Protein and glucose metabolism in response to hyperinsulinemia with and without hyperaminoacidemia in men with type 2 diabetes mellitus

Abstract

In poorly-controlled type 2 diabetes mellitus (T2DM), 24h whole-body protein turnover is elevated and net protein balance diminished compared to nondiabetic weight-matched controls. These alterations were partly attributed to insulin resistance of protein, particularly in hyperglycemic men, determined using hyperinsulinemic, euglycemic and isoaminoacidemic clamps. However the effect of intense glycemic control on this resistance is not known. Moreover, the effect of postprandial-level hyperaminoacidemia with concurrent hyperinsulinemia and hyperglycemia on glucose and protein metabolism in T2DM is not clearly established. Thus the objectives of this thesis are to determine in men with T2DM, at the whole-body and cellular levels whether i) hyperglycemia at 8.0 vs. 5.5 mmol/L worsens insulin sensitivity of protein, ii) protein metabolism is further impaired when insulin and amino acids are clamped at peak postprandial concentrations, iii) postprandial amino acids aggravate the insulin resistance of glucose metabolism, and iv) insulin sensitivity of glucose and protein is improved with intense control of T2DM. For objectives i) and ii), eight overweight or obese men with T2DM were studied with a hyperinsulinemic, hyperglycemic (8 mmol/L) clamp, first with plasma amino acids at postabsorptive (Hyper-2) then at postprandial concentrations (Hyper-3). Whole-body protein kinetics were assessed using 13C-leucine tracer method. Hyper-2 results were compared to those of diabetic men whose plasma glucose was lowered to 5.5 mmol/L and fasting aminoacidemia maintained during the hyperinsulinemic clamp (Hyper-1). For objective iii), the same protocol was used with whole-body glucose turnover assessed using 3 H-glucose. Vastus lateralis biopsies were obtained at baseline and during each step of the clamp to determine the phosphorylation states of Akt, mTOR, ribosomal protein (rp) S6, and insulin receptor substrate (IRS)-1. For objective iv), protein kinetics and glucose turnover in T2DM men were measured in 7 normoglycemic (A1C 6.1±0.2%) during Hyper-1 and Hyper-3 and 8 hyperglycemic (A1C 7.1±0.2 % p<0.01) during Hyper-2 and Hyper-3. Vastus lateralis biopsies were obtained at baseline and both steps of the clamp to determine the phosphorylation states of relevant proteins in the protein synthesis pathways. Results: 1) Hyper-2 is associated with a faster turnover rate but, similar net protein anabolism as Hyper-1; 2) Hyper-3 increases net protein synthesis to levels not different from healthy lean controls; 3) hyperaminoacidemia does not worsen insulin resistance of glucose metabolism; and 4) intense glycemic control does not improve insulin sensitivity of protein metabolism. The presence of insulin resistance of protein in T2DM men has important implications for dietary protein recommendations as it suggests their protein requirements may be greater than current DRIs. Further investigation is warranted to address this issue in women and whether other dietary interventions known to improve insulin resistance of glucose metabolism in T2DM would modulate that of protein metabolism.