| Summary: | Ca<sup>2+</sup> regulation in equine muscle is important for horse performance, yet little is known about this species-specific regulation. We reported recently that horse encode unique gene and protein sequences for the sarcoplasmic reticulum (SR) Ca<sup>2+</sup>-transporting ATPase (SERCA) and the regulatory subunit sarcolipin (SLN). Here we quantified gene transcription and protein expression of SERCA and its inhibitory peptides in horse gluteus, as compared to commonly-studied rabbit skeletal muscle. RNA sequencing and protein immunoblotting determined that horse gluteus expresses the <i>ATP2A1</i> gene (SERCA1) as the predominant SR Ca<sup>2+</sup>-ATPase isoform and the <i>SLN</i> gene as the most-abundant SERCA inhibitory peptide, as also found in rabbit skeletal muscle. Equine muscle expresses an insignificant level of phospholamban (PLN), another key SERCA inhibitory peptide expressed commonly in a variety of mammalian striated muscles. Surprisingly in horse, the RNA transcript ratio of <i>SLN</i>-to-<i>ATP2A1</i> is an order of magnitude <i>higher</i> than in rabbit, while the corresponding protein expression ratio is an order of magnitude <i>lower</i> than in rabbit. Thus, <i>SLN</i> is not efficiently translated or maintained as a stable protein in horse muscle, suggesting a non-coding role for supra-abundant <i>SLN</i> mRNA. We propose that the lack of SLN and PLN inhibition of SERCA activity in equine muscle is an evolutionary adaptation that potentiates Ca<sup>2+</sup> cycling and muscle contractility in a prey species domestically selected for speed.
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