Previous work showed that protein and mRNA levels of the "fast" isoform of the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA1) are markedly increased in unloaded slow-twitch soleus muscles, suggesting pretranslational control of gene expression [L. M. Schulte, J. Navarro, and S. C. Kandarian. Am. J. Physiol. 264 (Cell Physiol. 33): C1308-C1315, 1993]. However, because of the difficulty of measuring transcription rates from whole muscle, transcriptional activation of the SERCA1 gene with unloading has not been confirmed. Because SERCA1 pre-mRNA levels can reflect transcriptional activity, in the present study SERCA1 introns were sequenced to allow intron-directed RT-PCR measurement of SERCA1 pre-mRNA. These data were then compared with changes in SERCA1 mRNA expression in control and unloaded soleus muscles. After 2, 4, and 10 days of unloading, SERCA1 pre-mRNA and mRNA transcript levels increased significantly by two-, three-, and sevenfold, respectively (P < 0.01). Parallel increases in SERCA1 pre-mRNA and mRNA suggest transcriptional activation of the endogenous SERCA1 gene by muscle unloading. SERCA2, the cardiac/slow-twitch skeletal muscle isoform, was not markedly increased by unloading, and RNase protection assays showed no change in alternative splicing of SERCA1 or SERCA2 primary transcripts. With use of in vivo plasmid injection, the activity of a reporter gene driven by 3.6 kb of the SERCA1 5'-flanking region increased fivefold in 7-day-unloaded soleus muscles. Comparison of the magnitude of transcriptional activation of endogenous and constructed SERCA1 genes by unloading confirms the fidelity of using intronic RT-PCR to examine muscle gene transcription rates and suggests that cis-acting elements sufficient for regulating unloading-induced transcriptional activation are contained in this promoter construct.

reverse transcriptase-polymerase chain reaction; run-on assay

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