Chapter 54. Noninvasive Imaging of Insulin Action and Resistance in Humans.

Chapter 54.Noninvasive Imaging of

Insulin Action and Resistance in Humans

Introduction

• The composite effect of insulin on substrate metabolism– Divisible among different organs(liver, adipose tissue, muscle)– Divisible on the basis of diverse physiologic actions within an

organ(blood flow, capillary recruitment, transport in skeletal m)– Divisible on cellular basis

• In vitro or ex vivo system– Loss of vascular and neural networks– Isolation from the circulating internal milieu– Disruption of cytoskeletal structures and networks

• In vivo in animal, ideally in a noninvasive manner – in vivo homeostasis– NMR spectroscopy vs PET (Tab. 54.1)

Methods for Investigating Regional-Metabolism

• 60 년전 Himsworth : response to effect of insulin to influence glycemic excursion following IV dextroseIR

• 1960s, Rubin Andres – Quantitative measurement of insulin action – glucose clamp/euglycemic insulin infusion method

• Within particular metabolic pathways, with focus on particular organ system

4 ancillary study• Indirect calorimetry

– Measurement of glucose oxidation and fat oxidation– + clampInsulin stimulation of nonoxidative glucose metabolism(vs IR)

• Arteriovenous limb balance and organ balance– Site of insulin-stm glucose utilization skeletal m.(vs IR)– Robust effect of regional or systemic infusion of insulin – Relatively invasive, require technical expertise, not absolute approach for “isolating” skeletal muscle

metabolism, technical challenge• Flow measurement in limb balance

– Insulin stimulate increased blood flow in skeletal m.(vs IR)

• Radioactive and stable isotope tracer dilution method(glycerol)– Kinetic aspect of the appearance rate and utilization rates of a particular substrate systemic infor

mation & individual organ system– Nonlabled Endogenous glucose production: gluconeogenesis/glycogenolysis– Hepatic IR, glucose production from kidney, glycerol or FFA tracerlipolysis, insulin action on adipo

cyte• Tissue biopsy : skeletal m

– Enzyme activity, expression and phophorylation, content of lipid metabolites, glycogen and numerous intermediary metabolites, insulin signaling and other signaling pathways, glucose and fatty acid transporter proteins

Nuclear Magnetic Resonance Spectroscopy

• Exposed to a strong magnetatomic nuclei align with polaritynuclear spinexposed to oscillating radiofrequenciesshift in the angle within relaxation realign at the original angle release absorbed energy detected by receiver coil characteristic spetrum

• Presence of surrounding nuclei and electrons of other atoms and chemicals: “chemical shift” intracellualr and extracellular TG in skeletal m.

• Signal intensity is proportional to the tissue concentration: “phantom”standard/endogenous compound of known concentration

• Advantage – Not require the use of ionizing radiation, noninvasive, good tissue spe

cificity• Limitation

– Cost, availability of facilities, technical expertise, practical issue

isotopes in NMR spectroscopy

• 1H – 100% abundance– Strong signal from water signal to noise ratio– Intracellular TG content– Intramyocellular lipid content(IMCL): inverse correlation to whol

e-body insulin sensitivity• 34P

– Intracellular concentration of inorganic phophate, ATP, ADP, phosphocreatine, G6P

• 13C– 1.1% – Detection of skeletal m glycogen– Exogenous administration of 13C-glucose(Fig 54.1)

Ex. Of Application of NMR in Skeletal Muscle

Metabolism• 13C-glycogen content before&after exercise in healthy man

– Avison et al: significantly decreased after exercise and was reconstituted after a period of rest, reaching 80% of basal value in19 hours

– Price et al: during the recovery period, glycogen levels progressively increased but in a biphasic mode

• Slow component was inhibited by somatostatin infusion: fast phase-insulin independent/slow-insulin dependent (fig.54-2)

• 13C-glycogen synthesis in skeletal m of DM/normal– 13C-glycogen synthesis was 43% of normal in DM under hyperglyc

emic hyperinsulinemic clamp • whole body glucose disposal in human is predominantly accou

nted for by glycogen synthesis in skeletal m.• IR asso with a reduced state of glycogen synthesis

Ex. Of Application of NMR in Skeletal Muscle

Metabolism• Key rate-limiting step of stimulation by insulin of g

lucose utilization?– 31P-NMR using magnification of a substraction spectra

(stim-base) :G6P • Nl in insulin infusion G6P• IR • Rate-limiting step controlled by insulin is within the steps of gl

ucose uptake or glucose phosphorylation– Exercise intervention– Effect of insulin on the rate of glucose transport across

the sarcolemma: 13C-NMR• Intracelluar skeletal muscle glucose in obese DM

Ex. Of Application of NMR in Skeletal Muscle

Metabolism• Effect of FFA on IR

– FFA oxidation would compete with glucose for mitochondrial oxidation accumulation of metabolites in glycolytic pathway cellular glucose uptake

– FFA induced IR(infusion of lipid emulsion) skeletal m glycogen synthesis 50%

– G6P by FFA: FFA inhibit glucose disposal not by substrate competition for oxidation, but by inhibition of glucose transport across plasma memb or glucose phosphorylation by hexokinase

Ex. Of Application of NMR in Hepatic Metabolism

• Hepatic glycogen content : 13C-NMR– +3-3H-glucose infusion(glucose production): hepatic glycogenolysis r

ate 45% of endogenous glucose production in the first 10 hrs/ decreased to 4% after 50 hrs relative contribution of glucogenolysis in fasting

– Hepatic glycogen content after meal: net glycogen deposition represented 20% of the ingested carbohydrate content in meal

– T2DM : fasting hyperglycemia endogenous glucose production(glycogenolysis or gluconeogenesis)

• Gluconeogenesis systemic rate of glucose production(radioisotope tech)-hepatic glycogenolysis(NMR)

– T2DM : fasting liver glycogen / increased gluconeogenesis rather than glycogenolysis, account for increased postprandial liver glucose production

PET

Regions of interest(ROI)

• Across numerous planes(anatomic slices)/across as many frames(separate intervals or subdivisions of imaging time)

• Dynamic changes of tissue tracer activity

Metabolic Tracer activity• Not continous infusion but bolus injection• Most often arterial blood is sampled to determine

tracer activity– Several caveats

• Tracer metabolized in tissue, reenters the blood in another chemical forms

• Short half life, “decay-corrected”• Blood radioactivity from PET images-cardiac chamber

• Tissue activity across time arterial tracer activity measured task of analysis(artery:input/tissue:output)

• In area of DM– insulin action in skeletal muscle– Adipose tissue– Brain blood flow, cerebral metabolism, neurotransmitter ligan

ds– Cardiac metabolism

• Oncology imaging of tumor location and metabolism• High-resolution “micro CT”, “micro PET”

Applications of PET for Studies of Insulin Action in

Skeletal M.• 18F-FDG• Close correlation between PET assessments of insulin action and ind

ependent determinations obtained by limb balance and systemic measures

• Gender difference, effect of physical training, exercise, skeletal blood flow(15O-water & pharmacologic agents influencing vascular tone and blood flow) – Arterial flow is likely not a direct causal factor inmediating insulin action on gluc

ose utilizaton by skeletal m.• IR in T1DM: lower glucose uptake, greater heterogeneity in glucose up

take• Macroscopic index of overall 18F-FDG uptake

– Not separate estimation of relative contributions of glucose transport&phosphorylation

Dynamic PET• 3 Rate constant/ 3 –compartment model

– Robust insulin effect(10 ): k1(2 ) & k3 (6-8 )• Insulin dose-response

– At basal& low insulin: phosphorylation– Higher insulin: glucose transport– Insulin effect a redistribution of control between tranport & phosphorylation– Long chain acyl CoA in T2DM : Hexokinase II

• Leg glucose uptake(LGU) + PET– In IR, impairment of glucose phosphorylation is most apparent at low-to-mod insulin st

imulation

Cp Ce Cm

Optimizing the Modeling of dynamic PET Imaging of Skeletal

M.• Steps influence 18F-FDG metabolism

– Substrate delivery(tissue perfusion, capillary diffusion)– Transmembrane transport– Cellular trapping of glucose by phosphorylation

• 4 compartment/ 5 rate constant– 6-10 folds in glucose transport(lean vs obese)– Normal at supraphysiologic levels of insulin– Insulin act to redistribute control of glucose metabolis

m : rate-control effect of phosphorylation during fastingglucose transport during insulin-stimulated condition

Dynamic PET: effect of weight loss on IR

PET Imaging of Myocardial Metabolism in DM

• Coronary blood flow, ischemic myocardium, myocardial fuel metabolism independent of ischemia

• Relatively low rate of glucose utilization, instead high reliance on oxidation of fatty acid during fasting condition

• Insulin independent glucose uptake by myocardial contraction: insulin further stimulation(GLUT-4 translocation)

• Responsive to substrate competition– Infusion of heparin/lipid emulsion insulin-stm glucose

uptake

• Tissue heterogeniety in metabolism of FDG– Uptake of FDG: posterolateral wall of heart – “hibernating” myocardium

• Relative predominence of GLUT-1• Anaerobic ATP production via glycolysis

– Ischemic • normal/decreased/increased glucose metabolism• Concomitant determination of tissue perfusion

• Effect of insulin on myocardial perfusion and glucose metabolism– Myocardial glucose uptake in DM– Circulating insulin level (DM vs normal)– Insulin-stm myocardial blood flow

• Regional distribution, matching with areas of increased glucose utilization in T2DM

PET Imaging of Adipose Tissue Metabolism

• Depot of adiposity with IR and metabolic risk• Arteriovenous balance with abdominal subcuta

neous adipose tissue• 18F-FDG with PET

– Insulin-stm glucose uptake by adipose tissue– Visceral fat– Glucose uptake in adipose tissue by PET correlate

d well with whole-body glucose uptake

Conclusion

• The emergence of novel techniques-NMR, PET- have already contributed greatly to a clear understanding of the complex physiologic and molecular impairment of IR

• These nascent development using PET/NMR spectroscopy hold tremendous promise for future clinical and animal investigation in DM, obesity and IR