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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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If block by ivabradine depends on the voltage protocol used to activate the current. (A) If in control (cont) and after full block by ivabradine (3 μM, asterisks) induced by activation/deactivation protocols at −70 mV (top) and −100 mV (bottom) in two cells. (B) Action of the same drug concentration when applied during steady-state If activation at −70 mV (top) or −100 mV (bottom) in two cells.
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fig3: If block by ivabradine depends on the voltage protocol used to activate the current. (A) If in control (cont) and after full block by ivabradine (3 μM, asterisks) induced by activation/deactivation protocols at −70 mV (top) and −100 mV (bottom) in two cells. (B) Action of the same drug concentration when applied during steady-state If activation at −70 mV (top) or −100 mV (bottom) in two cells.

Mentions: To study the voltage dependence of the action of ivabradine on If, in view of the requirement of open channels for block to occur, we first investigated steady-state block at voltages negative to the activation threshold, where f-channels are open at least part of the time, by applying long activating steps. We found that the block exerted by the drug using this protocol was much smaller than that observed with activating/deactivating protocols (as in Figs. 1 and 2) with the same hyperpolarizing step. A comparison between the two different protocols is shown in Fig. 3. On the left (Fig. 3 A), 3 μM ivabradine applied during activation/deactivation protocols (−70/+5 mV, upper; −100/+5 mV, lower) caused a current reduction of 78.3% at −70 mV and 63.9% at −100 mV (compare control records with full-block records labeled by asterisks). These data are in accordance with the results in Fig. 1. Panels on the right (Fig. 3 B), on the other hand, refer to experiments where steps of several tens of seconds were applied to −70 (upper) and −100 mV (lower), and the drug applied after the current had reached steady-state activation. The current decrease with the latter protocol was much smaller (14.0% at −70 mV and 6.0% at −100 mV). The mean If block was 12.4 ± 1.9% at −70 mV (n = 4) and 6.4 ± 1.6% at −100 mV (n = 5) with the long-step protocols, as compared with the values of 77.4 ± 5.7% at −70 mV (n = 4) and 65.9 ± 2.4% at −100 mV (n = 19, as reported above) resulting from activating/deactivating protocols.


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

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

If block by ivabradine depends on the voltage protocol used to activate the current. (A) If in control (cont) and after full block by ivabradine (3 μM, asterisks) induced by activation/deactivation protocols at −70 mV (top) and −100 mV (bottom) in two cells. (B) Action of the same drug concentration when applied during steady-state If activation at −70 mV (top) or −100 mV (bottom) in two cells.
© Copyright Policy
Related In: Results  -  Collection

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

fig3: If block by ivabradine depends on the voltage protocol used to activate the current. (A) If in control (cont) and after full block by ivabradine (3 μM, asterisks) induced by activation/deactivation protocols at −70 mV (top) and −100 mV (bottom) in two cells. (B) Action of the same drug concentration when applied during steady-state If activation at −70 mV (top) or −100 mV (bottom) in two cells.
Mentions: To study the voltage dependence of the action of ivabradine on If, in view of the requirement of open channels for block to occur, we first investigated steady-state block at voltages negative to the activation threshold, where f-channels are open at least part of the time, by applying long activating steps. We found that the block exerted by the drug using this protocol was much smaller than that observed with activating/deactivating protocols (as in Figs. 1 and 2) with the same hyperpolarizing step. A comparison between the two different protocols is shown in Fig. 3. On the left (Fig. 3 A), 3 μM ivabradine applied during activation/deactivation protocols (−70/+5 mV, upper; −100/+5 mV, lower) caused a current reduction of 78.3% at −70 mV and 63.9% at −100 mV (compare control records with full-block records labeled by asterisks). These data are in accordance with the results in Fig. 1. Panels on the right (Fig. 3 B), on the other hand, refer to experiments where steps of several tens of seconds were applied to −70 (upper) and −100 mV (lower), and the drug applied after the current had reached steady-state activation. The current decrease with the latter protocol was much smaller (14.0% at −70 mV and 6.0% at −100 mV). The mean If block was 12.4 ± 1.9% at −70 mV (n = 4) and 6.4 ± 1.6% at −100 mV (n = 5) with the long-step protocols, as compared with the values of 77.4 ± 5.7% at −70 mV (n = 4) and 65.9 ± 2.4% at −100 mV (n = 19, as reported above) resulting from activating/deactivating protocols.

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