Role of Skeletal Muscle Nitric Oxide Production in Age-related Fatigue and Fatigability

Fatigue is highly prevalent and associated with future mortality in older individuals. Even
in non-disabled older persons, fatigue may be the primary reason for activity limitation.
However, understanding the etiology of fatigue in this population has been hampered by
differing or imprecise definitions of fatigue. As a result, the term fatigue has been
proposed to refer to the subjective experience of tiredness or lack of energy, whereas the
term fatigability should refer to the susceptibility to fatigue induced by activity of any
kind (mental, physical, etc). Skeletal muscle activity can contribute to the perception of
overall fatigue as well as produce a type of localized fatigue within skeletal muscle.
Skeletal muscle fatigue is defined as a decline in skeletal muscle performance resulting from
muscle activity.

We hypothesize that skeletal muscle NO-mediated responses are reduced with aging due to
decreased skeletal muscle NO production. NO is well-known to elicit vasodilation through
stimulation of cGMP signaling, and NO-mediated changes in muscle perfusion may influence both
skeletal muscle and overall fatigue. To measure skeletal muscle NO production, we will infuse
a stable isotope tracer of arginine, the precursor of NO, and measure its conversion across
the leg and in skeletal muscle to citrulline (which is another product of the reaction that
produces NO). If successful, this method will allow the study of relative changes in vascular
and muscle NO production that occur with aging and other conditions (e.g., hypertension,
Duchenne muscular dystrophy). We will also determine whether age-related differences in
muscle perfusion and NO-cGMP signaling exist between younger and older groups. As impaired
redox homeostasis and ryanodine receptor S-nitrosylation and phosphorylation have been
implicated in skeletal muscle fatigue, we will assess skeletal muscle redox homeostasis and
ryanodine receptor S-nitrosylation in these experiments. We hypothesize that aging will shift
muscle redox homeostasis to a more oxidized state.

Inclusion Criteria:

- 1. Age 20-35 yrs, and 60-80 yrs.

- 2. Ability to sign consent form (score >23 on the 30-item Mini Mental State
Examination, MMSE)

- 3. Stable body weight for at least 3 months

Exclusion Criteria:

- 1. Physical dependence or frailty (impairment in any of the Activities of Daily Living
(ADL), history of falls (>2/year) or significant weight loss in the past year)

- 2. Exercise training (>2 weekly sessions of moderate to high intensity aerobic or
resistance exercise)

- 3. Pregnancy

- 4. Significant heart, liver, kidney, blood or respiratory disease

- 5. Peripheral vascular disease

- 6. Diabetes mellitus or other untreated endocrine disease

- 7. Active cancer

- 8. Use of nitrates

- 9. Recent (within 6 months) treatment with anabolic steroids, or corticosteroids.

- 10. Alcohol or drug abuse

- 11. Severe depression (>5 on the 15-item Geriatric Depression Scale, GDS)

- 12. Cardiac abnormalities such as a cardiac shunt or previously diagnosed pulmonary
hypertension.

- 13. Systolic blood pressure <100 or >150, diastolic blood pressure <60 or >90.