| Summary: | Disuse atrophy of skeletal muscle is associated with a severe imbalance in cellular Ca<sup>2+</sup> homeostasis and marked increase in nuclear apoptosis. Nuclear Ca<sup>2+</sup> is involved in the regulation of cellular Ca<sup>2+</sup> homeostasis. However, it remains unclear whether nuclear Ca<sup>2+</sup> levels change under skeletal muscle disuse conditions, and whether changes in nuclear Ca<sup>2+</sup> levels are associated with nuclear apoptosis. In this study, changes in Ca<sup>2+</sup> levels, Ca<sup>2+</sup> transporters, and regulatory factors in the nucleus of hindlimb unloaded rat soleus muscle were examined to investigate the effects of disuse on nuclear Ca<sup>2+</sup> homeostasis and apoptosis. Results showed that, after hindlimb unloading, the nuclear envelope Ca<sup>2+</sup> levels ([Ca<sup>2+</sup>]<sub>NE</sub>) and nucleocytoplasmic Ca<sup>2+</sup> levels ([Ca<sup>2+</sup>]<sub>NC</sub>) increased by 78% (<i>p</i> < 0.01) and 106% (<i>p</i> < 0.01), respectively. The levels of Ca<sup>2+</sup>-ATPase type 2 (Ca<sup>2+</sup>-ATPase2), Ryanodine receptor 1 (RyR1), Inositol 1,4,5-tetrakisphosphate receptor 1 (IP<sub>3</sub>R1), Cyclic ADP ribose hydrolase (CD38) and Inositol 1,4,5-tetrakisphosphate (IP<sub>3</sub>) increased by 470% (<i>p</i> < 0.001), 94% (<i>p</i> < 0.05), 170% (<i>p</i> < 0.001), 640% (<i>p</i> < 0.001) and 12% (<i>p</i> < 0.05), respectively, and the levels of Na<sup>+</sup>/Ca<sup>2+</sup> exchanger 3 (NCX3), Ca<sup>2+</sup>/calmodulin dependent protein kinase II (CaMK II) and Protein kinase A (PKA) decreased by 54% (<i>p</i> < 0.001), 33% (<i>p</i> < 0.05) and 5% (<i>p</i> > 0.05), respectively. In addition, DNase X is mainly localized in the myonucleus and its activity is elevated after hindlimb unloading. Overall, our results suggest that enhanced Ca<sup>2+</sup> uptake from cytoplasm is involved in the increase in [Ca<sup>2+</sup>]<sub>NE</sub> after hindlimb unloading. Moreover, the increase in [Ca<sup>2+</sup>]<sub>NC</sub> is attributed to increased Ca<sup>2+</sup> release into nucleocytoplasm and weakened Ca<sup>2+</sup> uptake from nucleocytoplasm. DNase X is activated due to elevated [Ca<sup>2+</sup>]<sub>NC</sub>, leading to DNA fragmentation in myonucleus, ultimately initiating myonuclear apoptosis. Nucleocytoplasmic Ca<sup>2+</sup> overload may contribute to the increased incidence of myonuclear apoptosis in disused skeletal muscle.
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