Peripheral and central H1 histamine receptor occupancy by levocetirizine, a non-sedating antihistamine; a time course study in the guinea pig

BACKGROUND AND PURPOSE

The H(1) receptor occupancy (H1RO) in brain is an indicator of central side effects of antihistamines. Here, we determined the kinetics of central and peripheral H1RO by levocetirizine in relation to its brain and plasma concentration, and investigated the role of the blood-brain barrier in any delay in brain H1RO.

EXPERIMENTAL APPROACH

Concentration-time profiles in plasma and brain were obtained after 0.1 and 1 mg kg(-1) oral doses of levocetirizine in guinea pigs. H1RO in brain was measured ex vivo using [3H]-mepyramine and, in the periphery, by measuring the degree of inhibition of histamine-induced contractions of isolated guinea pig ileum.

KEY RESULTS

The concentration-time profile of levocetirizine indicated lower levels (partition coefficient, K(p)=0.06-0.08), higher t(max) (2-4 h vs 1-1.5 h) and longer terminal half-life (4-5.6 h vs 2.1-2.8 h) in brain than plasma. The H1RO at 0.1 and 1 mg kg(-1) were 75% and 97%, respectively, at 1 hr in the periphery and, in the brain, were <20% and 28-67% respectively, at all time points studied. Brain H1RO vs plasma concentrations profile showed a delay, but not when compared to brain concentrations.

CONCLUSIONS AND IMPLICATIONS

This study demonstrates an effective peripheral antihistamine effect of levocetirizine without central adverse effects at the dose close to human therapeutic dose. The slow increase in H1RO in the brain with time was caused by slow blood-brain barrier transport of levocetirizine. This demonstrates the importance of measuring time course of brain H1RO in relation to brain concentrations of drugs.

Contractile properties of various histaprodifen derivatives in guinea pig isolated ileum and trachea

We characterized the histamine H(1) receptor agonism of various histaprodifen derivatives in guinea pig isolated ileum and trachea in comparison with histamine. Based on their affinity (calculated pK(A) values for ileum and trachea, respectively), the compounds were ranked as follows: suprahistaprodifen (8.31/8.08) > N(alpha)-(4-phenylbutyl)histaprodifen (7.22/5.93) >or= histamine (5.79/5.19) approximately methylhistaprodifen (5.57/6.07). Based on their efficacy (calculated tau values for ileum and trachea, respectively), the compounds were ranked as follows: methylhistaprodifen (37.67/2.50) > histamine (5.64/1.80) > suprahistaprodifen (1.63/1.42) >or= N(alpha)-(4-phenylbutyl)histaprodifen (0.083/1.54). In the ileum, histamine and methylhistaprodifen showed a high histamine H(1) receptor reserve while suprahistaprodifen and N(alpha)-(4-phenylbutyl)histaprodifen are devoid of any histamine H(1 )receptor reserve. On the trachea, no histamine H(1 )receptor reserve was demonstrable with the four tested agonists. The kinetic of contraction/relaxation of the ileum was faster with histamine and methylhistaprodifen than with suprahistaprodifen and N(alpha)-(4-phenylbutyl)histaprodifen. Histamine contracted the trachea faster than histaprodifen derivatives. Levocetirizine antagonized contractions induced by histamine and histaprodifen derivatives in both tissues. The differences observed in the calculated pA(2) (7.60-8.29) and/or pD'(2) values (6.28-7.90) depending on the tissue and/or the agonist are discussed.

A new series of M3 muscarinic antagonists based on the 4-amino-piperidine scaffold

A series of 4-amino-piperidine containing molecules have been synthesized and structure-affinity relationship toward the M3-muscarinic receptor has been investigated. Chemical modulations provided molecules with K(i) for the human M3-R up to 1 nM with variable selectivity (3- to 40-fold) over the human M2-R. Compounds 2 (pA(2)=8.3, 8.6) demonstrates in vitro on guinea pig bladder and ileal strips potent anticholinergic properties and tissue selectivity.

Synthesis of Dioxigenated Systems. Preparation of Homologues of 1,4-Benzodioxin as Calcium Antagonists

A new compound (9), homologue of 1,4-benzodioxin, has been prepared and tested as calcium antagonist. Additionally, the intermediates 12 and 13 with structure of aminodiol, were also evaluated as anticalcium agents. All of them showed a moderate anticalcium activity.

Design and synthesis of substituted compounds containing the 1, 4-benzodioxin subunit. New potential calcium antagonists

New compounds possessing 1,4-benzodioxin or its saturated analogous heterocyclic system were synthesized and tested for calcium antagonist activity. Biological differences were seen between the different modifications applied. These compounds have been shown to be representative of a novel series of calcium channel antagonists.

Synthesis and Anticalcium Activity of New Compounds Containing the 2,3-Dihydro-1,4-dioxino[2,3-b]pyridine System

Summary: New compounds (3 and 4) possesing the 2,3-dihydro-1,4-dioxino[2,3-b]pyridine group were synthesized and tested as calcium antagonist agents. Both of them showed moderate anticalcium activity.

New substituted 1,4-benzoxazine derivatives with potential intracellular calcium activity

Substituted 1,4-benzoxazines bearing an amino side chain at the 2-position were prepared and were found to have a moderate activity on intracellular calcium. Of the compounds studied it was found that those which possess a homoveratrylamino moiety exhibited superior potency. The chain length and the nature of the amine (4-fluorophenylpiperazine, 4-fluorobenzhydryloxyethylamine, N-substituted homoveratrylamine) is discussed. The 4-benzyl-3, 4-dihydro-2-[3-[[2-(3,4-dimethoxyphenyl)ethyl]amino]propyl]-2H-1, 4-benzoxazine (3c) is the most potent derivative of the series with a ratio of IC50 values against PE (phenylephrine) and K+ of 2.1. Under these test conditions a ratio near 1 indicates potential intracellular calcium activity while a ratio greater than 100 an action on extracellular calcium influx.

In vitro calcium antagonistic and antioxidant effects of Org 13061 and its enantiomers, new potential antiatherosclerotic compounds

The calcium antagonistic and antioxidant properties of a new potential antiatherosclerotic agent, Org 13061 were compared with those of its (-) and (+) enantiomers (Org 13471 and Org 13581) In vitro and with appropriate reference drugs. Org 13061 antagonized contractions induced by potassium in rabbit aortic rings with an IC50 value of 0.50 microM and reduced the maximum rate of phase 0 depolarization (Vmax) of the 'slow' calcium-mediated transmembrane action potentials in cardiac tissue (IC25 = 0.82 microM). Similarly to reference drugs, Org 13061 was more selective in reducing vascular compared to cardiac contraction. In concentrations overlapping those exerting vasorelaxant actions, Org 13061 inhibited copper ion-induced human low density lipoprotein (LDL) peroxidation (0.1-1 microM) and inhibited lipid accumulation by rat aortic smooth muscle cells in culture (1-3 microM). Higher concentrations (3 microM) modestly inhibited proliferation of these cells. The (-) enantiomer was ten times more potent than the (+) enantiomer as a vasorelaxant but was equipotent in inhibiting lipid accumulation and LDL peroxidation (eg, lag phase of conjugated dienes formation increased by 29 and 61 min and by 22 and 56 min in response to 0.3 and 1 microM (-) and (+) enantiomers, respectively). The antioxidant probucol was approximately three times more potent than Org 13061 in inhibiting lipid accumulation but was 30 times less potent in antagonizing LDL peroxidation. The classical calcium channel blocking agents were totally ineffective on lipid accumulation (1-10 microM), whereas human LDL peroxidation was slightly reduced by nifedipine (0.1-3 microM) but unaltered by diltiazem (0.1-30 microM) and verapamil (0.1-3 microM). In conclusion, the racemic Org 13061 selectively blocks voltage-operated calcium channels (VOCs) in concentrations that also exert marked antioxidant activity. The (-) enantiomer is largely responsible for calcium channel block but as antioxidants, the enantiomers are equipotent. This mixed pharmacological profile of Org 13061, not shared by known calcium channel blocking agents, may be potentially useful in the treatment of atherosclerosis.

Comparison of amitriptyline metabolism in hepatocytes from streptozocin-induced diabetic rats and from non-diabetic rats

Biotransformation of amitriptyline (AMI) was studied at different intervals in freshly isolated hepatocytes from healthy or streptozocin-induced diabetic rats in order to investigate the influence of the diabetic state. Levels of free and conjugated AMI, demethylated and hydroxylated metabolites, were assessed by HPLC analysis. In hepatocytes isolated from diabetic rats, AMI was less completely metabolized and the demethylation reaction became more important than in non-diabetic rat hepatocytes. Although the proportions of hydroxylated metabolites decreased in diabetic rats, it always remained predominant. Furthermore, glucuronidation of metabolites was greater, especially for (Z)-10-hydroxynortriptyline in diabetic animals.

Lack of efficacy of 5-HT2A receptor antagonists to reduce brain damage after 3 minutes of transient global cerebral ischaemia in gerbils

Stimulation of the 5-HT2A receptors by serotonin has been reported to exert an excitatory effect on neocortical neurons in rats and mice, to facilitate ischaemia-induced release of excitatory amino acids and to mediate the vasomotor constrictor component of the response of blood vessels to 5-HT. 5-HT2A receptor antagonists have, therefore, been proposed as potential protectants against the effects of cerebral ischaemia. The aim of this study was to evaluate the effects of two relatively selective 5-HT2A receptor antagonists, ketanserin and ritanserin, on delayed hyperactivity and the ensuing neuronal degeneration induced by 3 minutes of bilateral carotid artery ligation in Mongolian gerbils. Effects were compared to that of flunarizine, which blocks calcium overload and served as a positive control in this paradigm. Temporal and/or rectal temperatures were measured and strictly controlled during the ischaemia and the early reperfusion phase. Locomotor activity was measured one day after the ischaemia and neuronal degeneration quantified 7 days later using an image analysis system (Quantimet 570, Leica). Global ischaemia in gerbils elicits hyperactivity associated with a delayed neuronal degeneration predominantly in the CA1 zone of the hippocampus. Ketanserin and ritanserin (3 and 10 mg/kg ip, twice daily for 3 days, pre- and postischaemia) did not protect the CA1 neurons against ischaemic damage. The postischaemic hyperactivity was inhibited only with the higher dose of ketanserin. As previously reported, flunarizine (30 mg/kg po) markedly reduced neuronal degeneration (-44.2%, p < 0.01) and totally abolished the ischaemia-induced hyperactivity. These data demonstrate that ketanserin and ritanserin are not effective protectants of the gerbil hippocampus against ischaemic damage when the body temperature of the animals is controlled, thus suggesting that 5-HT2A receptors are not directly implicated in the pathogenesis of global cerebral ischaemia in this model.

Failure of calcium channel blockade to reduce platelet-mediated cyclic flow variations in dogs with coronary stenosis and endothelial injury

Experimental canine coronary artery stenosis associated with endothelial injury results in a typical pattern of coronary flow characterized by gradual decreases in blood flow to almost zero values followed by abrupt restorations to original levels. Cyclic flow variations (CFVs) are the consequence of recurrent platelet aggregation at the site of the stenosis and subsequent dislodgement of the thrombus. The present study was designed to test the efficacy of diltiazem, nifedipine, and verapamil in inhibiting in vivo platelet aggregation as compared with that of aspirin and ketanserin, two potent reference compounds effective in this model. Except for aspirin, compounds were given as a slow intravenous infusion (i.v.) for 60 min to avoid hemodynamic changes. Diltiazem (0.01 mg/kg/min), nifedipine (3 micrograms/kg/min), and verapamil (0.01 mg/kg/min) were totally inactive against CFVs. A higher dose of verapamil (0.02 mg/kg/min) abolished CFVs in 3 of 4 dogs, but serious side effects were observed [atrioventricular (AV) block and death of 2 animals]. Aspirin (10 mg/kg bolus) caused complete inhibition of CFVs in 4 of 4 dogs, and ketanserin (0.01 mg/kg/min) abolished CFVs in 4 of 5 dogs. These data suggest that calcium channel blockade alone in contrast to cyclooxygenase inhibition or 5-HT2 antagonism cannot inhibit thrombus formation in this model.

Effects of the 5-hydroxytryptamine1A receptor agonists, 8-OH-DPAT, buspirone and flesinoxan, upon brain damage induced by transient global cerebral ischaemia in gerbils

The effects of the 5-hydroxytryptamine1A agonists, 8-OH-DPAT, buspirone and flesinoxan, on the delayed hyperactivity and on the ensuing neuronal degeneration induced by transient global cerebral ischaemia, were studied. In normothermic, male Mongolian gerbils, subjected to 3 min bilateral carotid artery ligation, the locomotor activity was measured 1 day after ischaemia. The neuronal damage was quantified 7 days later using an image analysis system. Buspirone (3 and 10 mg/kg, i.p.) and flesinoxan (1 and 3 mg/kg, i.p.), administered twice a day for 3 days both in pre- and post-ischaemic conditions, failed to significantly protect the CA1 zone of the hippocampus against neuronal damage. In contrast, 8-OH-DPat (1 and 3 mg/kg, i.p.) significantly reduced the neuronal degeneration. All compounds abolished the hyperactivity but there was no correlation between this parameter and the extent of the reduction in neuronal damage. The ineffectiveness of buspirone and flesinoxan was not the result of too low a dose - as evidenced by the complete inhibition of hyperactivity with both compounds and by the appearance of a serotonin behavior syndrome with flesinoxan - but is possibly related to a partial agonist activity at the 5-hydroxytryptamine1A receptor, as reported for buspirone. Further studies are necessary to explain the differences between these agonists.

Paracetamol potentiates isaxonine toxicity in vitro

The toxicity of isaxonine alone and in combination with the known glutathione depletor, paracetamol, was evaluated using rat hepatocyte primary cultures in vitro by measuring morphometric parameters and the leakage of intracellular lactate dehydrogenase into the culture medium. No cytotoxicity was observed with isaxonine at concentrations up to 10(-3) M, whereas paracetamol was cytotoxic at concentrations above 0.6 x 10(-3) M in the culture medium. Paracetamol cytotoxicity (0.6-3.3 x 10(-3) M) was enhanced in the presence of a non-cytotoxic concentration of isaxonine (10(-7) M). Furthermore cytotoxicity was observed when cells were exposed to a combination of non-cytotoxic concentrations of the paracetamol (0.3 x 10(-3) M) and isaxonine (10(-7) M). These findings demonstrate that isaxonine has no direct cytotoxic effect even at high concentrations. However co-administration of isaxonine with paracetamol greatly potentiates cytotoxicity. We suggest that this effect may be related to glutathione depletion within the cell but additional studies are required to verify this hypothesis.

Electrophysiological effects of Org 7797 in the closed-chest anaesthetized dog

1. The intravenous electrophysiological effects of a new antifibrillatory agent, Org 7797, were studied in closed chest anaesthetized dogs. Effects of fast sodium and slow calcium-mediated action potentials were also examined in guinea-pig isolated papillary muscle. 2. The major effects of a known antifibrillatory dose of Org 7797 (0.5 mg kg-1) were a protracted slowing of AV nodal conduction (for at least 20 min) and prolongation of the AV nodal functional refractory period. Conduction in the atria and His-Purkinje system (reflected by the St-A and HV intervals) were not significantly modified whilst ventricular conduction (reflected by the QRS interval) and the ventricular functional refractory period were only transiently prolonged. No other electrophysiological changes were seen. 3. A higher dose of Org 7797 (1.5 mg kg-1) slowed conduction at all levels of the myocardium (as evidenced by increases in the St-A, AH, HV and QRS intervals), slightly shortened cardiac repolarization (as assessed from JTc) and decreased Wenckebach rate. Atrial refractory periods were increased whereas effects on ventricular refractory periods were modest. 4. Neither heart rate nor sinus node recovery time were modified by either dose of Org 7797. 5. Org 7797, at a concentration (20 microM) which reduced Vmax of fast sodium-mediated action potentials in isolated papillary muscle by 83%, did not modify Vmax of slow calcium-mediated action potentials. It prolonged duration of the latter but did not modify that of the former. However, the plateau phase of both the 'fast' and especially the 'slow' action potentials was prolonged. 6. It is concluded that the main electrophysiological effects of a known antifibrillatory dose of Org 7797 in dogs with normal cardiac function are seen at the level of the AV node, actions which are unlikely to be explained by calcium channel block. Higher doses display a class Ic profile. This preferential action on the AV node may contribute to the control of ventricular rate during atrial fibrillation in the absence of infra-nodal conduction disturbances.7. These results contrast with those previously obtained in infarcted dogs and might further suggest that myocardial infarction enhances the Class I action of Org 7797.

An in vitro investigation of the relationships between potency, lipophilicity, cytotoxicity and chemical class of representative calcium antagonist drugs

Interrelationships that might exist between potency, lipophilicity and cytotoxicity of the chemically diverse calcium antagonist group of drugs have been examined in the present study. The potency of 11 representative calcium antagonists in inhibiting KCl-induced contractions in rabbit isolated aortic rings and their relative lipophilicity was determined using reversed phase HPLC. Their cytotoxicity in rat hepatocyte primary cultures was also determined. Cytotoxicity failed to correlate with potency, except for the highly lipophilic, non-selective, diphenylalkylamines (DPAs), suggesting that cytotoxicity was not caused by blockade of plasmalemmal voltage-operated calcium channels. Cytotoxicity moderately correlated with relative lipophilicity, the most lipophilic drugs also being the most cytotoxic. Relative lipophilicity may partly determine the cytotoxicity and pharmacological potency of Ca++ antagonists in a broad sense, but this correlation was not valid in each individual chemical series. We suggest that the higher cytotoxicity of the DPAs is at least partly due to a greater incorporation of the drugs into the hepatocyte plasmalemma compared to compounds in other chemical classes investigated. Further studies are required to elucidate the particular cytotoxic mechanisms involved.

Calcium antagonists can be classified using in vitro toxicity and potency indices

The toxicity of eleven calcium antagonists from different chemical families was determined in rat hepatocyte primary cultures. The calcium antagonist potency of the same compounds was also determined in isolated rabbit aortic rings contracted with high K+. The hepatocytotoxicity of the calcium antagonists was not directly linked to blockade of voltage-operated calcium channels, since there was no correlation between the rank order of hepatotoxicity and that for calcium antagonist potency. The toxicity and calcium antagonist potency of each calcium antagonist examined were used to calculate an in vitro therapeutic index value for each compound. It was observed that therapeutic indices fell into three distinct groups and we therefore propose that the in vitro therapeutic index can be used to subclassify the calcium antagonist group of drugs. The proposed classification corresponds very closely with one already suggested by Spedding on pharmacological grounds. In conclusion, the in vitro therapeutic index may provide a useful tool in the characterization and subclassification of novel calcium antagonist compounds.

Acetylcholine-induced relaxation of the rabbit aorta is preload-independent but markedly dependent upon the degree of excitatory agonist-induced tone

1. The aim of the present study was to investigate the relationship between endothelium-dependent relaxation evoked by acetylcholine, tissue preload and the degree of noradrenaline-induced tone in the isolated rabbit aorta. 2. In the aorta preload-response curves were bell-shaped with increasing preload augmenting responses to noradrenaline up to a certain point (optimal value) and then declining. Removal of the endothelium significantly increased responses to low concentrations of noradrenaline (less than 0.1 microM) but did not significantly affect the maximum response of the aorta to this amine or the preload-response curves generated at several concentrations of noradrenaline. 3. The optimal preload value was around 10 g for the aorta and changes in preload did not influence the sensitivity of the tissue to noradrenaline as assessed by pEC50 values to this agonist. 4. Acetylcholine (0.01-10 microM) evoked endothelium-dependent relaxations which in absolute terms increased as the tissue preload was increased. This relationship was much less evident when acetylcholine responses were measured in terms of the percentage inhibition of the respective noradrenaline contraction, when little or no change in the acetylcholine responses was noted. 5. When acetylcholine relaxations were expressed in terms of a percentage of the maximum noradrenaline-induced response evoked at each preload setting, the results confirmed that preload changes had little or no influence upon acetylcholine responses. 6. In contrast, tissue sensitivity to, and the extent of acetylcholine-induced relaxation, were markedly affected by the level of excitatory agonist-induced tone. As noradrenaline-induced tone increased, maximum responses and pIC50 values to acetylcholine were reduced.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effect of bepridil against veratrine-induced contracture in rat atria

In isolated stimulated rat atria, superfusion with veratrine caused a marked contracture (VIC) which was absent in calcium-free medium and which was inhibited by tetrodotoxin (IC50VIC of 1.38 microM). Lowering the extracellular calcium concentration from 2.5 to 0.5 or 0.1 mM reduced the veratrine-induced contracture and delayed its onset. Superfusion of bepridil (1-10 microM) for 60 min before and during veratrine exposure markedly slowed the onset of contracture, reduced the maximum response (IC50VIC = 2.11 microM) and facilitated recovery upon washout of the alkaloid. The direct negative inotropic effect (NIE) of bepridil (IC50NIE = 10.96 microM) resulted in an VIC/NIE ratio of 5.19 for this drug. The protective effects of bepridil were rate-independent and were not modified by the presence of atropine (1.4 microM) and propranolol (0.3 microM) in the medium. Diltiazem, verapamil and nifedipine only reduced veratrine-induced contracture at concentrations much higher than those producing a negative inotropic effect, giving them negative NIE/VIC ratios of 0.31, 0.08 and 0.08 respectively. Like bepridil, flunarizine had a positive NIE/VIC ratio (15.87, IC50VIC = 3.71 microM). The lack of effect of the quaternary derivative of bepridil CERM 11888 indicated that intracellular sites of action may be involved in the activity of bepridil on veratrine-induced contracture. Given that veratrine-induced changes may mimic some of the pathological changes occurring in ischaemia, the results suggest that bepridil and flunarizine may be more effective than L-type, slow calcium ion-channel blockers in protecting against calcium overload during ischaemia and reperfusion injury.

Comparison of in vivo acute lethal potency and in vitro cytotoxicity of 48 chemicals

The cytotoxicity of 48 compounds included in the MEIC (Multicenter Evaluation of In Vitro Cytotoxicity) list was determined in cultures of rat hepatocytes, McCoy, and MDBK cells. The average minimum concentration of each compound inducing cytotoxicity was measured in each cell type. The cytotoxicity values were then compared with published oral LD50 values for rats and mice. The logarithmic transformation of in vivo toxic doses and the corresponding in vitro cytotoxic concentrations showed a statistically significant correlation between the in vitro and in vivo values. The results show that an accurate in vivo LD50 dose could be predicted from in vitro data for at least 75% of the selected compounds. It is hoped that this finding will not only stimulate others to pursue in vitro technique but will eventually lead to elimination of the in vivo LD50 test.

Caffeine-induced contractions in rabbit isolated renal artery are differentially inhibited by calcium antagonists

Caffeine (1-60 mM) induced concentration-dependent, endothelium-independent phasic contractile responses in isolated rabbit renal artery ring preparations. For concentrations of caffeine over 2 mM, responses were mainly the result of intracellular calcium ion mobilization since they were relatively resistant to removal of calcium ions from the bathing medium. The L-type slow calcium channel blocker, nifedipine (10 microM), had no effect and high concentrations of verapamil and diltiazem (10-30 microM) only slight and inconsistent effects (not concentration-dependent) upon these caffeine responses. Likewise, the highly lipophilic calcium antagonists flunarizine and lidoflazine (3-30 microM) only slightly displaced caffeine concentration-response curves to the right and reduced the maximum response. These small inhibitory effects of flunarizine and lidoflazine were not augmented in a calcium-free medium. In contrast, the other lipophilic calcium antagonists, bepridil and fendiline (3-30 microM), produced marked, non-competitive type inhibition of caffeine responses, completely inhibiting responses to the alkaloid at the highest concentration. Furthermore, the inhibitory effects of bepridil and fendiline were markedly augmented in calcium-free medium. These results clearly differentiate bepridil and fendiline from the other calcium antagonists studied. In addition they provide further evidence for effects other than at the cell membrane which could theoretically contribute to the efficacy of bepridil and fendiline as anti-anginal agents.

Studies on the activity of bepridil as a scavenger of free radicals

Bepridil, a calcium antagonist with anti-anginal, anti-ischemic, and anti-arrhythmic properties was assessed for its ability to scavenge free radicals. Bepridil reduced the stable free radical 1,1-diphenyl-2-picrylhydrazil (DPPH) in the molar ratio 2:1 and, in this respect, was as active as the reference anti-oxidants hydroquinone and alpha-tocopherol. Allopurinol and SOD inhibited cytochrome c reduction in a hypoxanthine-xanthine oxidase superoxide generating system, whereas bepridil was ineffective. Deoxyribose degradation induced by the .OH radical was prevented by bepridil (IC50 = 0.050 mM). This ability to scavenge .OH was similar to that of dimethyl sulfoxide (DMSO) (IC50 = 0.056 mM) and more potent than that observed with mannitol and allopurinol (IC50 values of 0.74 mM and 0.92 mM, respectively). The powerful .OH scavenging activity of bepridil was confirmed in vivo on alloxan induced diabetes in mice. Bepridil exerted a marked protective effect at 0.150 mmol/kg whilst, ethanol and DMSO were active at the doses of 90 and 94 mmol/kg, respectively. These results demonstrate that bepridil is a potent .OH radical scavenger. This property may contribute to the therapeutic activity of this drug in myocardial ischaemia.

Electrophysiological effects of bepridil and its quaternary derivative CERM 11888 in closed chest anaesthetized dogs: a comparison with verapamil and diltiazem

1. The electrophysiological effects of bepridil, its quaternary derivative, CERM 11888 (methylpyrrolidinium bromide) (both 2.5 mg kg-1 i.v.) and those of verapamil and diltiazem (0.2 mg kg-1 i.v.) were studied in closed chest anaesthetized dogs at doses used in clinical studies. 2. The four drugs caused a bradycardia with the following order of potency: bepridil greater than CERM 11888 greater than diltiazem greater than verapamil. 3. All the compounds slowed conduction in the AV node, increased the refractory period (RP) and decreased Wenckebach rates with the following order: verapamil much greater than diltiazem greater than bepridil greater than CERM 11888. 4. Verapamil and diltiazem did not affect conduction or the RP in atria while bepridil weakly slowed the former and markedly increased the latter. CERM 11888 caused a lengthening of RP but this was a delayed effect. 5. In the ventricle, bepridil and CERM 11888 caused a small increase in the QRS and a more pronounced increase in the RP. Both compounds increased QTc but did not modify HV. Verapamil and diltiazem had no significant effects at the ventricular level. 6. Our results confirm that the main sites of action of calcium antagonists are the SA and AV nodes. Bepridil has a broader spectrum of activity and also acts at the atrial and ventricular levels. A comparison of the effects of bepridil with those of its quaternary derivative suggests the involvement of an intracellular action in the electrophysiological effects of bepridil.

Functional, behavioral, and histological changes induced by transient global cerebral ischemia in rats: effects of cinnarizine and flunarizine

Temporary cerebral ischemia (15 min) produced by "four-vessel occlusion" in the rat causes neurological disorders, changes in behavior (locomotor hyperactivity), and neuronal damage in the neocortex, striatum, and especially the CA1 zone of the hippocampus. We have studied the effects of two calcium overload blockers, flunarizine (50 mg/kg p.o. twice a day) and cinnarizine (100 mg/kg p.o. twice a day), on these alterations. Cinnarizine markedly improved the functional abnormalities of ischemia but had little or no effect upon the neuronal damage. In contrast, flunarizine provided far greater neuronal protection but with less obvious effects upon behavioral parameters. However, there was evidence of sedation 2 h after treating animals with this dose of flunarizine that might have masked any positive effect of the drug on behavior. We conclude that under the present experimental conditions, there is no correlation between the early and late behavioral changes observed following a temporary cerebral ischemic episode and the histological damage observed in certain vulnerable neurons, particularly in the hippocampus, 72 h after the insult.

Bepridil: a pharmacological reappraisal of its potential beneficial effects in angina and tissue protection following ischemia

In this review the pharmacologic properties of the calcium antagonist bepridil have been reexamined, particularly the evidence for an intracellular locus of action for the drug. Physicochemical properties of bepridil show it to be highly lipophylic, rapidly and extensively taken up, and accumulated in certain tissues. Combined electrophysiologic and mechanical studies have provided convincing, but indirect, evidence for an intracellular action of bepridil in cardiac muscle. Bepridil also fulfills, to a greater or lesser extent, certain important pharmacologic criteria necessary for evoking an intracellular action of a drug in cardiac and vascular smooth muscle: 1. Responses to agonists known to utilize intracellular calcium in the response are inhibited to a similar extent to depolarization-induced K+ responses. 2. Phasic and tonic responses to noradrenaline in vascular tissues are not, or are only to a minor extent, differentially antagonized. 3. Responses to the calcium ionophore A 23187 are antagonized. 4. Activity is retained following removal of the cell membrane by surfactants. 5. Isolated enzyme systems (e.g., calmodulin, myosin light-chain kinase) are affected by the drug at similar concentrations to those that are effective in whole cells or tissues. Finally results obtained with bepridil in ischemic myocardium have been reviewed to ascertain whether its broader pharmacologic spectrum over the calcium-entry blockers is associated with enhanced tissue protective properties. Positive results with bepridil in hypoxic myocytes and ischemic myocardium distinguishes this drug from the classical antianginal agents verapamil, nifedipine, and diltiazem. It is suggested that bepridil, because of its paucity of hemodynamic effects, may be of special therapeutic interest in the management of silent ischemia where cellular mechanisms leading to cytoprotection are more desirable than strong hemodynamic activity.

Comparative effects of bepridil, its quaternary derivative CERM 11888 and verapamil on caffeine-induced contracture in ferret hearts

1. The effects of bepridil, its quaternary derivative: CERM 11888 (methyl-pyrrolidinium bromide) (10(-7)-10(-5) M), and verapamil (10(-7)-10(-6) M) were compared on caffeine-induced contracture of isolated ventricular trabeculae of the ferret. 2. Bepridil diminished the amplitude of contracture in a concentration-dependent fashion, and this effect was significantly different from that of CERM 11888 which, like verapamil, only reduced the amplitude at the highest concentration used. 3. Bepridil (10(-6) M) significantly shortened the time to peak tension and accelerated the relaxation phase of contracture. This latter effect was different from that of CERM 11888. Verapamil (10(-6) M) also tended to accelerate the relaxation phase. At 10(-5) M these actions of bepridil on the time to peak and relaxation tended to reverse. 4. At all concentrations bepridil and verapamil reduced the rate of repriming of contracture and this effect of bedpridil was significantly different from that of its quaternary derivative which only showed a significant effect at 10(-5) M. 5. These results demonstrate a clear intracellular effect of bepridil in the ferret heart. Verapamil and CERM 11888 had only weak intracellular effects even at high concentrations. 6. Analysis of the results suggests that the main sites of action of bepridil in this model are the sarcoplasmic reticulum and one or two calcium compartments in the sarcolemma.

Potentiation of the gastric antisecretory activity of histamine H2-receptor antagonists by clebopride

The substituted benzamide, clebopride, at doses (0.03-3 mg kg-1 i.p.) that were without effect per se on the secretion of gastric acid in pylorus ligated (Shay) rats, potentiated the antisecretory effects of the histamine H2 receptor antagonists cimetidine and ranitidine in this model but not those of the muscarine receptor antagonist pirenzepine nor those of the proton pump inhibitor omeprazole. By contrast, clebopride was without influence on the inhibitory effects of cimetidine on pentagastrin-induced secretion in perfused stomach (Ghosh and Schild) preparations in anaesthetized rats. The significance of these findings is discussed in relation to the previously described potentiating effects of clebopride on the anti-ulcer activity of cimetidine in various experimental models, and the potential beneficial effects of such combined therapy in the clinic.

Synergistic interactions between piracetam and dihydroergocristine in some animal models of cerebral hypoxia and ischaemia

In pharmacological screening tests for activity against the cerebral insults of hypoxia and ischaemia induced by MgCl2 or decapitation in mice, the combination of piracetam and dihydroergocristine has been shown to produce synergistic effects in prolonging the survival time. This was not the case in the model of histiocytic anoxia induced by KCN. Using an optimal combination of piracetam and dihydroergocristine (533:1, Diemil) significant increases in cerebral resistance to hypercapnic anoxia and reductions in the duration of the ensuing electrical silence on the electrocorticogram have been demonstrated in the rat. The same combination was also effective in antagonizing the memory ablating effects of anoxia in rats subjected to electric footshocks during a standard passive avoidance response. The absence of clear effects on gross cerebral blood flow and metabolism, together with considerations of the known pharmacological properties of the two components of the combination and the effects of standard drugs in the models used, lead to the conclusion that the explanation of the observed synergism probably lies in complimentary actions at the level of the cerebral neurones and is independent of simple vasodilation.

Involvement of enteric neurones in the response of guinea-pig ileum preparations to metoclopramide

The role of myenteric neurones in mediating the stimulant effects of metoclopramide in vitro in the guinea-pig ileum has been investigated using the non-ionic surfactant Triton X-100. Histological examination of the ileum 30 days after application of Triton X-100 to the serosal surface demonstrated a marked reduction in the number of ganglion cells and nerve elements in the myenteric plexus. Longitudinal muscle-myenteric plexus (LM-MP) preparations from Triton X-100-treated animals were unresponsive to dimethylphenylpiperazinium and responded poorly or not at all to electrical field stimulation. Metoclopramide (30 microM) elicited small contractions in LM-MP preparations from control and sham-operated animals but failed to contract Triton X-100-treated tissues. However, tissues responded in a similar manner to exogenous acetylcholine (ACh). These results demonstrate the importance of a prejunctional site of action for metoclopramide in this tissue and suggest that contractile responses to the drug are mediated indirectly, probably by increased release of ACh from myenteric neurones.

Effect of diltiazem upon contractile responses to phenylephrine, cirazoline, Sgd 101/75, St 587 and B-HT 920 in rabbit aorta and dog saphenous vein preparations

Diltiazem (10 microM) did not significantly affect concentration-response curves to the full, relatively selective alpha 1-adrenoceptor agonists phenylephrine and cirazoline in rabbit aorta and dog saphenous vein preparations. The effects of these 2 agonists remained resistant to diltiazem even in tissues pretreated with phenoxybenzamine (0.03 or 0.1 microM, 20 min) to reduce the alpha-adrenoceptor reserve. Sgd 101/75 and St 587 were partial agonists in both vascular preparations. The concentration-response curves to these relatively selective alpha 1-adrenoceptor agonists were also unaffected, or only slightly attenuated, by diltiazem. B-HT 920 at low concentrations preferentially stimulated the dog saphenous vein preparation and only at high concentrations elicited small contractions of the rabbit aorta. The responses to B-HT 920 were mediated by alpha 2-adrenoceptors in the vein and by alpha 1-adrenoceptors in the aorta yet concentration-response curves to this agonist were significantly attenuated by diltiazem in both tissues. The results indicate that the resistance of certain alpha-adrenoceptor-mediated responses in vascular preparations to calcium entry blockers need not be associated with the presence of a significant receptor reserve and that calcium dependency of a response may be determined by the agonist.

A comparison of the stimulatory effects of metoclopramide and cinitapride in the guinea-pig isolated ileum

The pharmacological effects of a new benzamide derivative cinitapride, have been compared to those of metoclopramide in guinea-pig isolated ileum and longitudinal smooth muscle-myenteric plexus preparations treated with propranolol (3 microM). Cinitapride (EC50 = 0.74 microM) was 6 times more potent than metoclopramide (EC50 = 4.69 microM) in enhancing the twitch response of co-axially stimulated preparations and 11 times more potent in eliciting contractions in non-stimulated tissues, their respective EC50 values being 0.58 microM and 6.52 microM. These contractile effects of cinitapride and metoclopramide amounted to approximately 25% of the maximum response of the tissues to acetylcholine (1 microM). Neither cinitapride nor metoclopramide, in concentrations up to 10 microM, significantly affected concentration-response curves to exogenous acetylcholine or 5-hydroxytryptamine but both drugs elicited a concentration-dependent potentiation of the ileum responses to a fixed concentration (10 microM) of the ganglion stimulant dimethylphenylpiperazinium (DMPP). Analysis of the twitch-enhancing and contractile effects of cinitapride using a variety of drugs suggested that a common, prejunctional locus of action upon the cell bodies or axons of postganglionic, parasympathetic neurones of the myenteric plexus is involved in both of these responses. In hexamethonium (100 microM) and methysergide (0.1 microM)-treated longitudinal smooth muscle preparations desensitization or blockade of 5-hydroxytryptamine receptors using high concentrations of the same agonist (30 microM) or quipazine (10 microM) or the putative antagonists cocaine (30 microM) or tubocurarine (10 microM) produced small inhibitions (congruent to 20%) of the contractile responses to metoclopramide and cinitapride but did not affect twitch responses to these drugs. It is concluded that cinitapride is a more potent stimulant of guinea-pig intestinal smooth muscle than metoclopramide in vitro although the mechanism of action of both drugs appears to be similar and involves a prejunctional enhancement of acetylcholine release from intramural cholinergic neurones. Attempts to implicate a prejunctional facilitatory 5-hydroxytryptamine receptor in the mediation of the stimulant effects of these drugs were not conclusive and additional studies are required to fully explore this possibility.

Phenoxybenzamine-induced inhibition of cirazoline pressor responses in pithed rats pretreated with organic or inorganic calcium entry blocking drugs

In groups of propranolol-treated pithed rats pretreatment with either verapamil (1 mg/kg i.a., 20 min) or the inorganic calcium entry blocker (CEB), cobalt (23.8 mg/kg i.a., 20 min) reduced maximum obtainable pressor responses to the relatively selective alpha 2-adrenoceptor agonist B-HT 920 (0.1-1000 micrograms/kg i.v.) equally, by approximately 50%. Verapamil and cobalt at these doses had little or no effect upon pressor responses induced by the relatively selective alpha 1-adrenoceptor agonist cirazoline (0.1-1000 micrograms/kg i.v.). Phenoxybenzamine (0.1 mg/kg i.v., 15 min) displaced to the right and reduced by 44% the maximum obtainable pressor responses to cirazoline. Treatment of animals with the combination of either verapamil or cobalt followed by phenoxybenzamine, at the dose levels and pretreatment times given above, produced significantly greater inhibitions of cirazoline pressor responses (83% and 88% reduction in the maximum obtainable pressor responses to cirazoline respectively) than were observed following administration of phenoxybenzamine alone. Since yohimbine (1 mg/kg i.v.) did not significantly affect the residual responses to cirazoline following treatment with phenoxybenzamine the mechanism responsible for this interaction between CEBs and phenoxybenzamine is not mediated via postjunctional alpha 2-adrenoceptors. Additional studies are required to assess the involvement of a possible subtype of alpha 1-adrenoceptors which appear to mediate vascular responses sensitive to CEBs.

Calcium entry blocking drugs, 'calcium antagonists' and vascular smooth muscle function

The group of drugs known as "calcium antagonists' is under extensive investigation in experimental animals and man and a re-evaluation of their pharmacological properties is overdue. Recent proposals to adopt the more specific nomenclature of calcium entry blockers for some of these compounds (Vanhoutte & Bohr, 1981) should be supported since there is much confusion in the literature with this class of compound. In this review, which concentrates on vascular smooth muscle, only nifedipine, verapamil, their close chemical analogues and diltiazem are recognised as being relatively selective calcium entry blocking drugs. Whilst definitive evidence for calcium entry blockade must include the demonstration of a selective inhibition of Ca2+-influx into a tissue over a range of concentrations also inhibiting contraction, it is nevertheless possible to define several simple pharmacological criteria which may aid in the identification of such activity. These criteria include the selective antagonism of K+ and Ca2+-induced contractions, relative to those of noradrenaline in suitable vascular smooth muscle preparations and a selective inhibition of alpha 2- as opposed to alpha 1-adrenoreceptor mediated pressor responses in, for example, pithed rat preparations. Recent pharmacological and biochemical studies have identified 3 major subgroups of "calcium antagonist' drugs but the compounds within each subgroup varies with the technique adopted. It is therefore suggested that a combination of both pharmacological and ligand-binding studies be used for purposes of classification. Which mechanism, if any, of inhibiting calcium entry is therapeutically most desirable remains an important question for future research.

A comparison of the effects of verapamil and cinnarizine upon responses elicited by selective alpha 1- and alpha 2-adrenoceptor agonists in the autoperfused canine hindlimb

In an attempt to extend the hypothesis that activation of vascular postsynaptic alpha 2-adrenoceptors requires an influx of Ca2+ ions, the effects of 2 calcium entry blocking drugs verapamil and cinnarizine have been examined as inhibitors of the pressor responses to methoxamine and B-HT 920 in autoperfused dog hindlimb preparations. Verapamil (0.1-1 mg i.a.) selectively antagonized responses to B-HT 920 and had little or no effect upon responses to methoxamine, thus supporting this hypothesis. However cinnarizine, over the dose range studied (0.1-1 mg/kg i.a.) produced quantitatively similar inhibitions of the hindlimb responses to B-HT 920 and methoxamine. These results suggest that cinnarizine may have a different site of action to verapamil in resistance vessels of the dog hindlimb.

Peripheral dopamine receptors, potential targets for a new class of antihypertensive agents. Part II: Sites and mechanisms of action of dopamine receptor agonists

Relatively selective dopamine receptor agonists, like bromocriptine, lergotrile, pergolide and N,N-di-n-propyl-dopamine, lower arterial pressure in conscious spontaneously hypertensive rats and in several anesthetized animal preparations. This effect has been attributed to stimulation of dopamine receptors since it can be specifically antagonized by several dopamine receptor blocking agents (domperidone, haloperidol, pimozide, sulpiride). The two main mechanisms which can theoretically intervene in the antihypertensive effects of dopamine agonists are direct smooth muscle relaxation mediated by stimulation of post junctional DA1-dopamine receptors and the reduction of the neural release of norepinephrine resulting from activation of of DA2-dopamine receptors on ganglionic bodies or sympathetic nerve terminals. Other accessory mechanisms of undoubted interest might be a natriuretic effect or a decrease of aldosterone release. On the basis of the presently available pharmacological results in experimental animals, it is not unreasonable to advance the hypothesis that agonists of DA1- and DA2-dopamine receptors produce cardiovascular changes most compatible with an antihypertensive activity being due to a fall in peripheral resistance. However, before any of these compounds can become of therapeutic interest further research in this field is necessary to explore whether it is possible to minimize or even entirely avoid certain unwanted effects (vomiting, nausea, endocrinological alterations) that appear to be intimately associated particularly with those agents stimulating the DA2-dopamine receptors subtype. A more thorough pharmacological characterization of human dopamine receptors would be useful to provide an insight into whether novel chemical approaches can solve some of these problems. Finally, the ideal profits of future dopamine receptor agonists aimed at the treatment of elevated arterial pressure is discussed.

Peripheral dopamine receptors, potential targets for a new class of antihypertensive agents. Part I: Subclassification and functional description

The dopamine receptors of the peripheral cardiovascular system are not a pharmacologically uniform population. A number of studies indicate that they belong to at least two distinct subtypes for which it is proposed to adopt the name DA1- and DA2-dopamine receptors in an attempt to follow the nomenclature presently in fashion for several vascular receptors. Typical DA1-dopamine receptors are those occurring postjunctionally in the renal and mesenteric arterial beds where their stimulation mediates direct smooth muscle relaxation. Typical DA2-dopamine receptors are those present on postganglionic sympathetic neurons (axonal varicosities and perhaps ganglionic cell bodies) where their excitation leads, under appropriate physiological conditions, to a reduction of the neural release of norepinephrine. The latter effect can manifest itself by a passive fall in vascular resistance and heart rate. Other populations of dopamine receptors not yet well characterized pharmacologically but of theoretical interest as additional potential target sites for cardiovascular drugs might be present on nephrons and in the adrenal cortex. Their stimulation can mediate a natriuretic effect and a reduction of aldosterone release, respectively. The pharmacological evidence favoring the subclassification of cardiovascular dopamine receptors into two distinct subtypes is reviewed. Furthermore, the main agonists and antagonists of these receptors and the complexity of their pharmacological profile are mentioned. Part II of this minireview will be dedicated to the description of the sites and mechanisms of the antihypertensive action of dopamine receptor agonists.

Pre- and postsynaptic alpha adrenoceptor selectivity studies with yohimbine and its two diastereoisomers rauwolscine and corynanthine in the anesthetized dog

The selectivity of yohimbine and its two diastereoisomers rauwolscine and corynanthine for pre- and postsynaptic alpha adrenoceptors has been investigated in the anesthetized dog. Antagonism of the inhibitory effect of clonidine on the tachycardia produced by electrical stimulation of the ansa subclavia was used as a measure of presynaptic alpha-2 adrenoceptor blockade. Inhibition of the diastolic pressor response to phenylephrine in ganglion and beta blocked dogs was used as a measure of postsynaptic alpha-1 adrenoceptor blockade. All three of the isomers reduced, and at higher doses reversed, the inhibitory effect of clonidine Yohimbine and rauwolscine were equipotent in this respect and were approximately 100-fold more potent than corynanthine. However, all the isomers were equipotent as antagonists of the diastolic pressure response to phenylephrine. Yohimbine and rauwolscine were approximately 30 times more potent as alpha-2 adrenoceptor than alpha-1 adrenoceptor antagonists, whereas corynanthine was 10-fold more potent at alpha-1 adrenoceptors than at alpha-2 adrenoceptors. These results are in broad agreement with those previously reported from in vitro experiments showing yohimbine and rauwolscine to be preferential alpha-2 adrenoceptor antagonists and corynanthine to be a preferential alpha-1 adrenoceptor antagonist. It is concluded that the high affinity of the antagonists yohimbine and rauwolscine for alpha-2 adrenoceptors is responsible for their selectivity because at the level of blockade of postsynaptic alpha-1 adrenoceptors both isomers were equipotent with corynanthine.

In vitro studies on 6-fluoronoradrenaline at several peripheral sympathetic neuroeffector junctions

A comparison of the effects of noradrenaline and 6-fluoronoradrenaline has been made at several peripheral sympathetic neuroeffector junctions. In the rat vas deferens preparation in the presence of 1 microM cocaine, 6-fluoronoradrenaline was found to be about 9 times more potent than noradrenaline as an agonist at presynaptic inhibitory alpha 2- adrenoceptors. In the rabbit aorta, 6-fluoronoradrenaline had approximately one tenth of the potency of noradrenaline in stimulating the postsynaptic alpha 1-adrenoceptors. Furthermore 6-fluoronoradrenaline, in contrast to previous reports, appears to be a substrate for the neuronal uptake process since exposure to cocaine potentiated the inhibition of the twitch response of the vas deferens by 6-fluoronoradrenaline. In addition, 6-fluoronoradrenaline increased the spontaneous outflow of radioactivity from rabbit pulmonary artery strips prelabelled with 3H-noradrenaline and this increase was blocked by cocaine (30 microM). These results demonstrate that 6-fluoronoradrenaline is a preferential alpha 2-adrenoceptor agonist which is a substrate for the neuronal uptake process in peripheral sympathetically innervated smooth muscle preparations.

Preferential noradrenergic innervation of alpha-adrenergic receptors in vascular smooth muscle

In the isolated perfused hindlimb preparation of the dog, pressor responses to norepinephrine (NE) are mediated by postsynaptic alpha 1- and alpha 2-adrenergic receptors. Results obtained using preferential alpha 1- and alpha 2-adrenergic receptor antagonists suggest that the alpha 1-subtype is predominantly innervated while both alpha 1- and alpha 2-adrenergic receptor subtypes in vascular smooth muscle are accessible to circulating agonists and antagonists. Preliminary studies in the isolated perfused cat spleen support these in vivo findings in the dog. In contrast to vascular smooth muscle, the cat nictitating membrane appears to contain only alpha 1-adrenergic receptors postsynaptically.