title: Reflex sympathetic activation during static exercise is severely impaired in patients with myophosphorylase deficiency.
abstract: During static exercise, metabolites accumulate in the muscle interstitium where they stimulate chemosensitive afferent nerves that reflexly increase efferent muscle sympathetic nerve activity (MSNA) and blood pressure. In experimental animals, lactic acid potently stimulates the muscle metaboreflex, but its role in humans is more controversial. To determine if lactic acid is a critical mediator of metaboreflex activation in humans, we performed microelectrode recordings of MSNA in eight patients with myophosphorylase deficiency (McArdle's disease) who cannot metabolize intramuscular glycogen and do not generate lactic acid in exercising muscles. Each patient was matched with three healthy control subjects to maximize statistical power. In controls, 2 min of static handgrip performed at 33 % or 45 % of maximal voluntary contraction (MVC) produced intensity-dependent increases in MSNA (171 +/- 22 % and 379 +/- 95 %, respectively). In the patients, MSNA responses to static handgrip were markedly attenuated (33 +/- 14 % at 33 % MVC; 32 +/- 19 % at 45 % MVC; P < 0.05 vs. controls). Likewise, when static handgrip (30 % MVC) was performed to fatigue, MSNA increased by 366 +/- 73 % in controls but only by 51 +/- 14 % in patients (P < 0.05). Pressor responses to static handgrip were also attenuated in patients compared to controls, whereas heart rate responses were identical. In contrast to exercise, the MSNA responses to other reflex stimuli (the cold pressor test or Valsalva's manoeuvre) were similar in patients and controls. Together these data indicate that appropriate activation of glycogenolytic pathways is obligatory for normal metaboreflex-mediated sympathoexcitation during static exercise in humans.
authors.affiliation: Department of Internal Medicine, Division of Hypertension, University of Texas Southwestern Medical Center, Dallas, USA.
authors.forename: Paul J
authors.initials: PJ
authors.lastname: Fadel
authors.forename: Zhongyun
authors.initials: Z
authors.lastname: Wang
authors.forename: Meryem
authors.initials: M
authors.lastname: Tuncel
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Watanabe
authors.forename: Aamer
authors.initials: A
authors.lastname: Abbas
authors.forename: Debbie
authors.initials: D
authors.lastname: Arbique
authors.forename: Wanpen
authors.initials: W
authors.lastname: Vongpatanasin
authors.forename: Robert W
authors.initials: RW
authors.lastname: Haley
authors.forename: Ronald G
authors.initials: RG
authors.lastname: Victor
authors.forename: Gail D
authors.initials: GD
authors.lastname: Thomas
mesh_terms: D000328:Adult; D015992:Body Mass Index; D015444:Exercise / Q000502:physiology*; D005260:Female; D024982:Glycogen Phosphorylase, Muscle Form / Q000172:deficiency* / Q000502:physiology; D006012:Glycogen Storage Disease Type V / Q000503:physiopathology*; D018737:Hand Strength / Q000502:physiology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000294:innervation*; D012016:Reference Values; D012018:Reflex / Q000502:physiology*; D013564:Sympathetic Nervous System / Q000503:physiopathology*
chemical_list: D024982:Glycogen Phosphorylase, Muscle Form