movement to the cell membrane is blocked (reaction 1F).
2. Adipocytes → FFAs and TG (see
Fig. 39.16B ). Insulin stimulates expression of lipoprotein lipase (LPL) •Fig. 39.15 A, Glucose metabolism in an adipocyte during digestive (“D” reactions) and fasting (“F” reactions) phases. Reactions/pathways:1Dand1F(alsosee
Fig.39.14 legend);2D,glycolysis,pyruvatedehydrogenase/TCAcycle/OxPhos;3D,glycerol-3-phosphatedehydrogenase;4D,esterificationofFFAstoG3Ptoformtriglyceride.B, Lipid metabolism in adipocyte during digestive (“D” reactions) and fasting (“F” reactions) phases. Reactions/pathways:1D,synthesisoflipoproteinlipase(LPL)andLPLsecretionintosubcapillaryspace,bindingtoGPI-anchoredprotein(red box), andmigrationtoluminalsideofcapillaryendothelialcell;2D,lipolysisofchylomicronTGandreleasingfreeFFA(afterdigestion,chylomicronremnantisclearedfromcirculationbyliver);3D,activationofimportedFFAsbytransfertoacetylCoAtoformfattyacylCoA;4D,esterificationoffattyacylCoAstoG3PtoformTG(TGdropletsarecoveredandstabilizedbyperilipins[PL]);5D,dephosphorylationandinactivationofhormone-sensitivelipase(HSL),whichpromotesstorageofTG;1F,phosphorylationandactivationofHSL,whichcontributestocompletelipolysisofTG(ATGL,adipocyteTGlipase;HSL,diacylglyceridelipase;MGL,monoacylglyceridelipase);2F,finalstepinTGlipasebyMGLreleasesFFAandglycerol.