Conditioning causes an increase in glucose transporter-4 levels in mononuclear cells in sled dogs

Abstract

This study was designed to investigate the effects of physical conditioning on the expression of the insulin sensitive glucose transporter-4 protein (GLUT4) on mononuclear cells and HOMA-IR levels in dogs and compared to results reported in human skeletal muscle and the skeletal muscle of rodent models. Blood was sampled from conditioned dogs (n = 8) and sedentary dogs (n = 8). The conditioned dogs were exercised four months prior the experiment and were following a uniform training protocol, whereas the sedentary dogs were not. GLUT4 expression in mononuclear cells and plasma insulin levels were measured using commercially available enzyme-linked immunosorbent assay (ELISA). Blood glucose levels were determined using blood plasma. HOMA-IR was calculated using plasma insulin and blood glucose levels using the linear approximation formula. Our results indicate that the state of conditioning had a significant effect on the GLUT4 expression at the surface of mononuclear cells. HOMA-IR was also affected by conditioning in dogs. GLUT4 levels in mononuclear cells of sled dogs were inversely correlated with the homeostasis model assessment of insulin sensitivity. This study demonstrates that conditioning increases GLUT4 levels in mononuclear cells of sled dogs as it has been previously reported in skeletal muscle. Our results support the potential of white blood cells as a proxy tissue for studying insulin signaling and may lead to development of a minimally invasive and direct marker of insulin resistance. This may be the first report of GLUT4 in mononuclear cells in response to exercise and measured with ELISA.

Keywords: Conditioning; Exercise; GLUT4; Insulin sensitivity; Mononuclear cells.

Figure 1. GLUT4 concentrations on the surface of mononuclear cells of sled dogs is effected by long-term conditioning

GLUT4 concentrations were measured via enzyme linked immunoassay ELISA in mononuclear cells of conditioned (n=7) and sedentary (n=8) sled dogs. GLUT4 in mononuclear cells of conditioned dogs was 6690 ± 184 ng/g protein and in sedentary dogs was 4290 ± 773 ng/g protein. Means were statistically different (t-test, p<0.05).

Figure 2. Fasting insulin concentrations of conditioned and sedentary dogs indicate an apparent trend of improvement of insulin sensitivity after four months of conditioning

Insulin concentrations were measured via ELISA using the blood plasma in conditioned (n=7) and sedentary (n=7) sled dogs after an overnight fast. Insulin concentration of conditioned dogs was 3.3 ± 0.23 μU/mL and in sedentary dogs 3.8 ± 0.69 μU/mL. Although data show an apparent trend, means were not significantly different (t-test, p>0.05).

Figure 3. Fasting glucose concentrations of conditioned and sedentary dogs indicate an apparent trend of decreased blood glucose after four months of conditioning

Glucose levels were measured using the blood plasma in conditioned (n=7) and sedentary (n=7) sled dogs after an overnight fast. Glucose concentration of conditioned dogs was 96 ± 5 mg/dL and in sedentary dogs 110 ± 3 mg/dL. Means were significantly different (t-test, p<0.05).

Figure 4. The Homeostasis Model assessment to quantify insulin resistance (HOMA-IR) is significantly lower in conditioned dogs than in sedentary dogs

HOMA-IR was estimated with the linear model using fasting insulin and glucose levels. HOMA-IR was affected by conditioning in dogs. The four months conditioned dogs had significantly lower HOMA-IR (0.76 ± 0.10) as compared to the sedentary dogs (1.03 ± 0.24). Means were statistically different (t-test, p<0.05).

Figure 5. The relationship between GLUT4 levels and the homeostasis model assessment of insulin sensitivity HOMA-IR was investigated using Pearson correlation coefficient

HOMA-IR was estimated with the linear model using fasting insulin and glucose levels. Preliminary analysis was performed to ensure no violation of the assumption of normality and linearity. There was moderately close negative correlation between the two variables (•=conditioned dogs, □=sedentary dogs, r=−0.70, n=13, p=0.01).