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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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Inward current is required to relieve channel block by ivabradine. If block by 3 μM ivabradine was induced by a standard activation/deactivation protocol (−100/+5 mV); sample traces are shown on the left (cont, control; a, 30 s; and b, 120 s after drug perfusion; the latter corresponding to steady-state block). In the continuous presence of the drug, a prolonged (45 s) step to −100 mV was then applied while simultaneously adding 5 mM Cs+ (c); Cs+ was washed off at the end of the long step and the repetitive pulsing protocol resumed (traces d, 6 s and e, 36 s after Cs+ wash-out). Note that no block removal occurred during the prolonged hyperpolarization (compare records b and d in the inset).
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fig6: Inward current is required to relieve channel block by ivabradine. If block by 3 μM ivabradine was induced by a standard activation/deactivation protocol (−100/+5 mV); sample traces are shown on the left (cont, control; a, 30 s; and b, 120 s after drug perfusion; the latter corresponding to steady-state block). In the continuous presence of the drug, a prolonged (45 s) step to −100 mV was then applied while simultaneously adding 5 mM Cs+ (c); Cs+ was washed off at the end of the long step and the repetitive pulsing protocol resumed (traces d, 6 s and e, 36 s after Cs+ wash-out). Note that no block removal occurred during the prolonged hyperpolarization (compare records b and d in the inset).

Mentions: In Fig. 6, we applied a protocol similar to that in Fig. 4, but added 5 mM Cs+ to the perfusate during the prolonged (45 s) hyperpolarization to −100 mV, following steady-state block by 3 μM ivabradine achieved by a standard activation/deactivation protocol (−100/+5 mV: sample traces representing control [cont], mid [a], and full block [b] are shown on the left side). As expected, during perfusion with Cs+ the current was strongly reduced. At the end of the long hyperpolarizing step, Cs+ was washed out and the repetitive –100/+5 mV protocol resumed in the continuous presence of the drug. As apparent from the traces on the right side of Fig. 6, the current elicited by the first step to −100 mV following Cs+ perfusion had not recovered toward the control amplitude, but had kept the amplitude after full ivabradine block (compare trace d with trace b in the inset).


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

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

Inward current is required to relieve channel block by ivabradine. If block by 3 μM ivabradine was induced by a standard activation/deactivation protocol (−100/+5 mV); sample traces are shown on the left (cont, control; a, 30 s; and b, 120 s after drug perfusion; the latter corresponding to steady-state block). In the continuous presence of the drug, a prolonged (45 s) step to −100 mV was then applied while simultaneously adding 5 mM Cs+ (c); Cs+ was washed off at the end of the long step and the repetitive pulsing protocol resumed (traces d, 6 s and e, 36 s after Cs+ wash-out). Note that no block removal occurred during the prolonged hyperpolarization (compare records b and d in the inset).
© Copyright Policy
Related In: Results  -  Collection

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

fig6: Inward current is required to relieve channel block by ivabradine. If block by 3 μM ivabradine was induced by a standard activation/deactivation protocol (−100/+5 mV); sample traces are shown on the left (cont, control; a, 30 s; and b, 120 s after drug perfusion; the latter corresponding to steady-state block). In the continuous presence of the drug, a prolonged (45 s) step to −100 mV was then applied while simultaneously adding 5 mM Cs+ (c); Cs+ was washed off at the end of the long step and the repetitive pulsing protocol resumed (traces d, 6 s and e, 36 s after Cs+ wash-out). Note that no block removal occurred during the prolonged hyperpolarization (compare records b and d in the inset).
Mentions: In Fig. 6, we applied a protocol similar to that in Fig. 4, but added 5 mM Cs+ to the perfusate during the prolonged (45 s) hyperpolarization to −100 mV, following steady-state block by 3 μM ivabradine achieved by a standard activation/deactivation protocol (−100/+5 mV: sample traces representing control [cont], mid [a], and full block [b] are shown on the left side). As expected, during perfusion with Cs+ the current was strongly reduced. At the end of the long hyperpolarizing step, Cs+ was washed out and the repetitive –100/+5 mV protocol resumed in the continuous presence of the drug. As apparent from the traces on the right side of Fig. 6, the current elicited by the first step to −100 mV following Cs+ perfusion had not recovered toward the control amplitude, but had kept the amplitude after full ivabradine block (compare trace d with trace b in the inset).

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