HCN4

Nomenclature: HCN4

Family: Cyclic nucleotide-regulated channels

Annotation status:  image of a green circle Annotated and expert reviewed. Please contact us if you can help with updates.  » Email us

Gene and Protein Information
Species TM P Loops AA Chromosomal Location Gene Symbol Gene Name Reference
Human 6 1 1203 15q24-q25 HCN4 hyperpolarization activated cyclic nucleotide-gated potassium channel 4 18
Mouse 6 1 1201 9 C Hcn4 hyperpolarization-activated, cyclic nucleotide-gated K+ 4 28
Rat 6 1 1198 8q24 Hcn4 hyperpolarization activated cyclic nucleotide-gated potassium channel 4 21
Previous and Unofficial Names
HAC4
BCNG3
hyperpolarization activated cyclic nucleotide-gated potassium channel 4
hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 (HCN4)
hyperpolarization-activated, cyclic nucleotide-gated K+ 4
hyperpolarization-activated, cyclic nucleotide-gated potassium channel 4 (HCN4)
potassium/sodium hyperpolarization-activated cyclic nucleotide-gated channel 4
Database Links
Ensembl Gene
Entrez Gene
GeneCards
GenitoUrinary Development Molecular Anatomy Project
HomoloGene
Human Protein Reference Database
InterPro
KEGG Gene
OMIM
Orphanet Gene
PharmGKB Gene
PhosphoSitePlus
Protein Ontology (PRO)
RefSeq Nucleotide
RefSeq Protein
TreeFam
UniGene Hs.
UniProtKB
Wikipedia
Associated Proteins
Heteromeric Pore-forming Subunits
Name References
HCN1 23
HCN2 23
HCN3 23
Auxiliary Subunits
Name References
KCNE2 5,7
Trip8b 39
Other Associated Proteins
Name References
Calveolin proteins 1
β-adrenergic receptor 12
Ion Selectivity and Conductance
Species:  Human
Rank order:  K+ > Na+ > Ca2+ [17.4 pS]
References:  18-19,38
Ion Selectivity and Conductance Comments
Fractional Ca2+ current makes up only 0.60% of the net inward current [38].
Voltage Dependence
  V0.5 (mV)  τ (msec)  Reference  Cell type  Species 
Activation  -98.0 1000.0 – 387.0 7 Xenopus laevis oocyte Human
Inactivation  - -
Comments  V0.5 and τ-values are strongly influenced by experimental parameters such as temperature, pH and pulse protocol.
  V0.5 (mV)  τ (msec)  Reference  Cell type  Species 
Activation  -75.0 – -109.0 (median: -82.0) 2000.0 – 660.0 18,26,30,34 HEK 293 cells. Human
Inactivation  - -
Comments  V0.5 and τ-values are strongly influenced by experimental parameters such as temperature, pH and pulse protocol.
Activators (Human)
cAMP > cGMP
Activator Comments
cAMP and cGMP induce a shift of V0.5 by +10 to +25 mV [18,30,34]. cCMP shifts V0.5 of murine HCN4 by +10 mV [40].
Channel Blockers
Key to terms and symbols View all chemical structures Click column headers to sort
Ligand Sp. Action Affinity Units Concentration range (M) Holding voltage (mV) Reference
EC18 Hs - 5.4 pEC50 - -80.0 8
pEC50 5.4 (EC50 3.98x10-6 M) [8]
Holding voltage: -80.0 mV
cilobradine Hs Antagonist 6.0 pIC50 - -40.0 35
pIC50 6.0 [35]
Holding voltage: -40.0 mV
ivabradine Hs Antagonist 5.7 pIC50 - -40.0 35
pIC50 5.7 [35]
Holding voltage: -40.0 mV
zatebradine Hs Antagonist 5.7 pIC50 - -40.0 35
pIC50 5.7 [35]
Holding voltage: -40.0 mV
clonidine Mm Antagonist 5.0 pIC50 - -40.0 16
pIC50 5.0 [16]
Holding voltage: -40.0 mV
ZD7288 Hs Antagonist 4.7 pIC50 - -40.0 34
pIC50 4.7 [34]
Holding voltage: -40.0 mV
Cs+ Hs Antagonist 3.8 pIC50 - -40.0 34
pIC50 3.8 [34]
Holding voltage: -40.0 mV
View species-specific channel blocker tables
Channel Blocker Comments
Propofol inhibits and slows activation of the mouse HCN4 channel at clinically relevant concentrations [6].
Tissue Distribution
Heart (highest expression in the sino-atrial region), brain, testis.
Species:  Human
Technique:  Northern Blot
References:  18,30
Brain, heart.
Species:  Mouse
Technique:  Northern Blot
References:  28
Brain (high levels in thalamus and olfactory bulb).
Species:  Mouse
Technique:  In situ hybridisation
References:  22
Taste cells.
Species:  Mouse
Technique:  In situ hybridisation
References:  32
Heart.
Species:  Rat
Technique:  RNAse protection assay
References:  31
Retina.
Species:  Rat
Technique:  Immunohistochemistry
References:  24
Urinary bladder.
Species:  Rat
Technique:  RT-PCR, Western blot
References:  14
Physiological Functions
Transduction of sour taste.
Species:  Rat
Tissue:  Vallate papilla
References:  32
Development of cardiac pacemaker cells, heart rate control.
Species:  None
Tissue:  Pacemaker region of heart.
References:  27
Physiological Consequences of Altering Gene Expression
Knockout of the HCN4 gene results in embryonic lethality. Embryos are bradycardic and unable to speed up heart rate.
Species:  Mouse
Tissue:  Heart
Technique:  Knockout
References:  33
Tamoxifen-induced HCN4 deletion in the cardiac transduction system results in frequent sinus pauses and a reduced number of spontaneous pacemaker action potentials.
Species:  Mouse
Tissue:  Cardiac pacemaker cells.
Technique:  Knock-in of the tamoxifen-inducible CreER(T2) construct into the pacemaker channel HCN4 locus.
References:  15
Inducible and cardiac-specific HCN4 knockout (ciHCN4-KO) mouse model leads to progressive development of severe bradycardia (∼50% reduction of original rate) and AV block, eventually leading to heart arrest and death in about 5 days.
Species:  Mouse
Tissue:  Heart
Technique:  Tamoxifen (Tam)-inducible Cre-loxP system.
References:  3
Single point mutation HCN4 (R669Q/R669Q) results in death during embryonic development.
Species:  Mouse
Tissue:  Heart
Technique:  Gene knock-in
References:  13
Phenotypes, Alleles and Disease Models Mouse data from MGI

Show »

Allele Composition & genetic background Accession Phenotype Id Phenotype Reference
Hcn4tm1.1Jsr Hcn4tm1.1Jsr/Hcn4tm1.1Jsr
involves: 129/Sv * C57BL/6
MGI:1298209  MP:0001629 abnormal heart rate PMID: 14657344 
Hcn4tm1(cre/ERT2)Anlu|Hcn4tm1Jsr Hcn4tm1Jsr/Hcn4tm1(cre/ERT2)Anlu
involves: 129S1/Sv * 129X1/SvJ
MGI:1298209  MP:0003137 abnormal impulse conducting system conduction PMID: 18538341 
Hcn4tm1Rsei Hcn4tm1Rsei/Hcn4tm1Rsei
B6.Cg-Hcn4
MGI:1298209  MP:0005140 decreased cardiac muscle contractility PMID: 18219271 
Hcn4tm1Rsei Hcn4tm1Rsei/Hcn4tm1Rsei
B6.Cg-Hcn4
MGI:1298209  MP:0005333 decreased heart rate PMID: 18219271 
Hcn4+|Hcn4tm1Rsei Hcn4tm1Rsei/Hcn4+
B6.Cg-Hcn4
MGI:1298209  MP:0005333 decreased heart rate PMID: 18219271 
Hcn4tm1(cre/ERT2)Anlu|Hcn4tm1Jsr Hcn4tm1Jsr/Hcn4tm1(cre/ERT2)Anlu
involves: 129S1/Sv * 129X1/SvJ
MGI:1298209  MP:0005333 decreased heart rate PMID: 18538341 
Hcn4tm1Jsr|Myl2+|Myl2tm1(cre)Krc Hcn4tm1Jsr/Hcn4tm1Jsr,Myl2tm1(cre)Krc/Myl2+
involves: 129/Sv
MGI:1298209  MGI:97272  MP:0006207 embryonic lethality during organogenesis PMID: 14657344 
Hcn4tm1.1Jsr Hcn4tm1.1Jsr/Hcn4tm1.1Jsr
involves: 129/Sv * C57BL/6
MGI:1298209  MP:0006207 embryonic lethality during organogenesis PMID: 14657344 
Hcn4tm1Rsei Hcn4tm1Rsei/Hcn4tm1Rsei
B6.Cg-Hcn4
MGI:1298209  MP:0006207 embryonic lethality during organogenesis PMID: 18219271 
Hcn4+|Hcn4tm1(cre/ERT2)Anlu Hcn4tm1(cre/ERT2)Anlu/Hcn4+
Not Specified
MGI:1298209  MP:0002169 no abnormal phenotype detected PMID: 18538341 
Hcn4tm1(cre/ERT2)Anlu|Hcn4tm1Jsr Hcn4tm1Jsr/Hcn4tm1(cre/ERT2)Anlu
involves: 129S1/Sv * 129X1/SvJ
MGI:1298209  MP:0010520 sinoatrial block PMID: 18538341 
Clinically-Relevant Mutations and Pathophysiology
Disease:  Sinus node dysfunction
Drugs: 
Side effects:  Not established.
Therapeutic use:  Stable angina pectoris.
References:  2
Mutations not determined
Disease:  Brugada syndrome 8; BRGDA8
OMIM:  613123
Orphanet:  130
References: 
Mutations not determined
Disease:  Sick sinus syndrome 2, autosomal dominant; SSS2
OMIM:  163800
Orphanet:  166282, 334
Comments: 
References:  20,29,36
Click column headers to sort
Type Species Molecular location Description Reference
Missense Human D553N 36
Missense Human S672R 20,37
Truncation Human HCN4-573X 29
Disease:  Sinus bradycardia
References: 
Mutations not determined
Disease:  Familial sinus bradycardia (FSB)
References: 
Click column headers to sort
Type Species Molecular location Description Reference
Missense Human S672R A mutation identified in the cAMP binding domain (CNBD) of the human HCN4 channel, S672R, severely reduces the heart rate. All the individuals carrying the heterozygous mutation had rates lower than 60 bpm, while all individuals without the mutation had rates higher than 60 bpm. The S672R mutation induces a “cholinergic” type of effect, resulting in a 4.9 mV hyperpolarizing shift of the If activation curve in individuals carrying the heterozygous mutation. The consequent decrease in If current availability during diastole is therefore able to explain the slower heart rate of individuals with the mutation. 20,37
Missense Human A485V Sequencing of the HCN4 gene in all patients revealed a C to T transition at nucleotide position 1,454, which resulted in an alanine to valine change (A485V) in the ion channel pore found in most of their bradycardiac relatives, but not in 150 controls. Functional expression of the mutated ion channel in Xenopus oocytes and in human embryonic kidney 293 cells revealed profoundly reduced function and synthesis of the mutant channel compared to wild-type. 17
Missense Human G480R All affected family members with the missense mutation in the HCN4 channel (G480R) were asymptomatic with normal exercise capacity during long-term follow-up. Electrophysiological testing performed on 2 affected family members confirmed significant isolated sinus node dysfunction. In vitro experiments revealed an activation of the mutant channels at more negative voltages, a reduction of channel synthesis and a transport defect to the cell membrane. 25
Missense Human K530N 9
Disease:  Tachycardia-bradycardia syndrome and persistent atrial fibrillation (AF)
References: 
Click column headers to sort
Type Species Molecular location Description Reference
Missense Human K530N The index patient developed tachycardia-bradycardia syndrome and persistent atrial fibrillation (AF) in an age-dependent fashion. Pedigree analysis identified eight affected family members with a similar course of disease. Whole-cell patch clamp electrophysiology of HEK293 cells showed that homomeric mutant channels almost are indistinguishable from wild-type channels. In contrast, heteromeric channels composed of mutant and wild-type subunits displayed a significant hyperpolarizing shift in the half-maximal activation voltage. 9
Gene Expression and Pathophysiology
1.8-fold elevation of HCN4 mRNA in transgenic mice overexpressing the human β2-adrenoceptor; significant increase of HCN4 mRNA levels in hypertrophied cardiac myocytes.
Tissue or cell type:  Ventricular myocytes.
Pathophysiology:  Cardiac hypertrophy.
Species:  None
Technique: 
References:  10-11
3-fold increase in HCN4 gene expression in end-stage human heart failure.
Tissue or cell type:  Human ventricular myocytes.
Pathophysiology:  Heart failure.
Species:  Human
Technique: 
References:  4

References

Show »

1. Barbuti A, Scavone A, Mazzocchi N, Terragni B, Baruscotti M, Difrancesco D. (2012) A caveolin-binding domain in the HCN4 channels mediates functional interaction with caveolin proteins. J. Mol. Cell. Cardiol.53 (2): 187-95. [PMID:22659290]

2. Baruscotti M, Bucchi A, Difrancesco D. (2005) Physiology and pharmacology of the cardiac pacemaker ("funny") current. Pharmacol. Ther.107 (1): 59-79. [PMID:15963351]

3. Baruscotti M, Bucchi A, Viscomi C, Mandelli G, Consalez G, Gnecchi-Rusconi T, Montano N, Casali KR, Micheloni S, Barbuti A et al.. (2011) Deep bradycardia and heart block caused by inducible cardiac-specific knockout of the pacemaker channel gene Hcn4. Proc. Natl. Acad. Sci. U.S.A.108 (4): 1705-10. [PMID:21220308]

4. Borlak J, Thum T. (2003) Hallmarks of ion channel gene expression in end-stage heart failure. FASEB J.17 (12): 1592-608. [PMID:12958166]

5. Brandt MC, Endres-Becker J, Zagidullin N, Motloch LJ, Er F, Rottlaender D, Michels G, Herzig S, Hoppe UC. (2009) Effects of KCNE2 on HCN isoforms: distinct modulation of membrane expression and single channel properties. Am. J. Physiol. Heart Circ. Physiol.297 (1): H355-63. [PMID:19429827]

6. Cacheaux LP, Topf N, Tibbs GR, Schaefer UR, Levi R, Harrison NL, Abbott GW, Goldstein PA. (2005) Impairment of hyperpolarization-activated, cyclic nucleotide-gated channel function by the intravenous general anesthetic propofol. J. Pharmacol. Exp. Ther.315 (2): 517-25. [PMID:16033909]

7. Decher N, Bundis F, Vajna R, Steinmeyer K. (2003) KCNE2 modulates current amplitudes and activation kinetics of HCN4: influence of KCNE family members on HCN4 currents. Pflugers Arch.446 (6): 633-40. [PMID:12856183]

8. Del Lungo M, Melchiorre M, Guandalini L, Sartiani L, Mugelli A, Koncz I, Szel T, Varro A, Romanelli MN, Cerbai E. (2012) Novel blockers of hyperpolarization-activated current with isoform selectivity in recombinant cells and native tissue. Br. J. Pharmacol.166 (2): 602-16. [PMID:22091830]

9. Duhme N, Schweizer PA, Thomas D, Becker R, Schröter J, Barends TR, Schlichting I, Draguhn A, Bruehl C, Katus HA et al.. (2012) Altered HCN4 channel C-linker interaction is associated with familial tachycardia-bradycardia syndrome and atrial fibrillation. Eur. Heart J.,  [Epub ahead of print]. [PMID:23178648]

10. Fernández-Velasco M, Goren N, Benito G, Blanco-Rivero J, Boscá L, Delgado C. (2003) Regional distribution of hyperpolarization-activated current (If) and hyperpolarization-activated cyclic nucleotide-gated channel mRNA expression in ventricular cells from control and hypertrophied rat hearts. J. Physiol. (Lond.)553 (Pt 2): 395-405. [PMID:14514868]

11. Graf EM, Heubach JF, Ravens U. (2001) The hyperpolarization-activated current If in ventricular myocytes of non-transgenic and beta2-adrenoceptor overexpressing mice. Naunyn Schmiedebergs Arch. Pharmacol.364 (2): 131-9. [PMID:11534852]

12. Greene D, Kang S, Kosenko A, Hoshi N. (2012) Adrenergic regulation of HCN4 channel requires protein association with β2-adrenergic receptor. J. Biol. Chem.287 (28): 23690-7. [PMID:22613709]

13. Harzheim D, Pfeiffer KH, Fabritz L, Kremmer E, Buch T, Waisman A, Kirchhof P, Kaupp UB, Seifert R. (2008) Cardiac pacemaker function of HCN4 channels in mice is confined to embryonic development and requires cyclic AMP. EMBO J.27 (4): 692-703. [PMID:18219271]

14. He P, Deng J, Zhong X, Zhou Z, Song B, Li L. (2012) Identification of a hyperpolarization-activated cyclic nucleotide-gated channel and its subtypes in the urinary bladder of the rat. Urology79 (6): 1411.e7-13. [PMID:22446339]

15. Hoesl E, Stieber J, Herrmann S, Feil S, Tybl E, Hofmann F, Feil R, Ludwig A. (2008) Tamoxifen-inducible gene deletion in the cardiac conduction system. J. Mol. Cell. Cardiol.45 (1): 62-9. [PMID:18538341]

16. Knaus A, Zong X, Beetz N, Jahns R, Lohse MJ, Biel M, Hein L. (2007) Direct inhibition of cardiac hyperpolarization-activated cyclic nucleotide-gated pacemaker channels by clonidine. Circulation115 (7): 872-80. [PMID:17261653]

17. Laish-Farkash A, Glikson M, Brass D, Marek-Yagel D, Pras E, Dascal N, Antzelevitch C, Nof E, Reznik H, Eldar M et al.. (2010) A novel mutation in the HCN4 gene causes symptomatic sinus bradycardia in Moroccan Jews. J. Cardiovasc. Electrophysiol.21 (12): 1365-72. [PMID:20662977]

18. Ludwig A, Zong X, Stieber J, Hullin R, Hofmann F, Biel M. (1999) Two pacemaker channels from human heart with profoundly different activation kinetics. EMBO J.18 (9): 2323-9. [PMID:10228147]

19. Michels G, Er F, Khan I, Südkamp M, Herzig S, Hoppe UC. (2005) Single-channel properties support a potential contribution of hyperpolarization-activated cyclic nucleotide-gated channels and If to cardiac arrhythmias. Circulation111 (4): 399-404. [PMID:15687126]

20. Milanesi R, Baruscotti M, Gnecchi-Ruscone T, DiFrancesco D. (2006) Familial sinus bradycardia associated with a mutation in the cardiac pacemaker channel. N. Engl. J. Med.354 (2): 151-7. [PMID:16407510]

21. Monteggia LM, Eisch AJ, Tang MD, Kaczmarek LK, Nestler EJ. (2000) Cloning and localization of the hyperpolarization-activated cyclic nucleotide-gated channel family in rat brain. Brain Res. Mol. Brain Res.81 (1-2): 129-39. [PMID:11000485]

22. Moosmang S, Biel M, Hofmann F, Ludwig A. (1999) Differential distribution of four hyperpolarization-activated cation channels in mouse brain. Biol. Chem.380 (7-8): 975-80. [PMID:10494850]

23. Much B, Wahl-Schott C, Zong X, Schneider A, Baumann L, Moosmang S, Ludwig A, Biel M. (2003) Role of subunit heteromerization and N-linked glycosylation in the formation of functional hyperpolarization-activated cyclic nucleotide-gated channels. J. Biol. Chem.278 (44): 43781-6. [PMID:12928435]

24. Müller F, Scholten A, Ivanova E, Haverkamp S, Kremmer E, Kaupp UB. (2003) HCN channels are expressed differentially in retinal bipolar cells and concentrated at synaptic terminals. Eur. J. Neurosci.17 (10): 2084-96. [PMID:12786975]

25. Nof E, Luria D, Brass D, Marek D, Lahat H, Reznik-Wolf H, Pras E, Dascal N, Eldar M, Glikson M. (2007) Point mutation in the HCN4 cardiac ion channel pore affecting synthesis, trafficking, and functional expression is associated with familial asymptomatic sinus bradycardia. Circulation116 (5): 463-70. [PMID:17646576]

26. Qu J, Altomare C, Bucchi A, DiFrancesco D, Robinson RB. (2002) Functional comparison of HCN isoforms expressed in ventricular and HEK 293 cells. Pflugers Arch.444 (5): 597-601. [PMID:12194012]

27. Robinson RB, Siegelbaum SA. (2003) Hyperpolarization-activated cation currents: from molecules to physiological function. Annu. Rev. Physiol.65: 453-80. [PMID:12471170]

28. Santoro B, Liu DT, Yao H, Bartsch D, Kandel ER, Siegelbaum SA, Tibbs GR. (1998) Identification of a gene encoding a hyperpolarization-activated pacemaker channel of brain. Cell93 (5): 717-29. [PMID:9630217]

29. Schulze-Bahr E, Neu A, Friederich P, Kaupp UB, Breithardt G, Pongs O, Isbrandt D. (2003) Pacemaker channel dysfunction in a patient with sinus node disease. J. Clin. Invest.111 (10): 1537-45. [PMID:12750403]

30. Seifert R, Scholten A, Gauss R, Mincheva A, Lichter P, Kaupp UB. (1999) Molecular characterization of a slowly gating human hyperpolarization-activated channel predominantly expressed in thalamus, heart, and testis. Proc. Natl. Acad. Sci. U.S.A.96 (16): 9391-6. [PMID:10430953]

31. Shi W, Wymore R, Yu H, Wu J, Wymore RT, Pan Z, Robinson RB, Dixon JE, McKinnon D, Cohen IS. (1999) Distribution and prevalence of hyperpolarization-activated cation channel (HCN) mRNA expression in cardiac tissues. Circ. Res.85 (1): e1-6. [PMID:10400919]

32. Stevens DR, Seifert R, Bufe B, Müller F, Kremmer E, Gauss R, Meyerhof W, Kaupp UB, Lindemann B. (2001) Hyperpolarization-activated channels HCN1 and HCN4 mediate responses to sour stimuli. Nature413 (6856): 631-5. [PMID:11675786]

33. Stieber J, Herrmann S, Feil S, Löster J, Feil R, Biel M, Hofmann F, Ludwig A. (2003) The hyperpolarization-activated channel HCN4 is required for the generation of pacemaker action potentials in the embryonic heart. Proc. Natl. Acad. Sci. U.S.A.100 (25): 15235-40. [PMID:14657344]

34. Stieber J, Stöckl G, Herrmann S, Hassfurth B, Hofmann F. (2005) Functional expression of the human HCN3 channel. J. Biol. Chem.280 (41): 34635-43. [PMID:16043489]

35. Stieber J, Wieland K, Stöckl G, Ludwig A, Hofmann F. (2006) Bradycardic and proarrhythmic properties of sinus node inhibitors. Mol. Pharmacol.69 (4): 1328-37. [PMID:16387796]

36. Ueda K, Nakamura K, Hayashi T, Inagaki N, Takahashi M, Arimura T, Morita H, Higashiuesato Y, Hirano Y, Yasunami M, Takishita S, Yamashina A, Ohe T, Sunamori M, Hiraoka M, Kimura A. (2004) Functional characterization of a trafficking-defective HCN4 mutation, D553N, associated with cardiac arrhythmia. J. Biol. Chem.279 (26): 27194-8. [PMID:15123648]

37. Xu X, Marni F, Wu S, Su Z, Musayev F, Shrestha S, Xie C, Gao W, Liu Q, Zhou L. (2012) Local and global interpretations of a disease-causing mutation near the ligand entry path in hyperpolarization-activated cAMP-gated channel. Structure20 (12): 2116-23. [PMID:23103389]

38. Yu X, Duan KL, Shang CF, Yu HG, Zhou Z. (2004) Calcium influx through hyperpolarization-activated cation channels (I(h) channels) contributes to activity-evoked neuronal secretion. Proc. Natl. Acad. Sci. U.S.A.101 (4): 1051-6. [PMID:14724293]

39. Zolles G, Wenzel D, Bildl W, Schulte U, Hofmann A, Müller CS, Thumfart JO, Vlachos A, Deller T, Pfeifer A et al.. (2009) Association with the auxiliary subunit PEX5R/Trip8b controls responsiveness of HCN channels to cAMP and adrenergic stimulation. Neuron62 (6): 814-25. [PMID:19555650]

40. Zong X, Krause S, Chen CC, Krüger J, Gruner C, Cao-Ehlker X, Fenske S, Wahl-Schott C, Biel M. (2012) Regulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel activity by cCMP. J. Biol. Chem.287 (32): 26506-12. [PMID:22715094]

How to cite this page

Verena Hammelmann, Martin Biel.
Cyclic nucleotide-regulated channels: HCN4. Last modified on 27/11/2013. Accessed on 01/11/2014. IUPHAR/BPS Guide to PHARMACOLOGY, http://www.guidetopharmacology.org/GRAC/ObjectDisplayForward?objectId=403.