The effect of sorbinil, an aldose reductase inhibitor, on aortic function in control and streptozotocin-induced diabetic rats

1. The present study investigates the effect of treatment of 14-day streptozotocin-diabetic rats with the aldose reductase inhibitor, sorbinil, on changes ex vivo in aortic vasoconstriction and vasodilation. 2. Maximum contractile responses and aortic sensitivity to phenylephrine were significantly enhanced in aortae from 14-day diabetic rats, in accordance with our previous data. 3. Endothelium-dependent relaxations to carbachol were, in contrast, depressed, although endothelium-independent relaxations to forskolin and sodium nitroprusside were unaltered. 4. Sorbinil treatment of diabetic animals failed to prevent any of these diabetes-induced alterations in aortic function, and indeed exacerbated some of these alterations. In addition, sorbinil treatment caused altered aortic responses in control animals, which sometimes mirrored those found in diabetic animals. 5. It can be concluded that sorbinil may have actions in addition to, and independent of, polyol pathway inhibition. Thus, sorbinil may not be an effective tool for the investigation of aldose reductase inhibition within the vascular system of the rat.

Calmidazolium-induced rises in cytosolic calcium concentrations in Madin Darby canine kidney cells

The effect of calmidazolium on Ca(2+) signaling in Madin Darby canine kidney (MDCK) cells was investigated using fura-2 as a Ca(2+) probe. Calmidazolium at 2-5 microM increased [Ca(2+)](i) concentration dependently. The [Ca(2+)](i) rise induced by 2-5 microM calmidazolium comprised an immediate rise and a slow decay. External Ca(2+) removal partly inhibited the Ca(2+) signals, suggesting that calmidazolium activated external Ca(2+) influx and internal Ca(2+) release. In Ca(2+)-free medium, pretreatment with 3 microM calmidazolium abolished the Ca(2+) release induced by 1 microM thapsigargin, an endoplasmic reticulum Ca(2+) pump inhibitor, and, vice versa, pretreatment with thapsigargin inhibited calmidazolium-induced Ca(2+) release. This indicates that thapsigargin-sensitive Ca(2+) store was the source of calmidazolium-induced Ca(2+) release. Calmidazolium (3 microM) induced Mn(2+) quench of fura-2 fluorescence at 360 nm excitation wavelength, which was suppressed by 0.1 mM La(3+). Addition of 3 mM Ca(2+) increased [Ca(2+)](i) after pretreatment with 3-5 microM calmidazolium in Ca(2+)-free medium. This implies that calmidazolium activated concentration-dependent capacitative Ca(2+) entry. Calmidazolium (3 microM) augmented the capacitative Ca(2+) entry induced by 1 microM thapsigargin or 0.1 mM ATP by 38%. Calmidazolium (3 microM)-induced Ca(2+) release was blocked by pretreatment with 40 microM aristolochic acid and 2 microM U73122 (2 microM) to inhibit phospholipase A(2) and phospholipase, respectively, but pretreatment with 0.1 mM propranolol to inhibit phospholipase D had no effect. This suggests that calmidazolium induced internal Ca(2+) release in a manner dependent on phospholipases C- and A(2)-coupled events.

Phenolics: blocking agents for heterocyclic amine-induced carcinogenesis

Chemopreventive effects of synthetic and naturally occurring antioxidants on heterocyclic amine (HCA)-induced rat carcinogenesis and mechanisms of inhibition were assessed. In a medium-term liver bioassay, combined treatment with 0.03% 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and synthetic antioxidants such as 1-O-hexyl-2,3,5-trimethylhydroquinone (HTHQ), butylated hydroxyanisole (BHA), butylated hydroxutoluene (BHT), tert-butylhydroquinone (TBHQ) or propyl gallate, each at a dose of 0.25%, inhibited development of preneoplastic glutathione S-transferase placental form (GST-P) positive foci as compared with MeIQx alone, after initiation with diethylnitrosamine (DEN). Of these antioxidants, HTHQ showed the greatest activity. 8-Hydroxydeoxyguanosine (8-OHdG), a marker for DNA damage induced by active oxygen species, and malonedialdehyde and 4-hydroxynonenal levels were not largely influenced by the treatment with MeIQx or antioxidants, either alone or in combination. In the same medium-term liver bioassay, effects of some naturally occurring antioxidants, such as green tea catechins (GTC), hesperidin, chlorogenic acid, quercetin, rutin, curcumin, daidzin, ferulic acid and genistein were also examined. Of these antioxidants, only GTC tended to inhibit GST-P positive foci development, while quercetin, rutin, curcumin, daidzin, ferulic acid and genistein all exerted significant enhancing effects. Examination of HTHQ influence in a medium term liver bioassay with HCA Glu-P-1, in which the experimental period was extended for up to 26 weeks, also demonstrated a significant decrease in the incidence of liver tumours to 40% in the group treated with 0.5% HTHQ and 0.03% 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1) as compared with the Glu-P-1 alone value of 89%. Effects of HTHQ on colon carcinogenesis induced by 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) were evaluated in a two-stage colon carcinogenesis model using 1,2-dimethylhydrazine (DMH) as an initiator. At week 36, the multiplicity of colon tumours induced by 0.02% PhIP after DMH initiation (9.1+/-6.2/rat) was dose-dependently decreased by the combined treatment with 0.5% HTHQ (3.6+/-1.8, P < 0.001) and 0.125% HTHQ (6.2+/-3.2, not significant). Similarly, the incidence of mammary carcinomas in female F344 rats induced by oral administration of 0.02% PhIP (40%) for 52 weeks was significantly decreased by simultaneous treatment with 0.5% HTHQ (5%). Alpha-tocopherol and chlorophyllin only reduced the multiplicity of carcinomas. Analysis of the influence of HTHQ on metabolic activation of Glu-P-1 or PhIP after incubation with rat S9 mixture and NADPH by HPLC, revealed that each major metabolite was strongly reduced by the addition of HTHQ. Immunohistochemically detected PhIP-DNA adduct positive nuclei in the colon induced by continuous oral treatment with 0.02% PhIP for 2 weeks decreased by the combined treatment with 0.5 or 0.125% HTHQ. These results indicate that synthetic antioxidant HTHQ is a very strong chemopreventor of heterocyclic amine (HCA)-induced carcinogenesis and that depressed metabolic activation rather than antioxidant activity is responsible for the observed effect.

Moxonidine, a selective alpha2-adrenergic and imidazoline receptor agonist, produces spinal antinociception in mice

alpha2-Adrenergic receptor (AR)-selective compounds produce antihypertensive and antinociceptive effects. Moxonidine alleviates hypertension in multiple species, including humans. This study demonstrates that intrathecally administered moxonidine produces antinociception in mice. Antinociception was detected via the (52.5 degrees C) tail-flick and Substance P (SP) nociceptive tests. Moxonidine was intrathecally administered to ICR, mixed C57BL/6 x 129/Sv [wild type (WT)], or C57BL/6 x 129/Sv mice with dysfunctional alpha2aARs (D79N-alpha2a). The alpha2AR-selective antagonist SK&F 86466 and the mixed I1/alpha2AR-selective antagonist efaroxan were tested for inhibition of moxonidine-induced antinociception. Moxonidine prolonged tail-flick latencies in ICR (ED50 = 0.5 nmol; 0. 3-0.7), WT (0.17 nmol; 0.09-0.32), and D79N-alpha2a (0.32 nmol; 0. 074-1.6) mice. Moxonidine inhibited SP-elicited behavior in ICR (0. 04 nmol; 0.03-0.07), WT (0.4 nmol; 0.3-0.5), and D79N-alpha2a (1.1 nmol; 0.7-1.7) mice. Clonidine produced antinociception in WT but not D79N-alpha2a mice. SK&F 86466 and efaroxan both antagonized moxonidine-induced inhibition of SP-elicited behavior in all mouse lines. SK&F 86466 antagonism of moxonidine-induced antinociception implicates the participation of alpha2ARs. The comparable moxonidine potency between D79N-alpha2a and WT mice suggests that receptors other than alpha2a mediate moxonidine-induced antinociception. Conversely, absence of clonidine efficacy in D79N-alpha2a mice implies that alpha2aAR activation enables clonidine-induced antinociception. When clinically administered, moxonidine induces fewer side effects relative to clonidine; moxonidine-induced antinociception appears to involve a different alpha2AR subtype than clonidine-induced antinociception. Therefore, moxonidine may prove to be an effective treatment for pain with an improved side effect profile.

Inhibition of xenobiotic-induced genotoxicity in cultured precision-cut human and rat liver slices

In this study precision-cut liver slices have been used to evaluate the effects of the flavone tangeretin, the flavonoid glycoside naringin and the flavanone naringenin (the aglycone derived from naringin) on xenobiotic-induced genotoxicity. Liver slices were cultured for 24 h in medium containing [3H]thymidine and the test compounds and then processed for autoradiographic determination of unscheduled DNA synthesis (UDS). The cooked food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) markedly induced UDS in cultured human liver slices and both 2-acetylaminofluorene (2-AAF) and aflatoxin B1 (AFB1) induced UDS in cultured rat liver slices. Tangeretin (20 and 50 microM) was found to be a potent inhibitor of 5 and 50 microM PhIP-induced UDS in human liver slices, whereas 20 and 50 microM naringenin was ineffective and naringin only inhibited genotoxicity at a concentration of 1000 microM. In rat liver slices 50 microM tangeretin inhibited 10 and 50 microM 2-AAF-induced UDS, whereas 50 microM naringenin and 100 and 1000 microM naringin were ineffective. None of the three flavonoids examined inhibited 5 microM AFB1-induced UDS in rat liver slices. The inhibition of PhIP- and 2-AAF-induced UDS by tangeretin is probably attributable to the inhibition of the human and rat cytochrome P-450 isoforms which are responsible for the bioactivation of these two genotoxins. Although flavonoids can modulate xenobiotic-induced genotoxicity in human and rat liver slices, any protective effect is dependent on the particular combination of genotoxin and flavonoid examined. These results demonstrate that cultured precision-cut liver slices may be utilised as an in vitro model system to examine the modulation of xenobiotic-induced genotoxicity by flavonoids and other dietary components.

Reversal of sedative and clinicophysiological effects of epidural xylazine and detomidine with atipamezole and yohimbine in buffaloes (Bubalus bubalis)

The sedative and clinicophysiological effects of either xylazine (0.1 mg/kg) or detomidine (50 micrograms/kg) administered epidurally to groups of five buffaloes were reversed by the intravenous administration of either yohimbine (0.125 mg/kg) or atipamezole (10 micrograms/kg). Epidural xylazine or detomidine produced bradycardia, respiratory depression, ruminal stasis and hypothermia and these effects were completely reversed within five to 15 minutes after the intravenous administration of atipamezole or yohimbine, with no signs of excitement or resedation. Atipamezole reversed the effects of xylazine or detomidine more rapidly than yohimbine.

Agmatine inhibited tolerance to and dependence on morphine in guinea pig ileum in vitro

AIM

To observe effect of agmatine (Agm) on tolerance to and substance dependence on morphine (Mor) in guinea pig ileum longitudinal muscle (GPILM).

METHODS

The experiment was performed in electric field stimulation (EFS) test in vitro.

RESULTS

Mor inhibited twitch contractions of GPILM induced by EFS [IC50 = 140 (107-182) nmol.L-1]. Incubation of GPILM with Mor 270 nmol.L-1 for 8 h evoked a 37-fold increase in IC50 of Mor (tolerance) and a contractile response to naloxone (Nal, substance dependence). When the preparations were coincubated with Mor + Nal and Mor + Agm, Mor lost the ability to induce tolerance and inhibited the contractile responses of the preparations to Nal by 90% and 75%, respectively. These effects of Agm could be almost completely antagonized by idazoxan.

CONCLUSION

Agm prevented the development of tolerance to and substance dependence on Mor in GPILM in vitro by activation of imidazoline receptors.

The analgesic potency of dexmedetomidine is enhanced after nerve injury: a possible role for peripheral alpha2-adrenoceptors

This study investigated the analgesic potency and site of action of systemic dexmedetomidine, a selective alpha2-adrenoceptor (alpha2AR) agonist, in normal and neuropathic rats. Ligation of the L5-6 spinal nerves produced a chronic mechanical and thermal neuropathic hyperalgesia in rats. von Frey fibers and a thermoelectric Peltier device were used to measure mechanical and heat withdrawal thresholds over the hindpaw. Systemic dexmedetomidine dose-dependently increased the mechanical and thermal thresholds in the control animals (50% effective dose [ED50] 144 and 180 microg/kg intraperitoneally [i.p.], respectively). Neuropathic animals responded to much smaller doses of dexmedetomidine with mechanical and thermal ED50 values of 52 and 29 microg/kg i.p., respectively. There was no difference between the control and neuropathic animals with respect to dexmedetomidine-evoked sedation, as determined by decreased grid crossings in an open-field activity chamber (ED50 12 and 9 microg/kg i.p., respectively). Atipamezole, a selective alpha2AR antagonist, blocked the analgesic and sedative actions of dexmedetomidine inboth the neuropathic and control animals. However, L-659,066, a peripherally restricted alpha2AR antagonist, could only block the analgesic actions of dexmedetomidine in the neuropathic rats, with no effect in control animals. In conclusion, nerve injury enhanced the analgesic but not the sedative potency of systemic dexmedetomidine and may have shifted the site of alpha2 analgesic action to outside the blood-brain barrier.

IMPLICATIONS

We tested the analgesic efficacy of the alpha2 agonist dexmedetomidine in normal and nerve-injured rats. The analgesic potency of dexmedetomidine was enhanced after nerve injury with a site of action outside the central nervous system. Peripherally restricted alpha2 agonists may be useful in the management of neuropathic pain.

Reversal of the sedative and sympatholytic effects of dexmedetomidine with a specific alpha2-adrenoceptor antagonist atipamezole: a pharmacodynamic and kinetic study in healthy volunteers

BACKGROUND

Specific and selective alpha2-adrenergic drugs are widely exploited in veterinary anesthesiology. Because alpha2-agonists are also being introduced to human practice, the authors studied reversal of a clinically relevant dexmedetomidine dose with atipamezole, an alpha2-antagonist, in healthy persons.

METHODS

The study consisted of two parts. In an open dose-finding study (part 1), the intravenous dose of atipamezole to reverse the sedative effects of 2.5 microg/kg of dexmedetomidine given intramuscularly was determined (n = 6). Part 2 was a placebo-controlled, double-blinded, randomized cross-over study in which three doses of atipamezole (15, 50, and 150 microg/kg given intravenously in 2 min) or saline were administered 1 h after dexmedetomidine at 1-week intervals (n = 8). Subjective vigilance and anxiety, psychomotor performance, hemodynamics, and saliva secretion were determined, and plasma catecholamines and serum drug concentrations were measured for 7 h.

RESULTS

The mean +/- SD atipamezole dose needed in part 1 was 104+/-44 microg/kg. In part 2, dexmedetomidine induced clear impairments of vigilance and psychomotor performance that were dose dependently reversed by atipamezole (P < 0.001). Complete resolution of sedation was evident after the highest (150 microg/kg) dose, and the degree of vigilance remained high for 7 h. Atipamezole dose dependently reversed the reductions in blood pressure (P < 0.001) and heart rate (P = 0.009). Changes in saliva secretion and plasma catecholamines were similarly biphasic (i.e., they decreased after dexmedetomidine followed by dose-dependent restoration after atipamezole). Plasma norepinephrine levels were, however, increased considerably after the 150 microg/kg dose of atipamezole. The pharmacokinetics of atipamezole were linear, and elimination half-lives for both drugs were approximately 2 h. Atipamezole did not affect the disposition of dexmedetomidine. One person had symptomatic sinus arrest, and another had transient bradycardia approximately 3 h after receiving dexmedetomidine.

CONCLUSIONS

The sedative and sympatholytic effects of intramuscular dexmedetomidine were dose dependently antagonized by intravenous atipamezole. The applied infusion rate (75 microg x kg(-1) x min(-1)) for the highest atipamezole dose was, however, too fast, as evident by transient sympathoactivation. Similar elimination half-lives of these two drugs are a clear advantage considering the possible clinical applications.

Inhibition of PhIP mutagenicity by caffeine, lycopene, daidzein, and genistein

The heterocyclic amine 2-amino-1-methyl-6-phenylimidazo[4,5-beta]pyridine NPhIP) is a major dietary component in individuals eating cooked meats or fish. This heterocyclic amine requires biochemical activation, mainly through cytochrome P4501A2, and can be detoxified chiefly by 4'hydroxylation through other cytochromes, and be in turn converted through phase 2 enzymes to readily excreted conjugates. The active form of PhIP is mutagenic in Salmonella typhimurium TA98 and is a useful substrate to study the possible chemoprotective action of phytochemicals. We found that black and green tea depressed the mutagenicity of PhIP in dose-related fashion, and decaffeinated tea was less powerful an inhibitor. This led to the study of caffeine, that displayed effective dose-related inhibition of the mutagenicity of PhIP. Other antioxidants such as lycopene, the active antioxidant from tomatoes, and daidzein and genistein from soy products, also had a dose-related inhibition of the mutagenicity of PhIP. We conclude that PhIP is a good substrate found in several human foods to determine the protective effect of phytochemicals from vegetables, and beverages.

Potential role for imidazole in the rhythmic respiratory activity of the in vitro neonatal rat brainstem

We used the imidazole-binding agent, diethylpyrocarbonate (DEPC), to test the hypothesis that rhythmic respiratory activity of the in vitro neonatal rat brainstem-spinal cord preparation was functionally dependent on imidazole. Neural activity was recorded from spinal nerves (C1-C4) during superfusion with 95%O2/5%CO2 buffer at pH 7.3 and T = 26 degrees C. Superfusate containing DEPC (40 mM) caused cessation of rhythmic activity within minutes. In eight of 33 preparations, microinjection of DEPC (32 nmol) onto the ventral medullary surface (VMS) reduced burst amplitude by at least 50% within 10 min, and in 12 of 33 preparations, microinjection of DEPC produced neural apnea. Therefore, we conclude that proteins containing imidazole near the VMS are critically important for the maintenance of rhythmic respiratory activity in vitro. Furthermore, alphastat regulation of respiration may be an essential trait of this preparation.

1,5-Bis(4-amidinophenoxy)pentane (pentamidine) is a potent inhibitor of [3H]idazoxan binding to imidazoline I2 binding sites

The aromatic diamidine 1,5-bis(4-amidinophenoxy)pentane (pentamidine) is used for treatment and prophylaxis of Pneumocystis carinii pneumonia in patients with Acquired Immune Deficiency Syndrome. Clinical use of pentamidine has been restricted by significant toxicity, that includes hypotension, and hypoglycemia. Although clinical toxicity is well described, the mechanisms are still poorly understood. Competitive binding analyses using [3H]idazoxan as the radioligand, and cirazoline to define non-specific binding, demonstrate that pentamidine binds to an imidazoline I2 binding site on rat liver membranes with a Ki of 1.4+/-0.22 nM. The Ki indicates that pentamidine inhibits radioligand binding at imidazoline I2 sites with an affinity approximating the most potent known ligands and may be related to pentamidine toxicity. Moreover, pentamidine analogs inhibit radioligand binding with a range of affinities that vary according to their structure. Two candidate drugs, Compounds 5 and 6, are more active than pentamidine in the corticosteroid-suppressed rat model of P. carinii pneumonia, yet have different affinities for the imidazoline I2 site (Ki 5 = 50.1+/-1.06 nM and Ki 6 = approximately 3500 nM). Affinity for this site does not correlate with antimicrobial activity (r = 0.60; p = 0.09) or the calculated log of the octanol:water partition coefficient (ClogP) (r = -0.38; p = 0.22).

Inhibitory effects of antioxidants on formation of heterocyclic amines

It is important to search for effective antioxidants to suppress formation of mutagenic and carcinogenic heterocyclic amines (HCAs), like 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), because these HCAs are considered to be probable human carcinogens. The effects of various food-derived antioxidants on MeIQx formation were examined by their addition (0.2 mmol each) to mixtures of creatine (0.4 mmol), glycine (0.4 mmol) and glucose (0.2 mmol), and heating at 128 degreesC for 2 h. Glycine was replaced by l-phenylalanine in the case of PhIP formation. Among the 14 kinds of antioxidants tested, green tea catechins and the major component [(-)-epigallocatechin gallate], two flavonoids (luteolin and quercetin) and caffeic acid were found to clearly suppress the formation of both MeIQx and PhIP, being 3.2-75% of the level of the controls. These phenolic antioxidants also reduced the total mutagenicity of the heated mixtures. The results suggest that foodstuffs containing catechins, flavonoids and caffeic acid may suppress the formation of HCAs in cooked foods.

Immobilization of wild kinkajous (Potos flavus) with medetomidine-ketamine and reversal by atipamezole

As part of a wildlife rescue during the filling of a lake created by a hydroelectric dam (Petit Saut, French Guiana), 10 wild kinkajous (Potos flavus) were immobilized with medetomidine and ketamine for clinical examination and collection of biological samples. A mean (+/-SD) i.m. dose of 0.11+/-0.01 mg/kg medetomidine and 5.5+/-0.6 mg/kg ketamine rapidly induced complete immobilization (3.0+/-0.9 min) with good muscle relaxation and loss of corneal and pedal withdrawal reflexes. The duration and the quality of the anesthesia allowed procedures including minor surgery. Rectal temperature, heart and respiration rates, and relative oxyhemoglobin saturation (SpO2) were monitored at 5 min, 15 min, and 30 min after the medetomidine ketamine injection. Rectal temperature and heart rate significantly decreased during this time (P < 0.05). Low values of SpO2 (<90%) were recorded shortly after the injection. Hypoxemia partially resolved with time, confirmed by an increase in most SpO2 values. Atipamezole given i.m. at 5 mg/mg of medetomidine reversed the effects of the medetomidine in kinkajous. No adverse effects were observed during recovery. In group I, the antagonist was injected at 40.6+/-3.9 min. In group II, the animals showed signs of spontaneous recovery 37.9+/-6.9 min before antagonist injection at 52.2+/-6.1 min. Time from antagonist injection to ambulatory state was significantly shorter (P < 0.05) in group II (2.8+/-1.1 min) than in group I (6.9+/-1.2 min).

Observations on the use of medetomidine/ketamine and its reversal with atipamezole for chemical restraint in the mouse

Ketamine and medetomidine produced chemical restraint for minor procedures in mice. Male mice required 50 mg/kg ketamine, 10 mg/kg medetomidine intraperitoneally (i.p.), and females a higher dose of ketamine (75 mg/kg i.p.). The onset of restraint effects, judged by loss of righting reflex, was more rapid in males than females. The effects were reversed using atipamezole (1-2.5 mg/kg). Recovery following administration of atipamezole was more rapid in males than females. We conclude that ketamine/medetomidine, followed by reversal with atipamezole, is an effective technique for chemical restraint in the mouse.

Sedative effects of medetomidine and its reversal by atipamezole in llamas

OBJECTIVE

To determine a dose of medetomidine that will induce sedation in llamas, to assess effects of medetomidine sedation on arterial blood gas variables, and to determine efficacy of atipamezole in reversing medetomidine-induced sedation.

DESIGN

Prospective, randomized clinical trial.

ANIMALS

15 clinically normal adult llamas.

PROCEDURE

9 llamas received various doses of medetomidine (0.01, 0.02, or 0.03 mg/kg [0.005, 0.009, or 0.014 mg/lb] of body weight, i.m.). Heart and respiratory rates and sedative effects were recorded. Using the lowest dose that induced deep sedation, 6 different llamas were used to assess effects of medetomidine on arterial blood gas variables. These same 6 llamas were later given atipamezole (0.125 mg/kg [0.057 mg/lb], i.v.) 30 minutes after medetomidine injection. Heart and respiratory rates, sedative effects, and time from atipamezole injection to standing were recorded.

RESULTS

Sedation began 6.67 +/- 1.15 minutes (mean +/- SD) after medetomidine administration (0.03 mg/kg, i.m.). Arterial blood gas variables measured 30 and 60 minutes after injection were not different from baseline. Llamas that did not receive atipamezole remained recumbent for 91.50 +/- 24.68 minutes. After atipamezole administration, llamas were able to stand in 5.80 +/- 3.27 minutes.

CLINICAL IMPLICATIONS

Medetomidine induced light to deep sedation in a dose-dependent manner in clinically normal llamas. A dose of 0.03 mg/kg induced deep sedation with a short period of analgesia. Atipamezole rapidly reversed effects of medetomidine, and llamas recovered quickly and were soon able to stand.

Potential mechanisms of moxonidine-induced ocular hypotension: role of norepinephrine

In rabbit's aqueous humor, norepinephrine, epinephrine, dopamine and serotonin were detected simultaneously by a high performance liquid chromatography with electrochemical detection. Furthermore, the changes in catecholamine levels in aqueous humor were evaluated after topical application of moxonidine, an imidazoline1/alpha 2 receptor agonist, in the presence and absence of efaroxan. The level of norepinephrine in aqueous humor was reduced by moxonidine treatment. However, under the same set of conditions, there were no significant changes in the levels of dopamine, epinephrine or serotonin. Pretreatment with efaroxan antagonized moxonidine-induced suppression of norepinephrine levels. In other in vivo experiments, moxonidine caused a decrease in intraocular pressure which was antagonized by pretreatment with efaroxan. In the superior cervical ganglion preparation, norepinephrine release was increased 5-fold by the presence of a high K+ medium. The K(+)-evoked norepinephrine secretion was reduced by 55% by moxonidine. Pretreatment with efaroxan blocked the moxonidine-induced inhibition of norepinephrine release. It is concluded that inhibition of norepinephrine release from the superior cervical ganglion and suppression of aqueous norepinephrine levels contribute to the moxonidine-induced lowering of intraocular pressure. Moreover, the antagonism of moxonidine's in vivo and in vitro effects by efaroxan suggests the involvement of imidazoline1 receptors, but does not preclude activity on alpha 2 adrenoceptors.

Pharmacological profiles of a novel non-peptide angiotensin II type I receptor antagonist HR720 in vitro and in vivo

The pharmacological properties of 2-butyl-4-(methylthio)-1-[[2'-[[[(propylamino)carbonyl] amino]sulfonyl](1,1'-biphenyl)-4-yl]methyl]-1H-imidazole-5-carboxylate (HR720), a novel non-peptide angiotensin (Ang) II type I (AT1) receptor antagonist, were characterized in both in vitro and in vivo systems. In vitro autoradiography using 125I-[Sar1,Ile8]Ang II as a ligand revealed that HR720 competitively inhibited the specific binding of the ligand to the adrenal cortex. The IC50 value for the adrenal cortex was 1.5 x 10(-8) M, and the IC50 for medulla was 1.4 x 10(-6) M. Similar results were obtained in the adrenal cortex with CV-11974, a known potent AT1-receptor antagonist. Since AT1 receptors are known to predominate in the adrenal cortex and AT2-receptors in the adrenal medulla, it is considered that HR720 is highly selective for AT1 receptors. HR720 inhibited the Ang II-induced contraction of isolated rabbit aortic strips and human gastroepiploic arteries in a noncompetitive manner, pD'2=9.40 and 9.62 for rabbit aorta and human artery, respectively. With CV-11974, pD'2 values of 9.84 in isolated rabbit aorta and 10.00 in human artery were obtained. HR720 did not affect the norepinephrine-, serotonin- or KCl-induced contraction even at a concentration of 1 x 10(-5) M. In anesthetized hamsters, HR720 induced a dose-dependent inhibition of the pressure response to Ang II. The potency of HR720 to antagonize the Ang II-induced pressure response was similar to that of CV-11974. These results demonstrate that HR720 is a potent and selective AT1-receptor antagonist.

Evaluation of the effects of a specific alpha 2-adrenoceptor antagonist, atipamezole, on alpha 1- and alpha 2-adrenoceptor subtype binding, brain neurochemistry and behaviour in comparison with yohimbine

In the present study we evaluated the alpha 1- and alpha 2-adrenoceptor subtype binding, central alpha 2-adrenoceptor antagonist potency, as well as effects on brain neurochemistry and behavioural pharmacology of two alpha 2-adrenoceptor antagonists, atipamezole and yohimbine. Atipamezole had higher selectivity for alpha 2- vs. alpha 1-adrenoceptors than yohimbine regardless of the subtypes studied. Both compounds had comparable affinity for the alpha 2A-, alpha 2C- and alpha 2B-adrenoceptors, but yohimbine had significantly lower affinity for the alpha 2D-subtype. This may account for the fact that significantly higher doses of yohimbine than atipamezole were needed for reversal of alpha 2-agonist (medetomidine)-induced effects in rats (mydriasis) and mice (sedation and hypothermia). The effect on central monoaminergic activity was estimated by measuring the concentrations of transmitters and their main metabolites in whole brain homogenate. At equally effective alpha 2-antagonising doses in the rat mydriasis model, both drugs stimulated central noradrenaline turnover (as reflected by increase in metabolite levels) to the same extent. Atipamezole increased dopaminergic activity only slightly, whereas yohimbine elevated central dopamine but decreased central 5-hydroxytryptamine turnover rates. In behavioural tests, atipamezole (0.1-10 mg/kg) did not affect motor activity but stimulated food rewarded operant (FR-10) responding (0.03-3 mg/kg) whereas yohimbine both stimulated (1 mg/kg) and decreased (> or = 3 mg/kg) behaviour in a narrow dose range in these tests. In the staircase test, both antagonists increased neophobia, but in the two compartment test only yohimbine (> or = 3 mg/kg) decreased exploratory behaviour. The dissimilar effects of the antagonists on neurochemistry and behaviour are thought to be caused by non alpha 2-adrenoceptor properties of yohimbine. In conclusion, the alpha 2-antagonist atipamezole blocked all alpha 2-adrenoceptor subtypes at low doses, stimulated central noradrenergic activity and had only slight effects on behaviour under familiar conditions, but increased neophobia. The low affinity for the alpha 2D-adrenoceptor combined with its unspecific effects complicates the use of yohimbine as pharmacological tool to study alpha 2-adrenoceptor physiology and pharmacology.

Reversal of medetomidine-induced sedation in reindeer (Rangifer tarandus tarandus) with atipamezole increases the medetomidine concentration in plasma

The pharmacokinetics of two potent alpha 2-adrenoceptor agents that can be used for immobilization (medetomidine) and reversal (atipamezole) of the sedation in mammals, were studied in three reindeer (Rangifer tarandus tarandus) in winter and again in summer. Medetomidine (60 micrograms/kg) was injected intravenously (i.v.), followed by atipamezole (300 micrograms/kg) intravenously 60 min later. Drug concentrations in plasma were measured by HPLC. The administration of atipamezole resulted in an immediate 2.5-3.5 fold increase in the medetomidine concentration in plasma. Clearance for medetomidine (median 19.3 mL/min.kg) was lower than clearance for atipamezole (median 31.0 mL/min.kg). The median elimination half-lives of medetomidine and atipamezole in plasma were 76.1 and 59.9 min, respectively. The animals became resedated 0.5-1 h after the reversal with atipamezole. Resedation may be explained by the longer elimination half-life of medetomidine compared to atipamezole.

The effects of alpha-2 agonist, medetomidine and its antagonist, atipamezole on reaction and movement times in a visual choice reaction time task in monkeys

Alpha-2 adrenoceptor agonists have been shown to improve the working memory task performance of aged monkeys. Suggestions offered to explain this finding include improved short-term memory processing, slight sedation, and decreased distractiveness. Although sedation is evident at high doses, it may also contribute to the working memory task performance at low doses. The aim of the present work was to find out whether the positive effects of an alpha-2 agonist, medetomidine, on working memory performance could be explained by its sedative effects. This was accomplished by measuring the reaction and movement times of monkeys performing a visual choice reaction time task under the influence of medetomidine or its antagonist atipamezole. In the task a trial began with the monkey holding a central pad. After a short period one of two lateral light emitting diodes was turned on for 300 ms and the monkeys were trained to release the central bar and touch either of the bars, situated below the diodes, depending on the location of the stimulus. The reaction and movement times were significantly longer than on saline control only at the highest dose of medetomidine (10.0 micrograms/kg). At the lowest dose of atipamezole (0.01 mg/kg), the reaction times were significantly shorter than on saline control. The results of this study demonstrate that low doses of medetomidine, which have earlier been shown to improve working memory performance, do not induce sedation as measured by reaction and movement times.

Effects of SCA40 on bovine trachealis muscle and on cyclic nucleotide phosphodiesterases

While UK-93,928 (1-[[3-(6,9-dihydro-6-oxo-9-propyl-1H-purin-2-yl)-4-ethoxyphenyl] sulfonyl]-4-methylpiperazine; 5 nM-5 microM) was devoid of relaxant activity, benzafentrine, isoprenaline, levcromakalim and SCA40 (6-bromo-8-methylaminoimidazo[1,2-a]pyrazine-2-carbonitrile) each relaxed histamine (460 microM)-precontracted bovine isolated trachealis. Each of these relaxants was antagonised by a K+-rich (80 mM) medium. Except in the case of levcromakalim, nifedipine (1 microM) offset this antagonism. Charybdotoxin (100 nM) antagonised isoprenaline in a nifedipine-sensitive manner but did not antagonise SCA40 or benzafentrine. Iberiotoxin (100 nM) did not antagonise SCA40. Acting on tissue precontracted with carbachol, SCA40 potentiated isoprenaline but did not potentiate sodium nitroprusside. While levcromakalim (1 and 10 microM) induced hyperpolarisation, SCA40 (1 and 10 microM) induced little change in the membrane potential of bovine trachealis. In trachealis preloaded with 86Rb+, levcromakalim (1 and 10 microM) promoted efflux of the radiotracer while SCA40 (1 and 10 microM) had no effect. Tested as an inhibitor of isoenzymes of cyclic nucleotide phosphodiesterase, SCA40 was most potent against the type III, less potent against the type IV and least potent against the type I isoenzyme. It is concluded that neither inhibition of phosphodiesterase type V nor the promotion of BKCa channel opening explains the tracheal smooth muscle relaxant activity of SCA40. This compound relaxes bovine tracheal smooth muscle mainly by inhibiting phosphodiesterase isoenzyme types III and IV.