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

Calcium channel β subunits expressed in rat atria and atrial myocytes are identified and their expression quantified and compared to β subunit expression in the ventricle. mRNAs encoding all four know β subunits are expressed in atrial myocytes including the following splice variants: β_1a, β_2b, β_2c, β _2e and β_4d. The specific β₂ splice variants expressed in the atria (β_2b, β_2c, β_2e) differ from those previously reported from rat ventricle. β₂ isoform is the most abundant β mRNA expressed in the heart and the amount of both β₂ subunit mRNA and β ₂ subunit protein is significantly greater in the ventricles than in the atria. The expression of individual β₂ splice variants varies with age and within different chambers of the heart. The β _2b splice variant appears in both atria and ventricle in both young (4.5 week) and old (16 week) animals, the β_2c isoform is more highly expressed in young as compared to old animals and the β _2e splice variant is robustly expressed only in 4.5 week ventricle. β₄ mRNA expression is higher in atrial tissue than in ventricles and its expression decreases in older animals. In contrast, the abundance of the β₃ mRNA does not significantly change as a function of postnatal age. Antisense oligonucleotides targeting sequence common to all the β isoforms as well as that specific for β₂ significantly reduced HVA calcium current density in isolated atrial cells confirming that the β₂ subunits contribute to the regulation of HVA calcium current expression in the rat atria. The complexity of β isoform expression within the heart may provide a mechanism for functional fine-tuning of the cardiac HVA current.