Long-term If current blockade increases right atrial mRNA expression of HCN4, encoding proteins for If

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): This work was supported by a grant of the Romanian Ministry of Education and Research, CNCS - UEFISCDI, within PNCDI III Numerous long-term pharmacologic manipulations of cardiac ion channels have shown potential to modify the expression of genes encoding proteins for those channels. We aimed to investigate whether long-term pharmacologic inhibition of the hyperpolarization-activated inward current (If) using ivabradine affects the right atrial expression of genes encoding for hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, responsible for If. The right atrial mRNA expression of HCN1, HCN2, and HCN4, and of the neuron-specific HCN3 isoform was quantified in 6 control (Control) and 12 ivabradine-treated (IVA; 10 mg/kg, 4 weeks) adult male Wistar rats. The expression levels of the target genes were normalized with GAPDH housekeeping gene levels and compared between the two groups. Right atrial HCN1 and HCN2 expression levels were both similar (both p >0.05) between the IVA and the Control rats. There was also no significant difference in the right atrial expression of the neuron-specific HCN3 isoform between the two groups (p = 0.22). However, the right atrial expression of HCN4, the most highly expressed HCN isoform in the sinus node, was significantly higher in the ivabradine-treated compared to the non-treated rats (p = 0.02). Our study shows that chronic If blockade using ivabradine significantly up-regulates right atrial HCN4 expression. Although the examined samples contained both nodal and non-nodal tissue, since HCN4 is highly expressed in the nodal pacemaker cells and only sparsely in the remaining atrial myocardium, it is likely that the HCN4 changes observed in our study reflect sinus node alterations. HCN4 and a consequent If up-regulation could render the sinus node less sensitive to acute vagal inputs and protect against excessive vagal-induced bradycardia. Further studies will have to evaluate this hypothesis.