Brain concentrations of kynurenic acid after a systemic neuroprotective dose in the gerbil model of global ischemia.

1. Kynurenic acid (KYNA) is a kynurenine metabolite and a broad spectrum excitatory amino acid antagonist that has been shown to be neuroprotective in models of cerebral ischemia, when administered exogenously. However, the actual concentration required in the CNS to evoke significant neuroprotection has never been assessed. 2. The purpose of this study was to address this question in the gerbil model of forebrain ischemia. KYNA (400-1600 mg/kg) or vehicle were administered i.p. 15 min before 5 min bilateral carotid occlusion. 3. Seven days after reperfusion, ischemia-induced hippocampal nerve cell loss (95% in vehicle-treated) was significantly lower in KYNA-treated gerbils (65% and 52% at 1000 and 1200 mg/Kg, respectively, P < 0.01). Treatment with 1000 mg/kg produced brain KYNA concentrations that were dramatically elevated (135.9 and 42.3 microM in CSF and whole brain, vs 0.032 and 0.16 microM in controls, at 15 min after ischemia), as measured in a separate group of transcardially-perfused gerbils. Cerebral KYNA concentrations tended to return to basal values 2 hours after reperfusion. 4. These results indicate that KYNA has a marked neuroprotective effect in a model of forebrain ischemia. This activity is associated with KYNA concentrations in the brain and CSF that are compatible with the in vitro affinity of the compound for ionotropic glutamate receptors.

Phenylimidazolidin-2-one derivatives as selective 5-HT3 receptor antagonists and refinement of the pharmacophore model for 5-HT3 receptor binding.

A possible bioisosterism between the benzamido and the phenylimidazolidin-2-one moieties has been suggested on the basis of the similarity between the molecular electrostatic potential (MEP) of metoclopramide, a D2 receptor antagonist with weak 5-HT3 receptor antagonist properties, and zetidoline, a D2 receptor antagonist. Starting from this premise, a series of phenylimidazolidin-2-one derivatives bearing a basic azabicycloalkyl or an imidazolylalkyl moiety were synthesized and evaluated for 5-HT3 receptor radioligand binding affinity ([3H]-GR 43,694). In vitro 5-HT3 receptor antagonist activity was tested in the guinea pig ileum assay (GPI). A number of high-affinity ligands were shown to be potent 5-HT3 receptor antagonists in vivo as determined by inhibition of the Bezold--Jarisch reflex in the anesthetized rat. In general, the imidazolylalkyl derivatives were found to be more active than azabicycloalkyls. 1-(3,5-Dichlorophenyl)-3-[(5-methyl-1H-imidazol-4-yl)methyl]imidazoli din-2-one (58), in particular, displayed very high affinity for the 5-HT3 receptor (Ki of 0.038 nM) with a Kb of 5.62 nM in the GPI assay, being more potent than the reference compounds (ondansetron, tropisetron, granisetron, and BRL 46,470) tested. 58 showed an ID50 comparable to that of ondansetron (2.2 micrograms/kg i.v.) in the Bezold--Jarisch reflex. A molecular modeling study based on this structurally novel series of compounds allowed the refinement of previously reported 5-HT3 receptor antagonist pharmacophore models.

A comparative evaluation of thromboxane receptor blockade, thromboxane synthase inhibition and both in animal models of arterial thrombosis.

The combination of thromboxane (TX) synthase inhibition and prostaglandin (PG) H2/TXA2 receptor antagonism yields enhanced antithrombotic effects as compared with either intervention alone. However, it is not known whether the enhancing effect of TX synthase inhibition is expressed also in the presence of complete blockade of PGH2/TXA2 receptors. Thus we evaluated the antithrombotic effects of increasing doses of the PGH2/TXA2 receptor antagonist L 670596 alone and in combination with a dose of the TX synthase inhibitor FCE 22178 causing > 95% inhibition of platelet TXB2 production. In the dog model of electrically induced coronary thrombosis, occlusion time in control animals (n = 14) averaged 72 +/- 29 min. L 670596 alone dose-dependently antagonized platelet PGH2/TXA2 receptors and prolonged occlusion time. The addition of FCE 22178 displaced the dose-occlusion time relation of L 670596 in a parallel fashion without modifying receptor occupancy. In the rabbit model of copper coil-induced carotid artery thrombosis, occlusion was very rapid (14 +/- 4 min) in control animals (n = 17) and was not modified by either aspirin or FCE 22178. L 670596 caused a dose-related receptor blockade and prolongation of occlusion time. The association with FCE 22178 enhanced significantly the antithrombotic effect of L 670596 at all doses. We conclude that the full therapeutic potential of PGH2/TXA2 receptor antagonism is expressed at > 90% platelet receptor occupancy. The additive effect of TX synthase inhibition suggests that conversion of PGH2 to platelet-inhibitor and vasodilator prostaglandins might be of therapeutic importance, irrespective of the extent of PGH2/TXA2 receptor blockade.

Pharmacological characterization of FCE 27262, a combined thromboxane synthase inhibitor and PGH2/TXA2 receptor antagonist.

The literature supports the hypothesis that the association of a thromboxane (TX)A2 synthase inhibitor and a PGH2/TXA2 receptor antagonist has a superior antithrombotic effect when compared to both aspirin and single agent alone; a compound endowed with the dual mechanism of action might therefore be of therapeutic value for the management of thrombotic disorders. FCE 27262, an imidazol-1-yl-ethylideneaminooxypentanoic acid, displaces in vitro the binding of [3H]SQ 29,548 to washed human platelets (IC50 = 6.0 +/- 0.6 x 10(-8) M) and antagonizes human platelet aggregation induced by U 46619 in PRP with an IC50 (95% confidence limits) of 4.5(3.3-5.1) x 10(-7) M. It also selectively antagonizes the isolated vessel contraction induced by U 46619. In the rat aorta the Kb (95% confidence limits) was 1.6(0.6-4.3) x 10(-7) M. Additionally it inhibits in vitro TXB2 production in rat and human whole blood, the IC50 being, respectively, 5.9(3.3-9.6) x 10(-8) M and 3.8(2.9-5.0) x 10(-8) M. When administered orally to fed rats it also inhibits ex vivo TXB2 production in whole blood during clotting, the ID50 being 0.62(0.4-0.8) mg/kg. Both in vitro and ex vivo the effect of FCE 27262 on TXA2 synthase was selective, the production of PGE2, the product of a different isomerase from the common precursors, PG-endoperoxides, being concomitantly enhanced. In a canine model of electrically-induced coronary thrombosis, FCE 27262 (1 mg/kg i.v.) inhibits ex vivo TXB2 synthesis (> 95%), antagonizes U 46619-induced platelet aggregation and prolongs occlusion time (controls: 72 +/- 8 min, FCE 27262: 215 +/- 38 min; p < 0.01). In the same model both aspirin (5 mg/kg i.v.) and a pure PGH2/TXA2 receptor antagonist (L 670596), at a dose giving a similar degree of TXA2 synthase inhibition and receptor blockade, respectively, are significantly less effective. Thus, FCE 27262 combines thromboxane synthase inhibition and PGH2/TXA2 receptor antagonism in one molecule, resulting in enhanced antithrombotic activity. FCE 27262 thus may be an appropriate pharmacological tool to test the therapeutic potential of the dual mechanism of action.

Evidence for a direct vasoconstrictor effect of big endothelin-1 in the rat kidney.

Inhibition of endothelin-1 (ET-1)-converting enzyme has been suggested as a strategy for blocking ET-1-mediated vasoconstriction. However, it is unclear whether its putative substrate, bigET-1, is an inactive precursor. Thus, we compared in the rat the effects of ET-1 and bigET-1 on renal vascular resistance (RVR) in vitro (isolated perfused kidney, n = 15) and in vivo (Doppler shift technique, n = 23) when injected i.v. or in the rat renal artery (i.a.), before and after metalloprotease inhibition with phosphoramidon (30 mg/kg i.v.). In vitro, the ET-1/bigET-1 potency ratio for the RVR increase was 175; in vivo (i.v.) it was approximately 7 (ED50: 99 and 692 pmol/kg, respectively; P < 0.01). Unlike that of ET-1, the bigET-1 effect started slowly (peak effect at 15 min). On i.a. injection, the ED50 of ET-1 was lower but that of big ET-1 was unchanged (ED50: 28 and 706 pmol/kg, respectively). Moreover, the effect of i.a. bigET-1 on RVR was biphasic, with a dose-related rapid increase followed by a slowly developing further rise. Phosphoramidon completely inhibited the hemodynamic effects of i.v. bigET-1, but abolished only the second phase of the response when given i.a. It also significantly enhanced the effect of ET-1. We conclude that in the rat: (1) bigET-1 may affect RVR by both a direct effect and through phosphoramidon-sensitive conversion to ET-1; (2) the direct vasoconstrictor effect of bigET-1 might be expressed during endothelin-converting enzyme inhibition; (3) metalloproteases are involved in ET-1 degradation.

Proarrhythmic activity of intracoronary endothelin in dogs: relation to the site of administration and to changes in regional flow.

The endothelium-derived peptide, endothelin, has been shown to exert powerful constrictor activity in both isolated and in situ coronary arteries. Recent in vitro data on isolated cardiac myocytes suggest that the substance might also possess electrophysiologic properties. We investigated the possibility that endothelin (ET-1) may exert proarrhythmic effects when infused selectively in the coronary circulation of open-chest-anesthetized dogs. Animals were instrumented for the measurement of left anterior descending (LAD) or left circumflex (LCX) coronary artery blood flow, left systolic ventricular pressure (LSVP), dP/dtmax, mean arterial pressure (MAP), and epicardial electrocardiogram (ECG; three leads). Data were recorded during infusion (2 min) of saline (n = 5) or increasing doses of endothelin (5-80 pmol/kg) given selectively in either the LCX (n = 10) or the LAD (n = 10). When infused into the LCX, endothelin produced a dose-dependent decrease in flow (40 +/- 23% at 80 pmol/kg, mean +/- SD, p less than 0.01) with a concomitant increase in coronary resistance and a decrease in dP/dtmax and MAP. ECG changes typical of myocardial ischemia paralleled the decrease in flow and culminated in ventricular fibrillation at the highest dose (80% of dogs). Endothelin caused similar hemodynamic effects when infused in the LAD, but fatal arrhythmias occurred for lower doses and for little or no change in coronary blood flow. Thirty percent of the animals died at 10 and 60% died at 20 pmol/kg, doses that induced only a moderate decrease (8 +/- 7 and 21 +/- 12%, respectively) in LAD total blood flow. Ventricular tachycardia always preceded ventricular fibrillation and death.(ABSTRACT TRUNCATED AT 250 WORDS)