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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 causes inward rectification of If, which depends on E − Ef. Top: mean fully activated I/V relations measured from n = 7 cells exposed to both normal Tyrode solution (filled circles) and reduced (35 mM) Na+ concentration (open circles). I/V relations were measured as described previously (DiFrancesco et al., 1986), by applying pairs of steps, one to fully activate (1 s to −125 mV) and one to fully deactivate If (1 s to 15 mV), each followed by a step to the same test voltage, where the amplitude of the different current was measured. Mean ± SEM values are plotted. Linear fitting (straight lines) yielded reversal potentials (Ef) of −16.0 and −34.4 mV for normal and reduced Na+ concentration, respectively. Bottom: same curves, multiplied by fractional block values in normal and low Na+ solution as deduced from data in Fig. 7 at corresponding voltage and Na+ concentration. Strong rectification is apparent in the outward part of both curves, independently of the reversal potential. Lines through points.
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fig8: Ivabradine causes inward rectification of If, which depends on E − Ef. Top: mean fully activated I/V relations measured from n = 7 cells exposed to both normal Tyrode solution (filled circles) and reduced (35 mM) Na+ concentration (open circles). I/V relations were measured as described previously (DiFrancesco et al., 1986), by applying pairs of steps, one to fully activate (1 s to −125 mV) and one to fully deactivate If (1 s to 15 mV), each followed by a step to the same test voltage, where the amplitude of the different current was measured. Mean ± SEM values are plotted. Linear fitting (straight lines) yielded reversal potentials (Ef) of −16.0 and −34.4 mV for normal and reduced Na+ concentration, respectively. Bottom: same curves, multiplied by fractional block values in normal and low Na+ solution as deduced from data in Fig. 7 at corresponding voltage and Na+ concentration. Strong rectification is apparent in the outward part of both curves, independently of the reversal potential. Lines through points.

Mentions: An interesting feature in Fig. 7 is that in both curves, the region of steepest slope was located across the expected If reversal potential (Ef). In n = 7 cells, we measured fully activated I/V relations in normal and reduced external Na+ according to standard protocols (DiFrancesco et al., 1986); Ef obtained from linear fitting of mean I/V curves (see Fig. 8) was −16.0 mV in normal Tyrode solution and −34.4 mV in low Na+ solution, with a shift of 18.4 mV. These values are indicated by dotted lines in Fig. 7 B, and clearly intercept the I/V curves in their regions of steepest slope.


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

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

Ivabradine causes inward rectification of If, which depends on E − Ef. Top: mean fully activated I/V relations measured from n = 7 cells exposed to both normal Tyrode solution (filled circles) and reduced (35 mM) Na+ concentration (open circles). I/V relations were measured as described previously (DiFrancesco et al., 1986), by applying pairs of steps, one to fully activate (1 s to −125 mV) and one to fully deactivate If (1 s to 15 mV), each followed by a step to the same test voltage, where the amplitude of the different current was measured. Mean ± SEM values are plotted. Linear fitting (straight lines) yielded reversal potentials (Ef) of −16.0 and −34.4 mV for normal and reduced Na+ concentration, respectively. Bottom: same curves, multiplied by fractional block values in normal and low Na+ solution as deduced from data in Fig. 7 at corresponding voltage and Na+ concentration. Strong rectification is apparent in the outward part of both curves, independently of the reversal potential. Lines through points.
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Related In: Results  -  Collection

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fig8: Ivabradine causes inward rectification of If, which depends on E − Ef. Top: mean fully activated I/V relations measured from n = 7 cells exposed to both normal Tyrode solution (filled circles) and reduced (35 mM) Na+ concentration (open circles). I/V relations were measured as described previously (DiFrancesco et al., 1986), by applying pairs of steps, one to fully activate (1 s to −125 mV) and one to fully deactivate If (1 s to 15 mV), each followed by a step to the same test voltage, where the amplitude of the different current was measured. Mean ± SEM values are plotted. Linear fitting (straight lines) yielded reversal potentials (Ef) of −16.0 and −34.4 mV for normal and reduced Na+ concentration, respectively. Bottom: same curves, multiplied by fractional block values in normal and low Na+ solution as deduced from data in Fig. 7 at corresponding voltage and Na+ concentration. Strong rectification is apparent in the outward part of both curves, independently of the reversal potential. Lines through points.
Mentions: An interesting feature in Fig. 7 is that in both curves, the region of steepest slope was located across the expected If reversal potential (Ef). In n = 7 cells, we measured fully activated I/V relations in normal and reduced external Na+ according to standard protocols (DiFrancesco et al., 1986); Ef obtained from linear fitting of mean I/V curves (see Fig. 8) was −16.0 mV in normal Tyrode solution and −34.4 mV in low Na+ solution, with a shift of 18.4 mV. These values are indicated by dotted lines in Fig. 7 B, and clearly intercept the I/V curves in their regions of steepest slope.

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