The effect of fatigue on the caffeine sensitivity of skeletal muscle sarcoplasmic reticulum

Several studies have shown that the loss in tension during fatigue can be virtually reversed by exposure of the muscle to agents which evoke Ca²⁺ release from the SR. The purpose of this study was to determine whether the SR Ca²⁺ release mechanism of fatigued muscle is less sensitive to caffeine than that of rested muscle. Following a fatigue bout of electrically evoked tetanic pulses, the functioning of the SR of chemically skinned muscle fibers was determined by the sensitivity of the SR to increasing concentrations of caffeine. Measurements of tension and rate of tension development were made at the maximal Ca²⁺ activated contracture(pCa4.5), the maximal caffeine(25mM) activated contracture and at the caffeine threshold for contraction. All tension and rate values were normalized per cross sectional area and expressed as percents of the maximal calcium activated values. Results of the maximal Ca²⁺ and caffeine data suggest that the both control and fatigue fibers are similar in maximal tension and Ca²⁺ loading characteristics.

While no differences were found between rested or fatigued maximal Ca²⁺ or caffeine contractures, significant difference was found at the caffeine threshold (p<.05) with the fatigued muscle tending to contract at a higher caffeine concentration. This suggests that fatigued muscle is less sensitive to the caffeine stimulus for Ca²⁺ release from the SR.