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 Ca 2+ -transporting ATPase (SERCA) in horse gluteal muscle. Horses are bred for peak athletic performance but exhibit a high incidence of exertional rhabdomyolysis, with myosolic Ca 2+ suggested as a correlative linkage. To assess Ca 2+ 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 Ca 2+ -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 ratio ∼3-fold greater than rabbit SR. The Ca 2+ transport rate of horse SR vesicles is ∼2-fold greater than rabbit SR, suggesting horse myocytes have enhanced luminal Ca 2+ stores that increase intracellular Ca 2+ release and muscular performance. The absence of SLN inhibition of SERCA and the abundant expression of CASQ may potentiate horse muscle contractility and susceptibility to exertional rhabdomyolysis.