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 libra ry using a fragment of rat brain I-f cDNA. HAC I is composed of 1150 amino acid residues, and its cytoplasmic N- and C-terminal regions are longer tha n 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 H AC4 showed low homology to all HAC family members so far cloned. Northern b lotting revealed that HAC4 mRNA was the most highly expressed in the SA nod e among the rabbit cardiac tissues examined. The electrophysiological prope rties of HAC4 were examined using the whole cell patch-clamp technique. In COS-7 cells transfected with HAC4 cDNA, hyperpolarizing voltage steps activ ated slowly developing inward currents. The half-maximal activation was obt ained 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 -3 4.2 +/- 0.9 mV in 140 mM Na-o(+) and 5 mM K-o(+) versus 10 mM Na-i(+) and 1 45 mM K-i(+). These results indicate that HAC4 forms I-f in rabbit heart SA node.