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Funny current (or funny channel, or If, or IKf, or pacemaker current) refers to a specific current in the heart.

First described in the late 70s in Purkinje fibers and sinoatrial myocytes, the cardiac pacemaker “funny” (If) current has been extensively characterized and its role in cardiac pacemaking has been thoroughly investigated.[1][2][3]

The "funny", or "pacemaker" current is highly expressed in spontaneously active cardiac regions, such as the sinoatrial node (SAN, the natural pacemaker region), the atrio-ventricular node (AVN) and the Purkinje fibres of conduction tissue. Particularly unusual, the "funny" current is a mixed sodium-potassium current, inward and slowly activating on hyperpolarization at voltages in the diastolic range (normally from -60/-70 mV to -40 mV). When at the end of a sinoatrial action potential the membrane repolarizes below the If threshold (about -40/-50 mV), the "funny" current is activated and supplies inward current, which is responsible for starting the diastolic depolarization phase (DD); by this mechanism, the "funny" current controls the rate of spontaneous activity of sinoatrial myocytes, hence the cardiac rate.

Another unusual feature of If is its dual activation by voltage and by cyclic nucleotides. Cyclic adenosine monophosphate (cAMP) molecules bind directly to f-channels and increase their open probability[4]. cAMP dependence is a particularly relevant physiological property, since it underlies the If –dependent autonomic regulation of heart rate. Sympathetic stimulation raises the level of cAMP molecules which bind to f-channels and shift the If activation range to more positive voltages; this mechanism leads to an increase of the current at diastolic voltages and therefore to an increase of the steepness of DD and heart rate acceleration. Parasympathetic stimulation (which reduces cAMP) decreases the heart rate by the opposite action, that is by shifting the If activation curve towards more negative voltages (Fig 1).


A similar current, termed Ih, has also been described in different types of neurons where it has a variety of functions, including the contribution to control of rhythmic firing, regulation of neuronal excitability, sensory transduction, synaptic plasticity and more.

The molecular determinants of the pacemaker current belong to the Hyperpolarization-activated Cyclic Nucleotide-gated channels family (HCN, see HCN channel) of which 4 isoforms (HCN1-4) are known. Based on their sequence, HCN channels are classified as members of the superfamily of voltage-gated K+ (Kv) and CNG channels.[3][5]

Contents

Clinical significance

Ivabradine

Because of their relevance to generation of pacemaker activity and modulation of spontaneous frequency, f-channels are natural targets of drugs aimed to pharmacologically control heart rate. Several agents, called “heart rate reducing agents” act by specifically inhibiting f-channel function.[3] Ivabradine is the most specific and selective If inhibitor and the only member of this family that is now marketed for pharmacological treatment of chronic stable angina in patients with normal sinus rhythm who have a contraindication or intolerance to beta-blockers. Recent studies have also indicated that "funny" channel inhibition can be used to reduce the incidence of coronary artery disease outcomes in a subgroup of patients with heart rate ≥70 bpm[6][7].

Cardiovascular diseases represent a major cause of worldwide mortality, and the relevance of the genetic component in these diseases has recently become more apparent. Genetic alterations of HCN4 channels (the molecular correlate of sinoatrial f-channels) coupled to rhythm disturbances have been reported in humans. For example an inherited mutation of a highly conserved residue in the CNBD of the HCN4 protein (S672R) is associated with inherited sinus bradycardia.[8] In vitro studies indicate that the S672R mutation causes a hyperpolarizing shift of the HCN4 channel open probability curve of about 5 mV in heterozygosis, an effect similar to the hyperpolarizing shift caused by parasympathetic stimulation and able to explain a reduction of inward current during diastole and the resulting slower spontaneous rate.

Biological pacemakers, generally intended as cell substrates able to induce spontaneous activity in silent tissue, represent a potential tool to overcome the limitations of electronic pacemakers. One of the strategies used to generate biological pacemakers involves the use of cells inherently expressing or engineered to express funny channels. Different types of stem cells can be used for this purpose[5].

See also

References

  1. ^ Brown, H. F., DiFrancesco, D., Noble, S. J. (1979). How does adrenaline accelerate the heart? Nature, 280, 235-236.
  2. ^ DiFrancesco, D., Ojeda, C. (1980). Properties of the current if in the sino-atrial node of the rabbit compared with those of the current iK, in Purkinje fibres. Journal of Physiology, 308, 353-367.
  3. ^ a b c Baruscotti, M., Bucchi, A., DiFrancesco, D. (2005). Physiology and pharmacology of the cardiac pacemaker ("funny") current. Pharmacology & Therapeutics, 107, 59-79.
  4. ^ DiFrancesco, D., Tortora, P. (1991). Direct activation of cardiac pacemaker channels by intracellular cyclic AMP. Nature, 351, 145-147.
  5. ^ a b Barbuti, A., Baruscotti, M., DiFrancesco, D. (2007). The pacemaker current: from basics to the clinics. Journal of Cardiovascular Electrophysiology, 18, 342-347
  6. ^ Fox K, Ford I, Steg PG, Tendera M, Ferrari R; BEAUTIFUL Investigators (2008)Ivabradine for patients with stable coronary artery disease and left-ventricular systolic dysfunction (BEAUTIFUL): a randomised, double-blind, placebo-controlled trial. Lancet. 372(9641):807-16
  7. ^ Beautiful Study Group, Ferrari R, Ford I, Fox K, Steg PG, Tendera M.(2008)The BEAUTIFUL study: randomized trial of ivabradine in patients with stable coronary artery disease and left ventricular systolic dysfunction – baseline characteristics of the study population. Cardiology.110(4):271-82
  8. ^ Milanesi, R., Baruscotti, M., Gnecchi-Ruscone, T., DiFrancesco, D. (2006). Familial sinus bradycardia associated with a mutation in the cardiac pacemaker channel. The New England Journal of Medicine, 354, 151-157.

External links

  • DiFrancesco D, Tortora P. (1991). "Direct activation of cardiac pacemaker channels by intracellular cyclic AMP.". Nature. 351: 145–7. doi:10.1038/351145a0. PMID 1709448.  
  • DiFrancesco D (1986). "Characterization of single pacemaker channels in cardiac sino-atrial node cells". Nature. 324: 470–3. doi:10.1038/324470a0. PMID 2431323.  
  • 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): 151–157. doi:10.1056/NEJMoa052475. PMID 16407510.  
  • Barbuti A, Crespi A, Capilupo D, Mazzocchi N, Baruscotti M, DiFrancesco D. (2008). "Molecular composition and functional properties of f-channels in murine embryonic stem cell-derived pacemaker cells.". J Mol Cell Cardiol 46 (3): 343–51. doi:10.1016/j.yjmcc.2008.12.001. PMID 19135060.  
  • Barbuti A, Crespi A, Capilupo D, Mazzocchi N, Baruscotti M, DiFrancesco D. (2008). "Molecular composition and functional properties of f-channels in murine embryonic stem cell-derived pacemaker cells.". J Mol Cell Cardiol 46 (3): 343–51. doi:10.1016/j.yjmcc.2008.12.001. PMID 19135060.  
  • Bucchi A, Baruscotti M, DiFrancesco D (2002). "Current-dependent block of rabbit sino-atrial node I(f) channels by ivabradine.". J Gen Physiol 120 (1): 1–13. doi:10.1085/jgp.20028593. PMID 12084770.  
  • Accili E, Proenza C, Baruscotti M, DiFrancesco D. "From funny current to HCN channels: 20 years of excitation.". News Physiol Sci 17: 32–7. PMID 11821534.  

Given the above, do we have readily reproduced, clinically relevant evidence of a diastolic EKG? The encyclopedically agreed upon PQRS segments readily defines electrical systole. Does the remaining time interval (S-T) define electrical diastole? How can first order (sympathetic) depolarization be dampened to better illuminate a robust (parasympathetic) electrical diastole?

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