Background and aims Some of the most obvious peripheral tissue changes in Huntington's disease (HD) are found in skeletal muscle. Alterations in morphology, gene expression pattern, energy metabolism and differentiation have been described. However, it is still an open question to what extent these changes reflect cell autonomous effect of mutant huntingtin. As yet, surprisingly little information is available about contractile performance and excitation–contraction coupling in HD muscle. Recently we found that isometric contraction of fast twitch muscle in the R6/2 mouse model of HD exhibits significantly slower kinetics than WT muscle. To investigate the potential causes of the changes in muscle contraction we studied the kinetics of action potential triggered intracellular Ca2+ transients.
Methods Enzymatically dissociated interosseus muscle fibres of male R6/2 (11–13 weeks old) exhibiting disease symptoms and of age matched WT mice were primary cultured up to 2 days. Ca2+-transients were elicited by extracellular electrical stimulation. Ca2+ dependent fluorescence signals of Fura2-AM loaded cells exhibiting all or none responses were analysed. A kinetic model was applied to estimate Ca2+ removal and Ca2+ release.
Results In R6/2 fibres we observed significantly slower relaxation kinetics of Ca2+ transients elicited by single stimuli compared with WT. The mean time constant of relaxation was 31 ms±1 ms (SEM) for WT (n=25) and 53 ms±4 ms for R6/2 (n=73, p<0.01) at 25°C. Model analysis disclosed that these findings are compatible with a reduction by half of parvalbumin concentration and uptake rate constant in R6/2. Peak Ca2+ release flux was reduced to 45% of WT.
Conclusion In our experiments we demonstrate that R6/2 animals transgenic for mutant huntingtin show slowed Ca2+ kinetics in fast twitch muscle. The analysis identifies a reduction of sarcoreticular Ca2+ release and reuptake as well as a decrease in myoplasmic Ca2+ binding as likely sources for the changes observed in muscle contraction.
- calcium signalling
- skeletal muscle