Effects of fatigue and training on sarcoplasmic reticulum Ca regulation in human skeletal muscle

Li, Jia L., Xiao N. Wang, Steve F. Fraser, Michael F. Carey, Tim V. Wrigley, and Michael J. McKenna. Effects of fatigue and training on sarcoplasmic reticulum Ca2 regulation in human skeletal muscle. J Appl Physiol 92: 912–922, 2002. First published October 19, 2001; 10.1152/ japplphysiol.00643.2000.—Little is known about fatigue and training effects on sarcoplasmic reticulum (SR) function in human muscle, and we therefore investigated this in eight untrained controls (UT), eight endurance-trained (ET), and eight resistance-trained athletes (RT). Muscle biopsies (vastus lateralis) taken at rest and after 50 maximal quadriceps contractions (180°/s, 0.5 Hz) were analyzed for fiber composition, metabolites and maximal SR Ca2 release, Ca2 uptake, and Ca2 -ATPase activity. Fatigue reduced (P 0.05) Ca2 release (42.1 3.8%, 43.4 3.9%, 31.3 6.1%), Ca2 uptake (43.0 5.2%, 34.1 4.6%, 28.4 2.8%), and Ca2 ATPase activity (38.6 4.2%, 48.5 5.7%, 29.6 5.0%), in UT, RT, and ET, respectively. These decreases were correlated with fatigability and with type II fiber proportion (P 0.05). Resting SR measures were correlated with type II proportion (r 0.51, P 0.05). ET had lower resting Ca2 release, Ca2 uptake, and Ca2 -ATPase (P 0.05) than UT and RT (P 0.05), probably because of their lower type II proportion; only minor effects were found in RT. Thus SR function is markedly depressed with fatigue in controls and in athletes, is dependent on fiber type, and appears to be minimally affected by chronic training status.