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Am J Physiol Regul Integr Comp Physiol 281: R511-R518, 2001;
0363-6119/01 $5.00
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Vol. 281, Issue 2, R511-R518, August 2001

Force reduction uncoupled from pH and H2PO<UP><SUB>4</SUB><SUP>−</SUP></UP> in rat gastrocnemius in vivo with continuous 2-Hz stimulation

Julie H. Cieslar and Geoffrey P. Dobson

Department of Physiology and Pharmacology, School of Biomolecular and Molecular Sciences, James Cook University, Townsville, Queensland 4811, Australia

The aim of this study was to examine the effect of the products of ATP hydrolysis on the fatigue process in rat gastrocnemius in vivo. Adult male Sprague-Dawley rats (300-400 g) were anesthetized and ventilated in a custom-built cradle fitted with a force transducer that could be placed into a 7-T NMR magnet. The muscle was stimulated continuously at 2 Hz for 20 min (n = 7). Isometric twitch force increased in the first 4 min of stimulation accompanied by changes in twitch duration (20% increase in relaxation time). Prolonged relaxation was associated with changes in cytosolic pH (6.91 to 6.58), lactate (1.8 to 12.6 µmol/g wet wt), and H2PO<UP><SUB>4</SUB><SUP>−</SUP></UP> (7.57 to 13.99 mM). After 4 min, relaxation time, pH, lactate, and H2PO<UP><SUB>4</SUB><SUP>−</SUP></UP> returned toward control values as twitch force progressively decreased. No correlation was found between force decline (or twitch broadening) and total phosphate (3 to 23 mM), free [ADP] (18 to 95 µM), free [Mg2+] (0.58 to 0.96 mM), or free energy of ATP hydrolysis (-65 to -55 kJ/mol). We conclude that force decline is not due to increased pH and/or H2PO<UP><SUB>4</SUB><SUP>−</SUP></UP> but to fatigue of the fast-twitch fibers, possibly linked to glycogen depletion and/or failure of nerve impulse transmission in these fibers.

adenosine 5'-triphosphate; adenosine diphosphate; inorganic phosphate; muscle work; phosphorus-31 nuclear magnetic resonance


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