EFFECT OF LOW ATP CONCENTRATIONS ON FORCE RELAXATION IN RAT EDL MUSCLE FIBRES
Travis L. Dutka, Graham D. Lamb, Department of Zoology, La Trobe University, 3086 Vic.
During vigorous and/or prolonged activity, the [ATP] within the cytoplasm may decrease from ~7 mM to ~1 mM1 (even lower in areas with high ATP utilization and/or limited diffusion). In skinned amphibian skeletal muscle (at 0-5ºC), force relaxation gets progressively slower as [ATP] decreases2. We sought to examine if low [ATP] slows the rate of force relaxation in mammalian skeletal muscle.
Single mechanically skinned rat EDL fibres were connected to a force transducer, treated with Triton X-100 to destroy the sarcoplasmic reticulum, washed, then transferred to a standard K-HDTA solution (1mM free Mg2+; 8 mM total ATP; 10 mM creatine phosphate (CP) at pH 7.10, containing 0.5 mM EGTA, pCa 8). Individual fibres produced submaximal or maximal force when exposed to solutions with moderate Ca2+-buffering (0.2-1 mM CaEGTA-EGTA) at either pCa 6.0 or 4.5, containing 0.1, 0.5, 1, or 8 mM ATP (ATP exchanged with CP, with 1 mM free Mg2+). Relaxation of force was induced when the fibre was plunged into 50 mM EGTA (containing the same [ATP]). Force relaxation with 2 mM ATP present was not noticeably different compared to the control force relaxation (8 mM ATP). The time constant for force relaxation in the presence of 0.5 mM ATP was increased by ~ 50% compared to the control (8 mM ATP). When 100 µM ATP was present, the time constant for force relaxation was dramatically increased (in some cases lasting for secs), but not when exogenous creatine phosphokinase (CPK) was added, indicating that the [ATP] within the myofibrils was lower than 100 µM when the exogenous CPK was absent.
This data suggest, that under severe fatigue conditions where CP stores are exhausted and the [ATP] decreases sharply, force relaxation becomes dramatically slower, due to a decrease in the [MgATP] in the vicinity of the myosin heads.
(1) Karatzaferi C, de Haan A, Ferguson RA, van Mechelen W, Sargeant AJ. Pflügers Archiv. 2001;442: 467-474.
(2) Ferenczi MA, Goldman YE, Simmons RM. Journal of Physiology (London). 1983;350:519-543.