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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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Dependence of the ivabradine-induced If block by the current driving force. (A) Sample If traces from two cells showing the different degrees of steady-state block induced by 3 μM ivabradine at the same test potential of −30 mV with normal (140 mM, left) and reduced (35 mM, right) external Na+ concentration, as measured by activation/deactivation protocols (−100/−30 mV). Fractional block was ∼24% in normal and 54% in reduced Na+ concentration. Notice that at −30 mV, as expected, the deactivating If tail was inward in normal Na+, and outward in reduced Na+ conditions. (B) Comparison between mean fractional block curve in normal Tyrode solution (filled circles, as from Fig. 5) and in lowered Na+ (open circles). Each point of the curve in low Na+ represents the mean ± SEM from 3–6 cells. Vertical dotted lines correspond to the If reversal potentials measured from mean fully activated I/V relations from n = 7 cells in the two conditions (Ef = −16.0 mV in normal Tyrode and Ef = −34.4 mV in 35 mM Na+ as indicated). Arrows show the intercepts of the block curves with corresponding Ef values.
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fig7: Dependence of the ivabradine-induced If block by the current driving force. (A) Sample If traces from two cells showing the different degrees of steady-state block induced by 3 μM ivabradine at the same test potential of −30 mV with normal (140 mM, left) and reduced (35 mM, right) external Na+ concentration, as measured by activation/deactivation protocols (−100/−30 mV). Fractional block was ∼24% in normal and 54% in reduced Na+ concentration. Notice that at −30 mV, as expected, the deactivating If tail was inward in normal Na+, and outward in reduced Na+ conditions. (B) Comparison between mean fractional block curve in normal Tyrode solution (filled circles, as from Fig. 5) and in lowered Na+ (open circles). Each point of the curve in low Na+ represents the mean ± SEM from 3–6 cells. Vertical dotted lines correspond to the If reversal potentials measured from mean fully activated I/V relations from n = 7 cells in the two conditions (Ef = −16.0 mV in normal Tyrode and Ef = −34.4 mV in 35 mM Na+ as indicated). Arrows show the intercepts of the block curves with corresponding Ef values.

Mentions: In Fig. 7, the fractional block curve measured in the control Tyrode solution, as replotted from Fig. 5 (filled circles), is compared with that obtained with similar protocols in a low (35 mM) Na+ solution (open circles). The low Na+ curve still displays a region of steep slope and has an overall voltage-dependence similar to the curve in Tyrode, but is shifted to more negative voltages. The shift is such as to determine a large difference of blocking degrees between the two curves at intermediate voltages. For example, at −30 mV, the fractional block was 0.266 ± 0.014 in normal Na+, and 0.607 ± 0.016 in the low Na+ solution (see inset current traces).


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

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

Dependence of the ivabradine-induced If block by the current driving force. (A) Sample If traces from two cells showing the different degrees of steady-state block induced by 3 μM ivabradine at the same test potential of −30 mV with normal (140 mM, left) and reduced (35 mM, right) external Na+ concentration, as measured by activation/deactivation protocols (−100/−30 mV). Fractional block was ∼24% in normal and 54% in reduced Na+ concentration. Notice that at −30 mV, as expected, the deactivating If tail was inward in normal Na+, and outward in reduced Na+ conditions. (B) Comparison between mean fractional block curve in normal Tyrode solution (filled circles, as from Fig. 5) and in lowered Na+ (open circles). Each point of the curve in low Na+ represents the mean ± SEM from 3–6 cells. Vertical dotted lines correspond to the If reversal potentials measured from mean fully activated I/V relations from n = 7 cells in the two conditions (Ef = −16.0 mV in normal Tyrode and Ef = −34.4 mV in 35 mM Na+ as indicated). Arrows show the intercepts of the block curves with corresponding Ef values.
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Related In: Results  -  Collection

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fig7: Dependence of the ivabradine-induced If block by the current driving force. (A) Sample If traces from two cells showing the different degrees of steady-state block induced by 3 μM ivabradine at the same test potential of −30 mV with normal (140 mM, left) and reduced (35 mM, right) external Na+ concentration, as measured by activation/deactivation protocols (−100/−30 mV). Fractional block was ∼24% in normal and 54% in reduced Na+ concentration. Notice that at −30 mV, as expected, the deactivating If tail was inward in normal Na+, and outward in reduced Na+ conditions. (B) Comparison between mean fractional block curve in normal Tyrode solution (filled circles, as from Fig. 5) and in lowered Na+ (open circles). Each point of the curve in low Na+ represents the mean ± SEM from 3–6 cells. Vertical dotted lines correspond to the If reversal potentials measured from mean fully activated I/V relations from n = 7 cells in the two conditions (Ef = −16.0 mV in normal Tyrode and Ef = −34.4 mV in 35 mM Na+ as indicated). Arrows show the intercepts of the block curves with corresponding Ef values.
Mentions: In Fig. 7, the fractional block curve measured in the control Tyrode solution, as replotted from Fig. 5 (filled circles), is compared with that obtained with similar protocols in a low (35 mM) Na+ solution (open circles). The low Na+ curve still displays a region of steep slope and has an overall voltage-dependence similar to the curve in Tyrode, but is shifted to more negative voltages. The shift is such as to determine a large difference of blocking degrees between the two curves at intermediate voltages. For example, at −30 mV, the fractional block was 0.266 ± 0.014 in normal Na+, and 0.607 ± 0.016 in the low Na+ solution (see inset current traces).

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