Intracellular Ca2+ transients in mouse soleus muscle after hindlimb unloading and reloading

Abstract

The objective of this study was to determine whether altered intracellular Ca2+ handling contributes to the specific force loss in the soleus muscle after unloading and/or subsequent reloading of mouse hindlimbs. Three groups of female ICR mice were studied: 1) unloaded mice ( n = 11) that were hindlimb suspended for 14 days, 2) reloaded mice ( n = 10) that were returned to their cages for 1 day after 14 days of hindlimb suspension, and 3) control mice ( n = 10) that had normal cage activity. Maximum isometric tetanic force (Po) was determined in the soleus muscle from the left hindlimb, and resting free cytosolic Ca2+ concentration ([Ca2+]i), tetanic [Ca2+]i, and 4-chloro- m-cresol-induced [Ca2+]iwere measured in the contralateral soleus muscle by confocal laser scanning microscopy. Unloading and reloading increased resting [Ca2+]iabove control by 36% and 24%, respectively. Although unloading reduced Po and specific force by 58% and 24%, respectively, compared with control mice, there was no difference in tetanic [Ca2+]i. Po, specific force, and tetanic [Ca2+]iwere reduced by 58%, 23%, and 23%, respectively, in the reloaded animals compared with control mice; however, tetanic [Ca2+]iwas not different between unloaded and reloaded mice. These data indicate that although hindlimb suspension results in disturbed intracellular Ca2+ homeostasis, changes in tetanic [Ca2+]ido not contribute to force deficits. Compared with unloading, 24 h of physiological reloading in the mouse do not result in further changes in maximal strength or tetanic [Ca2+]i.