Sarcoplasmic reticulum function and muscle contractile character following fatiguing exercise in humans

Abstract

1. This study examined the alterations in calcium release, calcium uptake and calcium ATPase activity of skeletal muscle sarcoplasmic reticulum in response to a bout of intense dynamic knee extensor exercise, and the relationship between these changes and alterations in muscle contractile characteristics in the human quadriceps. 2. In biopsy samples taken from the vastus lateralis, sarcoplasmic reticulum calcium release and calcium uptake were significantly depressed (P < 0.01 and 0.05, respectively) immediately following the exercise with no alteration in the sarcoplasmic reticulum Ca2+-ATPase activity. 3. A 33 % reduction in the maximum voluntary isometric torque was found following the exercise, with reduced torques from electrically evoked isometric contractions at low frequencies of stimulation (10 and 20 Hz) but not at higher frequencies (50 and 100 Hz). 4. The depressed calcium release was correlated (P < 0.05) with a decreased ratio of torques generated at 20:50 Hz, indicating an involvement in low frequency fatigue; however, no correlations between the muscle relaxation times or rates of change of torque and calcium uptake were observed.

Figure 1

An example of an experimental record of Ca²⁺ fluxes over time from which the maximal rates of Ca²⁺ uptake and release from the SR were measured using the Ca²⁺ binding dye indo-1.

Figure 2. Typical histogram, from one subject, showing the peak torques produced throughout the isokinetic exercise

Note the slight recovery observed at the start of the second set of repetitions (i.e. repetition 91).

Figure 3. MVICs and MVIC_ts before exercise, in between the two sets, immediately post- and 3.5 h post-exercise

Values are means ±s.e.m. ** Significantly different from rest, P≤ 0.01; * P≤ 0.05; † significantly different from corresponding MVIC_t, P≤ 0.05.

Figure 4. Peak torques produced at 10, 20, 50 and 100 Hz pre- and immediately post-exercise

Values are means ±s.e.m. * Significantly different from pre-exercise, P ≤ 0.05.

Figure 5. Changes in Ca²⁺ release, Ca²⁺ uptake and Ca²⁺-ATPase activity as a percentage of pre-exercise after and 3.5 h after the exercise

Values are means ±s.e.m. ** Significantly different from pre-exercise, P≤ 0.01; *P≤ 0.05.

Figure 6

Calcium release versus 20:50 Hz ratio was significantly correlated (r = 0.50; P < 0.05).