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 (IhHCN2) activated on hyperpolarization. The average IhHCN2 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+, IhHCN2 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 IhHCN2 conductance and to a hyperpolarizing shift of reversal potential. External Cs+ ions (5 mM) blocked IhHCN2 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 IhHCN2 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 microM cAMP). These results indicate that although hHCN2 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.
Moroni
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A
Università di Milano, Dipartimento di Fisiologia e Biochimica Generali, Italy.
Barbuti
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A
Altomare
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C
Viscomi
C
C
Morgan
J
J
Baruscotti
M
M
DiFrancesco
D
D
D001683:Biological Clocks / Q000502:physiology*; D002460:Cell Line; D002586:Cesium / Q000494:pharmacology; D003001:Cloning, Molecular; D005576:Colforsin / Q000494:pharmacology; D000242:Cyclic AMP / Q000494:pharmacology; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D007473:Ion Channels / Q000235:genetics* / Q000378:metabolism*; D007700:Kinetics; D008564:Membrane Potentials / Q000502:physiology; D009124:Muscle Proteins; D009206:Myocardium / Q000737:chemistry; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000494:pharmacology; D015221:Potassium Channels; D012964:Sodium / Q000494:pharmacology
C578710:HCN2 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D009124:Muscle Proteins; D015221:Potassium Channels; D005576:Colforsin; D002586:Cesium; D012964:Sodium; D000242:Cyclic AMP; D011188:Potassium