Kinetic and ionic properties of the human HCN2 pacemaker channel

Human cDNA coding for the hyperpolarization-activated "pacemaker" channel HCN2 was expressed in Phoenix cells and yielded an inward current (I-hHCN2) activated on hyperpolarization. The average I-hHCN2 was half-activated at -83.1 mV and its kinetics could be described by second-order Hodgkin-Huxley gating. The time constant curve was bell-shaped and peaked at -82.2 mV. With 115 mM external Na+ and 30 mM external K+, I-hHCN2 reversed at -17.1 mV, and had a mean conductance of 5.6 nS. Reducing the external K+ or Na+ concentration led to a concentration-dependent reduction of the I-hHCN2 conductance and to a hyperpolarizing shift of reversal potential. External Cs+ ions (5 mM) blocked I-hHCN2 in a voltage-dependent way according to a Woodhull-type block model, at an electrical distance of 0.66 from the external membrane surface. and with a dissociation constant of 15 mM at 0 mV. Increasing cytoplasmic cAMP using forskolin increased I-hHCN2 by shifting the current activation curve to more positive voltages (11.7 mV). Exposure of the intracellular side of inside-out macro-patches to cAMP led to a depolarizing shift of the channel open probability curve (15.2 mV with 10 mu M cAMP). These results indicate that although h(HCN2) channels share several properties with native cardiac f-channels, differences also exist in permeability and block properties, suggesting that native channels may not be composed simply of homomeric constructs.