Regional metabolic rate of exogenous glucose in the isoprenaline and dobutamine stimulated canine myocardium as estimated by the 2-deoxy-D[1-14C]glucose method

Clenbuterol prevents epinephrine from antagonizing insulin-stimulated muscle glucose uptake

In the present study, we investigated the effects of chronic clenbuterol treatment on insulin-stimulated glucose uptake in the presence of epinephrine in isolated rat skeletal muscle. Insulin (50 microU/ml) increased glucose uptake in both fast-twitch (epitrochlearis) and slow-twitch (soleus) muscles. In the presence of 24 nM epinephrine, insulin-stimulated glucose uptake was completely suppressed. This suppression of glucose uptake by epinephrine was accompanied by an increase in the intracellular concentration of glucose 6-phosphate and a decrease in insulin-receptor substrate-1-associated phosphatidylinositol 3-kinase (IRS-1/PI3-kinase) activity. Clenbuterol treatment had no direct effect on insulin-stimulated glucose uptake. However, after clenbuterol treatment, epinephrine was ineffective in attenuating insulin-stimulated muscle glucose uptake. This ineffectiveness of epinephrine to suppress insulin-stimulated glucose uptake occurred in conjunction with its inability to increase the intracellular concentration of glucose 6-phosphate and attenuate IRS-1/PI3-kinase activity. Results of this study indicate that the effectiveness of epinephrine to inhibit insulin-stimulated glucose uptake is severely diminished in muscle from rats pretreated with clenbuterol.

Amylin influences insulin-stimulated glucose metabolism by two independent mechanisms

The effects of amylin on fiber type-specific muscle glucose metabolism under hyperglycemic (10 mmol/l) and hyperinsulinemic (2.1 nmol/l) conditions were investigated using a rat hindlimb perfusion system. Amylin concentration ranged from 1 to 100 nM. Efficacy for inhibition of glucose uptake traced with 2-deoxyglucose by amylin was demonstrated in all three fiber types. The incorporation of 2-deoxy-[3H]glucose tracer decreased from control values by 41% in fast oxidative (FO), 36% in fast glycolytic (FG), and 37% in slow oxidative (SO) muscle with 100 nM amylin. Amylin increased intracellular glucose 6-phosphate (G-6-P), and G-6-P was negatively correlated with 2-deoxyglucose uptake in both FO (r = -0.65; P < 0.01) and FG (r = -0.53; P < 0.01) muscle. Muscle glycogen concentration increased under control conditions and decreased in the presence of 100 nM amylin. Lactate arteriovenous efflux across the hindlimb increased significantly above control with 100 nM amylin (5.03 +/- 0.81 to 11.28 +/- 0.94 mumol.g-1.h-1). Adenosine 3',5'-cyclic monophosphate (cAMP) increased in FO and FG muscle with amylin. Salmon calcitonin-(8-32), an amylin antagonist, ameliorated the effect of amylin on all responses other than 2-deoxyglucose uptake and G-6-P concentration. These results suggest that amylin may work through at least two independent mechanisms, a cAMP-mediated effect on glycogen metabolism and a non-cAMP-mediated inhibition of glycolysis.

Glucose uptake and lumped constant variability in normal human hearts determined with [18F]fluorodeoxyglucose

BACKGROUND

Myocardial glucose uptake can be measured with [18F]fluoro-2-deoxyglucose (FDG) and positron emission tomography (PET). However, changes of myocardial metabolism may alter the ratio between the net rates of FDG and glucose uptake, known as the lumped constant. We tested the hypothesis that the variability of the lumped constant determined in animals explains the disagreement between human net myocardial glucose uptake calculated from aortocoronary sinus deficits and measured with PET.

METHODS AND RESULTS

In the three-compartment model of glucose transfer into cells, the lumped constant is a function of the relationship between the net and the unidirectional rates of uptake of glucose and glucose tracers such as FDG. Using this principle, validated in the human brain and the animal heart under experimental conditions, we estimated the lumped constant of the human heart by PET in 10 healthy men under several metabolic conditions established by altering the circulating insulin level during a euglycemic clamp and with somatostatin and heparin infusions. The lumped constant varied systematically between 0.44 and 1.35. At insulin levels below 100 pmol/L, free fatty acids were inversely related to serum insulin levels and the lumped constant increased linearly with serum insulin concentration. At insulin levels above 100 pmol/L, free fatty acids were suppressed and the lumped constant varied in inverse proportion to the insulin level. When the lumped constant was estimated in this manner, net myocardial glucose uptake agreed with that determined in previous measurements of blood flow and aortocoronary sinus deficit.

CONCLUSION

In the intact human organism, the cardiac lumped constant varies with the metabolic condition, as predicted from studies of the brain and animal heart under experimental conditions.