Lower skeletal muscle nutritive blood flow in older women is related to eNOS protein content

Oral administration of l-citrulline alters the vascular delivery of substances to rat skeletal muscles

Vascular endothelial function deteriorates with age and disease, and the production of vasodilator factors like nitric oxide (NO) decreases. The free amino acid l-citrulline increases vasodilation and blood flow through increased NO production. We examined the effects of oral l-citrulline administration on vascular delivery of substances to skeletal muscles. In Experiment 1, following oral l-citrulline administration and subsequent intravenous Evans blue dye (EBD) administration to rats, EBD levels delivered to skeletal muscles were measured after 60 min. In Experiment 2, plasma concentrations of amino acids and NOx, an indicator of vasodilation, were measured over time after oral l-citrulline administration. In Experiment 3, we measured EBD levels in skeletal muscles of streptozotocin-induced type 1 diabetic rats following l-citrulline administration. In these experiments, EBD levels in the soleus muscle were higher in the l-citrulline group than in the control group (19.9 ± 0.7 vs. 22.5 ± 1.9 μg/g tissue, p < 0.05). Plasma l-arginine, l-citrulline, and NOx levels were increased within 30 min after l-citrulline administration. EBD levels in the soleus and gastrocnemius muscles were higher in diabetic rats with l-citrulline administration (18.7 ± 2.2 vs. 25.0 ± 4.3 μg/g tissue, p < 0.05 and 8.0 ± 0.5 vs. 9.2 ± 0.8 μg/g tissue, p = 0.05, respectively). These data suggest that oral l-citrulline administration may increase the level of substances delivered to skeletal muscles by increasing the NO production in both normal and vascular endothelial dysfunction models.

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l-Citrulline (CIT) increases blood flow and induces vasorelaxation.

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CIT increased dye delivery to soleus but not gastrocnemius muscles in normal rats.

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CIT administration increased the blood levels of l-arginine and NOx.

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CIT led to higher dye delivery to soleus and gastrocnemius muscles in diabetic rats.

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CIT increase vascular delivery in skeletal muscles through increasing NO production.

Sex differences with aging in nutritive skeletal muscle blood flow: impact of exercise training, nitric oxide, and α-adrenergic-mediated mechanisms

The incidence of cardiovascular disease increases progressively with age, but aging may affect men and women differently. Age-associated changes in vascular structure and function may manifest in impaired nutritive blood flow, although the regulation of nutritive blood flow in healthy aging is not well understood. The purpose of this study was to determine if nitric oxide (NO)-mediated or α-adrenergic-mediated regulation of nutritive skeletal muscle blood flow is impaired with advanced age, and if exercise training improves age-related deficiencies. Nutritive blood flow was monitored in the vastus lateralis of healthy young and aged men and women via the microdialysis-ethanol technique prior to and following seven consecutive days of exercise training. NO-mediated and α-adrenergic-mediated regulation of nutritive blood flow was assessed by microdialysis perfusion of acetylcholine, sodium nitroprusside, N(G)-monomethyl-L-arginine, norepinephrine, or phentolamine. Pretraining nutritive blood flow was attenuated in aged compared with young women (7.39 ± 1.5 vs. 15.5 ± 1.9 ml·100 g(−1)·min(−1), P = 0.018), but not aged men (aged 13.5 ± 3.7 vs. young 9.4 ± 1.3 ml·100 g(−1)·min(−1), P = 0.747). There were no age-associated differences in NO-mediated or α-adrenergic-mediated nutritive blood flow. Exercise training increased resting nutritive blood flow only in young men (9.4 ± 1.3 vs. 19.7 ml·100 g(−1)·min(−1), P = 0.005). The vasodilatory effect of phentolamine was significantly reduced following exercise training only in young men (12.3 ± 6.14 vs. −3.68 ± 3.26 ml·100 g(−1)·min(−1), P = 0.048). In conclusion, the age-associated attenuation of resting nutritive skeletal muscle blood flow was specific to women, while the exercise-induced alleviation of α-adrenergic mediated vasoconstriction that was specific to young men suggests an age-associated modulation of the sympathetic response to exercise training.

Aging and the Skeletal Muscle Angiogenic Response to Exercise in Women

Whether aging lowers skeletal muscle basal capillarization and angiogenesis remains controversial. To investigate the effects of aging on skeletal muscle capillarization, eight young (YW) and eight aged (AW) women completed 8 weeks of exercise training. The response and relationships of muscle capillarization, interstitial vascular endothelial growth factor (VEGF), and microvascular blood flow to aerobic exercise training were investigated. Vastus lateralis biopsies were obtained before and after exercise training for the measurement of capillarization. Muscle interstitial VEGF protein and microvascular blood flow were measured at rest and during submaximal exercise at PRE, 1-WK, and 8-WKS by microdialysis. Exercise training increased (20%-25%) capillary contacts of type I, IIA, and IIB fibers in YW and AW. Interstitial VEGF protein was higher in AW than YW at rest and was higher in YW than AW during exercise independent of training status. Differences in muscle capillarization were not explained by secreted VEGF nor were differences in VEGF explained by microvascular blood flow. These results confirm that aging (57-76 years age range) does not impair the muscle angiogenic response to exercise training, although sex differences may exist in similarly trained women and men.

Blood flow restriction enhances post-resistance exercise angiogenic gene expression

PURPOSE

The objective of this study is to evaluate the effects of blood flow restriction (BFR) on muscle oxygenation during low-intensity resistance exercise as well as postexercise expression of molecules related to physiological angiogenesis.

METHODS

Using a randomized cross-over design, six apparently healthy young adults (22 ± 1 yr) performed 120 unilateral knee extensions at 40% of 1 repetition maximum with and without BFR (CNTRL). Near-infrared spectroscopy was used to measure oxygenation of the vastus lateralis during exercise. Serum and muscle expression of Post-Resistance vascular endothelial growth factor (VEGF) were determined preexercise, 4 h postexercise, and 24 h postexercise. Transcript (mRNA) expression of VEGF and other angiogenic genes was also determined.

RESULTS

BFR increased muscle hemoglobin (Hb) concentrations during exercise (14.4 ± 1.6 vs. 0.9 ± 1.6, P = 0.002), driven largely by an increase in deoxygenated Hb (11.0 ± 2.5 vs. 0.5 ± 1.1, P = 0.030). BFR also increased (P < 0.05) transcript expression of VEGF, VEGF-R2, hypoxia-inducible factor 1 alpha, inducible nitric oxide synthase (NOS), and neuronal NOS. The most dramatic change in response to BFR was an increase in VEGF mRNA at 4 h postexercise (4.1 ± 0.6 vs. 0.6 ± 0.2-fold change, P = 0.028). Compared with control, transcript expression of endothelial NOS, serum VEGF, or muscle protein expression of VEGF was not altered in response to BFR (P > 0.05).

CONCLUSION

Acute BFR increases postexercise expression of mRNA related to skeletal muscle angiogenesis, plausibly in response to changes in muscle Hb concentrations.

Muscle protein metabolism responds similarly to exogenous amino acids in healthy younger and older adults during NO-induced hyperemia

The combination of increasing blood flow and amino acid (AA) availability provides an anabolic stimulus to the skeletal muscle of healthy young adults by optimizing both AA delivery and utilization. However, aging is associated with a blunted response to anabolic stimuli and may involve impairments in endothelial function. We investigated whether age-related differences exist in the muscle protein anabolic response to AAs between younger (30 ± 2 yr) and older (67 ± 2 yr) adults when macrovascular and microvascular leg blood flow were similarly increased with the nitric oxide (NO) donor, sodium nitroprusside (SNP). Regardless of age, SNP+AA induced similar increases above baseline (P ≤ 0.05) in macrovascular flow (4.3 vs. 4.4 ml·min(-1)·100 ml leg(-1) measured using indocyanine green dye dilution), microvascular flow (1.4 vs. 0.8 video intensity/s measured using contrast-enhanced ultrasound), phenylalanine net balance (59 vs. 68 nmol·min(-1)·100 ml·leg(-1)), fractional synthetic rate (0.02 vs. 0.02%/h), and model-derived muscle protein synthesis (62 vs. 49 nmol·min(-1)·100 ml·leg(-1)) in both younger vs. older individuals, respectively. Provision of AAs during NO-induced local skeletal muscle hyperemia stimulates skeletal muscle protein metabolism in older adults to a similar extent as in younger adults. Our results suggest that the aging vasculature is responsive to exogenous NO and that there is no age-related difference per se in AA-induced anabolism under such hyperemic conditions.

Age-related anabolic resistance after endurance-type exercise in healthy humans

Age-related skeletal muscle loss is thought to stem from suboptimal nutrition and resistance to anabolic stimuli. Impaired microcirculatory (nutritive) blood flow may contribute to anabolic resistance by reducing delivery of amino acids to skeletal muscle. In this study, we employed contrast-enhanced ultrasound, microdialysis sampling of skeletal muscle interstitium, and stable isotope methodology, to assess hemodynamic and metabolic responses of older individuals to endurance type (walking) exercise during controlled amino acid provision. We hypothesized that older individuals would exhibit reduced microcirculatory blood flow, interstitial amino acid concentrations, and amino acid transport when compared with younger controls. We report for the first time that aging induces anabolic resistance following endurance exercise, manifested as reduced (by ∼40%) efficiency of muscle protein synthesis. Despite lower (by ∼40-45%) microcirculatory flow in the older than in the younger participants, circulating and interstitial amino acid concentrations and phenylalanine transport into skeletal muscle were all equal or higher in older individuals than in the young, comprehensively refuting our hypothesis that amino acid availability limits postexercise anabolism in older individuals. Our data point to alternative mediators of age-related anabolic resistance and importantly suggest correction of these impairments may reduce requirements for, and increase the efficacy of, dietary protein in older individuals.

Skeletal muscle nNOSμ protein content is increased by exercise training in humans

The major isoform of nitric oxide synthase (NOS) in skeletal muscle is the splice variant of neuronal NOS, termed nNOSμ. Exercise training increases nNOSμ protein levels in rat skeletal muscle, but data in humans are conflicting. We performed two studies to determine 1) whether resting nNOSμ protein expression is greater in skeletal muscle of 10 endurance-trained athletes compared with 11 sedentary individuals ( study 1) and 2) whether intense short-term (10 days) exercise training increases resting nNOSμ protein (within whole muscle and also within types I, IIa, and IIx fibers) in eight sedentary individuals ( study 2). In study 1, nNOSμ protein was ∼60% higher ( P < 0.05) in endurance-trained athletes compared with the sedentary participants. In study 2, nNOSμ protein expression was similar in types I, IIa, and IIx fibers before training. Ten days of intense exercise training significantly ( P < 0.05) increased nNOSμ protein levels in types I, IIa, and IIx fibers, a finding that was validated by using whole muscle samples. Endothelial NOS and inducible NOS protein were barely detectable in the skeletal muscle samples. In conclusion, nNOSμ protein expression is greater in endurance-trained individuals when compared with sedentary individuals. Ten days of intense exercise is also sufficient to increase nNOSμ expression in untrained individuals, due to uniform increases of nNOSμ within types I, IIa, and IIx fibers.

Relationship between body composition and skeletal muscle eNOS

OBJECTIVE

Owing to the relationship between nitric oxide related endothelial dysfunction, insulin resistance and cardiovascular disease in overweight individuals, we investigated if skeletal muscle endothelial nitric oxide synthase (eNOS) protein content and activity are lower in overweight than lean women.

DESIGN

A total of 19 women (age 26.0+/-1.7 years) underwent a resting muscle biopsy, body composition analysis by hydrostatic weighing and peak aerobic capacity determination using indirect calorimetry (Study 1). An additional separate set of six lean (< or = 25% fat) and six overweight (>25% fat) women were subsequently studied for the determination of eNOS activity, and to better control for absolute peak aerobic capacity between lean and overweight women (Study 2).

RESULTS

Skeletal muscle eNOS content was inversely related to percent body fat (r2 = 0.58, P < 0.01), and body mass index (r2 = 0.35, P < 0.05). Total eNOS activity was lower in overweight than lean women (2.09 +/- 0.22 vs 1.44 +/- 0.17 U, P < 0.05; n = 12), and was inversely related to percent body fat (r2 = 0.32, P = 0.05), and BMI (r 2 = 0.41, P < 0.05). Absolute and relative aerobic capacity were not independent predictors of skeletal muscle eNOS content (r2 = 0.11 and 0.26, respectively).

CONCLUSION

There is an inverse relationship between eNOS and percent body fat that may have implications for the previously reported reduced endothelial function and insulin sensitivity in overweight women.

NO-mediated alterations in skeletal muscle nutritive blood flow and lactate metabolism in fibromyalgia

The purpose of these investigations was to determine if differences exist in skeletal muscle nutritive blood flow and lactate metabolism in women with fibromyalgia (FM) compared to healthy women (HC); furthermore, to determine if differences in nitric oxide-mediated systems account for any detected alterations in blood flow and lactate metabolism and contribute to exertional fatigue in FM. FM (n = 8) and HC (n = 8) underwent a cycle ergometry test of aerobic capacity, a muscle biopsy for determination of nitric oxide synthase (eNOS, nNOS, iNOS) content, and microdialysis for investigation of muscle nutritive blood flow and lactate metabolism. During prolonged (3h) resting conditions, the ethanol outflow/inflow ratio (inversely related to blood flow) increased in FM over time compared to HC (P < 0.05). FM also exhibited a reduced nutritive blood flow response to aerobic exercise (P < 0.05). There was an increase in dialysate lactate in response to acetylcholine in FM, and to sodium nitroprusside in both groups, with a greater rise in dialysate lactate in FM (P < 0.05). The iNOS protein content was higher in FM and was negatively correlated with total exercise time (r(2) = 0.462, P < 0.05).

IN CONCLUSION

(1) There is reduced nutritive flow response to aerobic exercise and reduced maximal exercise time in FM that might relate to higher iNOS protein content and contribute to exertional fatigue in FM; (2) The increased dialysate lactate in FM in response to stimulation of NOS or a nitric oxide donor suggest that FM may be more sensitive than HC to the suppressive effect of nitric oxide on oxidative phosphorylation.