CALPAIN-II EXPRESSION IS INCREASED BY CHANGES IN MECHANICAL LEADING OF MUSCLE IN-VIVO

In the present investigation, we have tested the hypothesis that calpa in expression or activity in skeletal muscle is influenced by changes in mechanical loading in vivo. Muscle unloading for 10 days produced n o change in the concentrations of calpain I, or II, and no change in c alpain activation, as assessed by measurements of the proportion of ca lpain I or II isoforms that exhibited autoproteolytic modifications. H owever, muscle reloading for 2 days produced a 90% increase in calpain II concentration per unit wet weight of muscle relative to ambulatory controls. Although no change in the activation index for calpain I or II was identified for reloaded muscle, this index is an expression of the proportion of the total mass of each calpain isoform that is auto proteolyzed. Thus, there is also approximately a 90% increase in autol yzed calpain II in muscle experiencing increased loading than in contr ols. Northern analysis shows that the concentration of mRNA for calpai n II is increased in reloaded muscle, but no change in calpain II mRNA concentration in unloaded muscle. In situ reverse transcription polym erase chain reaction was used to confirm that nearly all calpain II mR NA in reloaded muscle is located in muscle fibers, with very little de tectable calpain II mRNA in non-muscle cells present in the tissue. To gether, these findings show that increased muscle loading causes a sel ective increase in the expression of calpain II isoform, thereby indic ating that its regulation is independent from other calpain isoforms. (C) 1997 Wiley-Liss, Inc.