Histidine-rich calcium binding protein (HRC) is a high capacity, low affinity Ca(2+) binding protein, specifically expressed in striated muscles of mammals. In rabbit skeletal and cardiac muscles, HRC binds to sarcoplasmic reticulum (SR) membranes via triadin, a junctional SR protein. Recently, a potential role in heart failure and arrhythmogenesis has been assigned to HRC due to its activity as regulator of SR Ca(2+) uptake and Ca(2+) release. HRC might play a particularly relevant role in the equine heart, given its slower resting heart rate (20-35 beats/min) and longer action potential duration (APD) (0.6-1.0 s) than are found in other mammals. The results from this study showed for the first time direct evidence that HRC protein in equine cardiac muscle was expressed in association with the SR membranes and that HRC transcriptional activity was three times higher in the ventricles compared to the atria. The predominance of HRC mRNA up-regulation in ventricular myocardium was specific to the horse heart, since a more even distribution between atria and ventricles was found in animals of similar body size or species, such as cattle or domestic donkeys.
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