Force reduction uncoupled from pH and H₂PO 4 − in rat gastrocnemius in vivo with continuous 2-Hz stimulation

Abstract

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 micromol/g wet wt), and H(2)PO (7.57 to 13.99 mM). After 4 min, relaxation time, pH, lactate, and H(2)PO 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 microM), free [Mg(2+)] (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 H(2)PO but to fatigue of the fast-twitch fibers, possibly linked to glycogen depletion and/or failure of nerve impulse transmission in these fibers.

Functional investigations of exercising muscle: a noninvasive magnetic resonance spectroscopy-magnetic resonance imaging approach

“…Indeed, during a 4-min maximal voluntary contraction exercise, a continuous force decrease has been reported, whereas P i and H 2 PO 4 -concentrations have reached a steady state after 2 min of exercise [11]. Similar results have been obtained in rat gastrocnemius muscle electrically stimulated with a continuous force decrease along the stimulation protocol, whereas H 2 PO 4 -concentration first increased and then decreased after 5 min of stimulation as a result of PCr resynthesis [78]. It must be pointed out that the discrepant results mentioned above may be related to different experimental protocols, thereby suggesting that the relationship between P i and/or H 2 PO 4 -and force would be dependent on exercise intensity and duration.…”

“…This phenomenon would decrease the proportion of force-producing cross-bridges, leading to a decline in force production. Attempts to show the inhibitory effect of P i on force under physiological conditions have mainly been performed using 31 P MRS [11,61,69,[76][77][78] and focused on P i and H 2 PO 4 -time-dependent changes, one the one hand, and force reduction, on the other hand [75]. A strong correlation has been demonstrated between H 2 PO 4 -accumulation and force reduction in human muscle during different types of experimental protocols including sustained or repeated contractions with maximal or submaximal intensities [69,76,77].…”

“…These results favour the hypothesis of H 2 PO 4 -being an important determinant of peripheral fatigue. However, the direct cause-effect relationship have been questioned on the basis of other noninvasive investigations focusing on the different time scales of both events [11,61,78]. Indeed, during a 4-min maximal voluntary contraction exercise, a continuous force decrease has been reported, whereas P i and H 2 PO 4 -concentrations have reached a steady state after 2 min of exercise [11].…”

In vivo MR investigation of skeletal muscle function in small animals

“…As an illustration, in vivo 31 P-MRS investigations in rat muscle depleted of PCr [29, 30] and in CK-knockout mice [31] have provided key information about the CK-PCr system. Study of the metabolic aspects of muscle fatigue is another issue that has been addressed using 31 P-MRS in small animals [8, [38][39][40][41]. Muscle fatigue remains indeed difficult to study in humans since many parameters such as weight, daily physical activity and motivation have to be taken into account.…”

“…Investigations are generally performed in rat or mouse gastrocnemius muscle [8,26-30, [39][40][41][42][43][44][45][46][47]. This muscle, which forms the calf of the lower hindlimb, is clearly distinct from the other muscles of the leg, and is directly accessible for MR coils.…”