Skeletal muscle neuronal nitric oxide synthase μ protein is reduced in people with impaired glucose homeostasis and is not normalized by exercise training
Introduction
Exercise training is a well-recognized means of improving glucose tolerance in humans [1]. This effect is consistent with the fact that skeletal muscle is the major site of insulin-stimulated glucose disposal in the body [2]. The 2 main mechanisms controlling glucose uptake into skeletal muscle are insulin and muscle contraction. Exercise training may exert its beneficial effects on glucose tolerance via both mechanisms; however, the precise signaling pathways involved are unclear. The nitric oxide (NO)/cyclic guanosine monophosphate pathway has been implicated in muscle contraction–mediated glucose uptake in humans [3], [4]. Nitric oxide also plays a role in insulin-stimulated muscle glucose uptake, although its effects are mainly hemodynamic [5], [6], [7] rather than directly on skeletal muscle glucose transport [8]. Nitric oxide is produced from l-arginine by the nitric oxide synthases (NOSs). Neuronal NOS (nNOS) μ is highly expressed in human skeletal muscle [9], [10]. Endothelial NOS (eNOS) protein is expressed to a lesser extent than nNOSμ in human skeletal muscle [9] and is found predominately in the endothelium [10]. There is very little inducible NOS (iNOS) protein expression in skeletal muscle of normal, healthy humans [9], [11].
It was recently shown that skeletal muscle iNOS protein was 4-fold higher in people with type 2 diabetes mellitus compared with healthy controls, whereas there was no difference in skeletal muscle eNOS protein levels [11]. No study has examined the level of skeletal muscle nNOSμ protein expression in people with impaired glucose tolerance or diabetes. Interestingly, mice lacking nNOSμ demonstrate peripheral (predominantly skeletal muscle) insulin resistance [12]. The purpose of the current study was to examine whether skeletal muscle nNOSμ protein expression is lower in individuals with impaired glucose homeostasis (prediabetes/type 2 diabetes mellitus) compared with healthy controls.
Intense short-term exercise training increases skeletal muscle nNOSμ protein expression in normal rats [13], [14], [15]. Although a human study found that skeletal muscle nNOSμ protein expression was not increased after 6 weeks of exercise training [16], we have found that skeletal muscle nNOSμ protein expression is significantly higher (approximately 60%) in endurance-trained compared with untrained individuals [17]. In the present study, we investigated whether an exercise training program that improves insulin sensitivity/glucose tolerance in people with impaired glucose homeostasis also increases skeletal muscle nNOSμ protein expression. We hypothesized that skeletal muscle nNOSμ protein would be lower in people with impaired glucose homeostasis than in healthy controls and that exercise training would increase skeletal muscle nNOSμ protein expression as well as insulin sensitivity/glucose tolerance.
Section snippets
Participants
Fourteen volunteers provided written informed consent for the study, which was approved by the Alfred Hospital and Monash University Ethics Committees and conducted in accordance with the Declaration of Helsinki of the World Medical Association. No subject was taking medication; and all were nonsmokers with a body mass index (BMI) <35 kg m−2, blood pressure ≤150/90 mm Hg, total cholesterol <6.5 mmol L−1, and triglycerides <4.0 mmol L−1. Fasting plasma glucose in conjunction with a 2-hour oral
Participants
There was no significant difference in age, sex balance, body mass, BMI, blood pressure, or lipid profile between the CON and DIAB groups (Table 1). Fasting plasma glucose, post-OGTT glucose and insulin (data not shown), and the HOMA insulin resistance index were all higher before training in DIAB than CON (P < .05, Table 2).
Exercise training
The CON group completed 19.1 ± 0.3 training sessions, whereas the DIAB group completed 19.3 ± 0.3 sessions. There was no significant difference in exercise training power
Discussion
This study demonstrated that patients with prediabetes/type 2 diabetes mellitus have 35% to 40% lower skeletal muscle nNOSμ protein expression than matched, healthy control individuals. Four weeks of exercise training improved glucose tolerance and indicators of insulin sensitivity but did not affect skeletal muscle nNOSμ protein expression in either the DIAB group or in the matched healthy control group. It has recently been shown that skeletal muscle iNOS protein content is elevated in people
Acknowledgments
This work was supported by grants from Diabetes Australia and the National Health and Medical Research Council of Australia.
The authors would like to thank the participants who took part in this study; Dr Zhiping Chen, Dr Robyn Murphy, and Dr Glenn Ward for technical assistance; and Dr Jean-Philippe Baguet for assistance with muscle biopsies and supervision of screening stress tests.
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