Influence of diabetes mellitus and insulin treatment on protein turnover in the rat

Type 2 Diabetes and Metformin Influence on Fracture Healing in an Experimental Rat Model

Persons with diabetes have a greater incidence of fractures compared with persons without diabetes. However, very little published information is available concerning the deleterious effect of late-stage diabetes on osseous structure and bone healing. The purpose of the present study was to evaluate the role of diabetes on fracture healing in a rat femur repair model. Thirty-six lean and diabetic Zucker rats were subdivided into 3 groups: (1) 12 lean rats as the control group; (2) 12 diabetic rats without blood glucose control (DM group); and (3) 12 diabetic rats treated with 300 mg/kg metformin to reduce the blood glucose levels (DM + Met group). Radiographs were taken every week to determine the incidence of bone repair and delayed union. All the rats were killed at 6 weeks after surgery. In both the sham-operated and the fractured and repaired femurs, significant decreases in the fracture-load/weight and marginal decreases in the fracture-load between the lean and DM groups were found. Metformin treatment significantly reduced the blood glucose and body weight 12 days postoperatively. Furthermore, a decrease in the fracture-load and fracture-load/weight in the repaired femurs was found in the DM + Met group. Diabetes impairs bone fracture healing. Metformin treatment reduces the blood glucose and body weight but had an adverse effect on fracture repair in diabetic rats. Further investigations are needed to reveal the mechanisms responsible for the effects of type 2 diabetes mellitus on bone and bone quality and the effect of medications such as metformin might have in diabetic bone in the presence of neuropathy and vascular disease.

Effects of severe diabetes and insulin on the femoral neck of the immature rat

The interactive effects of severe diabetes and insulin therapy on the geometrical, biomechanical, and histomorphological characteristics of the femoral neck were studied in rats that had streptozotocin-induced, insulin-dependent (Type-I) diabetes. Thirty-six female Sprague-Dawley rats (8 weeks of age) were assigned randomly to one of three groups: 12 to control (C), 12 to severe diabetes mellitus (SDM), and 12 to severe diabetes with insulin treatment (SDI). At the conclusion of 10 weeks, the femoral necks were loaded to failure via cantilever-bending tests, and the geometrical, structural, and material properties of the femoral neck were measured and correlated with fracture-surface cross sections. Decalcified cross sections of the femoral necks were analyzed histomorphometrically to determine the porosity, the bone-cell counts, and the bone spicule/marrow space ratio. Rats with severe insulin-dependent diabetes had significantly lower total body mass than did control rats, as well as significantly less femur mass, femur length, total-bone cross-sectional area, and cortical-shell cross-sectional area. Insulin therapy ameliorated some, but not all, of the detrimental effects of diabetes on femoral neck geometry. Compared with control and SDI rats, SDM rats had lower values for femoral neck structural properties, although differences in structural properties may have been related to retarded growth as well as to diabetes. SDM rats had a significantly lower bone spicule/marrow space ratio and number of osteoclasts than did either the control or SDI rats and had significantly greater porosity in the femoral neck cortex than did control rats. Decrements in femoral neck material properties--which were independent of differences in body size--were significantly and linearly correlated with severity of diabetes (as measured by blood glucose level).

Effect of physiologic hyperinsulinemia on skeletal muscle protein synthesis and breakdown in man

Although insulin stimulates protein synthesis and inhibits protein breakdown in skeletal muscle in vitro, the actual contribution of these actions to its anabolic effects in man remains unknown. Using the forearm perfusion method together with systemic infusion of L-[ring-2,6-3H]phenylalanine and L-[1-14C]leucine, we measured steady state amino acid exchange kinetics across muscle in seven normal males before and in response to a 2-h intraarterial infusion of insulin. Postabsorptively, the muscle disposal (Rd) of phenylalanine (43 +/- 5 nmol/min per 100 ml forearm) and leucine (113 +/- 13) was exceeded by the concomitant muscle production (Ra) of these amino acids (57 +/- 5 and 126 +/- 9 nmol/min per dl, respectively), resulting in their net release from the forearm (-14 +/- 4 and -13 +/- 5 nmol/min per dl, respectively). In response to forearm hyperinsulinemia (124 +/- 11 microU/ml), the net balance of phenylalanine and leucine became positive (9 +/- 3 and 61 +/- 8 nmol/min per dl, respectively (P less than 0.005 vs. basal). Despite the marked increase in net balance, the tissue Rd for both phenylalanine (42 +/- 2) and leucine (124 +/- 9) was unchanged from baseline, while Ra was markedly suppressed (to 33 +/- 5 and 63 +/- 9 nmol/min per dl, respectively, P less than 0.01). Since phenylalanine is not metabolized in muscle (i.e., its only fates are incorporation into or release from protein) these results strongly suggest that in normal man, physiologic elevations in insulin promote net muscle protein anabolism primarily by inhibiting protein breakdown, rather than by stimulating protein synthesis.

The effects of streptozocin-induced diabetes and weight-matched pair feeding on tumor growth and survival in Fischer rats

Sixty-one 150- to 180-g Fischer rats were assigned to one of four experimental groups: nondiabetic, tumor bearer (ND-TB, n = 15); diabetic, non-tumor bearer (D-NTB, n = 19); diabetic, tumor bearer (D-TB, n = 16); or nondiabetic, weight-matched control, tumor bearer (WM-TB, n = 11). The WM group was housed in metabolic cages and fed H2O ad libitum and rat chow to achieve appropriate carcass weight (comparable to D-NTB). Diabetes was induced with a single dose of streptozocin (STZ) 40 mg/kg body wt iv, 10 days prior to tumor inoculation. Viable methylcholanthrene-induced sarcoma cells (1 X 10(6) ) were injected into the right flank on Day O. Body weight and tumor volume were assessed every 3 days. On the day of death, the animal and excised tumor were weighed. Dividing the growth curves versus time into two phases based on tumor volumes 0-20 and 20-60 cm3 defined a significant early growth (0-20 cm3) delay of 5 days in the D-TB and WM groups vs the ND-TB group. The growth rate from 20-60 was not different in any group. Tumor growth retardation seen in STZ-induced diabetic animals can be mimicked and exceeded in a pair-fed, weight-matched control group. Diabetic and starved animals have smaller tumors and live longer than ad lib fed, nondiabetic animals. The delay in tumor growth is a reflection of the retardation in the early (0-20 cm3) growth rate.