Horse gluteus is a null-sarcolipin muscle with enhanced sarcoplasmic reticulum calcium transport

We have analyzed gene transcription, protein expression, and enzymatic activity of the Ca2+-transporting ATPase (SERCA) in horse gluteal muscle. Horses are bred for peak athletic performance but exhibit a high incidence of exertional rhabdomyolysis, suggesting Ca2+ as a correlative linkage. To assess Ca2+ regulation in horse gluteus, we developed an improved protocol for isolating horse sarcoplasmic reticulum (SR) vesicles. RNA-seq and immunoblotting determined that the ATP2A1 gene (protein product SERCA1) is the predominant Ca2+-ATPase expressed in horse gluteus, as in rabbit muscle. Gene expression was assessed for four regulatory peptides of SERCA, finding that sarcolipin (SLN) is the predominant regulatory peptide transcript expressed in horse gluteus, as in rabbit muscle. Surprisingly, the RNA transcription ratio of SLN-to-ATP2A1 in horse gluteus is an order of magnitude higher than in rabbit muscle, but conversely, the protein expression ratio of SLN-to-SERCA1 in horse gluteus is an order of magnitude lower than in rabbit. Thus, the SLN gene is not translated to a stable protein in horse gluteus, yet the supra-high level of SLN RNA suggests a non-coding role. Gel-stain analysis revealed that horse SR expresses calsequestrin (CASQ) protein abundantly, with a CASQ-to-SERCA protein ratio ∼3-fold greater than rabbit SR. The Ca2+ transport rate of horse SR vesicles is ∼2-fold greater than rabbit SR, suggesting that horse myocytes have enhanced luminal Ca2+ stores that increase intracellular Ca2+ release and muscular contractility. We hypothesize that the absence of SLN inhibition of SERCA and the abundant expression of CASQ may potentiate horse susceptibility to exertional rhabdomyolysis.