History of 'specific bradycardic agents'

The Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: from Biophysics to Pharmacology of a Unique Family of Ion Channels

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are important members of the voltage-gated pore loop channels family. They show unique features: they open at hyperpolarizing potential, carry a mixed Na/K current, and are regulated by cyclic nucleotides. Four different isoforms have been cloned (HCN1-4) that can assemble to form homo- or heterotetramers, characterized by different biophysical properties. These proteins are widely distributed throughout the body and involved in different physiologic processes, the most important being the generation of spontaneous electrical activity in the heart and the regulation of synaptic transmission in the brain. Their role in heart rate, neuronal pacemaking, dendritic integration, learning and memory, and visual and pain perceptions has been extensively studied; these channels have been found also in some peripheral tissues, where their functions still need to be fully elucidated. Genetic defects and altered expression of HCN channels are linked to several pathologies, which makes these proteins attractive targets for translational research; at the moment only one drug (ivabradine), which specifically blocks the hyperpolarization-activated current, is clinically available. This review discusses current knowledge about HCN channels, starting from their biophysical properties, origin, and developmental features, to (patho)physiologic role in different tissues and pharmacological modulation, ending with their present and future relevance as drug targets.

Alinidine prevents hypoperfusion-induced transmural flow redistribution in isolated paced rat hearts

1. The effect of alinidine on transmural flow redistribution during hypoperfusion was studied in the isolated rat heart. 2. Low flow perfusion of paced (5 Hz) rat Langendorff preparations with Tyrode's solution for 60 min resulted in loss of left ventricular pressure amplitude and contracture. 3. In the presence of 45.6 mumol/L alinidine in the hypoperfusion medium contractility was maintained and contracture was missing. 4. Myocardial sphere densities were assessed by infusion of fluorescent microspheres after 60 min of hypoperfusion. Histomorphometric evaluation of the left ventricular wall indicated marked redistribution of myocardial flow to the epicardium. Endo/epicardial quotients of sphere densities were significantly greater in alinidine-treated hearts. 5. Perfusion of hearts with 45.6 mumol/L alinidine for 30 min at a pressure of 80 cmH2O decreased coronary flow and contractility, but did not change the transmural quotient of sphere densities. 6. These results show that in isolated rat hearts with a ventricular wall of only 3-4 mm thickness severe hypoperfusion induces a shift in transmural flow distribution which is prevented by alinidine.

Antiischemic effects of alinidine in paced isolated rat hearts

Low-flow perfusion of paced (5 Hz) rat heart Langendorff preparation with Tyrode solution for 60 min caused a reduction or disappearance of left ventricular pressure amplitude. After 60 min of low-flow perfusion myocardial concentrations of high energy phosphates, as well as the energy charged potential, were diminished while lactate accumulated. Infusion of alinidine into the perfusate at low-flow conditions, resulted in a mean concentration of 15.0 +/- 0.60 microM, and prevented the mechanical and biochemical changes of low-flow perfusion, thus indicating maintenance of aerobic metabolism under hypoxic conditions. These heart rate-independent cardioprotective effects of the "specific bradycardic" drug alinidine are discussed in light of the adenosine antagonistic actions of this compound which should prevent coronary steal.