Hysteresis in human HCN4 channels: a crucial feature potentially affecting sinoatrial node pacemaking.
The hyperpolarization-activated cyclic nucleotide-gated (HCN) channels modulate and regulate cardiac rhythm and rate. It has been suggested that, unlike the HCN1 and HCN2 channels, the slower HCN4 channel may not exhibit voltage-dependent hysteresis. We studied the electrophysiological properties of human HCN4 (hHCN4) channels and its modulation by cAMP to determine whether hHCN4 exhibits hysteresis, by using single-cell patch-clamp in HEK293 cells stably transfected with hHCN4. Quantitative real-time RT-PCR was also used to determine levels of expression of HCNs in human cardiac tissue. Voltage-clamp analysis revealed that hHCN4 current (I(h)) activation shifted in the depolarizing direction with more hyperpolarized holding potentials. Triangular ramp and action potential clamp protocols also revealed hHCN4 hysteresis. cAMP enhanced I(h) and shifted activation in the depolarizing direction, thus modifying the intrinsic hHCN4 hysteresis behavior. Quantitative PCR analysis of human sinoatrial node (SAN) tissue showed that HCN4 accounts for 75% of the HCNs in human SAN while HCN1 (21%), HCN2 (3%), and HCN3 (0.7%) constitute the remainder. Our data suggest that HCN4 is the predominant HCN subtype in the human SAN and that I(h) exhibits voltage-dependent hysteresis behavior that can be modified by cAMP. Therefore, hHCN4 hysteresis potentially plays a crucial role in human SAN pacemaking activity.
Xiao
Yong-Fu
YF
Cardiac Rhythm Disease Management, Medtronic Inc., Mounds View, MN 55112, USA. yong-fu.xiao@medtronic.com
Chandler
Natalie
N
Dobrzynski
Halina
H
Richardson
Eric S
ES
Tenbroek
Erica M
EM
Wilhelm
Joshua J
JJ
Sharma
Vinod
V
Varghese
Anthony
A
Boyett
Mark R
MR
Iaizzo
Paul A
PA
Sigg
Daniel C
DC
D001683:Biological Clocks / Q000502:physiology*; D000242:Cyclic AMP / Q000502:physiology; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000502:physiology*; D055724:Electrophysiological Phenomena; D057809:HEK293 Cells; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D009124:Muscle Proteins / Q000502:physiology*; D018408:Patch-Clamp Techniques; D015221:Potassium Channels; D012849:Sinoatrial Node / Q000502:physiology*; D014162:Transfection
D054815:Cyclic Nucleotide-Gated Cation Channels; C119587:HCN4 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D009124:Muscle Proteins; D015221:Potassium Channels; D000242:Cyclic AMP