Regulation of sarcoplasmic reticulum calcium pump gene expression by hindlimb unweighting

Hindlimb unweighting (HU) causes upregulation of several muscle-specific genes responsible for the slow-to-fast transition in soleus skeletal muscle properties despite the profound muscle atrophy. The purpose of this study was to examine the expression of the fast and slow isoforms of the sarcoplasmic reticulum Ca²⁺-ATPase at the mRNA and protein level in the soleus muscle over a time course of HU and relate them to Ca²⁺-dependent ATPase activity and selected contractile properties. mRNA levels of the acetylcholine receptor (AChR) were measured to compare the signal of unweighting with denervation. Atrophy of the soleus muscles from tail-suspended rats was observed at all time points with muscle mass decreased by 52% at 28 days of HU (P < 0.05). Northern blot analysis showed the relative expression of the fast Ca²⁺ pump mRNA increased by 0, 250, 910, 1,340, and 4,050% over control levels at 1, 4, 7, 14, and 28 days of HU, respectively, whereas changes in slow mRNA were variable and modest in comparison. For the same time points, Western blot analysis showed relative expression of the fast Ca²⁺ pump protein increased by 30, 110, 320, 280, and 300% over control levels, whereas the slow-pump protein expression was unchanged except for a 75% decrease at 28 days of HU. Specific Ca²⁺-dependent ATPase activity was increased (P < 0.05) by 170% at 28 days of HU. Contractile properties measured in vitro at 14 and 28 days revealed time to peak tension and one- half relaxation time were shortened (P < 0.05) and a rightward shift in the tension-frequency curves in unloaded soleus muscles. In contrast to denervation, no increase in AChR mRNA was detected in the HU time course. These results indicate HU can dramatically upregulate the expression of the fast isoform of the Ca²⁺ pump and its related functional properties. Additionally, the triggering signal of unloading is distinct from the loss of innervation.