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Little is presently known about the effects of acute high-intensity exercise or training on release and uptake of Ca 2+  by the sarcoplasmic reticulum (SR). The aims here were to characterize this regulation in highly trained athletes following (1) repeated bouts of high-intensity exercise and (2) a period of endurance training including high-intensity sessions. Eleven cross-country skiers (25 ± 4 years, 65 ± 4 mL O 2 ⋅kg −1 ⋅min –1 ) performed four self-paced sprint time-trials (STT 1-4) lasting ≈ 4 min each (STT 1–4) and separated by 45 min of recovery; while 19 triathletes and road cyclists (25 ± 4 years, 65 ± 5 mL O 2 ⋅kg −1 ⋅min –1 ) completed 4 weeks of endurance training in combination with three sessions of high-intensity interval cycling per week. Release (μmol⋅g –1  prot⋅min –1 ) and uptake [tau (s)] of Ca 2+  by SR vesicles isolated from m.  triceps brachii  and m.  vastus lateralis  were determined before and after STT 1 and 4 in the skiers and in m.  vastus lateralis  before and after the 4 weeks of training in the endurance athletes. The Ca 2+  release rate was reduced by 17–18% in both limbs already after STT 1 (arms: 2.52 ± 0.74 to 2.08 ± 0.60; legs: 2.41 ± 0.45 to 1.98 ± 0.51,  P  < 0.0001) and attenuated further following STT 4 (arms: 2.24 ± 0.67 to 1.95 ± 0.45; legs: 2.13 ± 0.51 to 1.83 ± 0.36,  P  < 0.0001). Also, there was a tendency toward an impairment in the SR Ca 2+  uptake from pre STT1 to post STT4 in both arms and legs (arms: from 22.0 ± 3.7 s to 25.3 ± 6.0 s; legs: from 22.5 ± 4.7 s to 25.5 ± 7.7 s,  P  = 0.05). Endurance training combined with high-intensity exercise increased the Ca 2+  release rate by 9% (1.76 ± 0.38 to 1.91 ± 0.44,  P  = 0.009), without altering the Ca 2+  uptake (29.6 ± 7.0 to 29.1 ± 8.7 s;  P  = 0.98). In conclusion, the Ca 2+  release and uptake rates by SR in exercising limbs of highly trained athletes declines gradually by repetitive bouts of high-intensity exercise. We also demonstrate, for the first time, that the SR Ca 2+  release rate can be enhanced by a specific program of training in highly trained athletes, which may have important implications for performance parameters.

Effects of Acute Exercise and Training on the Sarcoplasmic Reticulum Ca2+ Release and Uptake Rates in Highly Trained Endurance Athletes