No abstract

A series of serotonin dimers of formula 4 in which two serotonin moeities are linked together through their 5-hydroxyl residue has been prepared and evaluated as 5-HT(1B/1D) receptor agonists. Binding experiments at cloned human 5-HT(1B), 5-HT(1D), and 5-HT(1A) receptors show that all of these dimers are very potent ligands at 5-HT(1B/1D) receptors with increased binding selectivity vs the 5-HT(1A) receptor when compared to serotonin. Studies of inhibition of the forskolin-stimulated c-AMP formation mediated by the human 5-HT(1B) receptor (formerly the 5-HT(1Dbeta) receptor) demonstrate that all of these serotonin dimers behave as full agonists. Among them, the piperazide derivatives of bis-serotonin, 4g,j, were also identified as very potent agonists in contracting the New Zealand white rabbit saphenous vein (pD2 = 7.6 in each case compared to 5.8 for sumatriptan). Results analysis supports the hypothesis that the important increase in potency of the serotonin dimers can be attributed to the presence of two serotonin pharmacophores in the same molecule, while the enhanced selectivity for 5-HT(1B/1D) receptor subtypes may be due to the position of the spacer attachment to serotonin.

We hypothesized that the slowly inactivating component of Na+ current, which is an integral part of the Na+ window current, is a major pathway for Na+ loading during myocardial ischemia. The putative protective effects of tetrodotoxin (TTX) and R-56865, at concentrations that selectively blocked the Na+ window current, as assessed by action potential plateau shortening without affecting maximum upstroke velocity (Vmax), were examined in isolated Langendorff-perfused guinea pig hearts subjected to 50 min of normothermic global ischemia and 60 min of reperfusion. In papillary muscles, TTX reduced action potential duration at > or = 10 nM but reduced Vmax only at > or = 1 microM. R-56865 (10 nM-10 microM) failed to affect Vmax but concentration dependently reduced action potential duration. Ischemia-induced cardiac dysfunction, including increases in left ventricular end-diastolic pressure and lactate dehydrogenase and creatine phosphokinase release at reperfusion, was attenuated by TTX and R-56865 (0.1-320 nM). Ischemic contracture (increase in left ventricular end-diastolic pressure) was abolished by drug concentrations as low as 1 nM, whereas higher concentrations (> 10 nM) of TTX and R-56865 were required to restore systolic function at reperfusion. TTX and R-56865 had little or no effect on hemodynamic variables. Evidence is provided of pronounced and direct cardioprotective effects of low concentrations of R-56865 and TTX in cardiac muscle during ischemia.(ABSTRACT TRUNCATED AT 250 WORDS)

No abstract

The clinical anti-anginal effectiveness of ranolazine is currently being evaluated. However, the mechanism of its anti-ischaemic action is still unclear. The aim of this work was to establish whether ranolazine exerts functional beta-adrenoceptor antagonist activity in the rat cardiovascular system. Radioligand binding studies were performed in rat hearts and guinea-pig lungs for beta1- and beta2-adrenoceptor affinity, respectively. Ranolazine had micromolar affinity for both beta,- and beta2-adrenoceptors (pKi5.8 and 6.3, respectively). Developed tension was measured in isolated rat left atria (electrically driven at 4 Hz) and cumulative concentration/response curves to (+/-)isoprenaline (0.01-1,000 nM) constructed. Ranolazine (0.32-10 microM) surmountably but weakly antagonised isoprenaline-induced positive inotropic responses, with an apparent pA2 of 5.85 (5.69-6.00) and a slope of -0.74 (-0.70 to -0.77). In bivagotomised, atropinised pithed rats, ranolazine per se evoked marked bradycardia at doses above 10 mg/kg i.v. (maximum variation at 80 mg/kg -125+/-15 bpm, n=6, P<0.001) by a mechanism apparently unrelated to blockade of beta1- or beta2-adrenoceptors. Cumulative incremental doses of (+/-)isoprenaline (0.63 ng/kg to 0.16 mg/kg i.v.) administered to pithed rats induced concomitant depressor and chronotropic responses. Animals received either vehicle (saline 0.9% i.v., n=12), atenolol (0.04-2.5 mg/kg i.v., n=6 per dose), ICI 118551 (0.01-0.63 mg/kg i.v., n=6 or 7 per dose), (+/-)propranolol (0.01-0.63 mg/kg i.v., n=6 per dose) or ranolazine (2.5-80 mg/kg i.v., n=6 or 7 per dose) 10 min prior to isoprenaline. Ranolazine dose-dependently and competitively antagonised isoprenaline-induced decreases in diastolic arterial pressure (DAP, dose ratio 12.2 with 80 mg/kg ranolazine) and increases in heart rate (HR, dose ratio 20.3 with 80 mg/kg ranolazine). Collectively, these results demonstrate that ranolazine behaves as a weak beta1- and beta2-adrenoceptor antagonist in the rat cardiovascular system.

1 We investigated whether contractile responses evoked by 5-HTlD receptor agonists were influenced by the endothelium (E) and nitric oxide (NO) in the rabbit isolated saphenous vein.2 Saphenous vein rings were set up for isometric tension recording in oxygenated (5% CO2 in 02) Krebs solution (pH 7.4) containing (10-6 M): idazoxan (1), indomethacin (10), ketanserin (0.1), prazosin (10), and NO nitro-L-arginine methyl ester (L-NAME; 0 or 10), a NO synthase inhibitor. In some experiments, the E was removed mechanically. 3 5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT) and sumatriptan (Sum) contracted rabbit saphenous vein rings in the potency order (pD2 range) of 5-CT(7.2-7.6) > 5-HT(6.2 -7.1)> Sum(5.0-5.8), irrespective of the presence or absence of the E or L-NAME (n = 9-37 per group) indicating that the potencies of the 3 agonists were not significantly affected by either the E or L-NAME. 4 Efficacy, as assessed by the maximal contractile response (E), was significantly greater for Sum compared to 5-HT and 5-CT with intact E irrespective of the presence (77 + 3, 62 + 3, and 50 + 3 mN respectively; P< 0.05 Sum versus 5-HT and 5-CT) or absence (26 + 3, 14 + 4, and 13 + 2 mN respectively; P<0.05 Sum versus 5-HT and 5-CT) of L-NAME. In E-denuded rings, the Em,,, values were all higher than in E-intact rings and did not differ between the 3 agonists (36 + 4, 37 + 4, and 36 + 5 mN for Sum, 5-HT and 5-CT, respectively; P>0.5 between the 3 agonists) indicating that an endothelium-derived relaxing factor (EDRF) counteracted the constrictor activities of the 5-HTID receptor agonists and raising the possibility that a component of the Sum-induced contractile responses was E-dependent. Without E, the presence of L-NAME did not significantly affect the Ema,, values of the 3 agonists (41+4, 41+5, and 41+4 mN for Sum, 5-HT, and 5-CT respectively; P> 0.5 between the 3 agonists) indicating that the NO synthase inhibited was of endothelial origin.5 Potentiation of the Ema, of the 3 agonists by L-NAME was significantly albeit partially reversed by Larginine (10-2 M) indicating that NO synthase was indeed inhibited by L-NAME. Furthermore, in the presence of E, potentiation of Em,,, of the 3 agonists by L-NAME was mimicked by methylene blue (l0-' M) providing further evidence that NO was involved in the attenuation by the E of the contractile responses induced by the 5-HT,D receptor agonists. 6 In the presence of an intact E and L-NAME, contractile responses elicited by 5-HT and Sum were competitively antagonized by the non-selective 5-HTlD receptor antagonist, methiothepin (pA2: 9.4 and 8.8; slopes: 0.66 and 0.81, respectively) and the highly selective 5-HTID receptor antagonist, GR 127935 (pA2: 9.0 in each case; slopes: 1.04 and 0.93, respectively) indicating that contractions were mediated through activation of a single population of 5-HTID receptors. Contractile responses elicited by 5-CT were also competitively antagonized by methiothepin and GR 127935, but non parallel rightward shifts of the concentration-respons...

Background-Thrombin plays a role in mediating ischemic injury and cardiac arrhythmias, but the mechanisms involved are poorly understood. Because voltage-gated sodium channels (VGSCs) have not previously been considered, putative effects of thrombin on VGSC function were investigated in human isolated cardiomyocytes. Methods and Results-Sodium current (I Na ) was recorded by the whole-cell patch-clamp method. Thrombin increased peak I Na amplitude in an activity-dependent manner, from 1 to 100 U/mL, with an apparent EC 50 of 91Ϯ16 U/mL. When tested at 32 U/mL, thrombin-increased I Na was abolished by tetrodotoxin (50 mol/L). Thrombin effects on I Na were reversible and repeatable, and 100 U/mL doubled peak I Na amplitude. Thrombin (32 U/mL) shifted I Na activation to hyperpolarized potentials without affecting steady-state inactivation, producing unusually large increases in window current. Hirudin (320 U/mL) or haloenol lactone suicide substrate (10 mol/L) failed to significantly affect these effects of thrombin. In current-clamped cardiomyocytes, thrombin (32 U/mL) depolarized resting membrane potential by 10 mV. Conclusions-Facilitation of VGSC activation causing large increases in window current is a major mechanism by which thrombin may promote ischemic sodium loading and injury.

No abstract