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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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Voltage dependence of steady-state f-channel block by 3 μM ivabradine. Measurements were made by two protocols. At voltages more negative than −40 mV (open circles) If was activated by long steps to test potentials and the drug perfused after steady-state activation had been reached; fractional block was measured as the ratio between blocked and control current amplitude. At voltages equal or more positive than −40 mV (filled circles), the membrane was held at the test voltage and a fixed activating voltage step (−100 mV*1.2 s) was applied repetitively (1/6 Hz); the drug was perfused during this protocol and fractional block measured for each test voltage as the ratio between blocked and control current at −100 mV at steady-state. Each point represents the mean ± SEM from 3–5 cells.
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fig5: Voltage dependence of steady-state f-channel block by 3 μM ivabradine. Measurements were made by two protocols. At voltages more negative than −40 mV (open circles) If was activated by long steps to test potentials and the drug perfused after steady-state activation had been reached; fractional block was measured as the ratio between blocked and control current amplitude. At voltages equal or more positive than −40 mV (filled circles), the membrane was held at the test voltage and a fixed activating voltage step (−100 mV*1.2 s) was applied repetitively (1/6 Hz); the drug was perfused during this protocol and fractional block measured for each test voltage as the ratio between blocked and control current at −100 mV at steady-state. Each point represents the mean ± SEM from 3–5 cells.

Mentions: We next proceeded to quantify the degree of block in a fuller range of activation/deactivation voltages by measuring the current decrease caused by 3 μM ivabradine at steady-state in the range −110 to 20 mV. At voltages below threshold (more negative than −40 mV), we applied long steps to the test voltage to activate the current at steady-state and then applied the drug until block developed fully, as in Fig. 3 B; in Fig. 5, mean fractional block values thus obtained are plotted as open circles.


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

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

Voltage dependence of steady-state f-channel block by 3 μM ivabradine. Measurements were made by two protocols. At voltages more negative than −40 mV (open circles) If was activated by long steps to test potentials and the drug perfused after steady-state activation had been reached; fractional block was measured as the ratio between blocked and control current amplitude. At voltages equal or more positive than −40 mV (filled circles), the membrane was held at the test voltage and a fixed activating voltage step (−100 mV*1.2 s) was applied repetitively (1/6 Hz); the drug was perfused during this protocol and fractional block measured for each test voltage as the ratio between blocked and control current at −100 mV at steady-state. Each point represents the mean ± SEM from 3–5 cells.
© Copyright Policy
Related In: Results  -  Collection

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

fig5: Voltage dependence of steady-state f-channel block by 3 μM ivabradine. Measurements were made by two protocols. At voltages more negative than −40 mV (open circles) If was activated by long steps to test potentials and the drug perfused after steady-state activation had been reached; fractional block was measured as the ratio between blocked and control current amplitude. At voltages equal or more positive than −40 mV (filled circles), the membrane was held at the test voltage and a fixed activating voltage step (−100 mV*1.2 s) was applied repetitively (1/6 Hz); the drug was perfused during this protocol and fractional block measured for each test voltage as the ratio between blocked and control current at −100 mV at steady-state. Each point represents the mean ± SEM from 3–5 cells.
Mentions: We next proceeded to quantify the degree of block in a fuller range of activation/deactivation voltages by measuring the current decrease caused by 3 μM ivabradine at steady-state in the range −110 to 20 mV. At voltages below threshold (more negative than −40 mV), we applied long steps to the test voltage to activate the current at steady-state and then applied the drug until block developed fully, as in Fig. 3 B; in Fig. 5, mean fractional block values thus obtained are plotted as open circles.

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