Location of the mechanism of the clonidine withdrawal tachycardia in rats

Withdrawal of chronic infusion of clonidine elicits severe tachycardia and short-lasting blood pressure elevations (upswings). Withdrawal of clonidine in low dosage (30 micrograms kg-1 day-1 i.c.v., 7 days) elicited a maximum of 10.9 +/- 0.5 upswings h-1. Cessation of s.c. infusion of clonidine (30 micrograms kg-1 day-1 7 days) evoked a maximum of 1.9 +/- 0.5 upswings h-1. After cessation of the two clonidine infusions no overshoot of heart rate occurred. Withdrawal of a higher dose of clonidine (300 micrograms kg-1 day-1 s.c., 7 days), however, induced tachycardia (from 302 +/- 8 to 433 +/- 8 beats min-1) and 7.6 +/- 1.4 upswings h-1. The administration of the alpha 2-adrenoceptor antagonist yohimbine precipitated withdrawal tachycardia in animals treated with oxymetazoline, a hydrophilic alpha-adrenoceptor agonist. Yohimbine (3 mg kg-1 i.p.) precipitated a severe rise in heart rate from 285 +/- 14 to 520 +/- 5 beats min-1 in oxymetazoline (300 micrograms kg-1 day-1 s.c., 7 days) treated rats and from 320 +/- 13 to 420 +/- 11 beats min-1 in saline-treated animals. Upswings were not induced by yohimbine treatment. It is concluded, that the blood pressure upswings after clonidine withdrawal are due to a central mechanism, whereas the mechanism of the overshoot of heart rate is located peripherally, probably at the cardiac presynaptic level.     

Lack of in vitro and in vitro effects of fenbendazole on phase I and phase II biotransformation enzymes in rats, mice and chickens.

Intraperitoneal administration of 10 mg fenbendazole/kg bw daily for 5 d caused no significant alterations in the activities of hepatic microsomal drug-metabolizing enzymes viz aminopyrine N-demethylase, aniline hydroxylase and cytosolic glutathione S-transferase in rats, mice and chickens. Similarly no significant difference in the amount of microsomal cytochrome P-450 and NADPH-cytochrome c reductase was found between control and treated animals. In vitro incubation of fenbendazole with rat, mouse and chicken microsomes suggests that the drug neither binds to microsomal protein cytochrome P-450 nor inhibits the activities of aminopyrine N-demethylase and aniline hydroxylase. Similarly in vitro addition of fenbendazole to cytosolic glutathione S-transferase from the above species did not alter the activity of this enzyme. The results indicate that fenbendazole does not alter the activity of hepatic microsomal monooxygenase system significantly in rats, mice and chickens at a dosage level of 10 mg/kg body weight. In vitro studies also indicate that fenbendazole does not interact with the hepatic microsomal monooxygenase system, indicating it is not a substrate for cytochrome P-450-dependent monooxygenase system.     

Stimulatory and inhibitory effects of dimethyl sulfoxide on microsomal aniline hydroxylase activity

Dimethyl sulfoxide (DMSO) at a single dose of 3 ml/kg body wt, administered i.p. to male rats, caused a significant increase in the hepatic microsomal aniline hydroxylase activity. However, the level of cytochrome P-450, the activities of NADPH-cytochrome c reductase, benzphetamine N-demethylase and aminopyrine N-demethylase were unchanged at 24 h post-treatment. DMSO interacted with control rat liver microsomes in vitro and produced a type II spectral change (peak at 420 nm and trough at 392 nm). On the other hand, liver microsomes from DMSO-treated rats gave qualitatively similar spectra, but with a higher magnitude of binding. Liver microsomes from DMSO-treated rats showed a 3.4-fold increase in Vmax for aniline hydroxylase, while Km was found to be the same when compared with control rat liver microsomes. In vitro addition of 6 mM DMSO to microsomal incubations from control and DMSO-treated rats caused a 9-fold and a 25-fold increase in Km, respectively, while Vmax values for aniline hydroxylase were unchanged. When DMSO (6 mM) was incubated with rat liver microsomes in the presence of NADPH, there was formation of formaldehyde. The results suggest an interaction of DMSO with microsomal cytochrome P-450.     

Inhibition of hepatic microsomal drug-metabolizing enzymes by imperatorin

The effect of imperatorin on hepatic microsomal mixed function oxidases (MFO) was investigated. On acute treatment, imperatorin (30 mg/kg, i.p.) caused a significant reduction in activities of hepatic aminopyrine N-demethylase, hexobarbital hydroxylase and aniline hydroxylase as well as cytochrome p-450 content in rats and mice. Kinetic studies on rat liver enzymes revealed that imperatorin appeared to be a competitive inhibitor of aminopyrine N-demethylase (Ki, 0.007 mM), whereas a non-competitive inhibitor of hexobarbital hydroxylase (Ki, 0.0148mM). Imperatorin also inhibited non-competitively aniline metabolism (Ki, 0.2mM). Imperatorin binds to phenobarbital-induced cytochrome p-450 to give a typical type 1 binding spectrum (max. 388nm, min 422nm). Multiple administrations of imperatorin (30 mg/kg, i.p. daily for 7 days) to mice shortened markedly the duration of hexobarbital narcosis and increased activities of hepatic aminopyrine N-demethylase and hexobarbital hydroxylase and the level of cytochrome p-450 whereas aniline hydroxylase activity was unaffected.     

Acute interaction of drugs. I. The effect of volatile anesthetics on the kinetics of aniline hydroxylase and aminopyrine demethylase in rat hepatic microsomes

The in vitro effect of halothane, methoxyflurane, diethyl ether, and chloroform on the Michaelis constant (K_m) and maximal velocity (V_max) of microsomal aniline hydroxylase and aminopyrine demethylase was determined. The microsomes were obtained from rats pretreated with phenobarbital or 3-methylcholanthrene as well as from untreated rats. The halogenated anesthetics increased both the K_m and V_max of aniline hydroxylase in microsomes from untreated rats and this effect was magnified in microsomes from phenobarbital-induced animals. The K_m and V_max of aniline hydroxylase was not stimulated above control levels by halogenated anesthetics in microsomes from methylcholanthrene-induced rats. These anesthetics tended to inhibit the aminopyrine demethylase by lowering the V_max. Enzyme induction did not alter this inhibition. Diethyl ether inhibited aniline hydroxylase and aminopyrine demethylase by lowering the V_max.     

Altered activity of hepatic mixed-function mono-oxygenase enzymes in streptozotocin-induced diabetic rats.

1. In streptozotocin-induced diabetic male rats, hepatic microsomal aminopyrine N-demethylase activity was depressed, whereas aniline hydroxylase activity and cytochrome P-450 content were increased over control values. 2. In diabetic female rats, hepatic microsomal aminopyrine N-demethylase activity, aniline hydroxylase activity, biphenyl 4-hydroxylase activity, and cytochrome P-450 content were increased over control values. 3. Insulin treatment of diabetic male and female rats antagonized all physical and biochemical abnormalities of the diabetic state; 4. Methyl analogues of streptozotocin did not produce a diabetic state when injected into female rats, and resulted in no changes in aminopyrine N-demethylase activity, aniline hydroxylase activity, or cytochrome P-450 content. 5. Insulin treatment of non-diabetic female rats resulted in slight decreases in aminopyrine N-demethylase and aniline hydroxylase activities, but no changes in cytochrome P-450 content. These observations suggest that insulin primarily influences drug metabolism of diabetic animals through correction of the insulin-deficient diabetic state.     

Stimulation of microsomal drug oxidation activities by incorporation into microsomes of purified NADPH-cytochrome c (P-450) reductase.

The effects of addition of purified NADPH-cytochrome c (P-450) reductase on microsomal activities of aniline hydroxylation, p-phenetidine O-deethylation and ethylmorphine and aminopyrine N-demethylations were investigated utilizing microsomes from untreated, phenobarbital-treated and 3-methylcholanthrene-treated rats. The purified reductase was incorporated into microsomes. The drug oxidation activities were increased by the fortification of microsomes with the reductase while the extent of increase in the activities varied with the substrate and microsomes employed. The most pronounced enhancement was seen in p-phenetidine O-deethylation, followed by aniline hydroxylation and aminopyrine and ethylmorphine N-demethylations. The enhancement was more remarkable in microsomes from rats treated with 3-methylcholanthrene or phenobarbital. alpha-Naphthoflavone inhibited p-phenetidine O-deethylation activity markedly when the reductase was incorporated into microsomes, indicating that a larger amount of a species of cytochrome P-450 sensitive to the inhibitor was capable of participating in the oxidation of this substrate in the presence of the added reductase. One of the two Km values seen with higher concentrations of aniline or aminopyrine was altered by the fortification of microsomes with the purified NADPH cytochrome c (P-450) reductase. From these results, we propose that NADPH-cytochrome c (P-450) reductase transfers electrons to the selected one or two of multiple species of cytochrome P-450 more preferentially depending upon the substrate and the concentration of the substrate in microsomal membranes.     

Dose-dependent pharmacokinetics of zoxazolamine in the rat

Zoxazolamine (ZX) is a model substrate frequently used in studies on (methylcholanthrene-inducible) hepatic cytochrome P-450 activity. The iv pharmacokinetics of ZX were studied in rats at four dose levels: 5 mg X kg-1 (n = 6), 25 mg X kg-1 (n = 6), 50 mg X kg-1 (n = 5), and 60 mg X kg-1 (n = 4). Concentrations of ZX in blood, as well as the urinary excretion of unchanged ZX and chlorzoxazone, were determined. The apparent systemic clearance (CLs,app) decreased with increasing dose from 52.6 +/- 3.9 at 5 mg X kg-1 to 9.3 +/- 0.4 ml X min-1 X kg-1 at 60 mg X kg-1. The apparent elimination half-life, t1/2,app, increased from 16.1 +/- 0.3 min to 141 +/- 28.5 min. There was only slight concentration dependency of plasma protein binding: 86.0 +/- 0.9% at 4.2 +/- 0.2 micrograms X ml-1 (n = 6) vs. 80.4 +/- 0.4% at 27.1 +/- 1.1 micrograms X ml-1 (n = 6). Since from clearance and protein binding data nonrestrictive clearance of ZX could be inferred, this small change in binding was regarded as irrelevant for the interpretation of pharmacokinetic data of ZX. The blood-plasma concentration ratio was larger than unity: 2.11 +/- 0.09 at 5.4 +/- 0.9 micrograms X ml-1, and 1.85 +/- 0.08 at 47.9 +/- 4.9 micrograms X ml-1 (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of morphine and clonidine on sulphobromophthalein disposition in mice

Levels of sulphobromophthalein (BSP) in plasma and liver were elevated by the opiate, morphine, and by the alpha 2-adrenoceptor agonist, clonidine. Neither morphine, 1 mg kg-1, nor clonidine, 0.01 mg kg-1, affected BSP levels significantly. When given together at these doses, they caused BSP levels in plasma and liver to be raised. At 20 mg kg-1, the effect of morphine on BSP levels was maximal, as was that of clonidine, 1.0 mg kg-1. However, the effect of these drugs given together on plasma BSP exceeded the maximal effect of either alone. Yohimbine, an alpha 2-adrenoceptor antagonist, did not affect BSP levels, nor did the opiate antagonist, naloxone. Each of these antagonists reversed the hepatobiliary effects of its respective agonist, as shown by return of BSP levels to those of saline-treated mice. Yohimbine did not reverse morphine, nor did naloxone reverse clonidine. The additive effects of morphine and clonidine and the specificities of their respective antagonists strongly suggest the involvement of discrete receptors mediating their essentially identical hepatobiliary effects.     

Distribution kinetics of FK-506, a novel immunosuppressant, after intravenous administration to rats in comparison with cyclosporin A.

The distribution kinetics of a novel potent immunosuppressant, FK-506 (FK) has been studied in comparison with cyclosporin A (CyA) both in vivo and in vitro using blood specimens. The infusion studies on FK, 5.0 mg kg-1 through the portal and femoral veins showed that the mean hepatic extraction ratio of FK was 27.9 per cent. The effect of clamping both the hepatic artery and the portal vein on the plasma disappearance profiles of FK, 5.0 mg kg-1, and CyA, 3.5 mg kg-1 was studied. The plasma disposition kinetics of CyA was almost the same as in the normal rats. However, the plasma FK levels were about 10 times higher than those obtained in the control group rats. This difference is attributed to the restricted initial distribution of FK to the liver, because the volume of the initial distribution space, V1, of FK was about 10 times smaller than that obtained in normal rats. In in vitro experiments, drug distribution was studied in blood samples (2.0 ml) spiked with FK or CyA, 1.0 micrograms ml-1. The plasma drug levels measured at 2 min after drug administration were 0.842 +/- 0.012 micrograms ml-1 and 0.769 +/- 0.047 micrograms ml-1 for FK and CyA, respectively. The distribution volume in the blood compartment, VB, was determined by dividing the spiked amount of drugs with these plasma concentrations. The VB was 2.38 +/- 0.04 ml for FK and 2.62 +/- 0.16 ml for CyA. There was no significant difference in VB between FK and CyA. The plasma free fraction, fp of the drugs was measured by the equilibrium dialysis method. For FK, the mean fp values (+/- SE) were 1.31 +/- 0.18 per cent (2.0 micrograms ml-1) and 1.93 +/- 0.18 per cent (5.0 micrograms ml-1). For CyA, the fp values were 4.85 +/- 0.36 per cent (1.0 micrograms ml-1) and 5.75 +/- 0.82 per cent (5.0 micrograms ml-1). The hydrophobicity parameter, logP' determined through the HPLC method was 0.386 for FK and 0.545 for CyA. Although FK was less hydrophobic than CyA, its protein binding was higher than CyA.     

Induction of hepatic microsomal drug metabolizing enzyme system by levamisole in male mice

To examine the effect of levamisole on the hepatic drug metabolizing enzyme system of mice, levamisole at a dose of 20 mg kg-1 day-1 was administered i.p. for 5 days. Compared with the control values, the levamisole treatment significantly increased the amount of cytochrome P450 and cytochrome b5 and the in-vitro activities of aminopyrine N-demethylase, benzphetamine N-demethylase and aniline hydroxylase. In contrast, there was no change in microsomal NADPH-cytochrome c reductase activity in-vitro or in the relative liver weight and microsomal protein content compared with the corresponding values for control mice. Furthermore, in-vivo induction of drug metabolism was demonstrated by decreased pentobarbitone sleeping times after levamisole pretreatment. These results indicate that certain hepatic microsomal mixed function oxidases of mice are induced by levamisole, the drug that is frequently used as an anthelmintic in veterinary medicine and as an immunostimulant drug in human medicine.     

Sex difference in responsiveness to Aztreonam of monooxygenase system in liver microsomes from rats

Effect of successive administration of Aztreonam on microsomal monooxygenase system was investigated in male and female Sprague-Dawley rats. The activities of benzphetamine N-demethylase, aminopyrine N-demethylase, p-nitroanisole O-demethylase and aniline hydroxylase in liver microsomes from male rats were decreased dose-dependently by Aztreonam. On the contrary, the activities in liver microsomes from female rats were slightly increased rather than decreased by the administration of Aztreonam. In addition, Aztreonam was found to decrease the specific content of microsomal cytochrome P-450 in male rats but not in female rats. The decreases in the activities observed in male rats were accompanied by a parallel decrease in the specific content of cytochrome P-450. Furthermore, the results of quantitation of P-450 (M-1), one of the male specific forms of cytochrome P-450, indicated that the administration of Aztreonam resulted in a dose-dependent decrease in the content of P-450 (M-1) in liver microsomes from male rats.     

Effects of short-term inhalation of cadmium oxides on rabbit pulmonary microsomal enzymes

Rabbits, guinea pigs, rats and mice were compared for the activity of benzo[a]pyrene hydroxylase. aminopyrine N-demethylase and aniline hydroxylase of pulmonary microsomes. The activity of the microsomal enzymes was highest in rabbits, followed by guinea pigs and then rats and mice. Effects of the inhalation of cadmium oxides (CdO) were studied on the pulmonary microsomal enzymes in male rabbits. Rabbits were exposed for 15 min to air containing microparticles of CdO at four different concentrations ranging from 6.4 ± 0.5 to 22.4 ± 0.4 mg Cd/m³. The animals were killed 24 hr after the inhalation of CdO. The lung weight of the animals was increased markedly at doses higher than 12.6 mg/m³, the increase attaining 50 per cent increase at the highest dose. The activity of benzo[a]pyrene hydroxylase was reduced significantly at the higher doses, the reduction being dose-dependent reaching 50 per cent reduction. The activity of aminopyrine N-demethylase and aniline hydroxylase was reduced moderately by CdO inhalation but the reduction was not dose-dependent. Time-course effects of CdO inhalation on the pulmonary microsomal enzymes were studied on the rabbits exposed for 15 min to air containing CdO at concentrations of 13.0 ± 0.3 mg Cd/m³. The animals were killed 0.5, 1, 2, 4, and 6 days after the inhalation. The body weight increase was less in the treated groups than in the controls and edematous lesions in the lung were observed in most of the treated animals. The lung weight was increased after the inhalation, reaching its plateau on day 4. The activity of the microsomal enzymes was reduced throughout the experimental period, the maximum reduction being obtained on day 2 after the inhalation except for aniline hydroxylase that was reduced at the same degree throughout the experimental period.     

Contrasting effects of ethylenethiourea on hepatic monooxygenases in rats and mice

The effects of ethylenethiourea (ETU) on the hepatic xenobiotic metabolizing system in rats and mice were investigated. Male rats and male mice were given oral doses of 50 and 75, or 50, 75, 100, 500, and 1,000 mg/kg for 3 days. The microsomal enzymes studied were aminopyrine N-demethylase, aniline hydroxylase, and cytochrome P-450. In rats, the activity of aminopyrine N-demethylase was reduced to values between 60 and 70% of controls 24 h after treatment. A decrease in aniline hydroxylase activity and cytochrome P-450 content was observed on the 3rd day after exposure. In mice, treatment with ETU resulted in an increase of cytochrome P-450 at all dose levels. The activity of aniline hydroxylase was significantly elevated in the groups receiving doses of 100 mg/kg and higher. Aminopyrine N-demethylase was unaffected by the treatment. The results suggest that there are qualitative differences between rats and mice after ETU exposure with respect to the response of the hepatic monooxygenases.     

Heterogeneous effects of ethylenebisdithiocarbamate (EBDC) pesticides on oxidative metabolism of xenobiotics

Comparative evaluation of the acute effects of ethylenebisdithiocarbamate (EBDC) fungicides, mancozeb and zineb on microsomal mixed function oxidases (MFO) revealed marked substrate-dependent inhibition of oxidative metabolism of aminopyrine, p-nitroanisole and aniline in rats sacrificed 4 hr after oral administration of 100 mg mancozeb or zineb/kg body weight. Mancozeb inhibited p-nitroanisole O-dealkylase and aniline hydroxylase to a greater degree than zineb, whereas the inhibition of aminopyrine N-demethylase by the two fungicides was quantitatively comparable. Interestingly, aryl hydrocarbon hydroxylase (AHH) which exhibited maximum inhibition with mancozeb, remained unaffected following zineb administration. The time-course and dose-dependence of MFO inhibition examined 1 and 4 hr after a single oral dose of 100 mg or 250 mg zineb/kg was expressed as a dose-dependent decline in the rate of xenobiotic biotransformation at 4 hr. In vitro interaction of zineb with MFO resulted in slightly greater inhibition of aminopyrine, p-nitroanisole and aniline while AHH exhibited more pronounced decrease with mancozeb. The magnitude of inhibition of aminopyrine N-demethylase and AHH was independent of the time of preincubation of fungicides with the enzyme. Kinetic studies indicated the non-competitive nature of AHH inhibition. Chronic oral treatment with mancozeb and zineb at a dose of 250 mg/kg for 4 weeks did not modify xenobiotic biotransformations except for a slight induction of aminopyrine N-demethylase by mancozeb.     

年龄对大鼠肝脏生物转化功能及膜流动性的影响

通过与青年(3～4月)及中年(14月)组比较,研究了老年(24月)大鼠肝脏生物转化酶活性改变及膜流动性变化。结果表明,老年大鼠肝微粒体P-450含量、NADPH-细胞色素C还原酶活性无明显改变,但氨基比林N-脱甲基酶、苯胺羟化酶活性明显降低,且微粒体及胞浆GST、胞浆GSH-Px活性也明显下降;同时肝微粒体膜脂区流动性明显降低,膜Ch/PL值显著增大。研究提示,微粒体膜脂质环境及流动性变化与上述生物转化功能改变可能有一定的联系。     

The role of alpha 2-adrenoceptors in the vasculature of the rat tail.

1. The effects of alpha 2-adrenoceptor agonists and antagonists on rat tail skin temperature (tts), an indicator of local cutaneous blood flow, were studied in conscious and anaesthetized rats and in the isolated, Krebs perfused, vascular bed of the rat tail. 2. In conscious rats, at an ambient temperature of 18.5-20 degrees C, tts was 21.0 +/- 0.2 degrees C and core (rectal) temperature (tc) was 38.2 +/- 0.04 degrees C (n = 126). The alpha 2-adrenoceptor antagonist, delequamine (RS-15385-197; 1 mg kg-1, s.c., n = 6), produced a rapid elevation in tts to 29.1 +/- 0.7 degrees C (P < 0.001 vs. saline-treated control group), attained 10 min after injection. tc fell slightly, by 1.0 +/- 0.1 degrees C. The tts response was dose-related over the dose-range tested (0.01-1 mg kg-1, s.c.), with an ED50 of 17 micrograms kg-1, s.c. (n = 6 per dose). 3. The maximum increases in tts in response to a dose of 1 mg kg-1, s.c. of alpha 2-adrenoceptor antagonists were as follows (n = 6 per drug): delequamine (+9.6 +/- 0.8 degrees C) > yohimbine (+9.0 +/- 1.0 degrees C) > WY-26703 (+7.9 +/- 1.3 degrees C) > piperoxan (+5.6 +/- 1.7 degrees C) > idazoxan (+4.6 +/- 1.3 degrees C) > imiloxan (+4.1 +/- 1.3 degrees C) > SKF 104078 (+2.0 +/- 1.9 degrees C) > BDF-6143 (+1.3 +/- 0.8 degrees C). 4. Prazosin (0.3 mg kg-1, s.c.), hydralazine (10 mg kg-1, s.c.) and nifedipine (3 mg kg-1, s.c.) did not increase tts, whereas propranolol (10 mg kg-1, s.c.) evoked a small increase in tts (+2.9 +/- 1.0 degrees C). Pentolinium (2-10 mg kg-1, s.c.) elicited a dose-related increase in tts, which was elevated by 4.4 +/- 1.3 degrees C after a dose of 10 mg kg-1; tc was reduced in a dose-related manner. Drug vehicles (1 ml kg-1, s.c.) had no effect on tts or tc.(ABSTRACT TRUNCATED AT 250 WORDS)     

Enhancement of Microsomal Aniline and Acetanilide Hydroxylation by Haemoglobin

1. Haemoglobin and myoglobin enhance rat liver microsomal p-hydroxylation of aniline and acetanilide. Microsomal N-demethylation of ethylmorphine and aminopyrine is not increased by haemoproteins.

2. The enhancement of microsomal p-hydroxylation is maximal at high substrate concentration and high haeme compound concentration.

3. Detergent-purified NADPH-cytochrome c reductase, free flavins and manganese ions considerably increase the haemoglobin-mediated, tissue-free hydroxylation of aniline. Microsomal aniline hydroxylation is not enhanced by haeme, ferric ion or albumin.

4. Catalase and cyanide ions are powerful inhibitors of haemoglobin-mediated aniline hydroxylation both in the presence and absence of tissue. Carbon monoxide inhibits the hydroxylase activity of the tissue-free system to a smaller extent than that of a system containing microsomes plus haemoglobin whereas p-chloromercuribenzoate inhibits only the flavoprotein-dependent hydroxylation of aniline mediated by haemoglobin.

5. Several possibilities of interactions between substrate, microsomes and haeme compounds are proposed.     

Effect of caffeine on the hepatic microsomal mixed function oxidase system during phenobarbital and benzo[a]pyrene treatment in rats

Simultaneous administration of caffeine (100 mg/kg, i.p., 3 days) and phenobarbital (80 mg/kg, i.p., 3 days) to adult male rats resulted in a significant decrease in hepatic cytochrome P-450 and acetanilide hydroxylase activity, compared to phenobarbital administration alone. While simultaneous administration of caffeine and benzo[a]pyrene (20 mg/kg, i.p., 2 days) increased acetanilide hydroxylase, compared to benzo[a]pyrene administration, no change was seen in the cytochrome P-450 concentration. In vitro addition of 2.5 mM caffeine to microsomal incubations from untreated, phenobarbital- and benzo[a]pyrene-treated rats inhibited aminopyrine N-demethylase activity. No significant difference was seen in the extent of aminopyrine N-demethylase inhibition due to the in vitro addition of caffeine to microsomes from untreated or phenobarbital-treated rats, whereas inhibition in microsomes from benzo[a]pyrene-treated rats was greater.     

Cardiovascular responses to central clonidine, alpha-methyldopa, and 6-hydroxydopamine in conscious normotensive and spontaneously hypertensive rats following naloxone

The possible involvement of endogenous opioid peptides in the cardiovascular responses observed following central alpha-adrenoceptor stimulation with clonidine, alpha-methyldopa (alpha-MD), and 6-hydroxydopamine (6-OHDA) was examined in conscious normotensive Wistar and spontaneously hypertensive (SHR) rats. Clonidine [2.5 micrograms intracisternally (i.c.)] produced rapid hypotension (-36 +/- 2 mm Hg) and bradycardia (-53 +/- 5 beats/min) in SHR that were similar to observations in animals given either naloxone (50 micrograms i.c. or 10 mg/kg i.p.) or appropriate saline control injections. Peripheral doses of naloxone (1-2 mg/kg) or saline did not further change arterial pressure or heart rate in either Wistar rats or SHR given alpha-MD (1.0 mg i.c.) 3 h earlier. In addition, central doses of naloxone (3 X 50 micrograms i.c.) given at hourly intervals did not affect the responses to alpha-MD. Central administration of 6-OHDA acutely releases noradrenaline which produces an initial fall in arterial blood pressure and heart rate. Intracisternal 6-OHDA (400 micrograms) produced similar time course and maximum circulatory effects in rats given naloxone (50 micrograms i.c. before and at each subsequent hour) as in saline-treated animals. Naloxone (1 mg/kg s.c.) significantly attenuated morphine-induced analgesia. These findings do not support a critical role of endogenous opioids in mediating the acute antihypertensive actions of clonidine and alpha-MD or in the cardiovascular responses produced by noradrenaline release following central 6-OHDA.