The effect of exercise-training on sarcoplasmic reticulum function in fast and slow skeletal muscle

The effect of endurance exercise on the capacity of crube homogenates (CH) to sequester Ca²⁺ was determined in the slow type I soleus, the fast type IIA deep region of the vastus lateralis (DVL), and the fast type IIB superficial region of the vastus lateralis (SVL). The Ca²⁺ uptake capacity was affected by exercise in a fiber type specific manner. The fast-twitch SVL showed a 35% decrease in the maximal rate of Ca²⁺ uptake (V_max) and a significantly lower K_m while the slow soleus and fast DVL were unaltered. The time course of Ca²⁺ uptake, and the peak amount of Ca²⁺ sequestered was not altered by exercise in any of the muscles studied. The homogenates from the exercise-trained soleus muscles exhibited an increased ability to retain Ca²⁺ and in this capacity became more like fast muscle.