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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 and block removal require open channels. (A) Time-course of If amplitude at −100 mV during an activation/deactivation protocol (−100/+5 mV, 1/6 Hz) from a holding potential of −35 mV. At the beginning of the perfusion with ivabradine (3 μM), the protocol was interrupted for 100 s while the cell was held at the holding potential. During this time no current reduction was observed. (B) In another cell, the same protocol was applied in the presence of the drug (3 μM) until full block developed. The protocol was interrupted and the cell held at −35 mV for 90 s while simultaneously removing the drug from the perfusing solution. During this time, no reversal of current inhibition occurred. Sample traces shown in the bottom panels were recorded at various times as indicated.
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fig2: If block by ivabradine and block removal require open channels. (A) Time-course of If amplitude at −100 mV during an activation/deactivation protocol (−100/+5 mV, 1/6 Hz) from a holding potential of −35 mV. At the beginning of the perfusion with ivabradine (3 μM), the protocol was interrupted for 100 s while the cell was held at the holding potential. During this time no current reduction was observed. (B) In another cell, the same protocol was applied in the presence of the drug (3 μM) until full block developed. The protocol was interrupted and the cell held at −35 mV for 90 s while simultaneously removing the drug from the perfusing solution. During this time, no reversal of current inhibition occurred. Sample traces shown in the bottom panels were recorded at various times as indicated.

Mentions: Experiments such as those in Fig. 1 indicate the presence of accumulation of inhibition during repetitive steps, a property that could reflect either relatively slow kinetics of drug-channel interaction, or that the f-channel block by ivabradine is use-dependent. To further investigate this aspect, in Fig. 2 we tried to establish whether the drug binds to f-channel closed and open configurations with different affinities. An If activation/deactivation protocol (same as in Fig. 1) was first applied; when perfusion with ivabradine started, the repetitive protocol was interrupted and the membrane voltage held for 100 s at −35 mV, where f-channels are closed.


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 and block removal require open channels. (A) Time-course of If amplitude at −100 mV during an activation/deactivation protocol (−100/+5 mV, 1/6 Hz) from a holding potential of −35 mV. At the beginning of the perfusion with ivabradine (3 μM), the protocol was interrupted for 100 s while the cell was held at the holding potential. During this time no current reduction was observed. (B) In another cell, the same protocol was applied in the presence of the drug (3 μM) until full block developed. The protocol was interrupted and the cell held at −35 mV for 90 s while simultaneously removing the drug from the perfusing solution. During this time, no reversal of current inhibition occurred. Sample traces shown in the bottom panels were recorded at various times as indicated.
© Copyright Policy
Related In: Results  -  Collection

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

fig2: If block by ivabradine and block removal require open channels. (A) Time-course of If amplitude at −100 mV during an activation/deactivation protocol (−100/+5 mV, 1/6 Hz) from a holding potential of −35 mV. At the beginning of the perfusion with ivabradine (3 μM), the protocol was interrupted for 100 s while the cell was held at the holding potential. During this time no current reduction was observed. (B) In another cell, the same protocol was applied in the presence of the drug (3 μM) until full block developed. The protocol was interrupted and the cell held at −35 mV for 90 s while simultaneously removing the drug from the perfusing solution. During this time, no reversal of current inhibition occurred. Sample traces shown in the bottom panels were recorded at various times as indicated.
Mentions: Experiments such as those in Fig. 1 indicate the presence of accumulation of inhibition during repetitive steps, a property that could reflect either relatively slow kinetics of drug-channel interaction, or that the f-channel block by ivabradine is use-dependent. To further investigate this aspect, in Fig. 2 we tried to establish whether the drug binds to f-channel closed and open configurations with different affinities. An If activation/deactivation protocol (same as in Fig. 1) was first applied; when perfusion with ivabradine started, the repetitive protocol was interrupted and the membrane voltage held for 100 s at −35 mV, where f-channels are closed.

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