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Current-dependent block of rabbit sino-atrial node I(f) channels by ivabradine.

Bucchi A, Baruscotti M, DiFrancesco D - J. Gen. Physiol. (2002)

Bottom Line: In this, the action of ivabradine on f-channels is similar to that reported of other rate-reducing agents such as UL-FS49 and ZD7288.Bound drug molecules do not detach from the binding site in the absence of inward current through channels, even if channels are open and the drug is therefore not "trapped" by closed gates.The use-dependence resulting from specific features of I(f) block by ivabradine amplifies its rate-reducing ability at high spontaneous rates and may be useful to clinical applications.

View Article: PubMed Central - PubMed

Affiliation: Department of General Physiology and Biochemistry, Laboratory of Molecular Physiology and Neurobiology, and INFM-Unità Milano Università, 20133 Milano, Italy.

ABSTRACT
"Funny" (f-) channels have a key role in generation of spontaneous activity of pacemaker cells and mediate autonomic control of cardiac rate; f-channels and the related neuronal h-channels are composed of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel subunits. We have investigated the block of f-channels of rabbit cardiac sino-atrial node cells by ivabradine, a novel heart rate-reducing agent. Ivabradine is an open-channel blocker; however, block is exerted preferentially when channels deactivate on depolarization, and is relieved by long hyperpolarizing steps. These features give rise to use-dependent behavior. In this, the action of ivabradine on f-channels is similar to that reported of other rate-reducing agents such as UL-FS49 and ZD7288. However, other features of ivabradine-induced block are peculiar and do not comply with the hypothesis that the voltage-dependence of block is entirely attributable to either the sensitivity of ivabradine-charged molecules to the electrical field in the channel pore, or to differential affinity to different channel states, as has been proposed for UL-FS49 (DiFrancesco, D. 1994. Pflugers Arch. 427:64-70) and ZD7288 (Shin, S.K., B.S. Rotheberg, and G. Yellen. 2001. J. Gen. Physiol. 117:91-101), respectively. Experiments where current flows through channels is modified without changing membrane voltage reveal that the ivabradine block depends on the current driving force, rather than voltage alone, a feature typical of block induced in inwardly rectifying K(+) channels by intracellular cations. Bound drug molecules do not detach from the binding site in the absence of inward current through channels, even if channels are open and the drug is therefore not "trapped" by closed gates. Our data suggest that permeation through f-channel pores occurs according to a multiion, single-file mechanism, and that block/unblock by ivabradine is coupled to ionic flow. The use-dependence resulting from specific features of I(f) block by ivabradine amplifies its rate-reducing ability at high spontaneous rates and may be useful to clinical applications.

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Ivabradine blocks If. (A) Structure of ivabradine. (B) Activating/deactivating steps (−100 mV*1.8 s/+5 mV*0.45 s) were applied every 6 s from a holding potential of −35 mV, and a given concentration of ivabradine perfused until full block developed. The time-course of If amplitude at −100 mV during block onset and removal is shown for three cells challenged with 0.3 (left), 3 (middle) and 30 μM ivabradine (right). Lower panels show current traces recorded just before and during block development (a to c). Zero current level drawn as a full line. C: dose–response relationship of If block by ivabradine from a total of n = 32 cells (mean ± SEM). Each cell was exposed to one drug dose only. Mean data points were fitted to the Hill equation y = 1/(1 + (IC50/x)h) where x is drug concentration, IC50 the half-block concentration and h the Hill factor (full line: best fitting values in text).
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fig1: Ivabradine blocks If. (A) Structure of ivabradine. (B) Activating/deactivating steps (−100 mV*1.8 s/+5 mV*0.45 s) were applied every 6 s from a holding potential of −35 mV, and a given concentration of ivabradine perfused until full block developed. The time-course of If amplitude at −100 mV during block onset and removal is shown for three cells challenged with 0.3 (left), 3 (middle) and 30 μM ivabradine (right). Lower panels show current traces recorded just before and during block development (a to c). Zero current level drawn as a full line. C: dose–response relationship of If block by ivabradine from a total of n = 32 cells (mean ± SEM). Each cell was exposed to one drug dose only. Mean data points were fitted to the Hill equation y = 1/(1 + (IC50/x)h) where x is drug concentration, IC50 the half-block concentration and h the Hill factor (full line: best fitting values in text).

Mentions: Whole-cell pipettes were filled with an intracellular-like solution containing (in mM): K-Aspartate, 130; NaCl, 10; EGTA-KOH, 5; CaCl2, 2; MgCl2, 2; ATP (Na-salt), 2; creatine phosphate, 5; GTP (Na-salt), 0.1; pH 7.2. In low (35 mM) Na+ solution, Na+ was replaced by an equimolar amount of choline chloride. Ivabradine (3-(3-{[((7S)-3,4-dimethoxybicyclo [4,2,0] octa-1,3,5-trien7-yl) methyl] methylamino} propyl)-1,3,4,5-tetrahydro-7,8-dimethoxy-2H-3-benzazepin-2-one hydrochloride; Fig. 1 A) was added to the extracellular solution by dissolving a stock solution (0.1–10 mM) to the final concentration desired. The drug was provided by the Institut de Recherches Internationales Servier, France. All experiments were performed at the controlled temperature of 32 ± 0.5°C. Currents were recorded and on-line filtered at a corner frequency of 1 KHz with an Axopatch 200B amplifier, and acquired using the pClamp 7.0 software (Axon Instruments, Inc.).


Current-dependent block of rabbit sino-atrial node I(f) channels by ivabradine.

Bucchi A, Baruscotti M, DiFrancesco D - J. Gen. Physiol. (2002)

Ivabradine blocks If. (A) Structure of ivabradine. (B) Activating/deactivating steps (−100 mV*1.8 s/+5 mV*0.45 s) were applied every 6 s from a holding potential of −35 mV, and a given concentration of ivabradine perfused until full block developed. The time-course of If amplitude at −100 mV during block onset and removal is shown for three cells challenged with 0.3 (left), 3 (middle) and 30 μM ivabradine (right). Lower panels show current traces recorded just before and during block development (a to c). Zero current level drawn as a full line. C: dose–response relationship of If block by ivabradine from a total of n = 32 cells (mean ± SEM). Each cell was exposed to one drug dose only. Mean data points were fitted to the Hill equation y = 1/(1 + (IC50/x)h) where x is drug concentration, IC50 the half-block concentration and h the Hill factor (full line: best fitting values in text).
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2238187&req=5

fig1: Ivabradine blocks If. (A) Structure of ivabradine. (B) Activating/deactivating steps (−100 mV*1.8 s/+5 mV*0.45 s) were applied every 6 s from a holding potential of −35 mV, and a given concentration of ivabradine perfused until full block developed. The time-course of If amplitude at −100 mV during block onset and removal is shown for three cells challenged with 0.3 (left), 3 (middle) and 30 μM ivabradine (right). Lower panels show current traces recorded just before and during block development (a to c). Zero current level drawn as a full line. C: dose–response relationship of If block by ivabradine from a total of n = 32 cells (mean ± SEM). Each cell was exposed to one drug dose only. Mean data points were fitted to the Hill equation y = 1/(1 + (IC50/x)h) where x is drug concentration, IC50 the half-block concentration and h the Hill factor (full line: best fitting values in text).
Mentions: Whole-cell pipettes were filled with an intracellular-like solution containing (in mM): K-Aspartate, 130; NaCl, 10; EGTA-KOH, 5; CaCl2, 2; MgCl2, 2; ATP (Na-salt), 2; creatine phosphate, 5; GTP (Na-salt), 0.1; pH 7.2. In low (35 mM) Na+ solution, Na+ was replaced by an equimolar amount of choline chloride. Ivabradine (3-(3-{[((7S)-3,4-dimethoxybicyclo [4,2,0] octa-1,3,5-trien7-yl) methyl] methylamino} propyl)-1,3,4,5-tetrahydro-7,8-dimethoxy-2H-3-benzazepin-2-one hydrochloride; Fig. 1 A) was added to the extracellular solution by dissolving a stock solution (0.1–10 mM) to the final concentration desired. The drug was provided by the Institut de Recherches Internationales Servier, France. All experiments were performed at the controlled temperature of 32 ± 0.5°C. Currents were recorded and on-line filtered at a corner frequency of 1 KHz with an Axopatch 200B amplifier, and acquired using the pClamp 7.0 software (Axon Instruments, Inc.).

Bottom Line: In this, the action of ivabradine on f-channels is similar to that reported of other rate-reducing agents such as UL-FS49 and ZD7288.Bound drug molecules do not detach from the binding site in the absence of inward current through channels, even if channels are open and the drug is therefore not "trapped" by closed gates.The use-dependence resulting from specific features of I(f) block by ivabradine amplifies its rate-reducing ability at high spontaneous rates and may be useful to clinical applications.

View Article: PubMed Central - PubMed

Affiliation: Department of General Physiology and Biochemistry, Laboratory of Molecular Physiology and Neurobiology, and INFM-Unità Milano Università, 20133 Milano, Italy.

ABSTRACT
"Funny" (f-) channels have a key role in generation of spontaneous activity of pacemaker cells and mediate autonomic control of cardiac rate; f-channels and the related neuronal h-channels are composed of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel subunits. We have investigated the block of f-channels of rabbit cardiac sino-atrial node cells by ivabradine, a novel heart rate-reducing agent. Ivabradine is an open-channel blocker; however, block is exerted preferentially when channels deactivate on depolarization, and is relieved by long hyperpolarizing steps. These features give rise to use-dependent behavior. In this, the action of ivabradine on f-channels is similar to that reported of other rate-reducing agents such as UL-FS49 and ZD7288. However, other features of ivabradine-induced block are peculiar and do not comply with the hypothesis that the voltage-dependence of block is entirely attributable to either the sensitivity of ivabradine-charged molecules to the electrical field in the channel pore, or to differential affinity to different channel states, as has been proposed for UL-FS49 (DiFrancesco, D. 1994. Pflugers Arch. 427:64-70) and ZD7288 (Shin, S.K., B.S. Rotheberg, and G. Yellen. 2001. J. Gen. Physiol. 117:91-101), respectively. Experiments where current flows through channels is modified without changing membrane voltage reveal that the ivabradine block depends on the current driving force, rather than voltage alone, a feature typical of block induced in inwardly rectifying K(+) channels by intracellular cations. Bound drug molecules do not detach from the binding site in the absence of inward current through channels, even if channels are open and the drug is therefore not "trapped" by closed gates. Our data suggest that permeation through f-channel pores occurs according to a multiion, single-file mechanism, and that block/unblock by ivabradine is coupled to ionic flow. The use-dependence resulting from specific features of I(f) block by ivabradine amplifies its rate-reducing ability at high spontaneous rates and may be useful to clinical applications.

Show MeSH
Related in: MedlinePlus