Distribution and relative expression levels of calcium channel beta subunits within the chambers of the rat heart.

Calcium channel beta subunits expressed in rat atria and atrial myocytes are identified and their expression quantified and compared to beta subunit expression in the ventricle. mRNAs encoding all four know beta subunits are expressed in atrial myocytes including the following splice variants: beta1a, beta2b, beta2c, beta2e and beta4d. The specific beta2 splice variants expressed in the atria (beta2b, beta2c, beta2e) differ from those previously reported from rat ventricle. Beta2 isoform is the most abundant beta mRNA expressed in the heart and the amount of both beta2 subunit mRNA and beta2 subunit protein is significantly greater in the ventricles than in the atria. The expression of individual beta2 splice variants varies with age and within different chambers of the heart. The beta2b splice variant appears in both atria and ventricle in both young (4.5 week) and old (16 week) animals, the beta2c isoform is more highly expressed in young as compared to old animals and the beta2e splice variant is robustly expressed only in 4.5 week ventricle. Beta4 mRNA expression is higher in atrial tissue than in ventricles and its expression decreases in older animals. In contrast, the abundance of the beta3 mRNA does not significantly change as a function of postnatal age. Antisense oligonucleotides targeting sequence common to all the beta isoforms as well as that specific for beta2 significantly reduced HVA calcium current density in isolated atrial cells confirming that the beta2 subunits contribute to the regulation of HVA calcium current expression in the rat atria. The complexity of beta isoform expression within the heart may provide a mechanism for functional fine-tuning of the cardiac HVA current.