SKELETAL-MUSCLE CALCIUM-ACTIVATED NEUTRAL PROTEASE (CALPAIN) WITH EXERCISE

The purpose of this study was to investigate whether exercise could induce calpain activation by altering the Ca2+ required for half-maximal activity (pCa50) and/or susceptibility of digestible muscle protein substrates. Rats (225 g) were assigned to control, exercise (25 m/min, 0% grade), and 24-h recovery groups. Exercise resulted in a generalized 48 +/- 18% loss of muscle glycogen and a twofold increase in plasma creatine kinase levels (P less-than-or-equal-to 0.05). Exercise increased total caseinolysis of diethylaminoethyl Sepharose-prepared low (u) and high (m) Ca2+ calpain isoforms by 22 and 30%, respectively (P less-than-or-equal-to 0.05). The pCa50 of u- and m-calpain with exercise increased from 5.98 +/- 0.12 to 6.20 +/- 0.15 (P greater-than-or-equal-to 0.05) and from 3.63 +/- 0.10 to 3.90 +/- 0.16 (P greater-than-or-equal-to 0.05), respectively. In vitro, calpain-mediated degradation/disappearance rates (i.e., percentage of protein degraded in 10 min) for control tropomyosin and alpha-actinin were 69 and 30% compared with 92 and 61% after exercise (P less-than-or-equal-to 0.05). The results of this study confirm that level running increases total nonlysosomal Ca2+ specific protease activity, which may promote exercise-induced muscle damage or fatigue.