title: Muscle glycogenolysis and H+ concentration during maximal intermittent cycling.
abstract: The relationships between muscle glycogenolysis, glycolysis, and H+ concentration were examined in eight subjects performing three 30-s bouts of maximal isokinetic cycling at 100 rpm. Bouts were separated by 4 min of rest, and muscle biopsies were obtained before and after bouts 2 and 3. Total work decreased from 20.5 +/- 0.7 kJ in bout 1 to 16.1 +/- 0.7 and 13.2 +/- 0.6 kJ in bouts 2 and 3. Glycogenolysis was 47.2 and 15.1 mmol glucosyl U/kg dry muscle during bouts 2 and 3, respectively. Lower accumulations of pathway intermediates in bout 3 confirmed a reduced glycolytic flux. In bout 3, the work done represented 82% of the work in bout 2, whereas glycogenolysis was only 32% of that in bout 2. Decreases in ATP and phosphocreatine contents were similar in the two bouts. Muscle [H+] increased from 195 +/- 12 to 274 +/- 19 nmol/l during bout 2, recovered to 226 +/- 8 nmol/l before bout 3, and increased to 315 +/- 24 nmol/l during bout 3. Muscle [H+] could not be predicted from lactate content, suggesting that ion fluxes are important in [H+] regulation in this exercise model. Low glycogenolysis in bout 3 may be due to an inhibitory effect of increased [H+] on glycogen phosphorylase activity. Alternately, reduced Ca2+ activation of fast-twitch fibers (including a possible H+ effect) may contribute to the low overall glycogenolysis. Total work in bout 3 is maintained by a greater reliance on slow-twitch fibers and oxidative metabolism.
authors.affiliation: School of Human Biology, University of Guelph, Ontario, Canada.
authors.forename: L L
authors.initials: LL
authors.lastname: Spriet
authors.forename: M I
authors.initials: MI
authors.lastname: Lindinger
authors.forename: R S
authors.initials: RS
authors.lastname: McKelvie
authors.forename: G J
authors.initials: GJ
authors.lastname: Heigenhauser
authors.forename: N L
authors.initials: NL
authors.lastname: Jones
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000328:Adult; D001642:Bicycling; D005260:Female; D006003:Glycogen / Q000378:metabolism*; D006801:Humans; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D008297:Male; D009132:Muscles / Q000378:metabolism*; D010725:Phosphocreatine / Q000378:metabolism; D013177:Sports; D013997:Time Factors
chemical_list: D010725:Phosphocreatine; D006859:Hydrogen; D000255:Adenosine Triphosphate; D006003:Glycogen