Use-dependent inhibition of hHCN4 by ivabradine and relationship with reduction in pacemaker activity.
OBJECTIVE
Ivabradine, a specific and use-dependent I(f) inhibitor, exerts anti-ischaemic activity purely by reducing heart rate. The aim of this work was to characterize its effect on the predominant HCN channel isoform expressed in human sino-atrial nodes (hSAN), to determine its kinetics in HCN channels from multicellular preparations and rate-dependency of its action.


METHODS
RT-PCR analysis of the four HCN channel isoforms was carried out on RNAs from hSAN. Patch-clamp and intracellular recordings were obtained from CHO cells stably expressing hHCN4 and isolated SAN, respectively. Beating rate of rat isolated atria was followed using a transducer.


RESULTS
hHCN4 mRNAs were predominant in hSAN. Ivabradine induced a time-dependent inhibition of hHCN4 with an IC(50) of 0.5 microM. In rabbit SAN, ivabradine progressively reduced the frequency of action potentials: by 10% after 3 h at 0.1 microM, by 14% after 2 h at 0.3 microM and by 17% after 1.5 h at 1 microM. After 3h, ivabradine reduced the beating rate of rat right atria with an IC(30) of 0.2 microM. The onset of action of ivabradine was use-dependent rather than time-dependent with slower effects than caesium, an extracellular I (f) blocker. Ivabradine 3 microM decreased the frequency of action potentials in SAN from guinea-pig, rabbit and pig by 33%, 21% and 15% at 40 min, respectively.


CONCLUSIONS
The use-dependent inhibition of hHCN4 current by ivabradine probably contributes to its slow developing effect in isolated SAN and right atria and to its increased effectiveness in species with rapid SAN activity.
Thollon
C
C
Division Pathologies Cardiaques et Vasculaires, Institut de Recherches Servier, 11 rue des Moulineaux, Suresnes 92150, France. catherine.thollon@fr.netgrs.com
Bedut
S
S
Villeneuve
N
N
Cogé
F
F
Piffard
L
L
Guillaumin
J-P
JP
Brunel-Jacquemin
C
C
Chomarat
P
P
Boutin
J-A
JA
Peglion
J-L
JL
Vilaine
J-P
JP
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D001552:Benzazepines / Q000494:pharmacology*; D001683:Biological Clocks / Q000187:drug effects*; D016466:CHO Cells; D006224:Cricetinae; D003412:Cricetulus; D054815:Cyclic Nucleotide-Gated Cation Channels; D005260:Female; D006339:Heart Rate / Q000187:drug effects*; D006801:Humans; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels / Q000037:antagonists & inhibitors* / Q000235:genetics; D000077550:Ivabradine; D008297:Male; D009124:Muscle Proteins / Q000037:antagonists & inhibitors* / Q000235:genetics; D015221:Potassium Channels; D012333:RNA, Messenger / Q000235:genetics; D011817:Rabbits; D051381:Rats; D017208:Rats, Wistar; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013552:Swine
D001552:Benzazepines; D054815:Cyclic Nucleotide-Gated Cation Channels; C119587:HCN4 protein, human; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D009124:Muscle Proteins; D015221:Potassium Channels; D012333:RNA, Messenger; D000077550:Ivabradine