Hyperpolarization-activated ion channel with very slow activation and inactivation exhibiting weak selectivity for potassium over sodium ions. May contribute to the native pacemaker currents in heart (If) that regulate the rhythm of heart beat. May contribute to the native pacemaker currents in neurons (Ih). May mediate responses to sour stimuli. Activated by cAMP. cAMP binding causes a conformation change that leads to the assembly of an active tetramer and channel opening. Homotetramer. The potassium channel is composed of a homo- or heterotetrameric complex of pore-forming subunits (By similarity). Highly expressed in the heart sinoatrial node (SAN). Not detected in atrium, ventricle, forebrain or cerebellum. Detected at very low levels in total brain. The segment S4 is probably the voltage-sensor and is characterized by a series of positively charged amino acids at every third position. Inhibited by extracellular cesium ions. Belongs to the potassium channel HCN family.