Glibenclamide, a specific inhibitor of ATP-sensitive K+ channels, inhibits coronary vasodilation induced by angiotensin II-receptor antagonists

The purpose of our study was test the hypothesis that endogenous angiotensin II contributes to the basal coronary artery tone by acting at vascular ATP-sensitive K+ (K+ATP) channels. Coronary blood flow (CBF) and other hemodynamic parameters were measured in anesthetized dogs. Intracoronary infusion of the selective antagonists of angiotensin II AT1 receptors (L-158,809 and E4177) increased CHF without affecting other hemodynamic parameters, indicating that endogenous angiotensin II caused coronary vaso-constriction through the AT1 subtype receptors. Coronary vasodilation in response to AT1 receptor antagonists was blunted by pretreatment with glibenclamide (a specific inhibitor of K+ATP channels; p < 0.01) but not by either an adenosine-receptor antagonist or an inhibitor of nitric oxide synthesis. Coronary vasodilation in response to AT1-receptor antagonists was partly reduced (p < 0.01) by PD-123319 (the AT2-receptor antagonist). Glibenclamide had no effect on coronary vasodilation induced by sodium nitroprusside. These results indicate that in dogs in vivo, coronary vasodilation in response to AT 1-receptor antagonists inhibited markedly by glibenclamide and partly by PD-123319, suggesting that endogenous angiotensin II contributes to the maintenance of basal coronary vascular tone by acting at K+ATP channels through its receptors.

Reversible immobilization of free-ranging polar bears with medetomidine-zolazepam-tiletamine and atipamezole

The objective of this study was to determine if the potent alpha 2 agonist, medetomidine, and its specific antagonist, atipamezole, could be effectively used to immobilize polar bears (Ursus maritimus). Specifically, our goal was to develop a drug combination containing medetomidine that addressed some of the problems such as prolonged recovery time, non-reversibility, and poor analgesia that have been identified with the currently preferred drug combination, zolazepamtiletamine (Telazol or Zoletil). During 1995 and 1996, 51 free-ranging polar bears along the western coast of Hudson Bay, Canada, were immobilized with a combination of medetomidine, zolazepam, and tiletamine (MZT). Immobilization with MZT was characterized by a short induction time, low volume, reliable and predictable immobilization and reversibility, adequate analgesia, and relative safety in handling for field personnel. Few adverse physiological effects were observed in any target animals with the exception of a single bear which convulsed and died shortly after it was reversed from anesthesia with atipamezole. We conclude that MZT is an effective drug combination for immobilizing polar bears. However, because of an unexplained mortality, further investigation of the physiological effects of MZT and atipamezole is warranted.

Anti-obesity effect of MPV-1743 A III, a novel imidazoline derivative, in genetic obesity

MPV-1743 A III ((+/-)-4-(5-fluoro-2,3-dihydro-1H-inden-2-yl)-1H-imidazole) is a novel imidazoline derivative. In this study, it was shown to bind with high affinity to alpha2-adrenoceptor subtypes alpha2A (IC50) = 0.66 +/- 0.06 nM), alpha2B (IC50) = 3.8 +/- 0.53 nM), alpha2C (IC50) = 3.1 +/- 0.61 nM) in the recombinant S115 cells and to alpha2D (IC50 = 0.94 +/- 0.10 nM) in the rat submandibular gland. MPV-1743 A III also showed remarkably high affinity to alpha1-adrenoceptors (IC50 = 150 +/- 12 nM) in the rat cerebral cortex and to imidazoline I2b-binding sites (IC50) = 150 +/- 5.0 nM) in the rat liver. The functional alpha2-adrenoceptor antagonistic effect of MPV-1743 A III was demonstrated by studying the ability of orally administered MPV-1743 A III to reverse and prevent the alpha2-adrenoceptor agonist detomidine-induced mydriasis in rat. The anti-obesity effect of MPV-1743 A III was investigated in genetically obese (fa/fa) Zucker rats in two different phases of obesity. Chronic treatment with MPV-1743 A III (0.3 3 mg/kg per day p.o. for 3 weeks) dose dependently decreased weight gain in early-phase obesity. In fully established obesity, GDP binding to mitochondria and expression of uncoupling protein mRNA were increased in brown adipose tissue by MPV-1743 A III indicating an activation of non-shivering thermogenesis. The present study shows that MPV- 1743 A III has a modest anti-obesity effect in the genetic rodent model of obesity. The relative importance of alpha2- and alpha1-adrenoceptors and imidazoline I2b-binding sites in mediating the effects of MPV-1743 A III needs further evaluation.

Pial arteriolar constriction to alpha 2-adrenergic agonist dexmedetomidine in the rat

Dexmedetomidine (Dex) is an alpha 2-adrenergic agonist that decreases cerebral blood flow (CBF) when administered systemically. It is unclear whether cerebral vasoconstriction is mediated by a local effect on cerebral vessels or by a remote neural mechanism. In the present study, we compared the pial arteriole responses to locally and systemically administered Dex with and without local application of the specific alpha 2-adrenergic antagonist atipamezole. Six groups of male rats (n = 7 each) were anesthetized with isoflurane and prepared for measurements of small (20-39 microns), medium (40-59 microns), and large (60-79 microns) pial arteriole diameter by intravital microscopy or for regional CBF measurement by the radiolabeled-microsphere method. Local application of Dex caused dose-dependent constriction that was significant starting at 10(-8) M for small and medium-sized arterioles and at 10(-7) M for large arterioles. Constriction to 10(-5) M in small [21 +/- 2% (SE)], medium (21 +/- 2%), and large (15 +/- 1%) arterioles was almost completely blocked by local application of 10(-4) M atipamezole. Intravenous administration of Dex at 1 microgram/kg decreased CBF and caused modest arteriolar constriction that began to resolve 8 min after administration. A dose of 10 micrograms/kg constricted arterioles of all sizes with constriction beginning to resolve after approximately 10 min. Local application of atipamezole (10(-4) M) slightly blunted the response to 1 micrograms/kg of intravenous Dex but did not substantially limit constriction after 10 micrograms/kg. These data demonstrate that pial arterioles are capable of substantial constriction to Dex by a local alpha 2-adrenergic mechanism. However, the inability of locally applied atipamezole to substantially inhibit the vasoconstrictor response to systemically administered Dex suggests that Dex might also cause vasoconstriction indirectly through actions at other sites in the central nervous system.

Chemoprotection against the formation of colon DNA adducts from the food-borne carcinogen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in the rat

The mutagenic heterocyclic aromatic amine, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), is a pyrolysis product in cooked foods that has been shown to be a rat colon carcinogen and has been implicated in the etiology of human colon cancer. In order to identify chemoprotection strategies that could be carried out in humans, a pilot study was conducted in which PhIP-DNA-adduct levels were quantified in the colons of male F344 rats that had been subjected to 16 different putative chemoprotection regimens, followed by a gavage of PhIP (50 mg/kg) and sacrifice 24 h later. The 16 treatments (Oltipraz, benzylisothiocyanate, diallyl sulfide, garlic powder, ethoxyquin, butylated hydroxyanisole, glutathione, indole-3-carbinol, alpha-angelicalactone, kahweol/cafestol palmitates, quercetin, green tea, black tea, tannic acid, amylase-resistant starch, and physical exercise) comprised sulfur-containing compounds, antioxidants, flavonoids, diterpenes, polyphenols, high dietary fiber, etc. The strongest inhibition of PhIP-DNA adduct formation in the colon was observed upon pretreatment with black tea, benzylisothiocyanate, and a mixture (1:1) of kahweol:cafestol palmitates, which resulted in 67, 66, and 54% decreases in colon PhIP-DNA adduct levels, as compared with controls. Preliminary studies on their mechanism of action indicated that only kahweol:cafestol caused a substantial induction of glutathione S-transferase isozymes (GSTs) that are thought to be important in the detoxification of PhIP. Notably, this induction occurred in the liver rather than in the colon.

Carcinogenicity of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in the rat

A total of 10 highly-mutagenic heterocyclic amines have been identified to be carcinogenic in rodents. Among these, 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), generally the most abundant with normal cooking procedures, induces mammary and colon carcinomas in rats in a clear dose-dependent manner. In a two-generation exposure (transplacental and trans-breast milk) experiment using Sprague-Dawley rats, an increased risk of mammary adenocarcinoma development was found in the second generation. Excretion of PhIP into the milk and transfer of PhIP to fetuses and neonates with resultant hepatic PhIP-DNA adduct formation were also confirmed. On the other hand, PhIP mammary carcinogenesis was significantly inhibited by coadministration of chlorophyllin or a synthetic antioxidant, 1-O-hexyl-2,3,5-trimethylhydroquinone, in long-term experiments using female F344 rats. The available findings strongly suggest that this food-derived carcinogen might be of importance as an environmental factor in the production of human cancers and that its carcinogenicity could be largely avoided by reducing intake of such compounds or by adoption of appropriate chemopreventive measures.

Differential effects of two intraventricularly injected alpha 2 agonists, ST-91 and dexmedetomidine, on electroencephalogram, feeding, and electromyogram

Central alpha 2 agonists induce feeding in animals, electroencephalographic (EEG) synchronization, and sedation. Recent observations suggest that the investigational compound ST-91 and dexmedetomidine (DMET) may interact with distinct alpha 2 subclasses at spinal sites. We examined these drugs in brain. To accomplish these aims, ST-91, DMET, or methoxamine (METH), an alpha 1 agonist, were administered into the cerebral ventricles of unanesthetized rats prepared with chronic intracerebroventricular (ICVT) cannulae and cortical EEG electrodes. Behavior, EEG, electromyography (EMG), and feeding were assessed. We found that DMET resulted in dose-dependent (1-32 nmol) sedation, EEG synchronization, and a reduced gastrocnemeus EMG, while ST-91 (up to 153 nmol) had no effect. In contrast, ST-91 (1-153 nmol) resulted in a dose-dependent evocation of feeding, while the effects of DMET on feeding, if any, were obscured by the sedation. All effects were antagonized in a dose-dependent fashion by ICVT yohimbine (an alpha 2 antagonist). ICVT atipamezole (an alpha 2 antagonist), but not prazosin (an alpha 1 antagonist), reversed the effects of DMET. With ST-91, both atipamezole and prazosin had modest, but significant, antagonistic effects. ICVT METH had no effect. The differential physiological actions of these two intracerebroventricularly injected drugs, in concert with previous pharmacological studies, suggest two distinct subclasses of yohimbine-sensitive alpha 2 adrenergic receptors in the brain.

Formation of the mutagenic/carcinogenic imidazoquinoxaline-type heterocyclic amines through the unstable free radical Maillard intermediates and its inhibition by phenolic antioxidants

Generation of the imidazoquinoxaline-type heterocyclic amines in the heated model system composed of glucose/glycine/creatinine in aqueous diethylene glycol was effectively prevented by phenolic antioxidants, butylated hydroxyanisole (BHA), propyl gallate (PG), sesamol, esculetin and epigallocatechin gallate (EGCG) in a dose-dependent manner. Generation of the mutagens in heated-and-dried bonito meat was effectively prevented on pretreatment with EGCG or green tea extract. Electron spin resonance (ESR) studies showed that the heated model mixture of glucose/glycine generated the unstable pyrazine cation radical, and its formation was inhibited by BHA, sesamol and EGCG. ESR-spin trapping studies using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN) showed that the heated model mixture of glucose/glycine or glucose/glycine/creatinine generated unstable carbon-centred radical(s), and their formation was effectively inhibited by BHA, sesamol and EGCG. It is likely that the unstable free radical Maillard intermediates played an important role in the formation of the imidazoquinoxaline-type heterocyclic amines, and the phenolic antioxidants effectively scavenged the radical species to prevent the mutagen formation.

Presynaptic imidazoline receptors and alpha 2-adrenoceptors in the human heart: discrimination by clonidine and moxonidine

The involvement of presynaptic alpha 2-autoreceptors and imidazoline receptors in the modulation of noradrenaline release was investigated in strips from human atrial appendages preincubated with [3H]noradrenaline and superfused with medium containing desipramine and corticosterone. Electrical impulses were applied transmurally at 2 Hz. The imidazoline derivatives moxonidine and clonidine reduced to evoked tritium overflow in a concentration-dependent manner. Moxonidine was 18-fold more potent than clonidine. The concentration-response curve for moxonidine, but not for clonidine was shifted to the right by the alpha 2-adrenoceptor antagonist rauwolscine. The apparent pA2 value of rauwolscine against moxonidine was 8.41. An inhibitory effect was also observed for the imidazoline derivative BDF 6143 (4-chloro-2-(2-imidazolin-2-ylamino)-isoindoline), a mixed alpha 2-adrenoceptor antagonist/imidazoline receptor agonist; BDF 6143 was about 2-fold more potent than clonidine. Rauwolscine (1 microM) did not substantially shift the concentration-response curve of BDF 6143. It is concluded that noradrenaline release in the human atrium is inhibited not only via presynaptic alpha 2-autoreceptors but also via presynaptic non-I1, non-I2 imidazoline receptors. The failure of rauwolscine to antagonize the inhibitory effect of clonidine suggests that clonidine preferentially stimulates the presynaptic imidazoline receptors; the alpha 2-adrenoceptor component of its action is probably suppressed by an inhibitory interaction between the two receptors. In contrast, moxonidine acts via presynaptic alpha 2-autoreceptors, leaving the presynaptic imidazoline receptor unaffected.

A comparison of the antimutagenic potential of green, black and decaffeinated teas: contribution of flavanols to the antimutagenic effect

The present study was undertaken to compare the antimutagenic activity of aqueous extracts, at the concentrations used for human consumption, from green, black and decaffeinated black tea. Antimutagenic potential was evaluated against three indirect-acting dietary carcinogens, Glu-P-1, benzo(a)pyrene and nitrosopyrrolidine. All three types of tea gave rise to strong and concentration-dependent suppression of the mutagenicity of the three premutagens in the presence of an activation system. No major difference in the antimutagenic potential of the three types of tea could be discerned. Black tea, decaffeinated black tea and, to a lesser extent, green tea also antagonized the mutagenicity of the direct-acting mutagen 9-aminoacridine. All three types of tea inhibited markedly the NADPH-dependent reduction of cytochrome c and the O-dealkylations of ethoxy-, methoxy- and, to a much lesser extent, pentoxy-resorufin. When the microsomal metabolism was terminated, after the metabolic activation of the premutagens, incorporation of the aqueous tea extracts into the activation system caused a concentration-dependent suppression of mutagenic response. No significant difference in the antimutagenic activity of the three types of tea in this system was evident. Bearing in mind the much higher concentration of flavanols in green tea compared with the black teas, it may be concluded either that these compounds are unlikely to be the major tea components responsible for the antimutagenic, and possibly anticarcinogenic, properties of tea or that their fermentation products are similarly active.

Antagonistic effects of atipamezole on medetomidine-induced sedation in horses

The antagonistic effects of atipamezole (20, 40, 60, 80, and 100 micrograms/kg i.v.) on medetomidine (10 micrograms/kg i.v.)-induced sedation were evaluated in horses. Although 20 and 40 micrograms/kg of atipamezole were not sufficient to reverse the sedation, 60 micrograms/kg did effectively reverse the sedation. Atipamezole at 80 micrograms/kg was more potent, and significantly shortened the duration of sedation without any apparent side effects, but a higher dose of 100 micrograms/kg was not more effective than 80 micrograms/kg. The possible use of atipamezole as a reversal agent may enhance the value and availability of medetomidine as a chemical restraint agent in horses.

Medetomidine sedation in dogs and cats: a review of its pharmacology, antagonism and dose

Medetomidine is a relatively new sedative analgesic in dogs and cats but some precautions are required when using it. It is a potent alpha 2-adrenoceptor agonist and stimulates receptors centrally to produce dose-dependent sedation and analgesia and receptors centrally and peripherally to cause marked bradycardia and decrease the cardiac output. While hypotension occurs frequently, higher doses of the sedative can raise the blood pressure due to an affect on peripheral receptors. Slowing of the respiratory rate is a frequent effect of medetomidine with some dogs showing signs of cyanosis. Other actions that follow medetomidine use are slowing of gastrointestinal motility, hypothermia, changes to endocrine function and, occasionally, vomiting and muscle twitching. The clinical use of medetomidine in dogs and cats is discussed. Recommended dose rates are presented along with precautions that should be taken when it is used alone for sedation, as an anaesthetic premedicant or in combination with ketamine, propofol or opioids. Hypoxaemia occurs frequently in dogs given medetomidine and propofol. The actions of medetomidine can be rapidly reversed with the specific alpha 2-adrenoceptor antagonist, atipamezole, which is an advantage because undesirable and sedative actions of medetomidine can be terminated.

[Rational hypertension treatment]

The primary objective of antihypertensive treatment is to prevent the involvement of target-organs, including hypertensive vascular disease of the kidney or left ventricular hypertrophy. Antihypertensive treatment should not worsen other cardiovascular risk factors (e.g. lipids) or impair quality of life. Contemporary efforts to optimize antihypertensive therapy are focused on single-drug therapy and on individualizing treatment according to patients age, sex, race and the presence of concomitant illnesses and therapies, in order to improve compliance and reduce overall cardiovascular morbidity and mortality. Several antihypertensive drugs such as ACE-inhibitors, beta-adrenergic-receptor antagonists, calcium channel blockers, diuretics, alpha-adrenergic-receptor antagonists, and newer substances such as imidazoline-receptor antagonists and angiotensin-II antagonists are discussed.

[Imidazoline receptors and use of drug blocking receptors I1]

Imidazoline-preferring receptors are important in the pathophysiology of hypertension. The selective l1-receptor agonists are moxonidine, cimetidine and rilmenidine. Some clinical studies indicate the usefulness of moxonidine therapy in hypertension, arrhythmias and acute myocardial infarction.

Effect of endogenous angiotensin II on renal nerve activity and its arterial baroreflex regulation

To determine the effects of physiological alterations in endogenous angiotensin II (ANG II) activity on basal renal sympathetic nerve activity and its arterial baroreflex regulation, the effect of ANG II receptor (AT1) blockade with losartan was examined in conscious rats consuming low, normal, or high sodium diet that were instrumented for the simultaneous measurement of arterial pressure and renal sympathetic nerve activity. Intravenous losartan decreased arterial pressure in low (-27 +/- 4 mmHg) and normal (-15 +/- 2 mmHg) but not in high sodium diet rats (-5 +/- 2 mmHg). When arterial pressure had been restored to the prelosartan value with methoxamine infusion, renal sympathetic nerve activity was decreased in low (-27 +/- 4%) and normal (-20 +/- 3%) but not in high sodium diet rats (-5 +/- 2%). Arterial baroreflex regulation of renal sympathetic nerve activity was shifted to a lower pressure (arterial pressure at midrange) in low (-8 +/- 2 mmHg) and normal (-7 +/- 2 mmHg) but not in high sodium diet rats (0 +/- 2 mmHg). Intracerebroventricular losartan did not significantly decrease arterial pressure but decreased renal sympathetic nerve activity in low (-28 +/- 5%) and normal (-20 +/- 4%) but not in high sodium diet rats (-2 +/- 2%). Arterial baroreflex regulation of renal sympathetic nerve activity was shifted to a lower pressure (arterial pressure at midrange) in low (-7 +/- 2 mmHg) and normal (-5 +/- 1 mmHg) but not in high sodium diet rats (0 +/- 2 mmHg). These results indicate that physiological alterations in endogenous ANG II activity tonically influence basal levels of renal sympathetic nerve activity and its arterial baroreflex regulation.

Role of serotonergic neurotransmission in the hypnotic response to dexmedetomidine, an alpha 2-adrenoceptor agonist

The role of serotonergic pathways in the hypnotic response to dexmedetomidine was examined in neurochemical and behavioral studies. Following acute administration of dexmedetomidine, loss of righting reflex and changes in serotonin (5-hydroxytryptamine, 5-HT) and norepinephrine turnover in different brain regions (locus coeruleus and hippocampus) were assessed. In separate experiments, the effect of dexmedetomidine on 5-HT turnover was measured in rats rendered tolerant to the hypnotic effects of dexmedetomidine. These neurochemical data were complemented by a study of dexmedetomidine-induced hypnotic response in the presence of a 5-HT2 receptor agonist and antagonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and ritanserin, respectively. Dexmedetomidine (1-500 micrograms.kg-1) dose dependently reduced 5-HT and norepinephrine turnover in both the locus coeruleus and hippocampus. The decrease in 5-HT turnover more closely correlated with the dose-response curve for loss of righting reflex, a behavioral measure of hypnosis, than did the norepinephrine turnover. In previous studies with chronic administration of dexmedetomidine (3 micrograms.kg-1.h-1 for 7 days), the norepinephrine turnover effect of acute dexmedetomidine (30 micrograms.kg-1) persisted while the hypnotic effect was blunted. Following the same regimen, the drug's ability to diminish 5-HT turnover was also blunted. This biochemical evidence for the role of 5-HT in sleep was supported by the behavioral evidence that dexmedetomidine (100 micrograms.kg-1 i.p. or 7 micrograms.0.2 microliter-1 locus coeruleus)-induced hypnosis was dose dependently blocked by DOI (0.08-0.32 mg.kg-1 i.p.). The selectivity of this effect was demonstrated by the finding that ritanserin (0.16 mg.kg-1 i.p.) pretreatment blocked the effects of DOI (0.16 mg.kg-1 i.p.) on dexmedetomidine (100 micrograms.kg-1 i.p. or 7 micrograms.0.2 microliter-1 locus coeruleus)-induced loss of righting reflex. In conclusion, these findings suggest that the hypnotic effect of the alpha 2-adrenoceptor agonist, dexmedetomidine, is not mediated solely by changes in noradrenergic neurtransmission, but instead is strongly associated with a decrease in serotonergic neurotransmission and correspondingly diminished by stimulation of 5-HT2 receptors.

Inhibition of 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) mutagenicity by black and green tea extracts and polyphenols

Solutions of lyophilized preparations of standard black and green tea extracts were made and tested over a range of six concentrations as inhibitors of the mutagenicity caused by the fool mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in the Salmonella typhimurium TA98 assay containing S9 fraction from rats induced with alpha-naphthoflavone and phenobarbital. Extracts of both black and green tea were equally good inhibitors of mutagenicity. Purified polyphenols were prepared from tea extracts by solvent extraction. The polyphenols of black tea were more potent inhibitors of mutagenicity than the polyphenols of green tea. These findings suggest that black tea may have similar health-promoting properties to those reported previously for green tea.

Antagonism by idazoxan at low dose but not high dose, of the natriuretic action of moxonidine

1. Recent studies concerning the imidazoline receptor have utilized idazoxan as a specific imidazoline receptor antagonist. The aim of the present study was to describe the in vivo effects of various doses of idazoxan on renal function, in the presence and absence of moxonidine, an I1 imidazoline receptor agonist. 2. In anaesthetized, unilaterally nephrectomized (7 to 10 days) Sprague Dawley rats, an intrarenal infusion of moxonidine (3 nmol kg-1 min-1) increased urine flow rate, sodium excretion and osmolar clearance without altering free water clearance. Pretreatment with intravenous idazoxan at 0.1 and 0.3 mg kg-1 produced a dose-related decrease in the renal actions of moxonidine. However, a higher dose of idazoxan (1 mg kg-1) was not as effective as the 0.3 mg kg-1 dose in blocking the effects of moxonidine. 3. In a separate series of experiments, the direct renal actions of idazoxan alone were investigated. Idazoxan at 0.3 mg kg-1 failed to alter urine flow rate and sodium excretion. However, idazoxan at 1 mg kg-1 produced a significant increase in urine flow rate and sodium excretion in association with an increase in osmolar clearance. 4. These results do not prove but are consistent with low doses of idazoxan antagonizing the sites stimulated by moxonidine (renal imidazoline receptors). However, at higher doses, idazoxan may function as a partial agonist and/or interact with other receptors to increase urine flow rate, independent of imidazoline receptor blockade. These studies underscore the importance of the dose of idazoxan administered when this antagonist is used as a tool to investigate imidazoline receptors.

Mechanisms of dexmedetomidine-induced cerebrovascular effects in canine in vivo experiments

Dexmedetomidine decreases cerebral blood flow without significantly affecting cerebral oxygen consumption in anesthetized dogs. To assess the direct cerebrovascular effects of dexmedetomidine, we investigated the responses of vasomotor tone to topical application of dexmedetomidine to pial vessels in vivo, using a parietal cranial window. Forty-one dogs were anesthetized with pentobarbital. In 20 dogs, we topically applied six concentrations of dexmedetomidine solution (10(-8), 10(-7), 10(-6), 10(-5), 10(-4), 10(-3) M) and directly measured pial arterial and venous diameters. In 10 dogs, the inhibitory effects of pretreatment of pial vessels with 10(-5) M yohimbine were examined after the application of 10(-5) dexmedetomidine. In the remaining 11 dogs, the effects of 10(-3) M dexmedetomidine were evaluated in the presence of N omega-nitro-L-arginine methyl ester (L-NAME), glibenclamide, or propranolol. Dexmedetomidine significantly constricted pial arteries and veins in a concentration-dependent manner (10(-7) M to 10(-4) M; P < 0.05). Yohimbine blocked dexmedetomidine-induced constriction of pial vessels (both large and small arteries and large veins P < 0.0001; small veins P < 0.005). However, when the highest concentration of dexmedetomidine (10(-3) M) was administered under the window, pial vessel diameter was not significantly altered. In the presence of glibenclamide, 10(-7) and 10(-3) M dexmedetomidine induced a significant decrease in pial arterial diameter compared with 10(-7) and 10(-3) M dexmedetomidine solution alone, respectively (P < 0.05). L-NAME or propranolol did not affect the dexmedetomidine-induced constriction.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory effects of 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), green tea catechins and other antioxidants on 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1)-induced rat hepatocarcinogenesis and dose-dependent inhibition by HTHQ of lesion induction by Glu-P-1 or 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx)

The effects of 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), green tea catechins (GTC), alpha-tocopherol, beta-carotene, chlorophyllin, phenylethylisothiocyanate (PEITC), 3-O-ethylascorbic acid (EAsA), 3-O-dodecylcarbomethyl ascorbic acid (DAsA), n-tritriacontane-16,18-dione (TTAD) and d-limonene on 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1)- or dimethylnitrosamine (DMN)-induced hepatocarcinogenesis, and the dose dependence of HTHQ inhibition of Glu-P-1- or 2-amino-3,8-dimethylimidazo [4,5-f]quinoxaline (MeIQx)-influence on lesion development were examined in a rat medium-term liver bioassay system featuring diethylnitrosamine initiation and partial hepatectomy. At the end of week 8, the number and total area of glutathione S-transferase placental form (GST-P) positive liver foci in rats treated with 0.03% Glu-P-1 alone were increased significantly (46.8 +/- 11.0 and 12.0 +/- 5.6 respectively) as compared to the control values (3.8 +/- 1.6 and 0.4 +/- 0.2). Combined treatment with 1% HTHQ remarkably reduced both of these parameters (8.1 +/- 2.1 and 0.6 +/- 0.2). GTC (1%), PEITC (0.1%), beta-carotene (0.1%) and DAsA (1%) also demonstrated inhibition but less than HTHQ. On the other hand, these antioxidants did not influence development of foci initiated by 0.002% DMN. In the dose-response study, up to 0.125% HTHQ significantly reduced the effects of 0.02% Glu-P-1 or 0.03% MeIQx on the number and area of foci. These results indicate that several antioxidants exert chemopreventive effects against heterocyclic amine (HCA)-induced hepatocarcinogenesis, and particularly HTHQ which thus deserves further attention as a chemopreventor in the contest of the environmentally important HCA group of carcinogens.

Effects of cimetidine on pharmacokinetics and pharmacodynamics of losartan, an AT1-selective non-peptide angiotensin II receptor antagonist

This was a 2-period randomized, crossover study in 8 healthy males to determine the effects of cimetidine (400 mg q.i.d. for 6 days) on the pharmacokinetics and pharmacodynamic effects of the angiotensin II receptor antagonist, losartan (100 mg). Cimetidine increased the AUC for losartan 18% without affecting the AUC for E-3174, the active metabolite of losartan. The increase in plasma renin activity following losartan was not affected by cimetidine (maximum mean increases 12.6 and 12.1 ng Ang I.ml-1.h-1 without and with cimetidine, respectively). These results indicate that cimetidine does not appear to alter the pharmacokinetics or pharmacodynamics of losartan to a clinically significant extent.

Further characterization of the receptor mechanism involved in the antidysrhythmic effect of dexmedetomidine on halothane/epinephrine dysrhythmias in dogs

BACKGROUND

alpha 2 Adrenoceptors in the central nervous system mediate various physiologic processes, including cardiovascular control. Recently, some of these actions have been reported to be mediated by a nonadrenergic receptor, namely an imidazoline receptor. The authors previously reported that dexmedetomidine, a selective alpha 2 agonist, prevents the genesis of halothane-epinephrine dysrhythmias through a central mechanism. Because dexmedetomidine also binds to imidazoline receptors, we performed the current study to examine the precise receptor mechanism involved in the antidysrhythmic property of dexmedetomidine.

METHODS

Adult mongrel dogs were anesthetized with halothane (1.3%) and monitored continuously for systemic arterial pressure and premature ventricular contractions. The dysrhythmogenic dose of epinephrine was defined as the smallest dose producing four or more premature ventricular contractions within 15-s period. We examined the antidysrhythmic action of dexmedetomidine in the presence of two kinds of alpha 2 antagonists, that is, agents that label imidazoline receptors and exert a pharmacologic action through imidazoline receptors (idazoxan and atipamezole) and agents that are nonimidazoline compounds and are lacking in pharmacologic action through imidazoline receptors (rauwolscine and L-659,066). They were given cerebroventricularly.

RESULTS

Idazoxan and atipamezole significantly inhibited the antidysrhythmic action of dexmedetomidine, whereas rauwolscine and L-659,066 did not.

CONCLUSIONS

Because alpha 2 antagonists having imidazoline or imidazole structures inhibited the antidysrhythmic action of dexmedetomidine, and the inhibition produced by the non-imidazoline alpha 2 antagonists was not significant, imidazoline receptors in the central nervous system are more responsible for the antidysrhythmic action of dexmedetomidine than are alpha 2 adrenoceptors.

Pharmacology of CS-866, a novel nonpeptide angiotensin II receptor antagonist

CS-866, (5-methyl-2-oxo-1,3-dioxolen-4-yl)methoxy-4-(1-hydroxy-1- methylethyl)-2-propyl-1-(4-[2-(tetrazol-5-yl)-phenyl]phenyl)met hylimidazol- 5-carboxylate, a prodrug type angiotensin receptor antagonist, is deesterified to the active acid, RNH-6270. RNH-6270 inhibited [125I]angiotensin II binding to bovine adrenal cortical membranes (angiotensin AT1 receptors) with an IC50 value of 7.7 nM, but not [125I]angiotensin II binding to bovine cerebellar membranes (angiotensin AT2 receptors), indicating the selectivity of the compound for angiotensin AT1 receptors. In guinea pig aortas, RNH-6270 reduced the maximal response of the concentration-contractile curve for angiotensin II (pD'2 = 9.9), but had no effect on the contractile response induced by phenylephrine or KCl. In conscious rats, intravenously injected RNH-6270 inhibited angiotensin II-induced pressor responses in a dose-dependent manner, and orally administered CS-866 produced a long-lasting inhibition of angiotensin II pressor responses. SK&F-525A, a P-450 inhibitor, suppressed the angiotensin II inhibitory effect of losartan, but not that of CS-866. These results demonstrate that RNH-6270 is a potent and AT1-selective angiotensin receptor antagonist and that, after oral administration, CS-866 has a long-lasting angiotensin II inhibitory action which is not affected by drug metabolizing enzymes in the liver.

Alpha-2 adrenergic modulation of sleep: time-of-day-dependent pharmacodynamic profiles of dexmedetomidine and clonidine in the rat

Alpha adrenergic agonists such as clonidine are widely used for their antihypertensor effects, but they also cause sedation. The mechanisms underlying soporific effects of such compounds are poorly understood, but appear to involve the alpha-2 adrenergic receptor sub-type. To further investigate the role of this receptor in sleep-wake regulation, rats received injections i.p. either during their peak of activity (circadian time CT-18: 6 hr after lights out) or near the mid-point of their sleep-dominated phase (CT-5: 5 hr after lights on) with either the highly selective alpha-2 agonist dexmedetomidine (dMED) 0.02 to 0.04 mg/kg or the less selective alpha-2 agonist, clonidine 0.04 to 0.08 mg/kg, or vehicle. Clonidine and dMED showed remarkable overall similarities in their soporific profiles. Except for the lower dose of clonidine, both CT-5 and CT-18 treatments increased the percent of time spent in non-REM (NREM) sleep. The increase in NREM was followed by a reduction of NREM sleep that was accompanied by locomotor activity and body temperature above control levels. After CT-5 treatments, this period of reduced NREM sleep was followed by a secondary increase in NREM 7 to 10 hr posttreatment. REM sleep was markedly reduced for 9 to 10 hr after all treatments at both times of day, with elevated REM levels 18 to 30 hr posttreatment. Pre-treatment with the selective alpha-2 antagonist atipamezole (0.5 mg/kg) reversed the effects of CT-18 dMED 0.04 mg/kg except REM sleep suppression, which was only partially reversed. The NREM-inducing potency of dMED 0.02 mg/kg was greater when administered at CT-18 than at CT-5. Taken together with other evidence, these findings suggest that the profound NREM-inducing effects of dMED may be mediated by postsynaptic alpha-2 adrenoceptors. Furthermore, the pharmacodynamic action of alpha-2 adrenergic agonists, like many other sedative hypnotics (e.g., benzodiazepines), produce a hysteresis in sleep-wake regulation characterized by "rebound" waking after drug-induced sleep.