Background and purpose: 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 ratedependency of its action. Experimental approach: 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. Key results: hHCN4 mRNAs were predominant in hSAN. Ivabradine induced a time-dependent inhibition of hHCN4 with an IC 50 of 0.5 mM. In rabbit SAN, ivabradine progressively reduced the frequency of action potentials: by 10% after 3 h at 0.1 mM, by 14% after 2 h at 0.3 mM and by 17% after 1.5 h at 1 mM. After 3h, ivabradine reduced the beating rate of rat right atria with an IC 30 of 0.2 mM. 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 mM decreased the frequency of action potentials in SAN from guinea-pig, rabbit and pig by 33%, 21% and 15% at 40 min, respectively. Conclusions and implications: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.

Mild hyperhomocysteinemia is prevalent in the general population and has been linked to endothelial dysfunction and high arterial pressure (AP) in clinical studies. The present study was designed to determine whether a rise in AP was induced by mild hyperhomocysteinemia and whether the potential rise in AP is secondary or prior to endothelial dysfunction. Experiments were performed in a rat model of mild hyperhomocysteinemia induced by oral administration of homocysteine for 1–4 months. Aortic endothelial dysfunction was observed 2 months after homocysteine treatment while endothelium-independent vasodilation was normal. In parallel, homocysteine treatment increased phenylephrine-induced contraction in aortas with endothelium, but did not modify the contraction in aortas without endothelium, suggesting a decrease of basal NO production. In conscious unrestrained rats, AP was not significantly different 1, 2, 3 and 4 months after homocysteine treatment. In correlation, endothelial function of a resistance vessel (mesenteric artery), mainly non-NO nonprostanoid factor mediated, was preserved, indicating that homocysteine treatment only affected the NO pathway. In conclusion, mild hyperhomocysteinemia alone is not sufficient to elevate arterial blood pressure, at least in the rat model. Aortic endothelial dysfunction produced by mild hyperhomocysteinemia is independent of hemodynamic factors.