The method may include administering to a subject in need thereof an effective amount of an HCN polynucleotide. The HCN polynucleotide includes a nucleotide sequence encoding an HCN polypeptide having channel activity. The amino acid sequence of the HCN polypeptide and the amino acid sequence of a reference polypeptide have at least 80% identity, where the reference polypeptide begins with an amino acid selected from amino acids 92-214 and ends with an amino acid selected from amino acids 723-1188 of SEQ ID NO:8. An example of a reference polypeptide is amino acids 214-723 of SEQ ID NO:8. The HCN polynucleotide may be DNA or RNA.