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.