Downregulation of HCN isoforms and Na+- and Ca2+-channel subunits: HCN channels are responsible for If, an important contributor to spontaneous phase-4 diastolic depolarization in PFs.14 HCN1 and HCN4 showed reduction in HF, in LPFs and RPFs (Figure 3A and 3B). This evidence concerns the gene HCN4 and hydrops fetalis.