Pacemaker channels produce an instantaneous current.
Spontaneous rhythmic activity in mammalian heart and brain depends on pacemaker currents (I(h)), which are produced by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Here, we report that the mouse HCN2 pacemaker channel isoform also produced a large instantaneous current (I(inst(HCN2))) in addition to the well characterized, slowly activating I(h). I(inst(HCN2)) was specific to expression of HCN2 on the plasma membrane and its amplitude was correlated with that of I(h). The two currents had similar reversal potentials, and both were modulated by changes in intracellular Cl(-) and cAMP. A mutation in the S4 domain of HCN2 (S306Q) decreased I(h) but did not alter I(inst(HCN2)), and instantaneous currents in cells expressing either wild type HCN2 or mutant S306Q channels were insensitive to block by Cs(+). Co-expression of HCN2 with the accessory subunit, MiRP1, decreased I(h) and increased I(inst(HCN2)), suggesting a mechanism for modulation of both currents in vivo. These data suggest that expression of HCN channels may be accompanied by a background conductance in native tissues and are consistent with at least two open states of HCN channels: I(inst(HCN2)) is produced by a Cs(+)-open state; hyperpolarization produces an additional Cs(+)-sensitive open state, which results in I(h).
Proenza
Catherine
C
Ion Channel Laboratory, School of Kinesiology, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada.
Angoli
Damiano
D
Agranovich
Eugene
E
Macri
Vincenzo
V
Accili
Eric A
EA
D000818:Animals; D016466:CHO Cells; D002462:Cell Membrane / Q000378:metabolism; D002713:Chlorine / Q000378:metabolism; D006224:Cricetinae; D000242:Cyclic AMP / Q000378:metabolism; D004553:Electric Conductivity; D004594:Electrophysiology; D049452:Green Fluorescent Proteins; D006863:Hydrogen-Ion Concentration; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007150:Immunohistochemistry; D007473:Ion Channels / Q000737:chemistry* / Q000502:physiology*; D008164:Luminescent Proteins / Q000378:metabolism; D051379:Mice; D018613:Microscopy, Confocal; D009124:Muscle Proteins; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D011188:Potassium / Q000378:metabolism; D015221:Potassium Channels; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D013997:Time Factors; D014162:Transfection
C578711:Hcn2 protein, mouse; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D008164:Luminescent Proteins; D009124:Muscle Proteins; D015221:Potassium Channels; D011993:Recombinant Fusion Proteins; D049452:Green Fluorescent Proteins; D002713:Chlorine; D000242:Cyclic AMP; D011188:Potassium