Prolonged exercise potentiates sarcoplasmic reticulum Ca2+ uptake in rat diaphragm

The effects of a single bout of prolonged treadmill exercise [mean=81 (13) min] on sarcoplasmic reticulum (SR) Ca(2+) release, uptake and ATPase activity were determined in the costal region of rat diaphragm (D) and red gastrocnemius (RG). Glycogen depletion measurements made immediately following exercise suggested that treadmill running substantially recruited the fibers throughout both muscles. SR Ca(2+) ATPase activity, measured in isolated SR vesicles, decreased in the RG by 33% but remained unchanged in D in response to the exercise bout. This effect in RG was matched by a 37% decline in Ca(2+) uptake and a 28% depression in Ca(2+) release when measured in muscle homogenates. Conversely, Ca(2+) uptake increased between 157% and 263% in the D in the absence of any change in Ca(2+) release. These data show that the attenuation of SR function that has been consistently observed in limb muscle over the last several decades is absent in diaphragm despite the fact that its fibers appear to experience sufficient activity to deplete their glycogen. In fact, the large increase in Ca(2+) uptake in D shows that prolonged activity actually potentiates the ability of SR vesicles to sequester Ca(2+) in the absence of any increase in energy cost. Thus, it appears necessary to re-evaluate the role of exercise in regulating Ca(2+) sequestration by the SR as different muscles may respond in ways that are dictated by their function.