INCREASED HEXOSAMINE AVAILABILITY SIMILARLY IMPAIRS THE ACTION OF INSULIN AND IGF-1 ON GLUCOSE DISPOSAL

Prolonged glucosamine (GlcN) infusion increases the skeletal muscle he xosamine concentration and induces peripheral insulin resistance In co nscious rats, IGF-1 and insulin share common steps in signal transduct ion, and the action of IGF-1 on carbohydrate metabolism is preserved i n certain insulin-resistant states, In our study, we attempted to deli neate whether increased GlcN availability also impairs the effects of IGF-1 on glucose uptake (R(d)), glycolysis, and glycogen synthesis, We performed euglycemic IGF-1 (5 and 15 mu g . k(-1) . min(-1)) and insu lin (3 and 18 mU mg . kg(-1) . min(-1)) clamp studies at 0-2 h and 5-7 h in conscious rats (n = 44) during saline or GlcN infusions, GlcN in fusion raised plasma GlcN levels to similar to 2.0 mmol/l and skeletal muscle uridinediphospho-n-acetylglucosamine to 80-150 nmol/g (approxi mately three- to fivefold over basal), During physiological hyperinsul inemia (3 mU . kg(-1) . min(-1), plasma insulin congruent to 50 mu U/m l), GlcN infusion caused comparable decreases in R(d) (15.7 +/- 1.0 [5 -7 h] vs. 21.7 +/- 2.3 [0-2 h] mg . kg(-1) . min(-1); P < 0.01) and gl ycogen synthesis (5.4 +/- 0.5 [5-7 h] vs. 10.4 +/- 1.9 [0-2 h] mg . kg (-1) . min(-1); P < 0.005). Furthermore, GlcN markedly decreased R(d) by 7.8 +/- 1.2 mg . kg(-1) . min(-1) (18.7 +/- 0.7 [5-7 h] vs. 26.5 +/ - 1.3 [0-2 h] mg . kg(-1) . min(-1); P < 0.001 vs. control) during IGF -1 (5 mu g . kg(-1) . min(-1)) clamp studies. This decline was associa ted with a 26% decrease in the steady-state concentration of skeletal muscle Glc-6-P (286 +/- 45 vs. 386 +/- 36 nmol/g; P < 0.01) and was pr imarily caused by impaired glycogen synthesis (6.7 +/- 0.5 [5-7 h] vs. 13.9 +/- 0.9 [0-2 h] mg . kg(-1) . min(-1); P < 0.005). The effects o f GlcN infusion on glucose disposal (percentage decrease in R(d)) were correlated (r(2) = 0.803; P < 0.01) with the skeletal muscle concentr ation of UDP-GlcNAc. To investigate whether IGF-1 can overcome GlcN-in duced insulin resistance, GlcN and insulin (18 mU . kg(-1) . min(-1)) were infused for h during euglycemic clamps, and IGF-1 (15 mu g . kg(- 1) . min(-1)) was superimposed during the final 2 h. GlcN infusion ind uced severe impairment of insulin action on R(d) (39.4 +/- 3.2 [4-5 h] vs. 49.8 +/- 3.6 [1-2 h] mg . kg(-1) . min(-1); P < 0.05), which the addition of IGF-1 failed to improve (35.9 +/- 2.3 [6-7 h] vs. 39.4 +/- 3.2 [4-5 h] mg . kg(-1) . min(-1); P > 0.1). In summary, GlcN induced severe resistance to the actions of both insulin and IGF-1 on glucose uptake and glycogen synthesis, and IGF-1 was unable to overcome GlcN- induced insulin resistance. Thus, it is likely that GlcN causes periph eral insulin resistance acting at a site common to both IGF-1 and insu lin signaling pathways.