Synopsis

Keywords: Muscle, Diabetes

In this study, metabolic responses to 10-week treadmill exercise on obese Zucker rats were examined using ³¹P magnetic resonance spectroscopy. Exercise resulted in improvement in glucose tolerance and aerobic capacity (VO_2max) while retaining mitochondrial oxidative capacity (MOC) and creatine kinase (CK) activity. Sedentary rats, however, showed poor glucose tolerance and reduced VO_2max, which were also marked by significant changes in their MOC and CK activity.

Introduction

Exercise has the benefit of improving insulin sensitivity in type 2 diabetes (T2D), as it contributes to mitochondrial biogenesis and improvement in mitochondrial function in skeletal muscle.^1,2 Nevertheless, there are reports of T2D patients who have not reaped the benefit of exercise, and the mechanism and predictors behind training responses are still poorly understood.^{3 31}P-Magnetic resonance spectroscopy (³¹P-MRS) provides a noninvasive method to evaluate high-energy phosphate metabolism in response to exercise training. In this study, we evaluated the effect of exercise on metabolic responses in obese Zucker rat, a rat model of obesity and insulin resistance, before and after 10-week exercise training. Glucose tolerance and aerobic capacity (VO_2max) were evaluated to assess changes in whole-body metabolic responses, while ³¹P-MRS methods were employed to evaluate changes in metabolic responses such as mitochondrial oxidative capacity (MOC) and creatine kinase (CK) activity. Our results revealed that obese rats responded positively to exercise, leading to improved glucose tolerance and VO_2max.

Method

Experimental protocol: Six- to seven-week old obese Zucker rats were randomly assigned to sedentary or exercise groups (n=9 per group). Rats in the exercise groups were engaged in a 10-week treadmill running program. Pre- and post-training characterizations were performed on all rats, including oral glucose tolerance test (OGTT), measurement of maximal oxygen consumption rate (VO_2max), and MRI/MRS evaluation of muscle energetics and fat-water content.
MRI/MRS methods: MRI/MRS studies were performed on a 9.4T preclinical scanner. The rats were anesthetized with ~2% isoflurane and placed in a lateral position. One of the hindlimbs was secured in a ³¹P volume coil. Two needle electrodes were inserted subcutaneously to stimulate the sciatic nerve. An electrical stimulation was applied for 5 min to induce high-intensity muscle contraction. Fat and water content were measured using iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) with three asymmetric echo delays (π/6, 5π/6, and 3π/2 radian shift).⁴ A total of eight slices covering the entire hindlimb were acquired with an in-plane resolution of 0.25 x 0.25 mm² and a slice thickness of 2 mm.
Dynamic ³¹P-MRS data were acquired during stimulation-induced muscle contractions. PCr recovery rate, an index of mitochondrial oxidative capacity, was quantified by fitting the PCr recovery curve after the stimulation to a monoexponential model.
CK rate constant (k_f^CK) was measured by ³¹P magnetic resonance spectroscopic fingerprinting (³¹P-MRSF) method described previously.^5,6 A total of 30 fingerprints were acquired in 10 min. The fingerprints were averaged and k_f^CK by matching the averaged fingerprint to a dictionary of simulated fingerprints.

Result

Glucose tolerance in sedentary and exercise rats were compared before and after the training by quantifying the area under the glucose clearance curve (AUC) (Figure 1A). Compared to the baseline, rats in the sedentary group developed significantly impaired glucose clearance after 10 weeks (p < 0.05), suggesting the development of insulin resistance. In contrast, rats in the exercise group did not show decreased capacity in glucose clearance. Sedentary rats also showed a decrease in VO_2max (p < 0.05) while those in the exercise group showed preserved VO_2max (Figure 1B).
Figure 2 shows the results of muscle and fat quantification from IDEAL MRI. Rats in the exercise group exhibited a significant increase in muscle volume while sedentary rats showed a trend of increased muscle volume that did not reach statistical significance (Figure 2 B). Exercise rats displayed a trend in the increase in subcutaneous fat volume but did not reach statistical significance, while sedentary rats exhibited no difference (Figure 2 C). Both sedentary and exercise rats showed a trend of increased fat fraction after 10 weeks but did not reach statistical significance. Further, there was no significant difference in fat fraction between sedentary and exercise rats (Figure 2 D).
Figure 3 shows muscle energetics quantified by ³¹P-MRS and ³¹P-MRSF. Sedentary rats displayed ~25% reduction in PCr recovery rate compared to the baseline (0.73 ± 0.15 to 0.55 ± 0.14 min^-1, p < 0.05). However, rats in the exercise group only showed a slight reduction (~ 11%) in PCr recovery rate that did not reach statistical significance (0.76 ± 0.22 to 0.67 ± 0.13 min^-1, p = 0.4). Finally, sedentary rats also showed a 14% increase in CK rate constant after 10 weeks (0.44 ± 0.05 to 0.50 ± 0.04 s^-1, p < 0.05), while the CK rate constant in the exercise group remained unaltered after 10-week training.

Discussion and conclusion

In this study, we investigated the effect of exercise training on energy metabolism and glucose tolerances of obese Zucker rats. Our results show that exercise training can delay the development of metabolic dysfunction, glucose intolerance, as well as the decrease in aerobic capacity.

Acknowledgements

This work was supported by a grant from the National Institute of Health (R01 EB23704).