EFFECTS OF TRAINING AND EXTRACELLULAR CALCIUM LOAD ON SARCOPLASMIC RETICULUM CALCIUM STORAGE AND FRACTIONAL RELEASE 1316:

Exercise training results in an improvement in cardiac performance but no data has examined if training alters the cardiac sarcoplasmic reticulum calcium (SR Ca2+) stores and fractional release (FR) of SR Ca2+ stores under increasing Ca2+ loads. Female Wistar rats were divided into sedentary and a swimming group that trained for 60 min/day, 5 days/wk for 40 wks. Following swim training we determined twitch indices and the storage/fractional release of SR Ca2+ in isolated right ventricular trabeculae under 0.5, 1 and 1.5mM extracellular Ca2+ loads. Swim training increased (P<0.05) trabeculae peak developed tension and rates of contraction and relaxation at all Ca2+ loads. Utilizing rapid cooling contractures (RCCs) as an indirect measure of SR Ca2+ stores, swim training increased SR Ca2+ stores at all Ca2+ loads (P<0.05). Increasing the Ca2+ load did not increase SR Ca2+ stores in sedentary animals. Training increased the FR of SR Ca2+ and elevating the Ca2+ load increased the FR of SR Ca2+ in trained animals from 77 to 85 to 90% and in sedentary animals from 51 to 65 to 70%. These data indicate that improvements in cardiac performance with exercise training and increasing extracellular Ca2+ loads may be attributed to increases in SR Ca2+ stores and/or the FR of SR Ca2+, while in sedentary animals may only be the result of increases in the FR of SR Ca2+.