Molecular characterization of the hyperpolarization-activated cation channel in rabbit heart sinoatrial node

We cloned a cDNA (HAC4) that encodes the hyperpolarization-activated cation channel (I(f) or I(h)) by screening a rabbit sinoatrial (SA) node cDNA library using a fragment of rat brain I(f) cDNA. HAC4 is composed of 1150 amino acid residues, and its cytoplasmic N- and C-terminal regions are longer than those of HAC1-3. The transmembrane region of HAC4 was most homologous to partially cloned mouse I(f) BCNG-3 (96%), whereas the C-terminal region of HAC4 showed low homology to all HAC family members so far cloned. Northern blotting revealed that HAC4 mRNA was the most highly expressed in the SA node among the rabbit cardiac tissues examined. The electrophysiological properties of HAC4 were examined using the whole cell patch-clamp technique. In COS-7 cells transfected with HAC4 cDNA, hyperpolarizing voltage steps activated slowly developing inward currents. The half-maximal activation was obtained at -87.2 ± 2.8 mV under control conditions and at -64.4 ± 2.6 mV in the presence of intracellular 0.3 mM cAMP. The reversal potential was - 34.2 ± 0.9 mV in 140 mM Na⁺(o) and 5 mM K⁺(o) versus 10 mM Na⁺(i) and 145 nM K⁺(i). These results indicate that HAC4 forms I(f) in rabbit heart SA node.