Clonidine-induced inhibition of sympathetic nerve activity: No indication for a central presynaptic or an indirect sympathomimetic mode of action

Summary The effects of clonidine on blood pressure, heart rate, contractile state of the nictitating membranes, spontaneous sympathetic nerve activity and response of sympathetic nerves to hypothalamic stimulation were compared in normal anaesthetized cats and in anaesthetized cats pretreated with reserpine and -methyl-p-tyrosine. The pretreatment lowered the noradrenaline content of various parts of the brain to less than 5 ng/g, i.e. to less than 1–3% of that of the controls. Under the conditions of this severe noradrenaline depletion, blood pressure and heart rate were low and spontaneous sympathetic nerve activity consisted of continous, high-amplitude discharges which contrasted with the low-amplitude bursts of activity—synchronous with the respiration—of the controls. In contrast to the controls, clonidine did not lower blood pressure and heart rate in the cats with noradrenaline depletion; however, the clonidine-induced contractions of the nictitating membranes were of similar magnitude and duration in both groups of animals. The efficacy of clonidine in reducing or abolishing spontaneous sympathetic nerve activity and in inhibiting the response of sympathetic nerves to hypothalamic stimulation was equal in controls and in cats with noradrenaline depletion, its potency being 3-fold higher in the former. The results indicate a direct stimulation of -adrenoceptors by clonidine both in the periphery and in the central nervous system and make it unlikely that the central effect of clonidine on blood pressure is due to a release of noradrenaline from central adrenergic neurones. It is further concluded that clonidine activates an adrenergic mechanism in the central nervous system by stimulation of postsynaptic -adrenoceptors. The inhibition of such a mechanism as a consequence of a diminished noradrenaline release due to stimulation of presynaptic -adrenoceptors—as proposed from in vitro experiments—seems to be of no importance for the central effect of clonidine on sympathetic nerve activity and blood pressure.Preliminary results have been presented at the 15th Spring Meeting of the German Pharmacological Society in Mainz (Haeusler, 1974a).     

Pre- and postsynaptic effects of yohimbine stereoisomers on noradrenergic transmission in the pulmonary artery of the rabbit

Summary Effects on noradrenergic neurotransmission of five stereoisomers of yohimbine and of the closely related compound yohimbol were studied in strips of the pulmonary artery of the rabbit. In some experiments the tissue was preincubated with ³H-noradrenaline. Three effects were observed. Firstly, antagonism to the contractile effect of noradrenaline and of sympathetic nerve stimulation; the antagonism reflected competitive blockade of postsynaptic -adrenoceptors. Secondly, an increase in the stimulation-evoked overflow of total tritium and ³H-noradrenaline; the increase appeared to be due to blockade of presynaptic -adrenoceptors. Thirdly, an increase in the basal outflow of ³H-3,4-dihydrophenylglycol, presumably by impairment of the vesicular storage of ³H-noradrenaline. According to their relative potencies in eliciting these effects, the drugs could be divided into three groups. Rauwolscine, -yohimbine and yohimbol preferentially blocked the presynaptic -adrenoceptor; rauwolscine and -yohimbine, like yohimbine, at low concentrations increased the contractile response to sympathetic nerve stimulation. Corynanthine preferentially blocked the postsynaptic -adrenoceptor. Pseudoyohimbine and 3-epi--yohimbine were very weak antagonists at either receptor; they mainly accelerated the basal outflow of ³H-3,4-dihydroxyphenylglycol.From these results and those of a previous study it is concluded that, in a series of twelve -adrenolytic drugs, rauwolscine shows the greatest preference for presynaptic and corynanthine the greatest preference for postsynaptic -adrenoceptors. In view of the chemical similarity of the two compounds these opposite properties are striking. Corynanthine and rauwolscine might be useful tools for the subclassification of -adrenoceptors.     

α-Adrenoceptor activation of nictitating membrane and iris in cats

Summary Intra-arterial administration of (+)- and (–)isomers of adrenaline and noradrenaline produced doserelated contraction of the nictitating membrane (NM) and dilation of the pupil in anesthetized cats. The relative potencies were (–)-adrenaline > (+)-adrenaline = (–)-noradrenaline > (+)-noradrenaline. Observations of the effects of -adrenoceptor antagonists on (–)-noradrenaline activation of these two effectors were made simultaneously. All of the ₁-adrenoceptor antagonists tested produced a doserelated blockade of the NM with the relative potencies being prazosin > WB-4101 > phentolamine > phenoxybenzamine. In contrast, the iris dilator was blocked by WB-4101 and phenoxybenzamine but was refractory to antagonism by doses of prazosin and phentolamine that reduced the (–)-noradrenaline evoked NM response by 75–80% in the same animals. The ₂-adrenoceptor antagonist, yohimbine, produced significant inhibition of the NM only at high dose (1 mg/kg) but even at this level had no effect on pupil diameter. These results suggest that activation of the NM by exogenous noradrenaline is due solely to stimulation of ₁-adrenoceptors. ₂-adrenoceptors do not seem to significantly contribute to noradrenaline induced activation of either the NM or iris dilator muscle in vivo. In contrast, the -adrenoceptors on the iris dilator muscle that are stimulated by exogenous noradrenaline can not easily be classified pharmacologically as either ₁- or ₂-adrenoceptors.This work was supported by Grant OK8710676 from the National Science Foundation and by Research to Prevent Blindness, Inc     

Depression of exploratory activity by clonidine in rats as a model for the detection of relative pre- and postsynaptic central noradrenergic receptor selectivity of α-adrenolytic drugs

Summary The effects of various -adrenoceptorblocking drugs on the depression of exploratory activity (ambulation and rearing) induced by 0.1 mg/kg i.p. clonidine were investigated in the rat. In parallel experiments, the effects of the same drugs on pre- and postsynaptic -receptors were determined in vitro (field-stimulated cortex slices and isolated vas deferens of the rat, respectively). Tolazoline, esproquine, yohimbine and piperoxan distinctly antagonized the inhibition of exploration produced by clonidine. All these drugs were found to possess relatively higher selectivity for the presynaptic -receptors, as judged by the ratios of the concentrations inducing a 50% increase in field-stimulated ³H-noradrenaline-overflow and the concentrations required to shift the EC₅₀ for the antagonism of noradrenaline-induced contractions of the vas deferens to the right by a factor of 2 (pA₂, ratio <1): In contrast, phentolamine and phenoxybenzamine which showed preferential postsynaptic -receptor blocking activity (ratio>1), potentiated rather than antagonized the effects of clonidine. Mianserin, although preferentially blocking the postsynaptic receptors, had no effect on clonidine-induced hypoactivity up to the high dose of 100 mg/kg i.p., probably because of its additional NA-uptake-inhibiting properties. The antagonism of clonidine by the selective presynaptic -receptor blockers was observed within a limited dose-range. Increasing the doses above an optimal level, which varied from one compound to another, resulted in a decrease in the effect. It is suggested that this phenomenon reflects the counter-balancing postsynaptic -adrenoceptor blockade occuring at higher concentrations of these drugs. In general, the results show a fairly good correlation between antagonism of clonidine in vivo and preferential blockade of presynaptic -receptors in vitro. Clonidine-induced suppression of exploration therefore seems to be a valuable model for the investigation of drug interactions with -adrenergic receptors in the central nervous system.These results were presented in part at the Spring Meeting of German Pharmacological Society, Mainz, 16–18 March, 1977     

Alpha2-adrenoceptor-mediated responses to so-called selective alpha1-adrenoceptor agonists after partial blockade of alpha1-adrenoceptors

Summary In dog saphenous vein — a tissue possessing both postsynaptic ₁- and ₂-adrenoceptors — the effects of two selective ₁-adrenoceptor agonists (phenylephrine and methoxamine) were compared with that of the selective ₂-adrenoceptor agonist, UK-14,304, before and after phenoxybenzamine. Furthermore, the influence exerted by prazosin, yohimbine and verapamil on the effects of these agonists was also studied before and after phenoxybenzamine. In the absence of phenoxybenzamine, prazosin (56 nmol/l) caused a parallel shift of the concentration-response curves of both phenylephrine and methoxamine to the right (by 0.94 and 1.1 log units, respectively) and had no effect on the concentration-response curve of UK-14,304, while 20 nmol/l yohimbine caused a marked parallel shift of the concentration-response curve of UK-14,304 to the right (by 1.18 log units) and caused only minor displacements of those of phenylephrine and methoxamine (by 0.2 and 0.33 log units, respectively). After exposure of the strips to 30 nmol/l phenoxybenzamine, prazosin (56 nmol/l) caused small shifts of the concentration-response curves of both phenylephrine (by 0.36 log units) and methoxamine (by 0.31 log units) and did not change that of UK-14,304, while yohimbine (20 nmol/l) caused pronounced parallel shifts of the concentration-response curves (to the right) of all the agonists: phenylephrine (by 1.0 log units), methoxamine (by 0.93 log units) and UK-14,304 (by 1.28 tog units). When UK-14,304 was added to the bath during a sub-maximal contraction to phenylephrine it caused a further contraction almost up to the maximum; if this procedure was repeated after phenoxybenzamine (30 nmol/1), there was no further contraction to UK-14,304.In the absence of phenoxybenzamine, verapamil (5 mol/l) caused a parallel shift of the concentration-response curve of phenylephrine (or methoxamine) to the right and a non-parallel shift (with marked depression of the maximal effect) of that of UK-14,304. However, after phenoxybenzamine (30 nmol/l), the same concentration of verapamil caused non-parallel shifts of the concentration-response curves of the three agonists to the right with about equal depression of the maximal effects. We conclude that, after removal of ₁-adrenoceptor reserve by phenoxybenzamine, the responses to selective ₁-adrenoceptor agonists are predominantly ₂-adrenoceptor-mediated. This may explain why under these conditions, the selective ₁-and ₂-adrenoceptor agonists are equally antagonized by calcium entry blockers.This work was supported by a grant from the University of Porto (Subsidio para acção de investigação no. 36/85) Send offprint requests to S. Guimarães at the above address     

Reserpine and alpha-methyldopa in the treatment of tardive dyskinesia

Thirty inpatients with evidence of tardive dyskinesia secondary to antipsychotic medications participated in this double-blind, controlled, randomized study comparing reserpine, -methyldopa and placebo. Reserpine at doses of 0.75–1.5 mg daily, or -methyldopa at doses of 750–1,500 mg daily, produced a statistically significant improvement in tardive dyskinesia symptomatology compared to the results obtained with placebo.Paper presented at the New Clinical Drug Evaluation Unit Program, Key Biscayne, Florida, May 22–24, 1979     

The effectiveness of yohimbine in blocking rat central dopamine autoreceptors in vivo

Summary The influence of various -adrenoceptor antagonists (10 mg/kg i.p.) upon the rate of turnover of dopamine (DA) in the rat brain was investigated. Taking the levels of the DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) as a measure of the rate of DA turnover, it was found that prazosin and phenoxybenzamine decreased, whereas piperoxane and yohimbine increased the turnover rate both in the corpus striatum and in the tuberculum olfactorium. Azapetine, phentolamine and tolazoline as well as the -adrenoceptor antagonist propranolol were without a significant effect, whereas the DA antagonist haloperidol increased DOPAC and HVA levels and decreased the levels of DA itself.The possibility that the yohimbine-induced increase in the DA turnover rate was produced by a direct blockade of DA autoreceptors, was investigated under conditions where influences other than those elicited via DA autoreceptors are thought to be eliminated, i.e. in rats treated with reserpine or -butyrolactone (GBL). In rats that were pretreated with reserpine, yohimbine (10 mg/kg i.p.) was found to be ineffective in antagonizing the reduction of the accumulation of 3,4-dihydroxyphenylalanine (DOPA) following decarboxylase inhibition, that was produced by the DA agonist apomorphine (2.0 mg/kg i.p.). In rats pretreated with reserpine, yohimbine (10 mg/kg i.p.) was also ineffective in antagonizing the reduction of the DOPAC and HVA levels produced by apomorphine (2.0 mg/kg i.p.), but it was effective in antagonizing the reduction of the HVA level that was produced by the selective DA autoreceptor agonist N,N-di-n-propyl-7-hydroxy-2-aminotetralin (DP-7-AT, 1.0 mg/kg i.p.). In rats treated with GBL and a decarboxylase inhibitor, apomorphine (2.0 mg/kg i.p.) and DP-7-AT (1.0 mg/kg i.p.) induced a maximal suppression of the GBL-induced increase in the accumulation of DOPA. The effects of both apomorphine and DP-7-AT were partially inhibited by yohimbine (10 mg/kg i.p.). In inhibiting the effect of DP-7-AT, yohimbine appeared to be 100–200 times less effective than the DA antagonist haloperidol when both antagonists were administered at a fixed pretreatment time (1 h). It is concluded that yohimbine does indeed possess direct central DA autoreceptor blocking properties in vivo, and that this has to be taken into consideration if yohimbine is used as a pharmacological tool in order to achieve a selective blockade of ₂-adrenoceptors.     

Activation of histamine H3 receptors produces presynaptic inhibition of neurally evoked cat nictitating membrane responses in vivo

Summary This study was undertaken in order to determine the potential role of prejunctional histamine H₃ receptors in an in vivo adrenergic model system. Frequency-dependent nictitating membrane responses were elicited by sympathetic nerve stimulation in anesthetized cats. Systemic administration of the selective histamine H₃ receptor agonist, (R)--methylhistamine (RMeHA) produced a dose-related depression of amplitude of the evoked nictitating membrane responses with a threshold of about 10 g/kg and maximal effect (50% depression at the lowest frequency; 0.5 Hz) seen at 100–300 gg/kg. Responses obtained with low frequency stimulation were more sensitive to depression by RaMeHA than were responses evoked with higher frequencies of stimulation. Larger doses of RaMeHA given to the same animals, failed to produce additional inhibition.RMeHA depressed the amplitude of nictitating membrane responses evoked by either pre- or postganglionic nerve stimulation to an equivalent degree. This depressant action of RMeHA was antagonized by pretreatment with the specific histamine H₃ antagonist, thioperamide (3 mg/kg), but not by combined pretreatment with histamine H₁ and H₂ blockers chlorpheniramine (300 Ftg/kg) and cimetidine (5 mg/kg). Intravenous administration of adrenaline (1–30 wg/kg) also produced graded nictitating membrane responses that were not altered by subsequent administration of RMeHA.These results suggest that histamine H₃ receptors are involved in the modulation of neurally evoked noradrenaline release in the cat nictitating membrane by an inhibitory presynaptic action. The most likely site of drug action appears to be at the neuroeffector junction as no appreciable ganglionic effect of RMeHA was observed in this in vivo model system. Send offprint requests to M. C. Koss at the above address     

On the role of α-methyldopamine in the antihypertensive effect of α-methyldopa

Summary In renal hypertensive rats the cerebral concentration of -methyldopa, -methyldopamine, -methylnoradrenaline, dopamine and noradrenaline as well as the blood-pressure were determined simultaneously. The antihypertensive effect followed a time course identical to that of the increase in the cerebral concentration of -methyldopamine and of the decrease in the concentration of dopamine, whereas lowering of blood pressure on the one hand, and changes in the levels of -methylnoradrenaline and noradrenaline, on the other, were not related to each other. Dose-response relationships showed the same correlations and lack of correlations, respectively.These results suggest that non--hydroxylated catecholamines play a major role in mediating the antihypertensive effect of -methyldopa or, alternatively, that only the newly biosynthesized -methyl-noradrenaline is effective in lowering blood pressure.A preliminary communication has been presented at the Spring Meeting 1973 of the German Pharmacological Society at Mainz (Waldmeier et al., 1973).     

Effects of ring substitution on the pre- and postjunctional alpha-adrenergic activity of aryliminoimidazolidines

Summary The pre- and postjunctional -adrenergic agonist potency of a series of aryliminoimidazolidines was determined in the isolated rabbit ear artery. This series included clonidine, an antihypertensive agent thought to act by stimulating brainstem -receptors and known to be a preferentially prejunctional -adrenergic agonist. Although all of the compounds acted preferentially on the prejunctional -adrenoceptor, ring substitution had a dramatic effect on both potency and the degree of selectivity. 2-(3,4-Dihydroxyphenylimino) imidazolidine was both the most potent and most selective prejunctional -agonist in this series.     

Smooth muscle of rabbit aorta contains α1- but not α2-Adrenoceptors

Summary We have investigated the residual contractile response to noradrenaline remaining after phenoxybenzamine (3×10^–7 mol/l) in rabbit aorta, since it has been reported that phenoxybenzamine at low doses completely and irreversibly blocks ₁- but not ₂-adrenoceptors. The contraction elicited by noradrenaline slowly recovered with time after it had been almost abolished by phenoxybenzamine. This residual response was blocked by the ₁-selective antagonist prazosin (3×10^–8 mol/l) but not by the ₂-selective antagonist rauwolscine (3×10^–7 mol/l). The results confirm that the smooth muscle of rabbit aorta contains ₁- not ₂-adrenoceptors.     

In vivo studies on alpha-adrenergic receptor subtypes in human veins

Summary We studied in vivo responsiveness of venous ₁ and ₂-adrenoceptors, measuring the diameter changes in superficial veins in response to -adrenergic agonists and antagonists in healthy human volunteers. The dorsal hand vein technique was used because it permits complete dose-response studies of venous constriction without confounding reflex alterations.Local infusions of all agonists studied induced dose-dependent contraction of the hand vein; the maximal effects (Emax) were: norepinephrine (88% ± 10%), methox amine (97% ± 5%), phenylephrine (95% ± 6%), clonidine (54% ± 12%), and azepexole (68% ± 26%). Clonidine reduced the norepinephrine-induced venoconstriction by 11% ± 10%. Oral doses of 1 mg prazosin antagonized the venoconstriction induced by norepinephrine, methoxamine, and clonidine, but not by azepexole. Yohimbineantagonism was observed against all agonists studied. Inhibition by yohimbine of clonidine-induced venoconstriction was irreversible over 60–180 min.Results show that the in vivo effects on veins of -adrenergic agonists are in good agreement with results from in vitro experiments. Agonists with ₁- and ₂-adrenoceptor subtype selectivity cause venoconstriction in vivo, but ₂-receptor mediated constriction is intrinsically weaker. Clonidine acts as a partial antagonist against norepinephrine, presumably on postsynaptic ₂-receptors. At high doses, ₂-adrenoceptor subtype selectivity of clonidine and yohimbine appear to be partially lost in vivo. Send offprint requests to H. G. Eichler at the above address     

Effects of morphine on sleep in the cat

The effects of morphine sulfate, 300 g/kg s.c., on the sleep of cats was studied by electroencephalographic techniques. In contrast to placebo experiments the animals were awake for approximately 6 h after administration of morphine; the return of regular sleep patterns occurred after about 11 h. A rebound increase in rapid eye movement (REM) sleep time and percentage was noted during the 11th through the 17th hour of the study. Sleep following manual sleep deprivation for 10 h showed a rebound increase in REM and non-rapid eye movement (NREM) sleep time. NREM sleep rebound after manual sleep deprivation exceeded that occurring after morphine. The alerting actions of morphine could be blocked by naloxone, 100 g/kg s.c., for about 90 min. Naloxone alone increased REM sleep time and percentage. Single (84 mg/kg) or multiple (51 mg/kg for 4 injections) doses of dl--methyltyrosine i.p. did not block the alerting action or REM sleep rebound caused by morphine. 5-Hydrotryptophan (30 mg/kg) i.p. did not antagonize the alerting action of morphine.Publication No. 1044 of the Division of Basic Health Sciences of Emory University. A preliminary report of this work was presented at the Fall Meeting of the American Society for Pharmacology and Experimental Therapeutics, Pittsburgh, Pennsylvania, August, 1969. This work was supported by USPHS grant MH12870-04.A predoctoral fellow of Graduate Pharmacology Training Grant 2T1-GM179.     

Binding of 3H-clonidine to an α-adrenoceptor in membranes of guinea-pig ileum

Summary The binding of ³H-clonidine to membrane particles from guinea-pig ileum was investigated. The specific binding, i.e. the binding that could be inhibited by high concentrations of unlabeled clonidine or noradrenaline, was of high affinity, K _D3 nM. The number of sites was approximately 25 fmol/mg protein. Rate constants of association and dissociation were 5.3×10⁷ M^–1 min^–1 and 0.18 min^–1, respectively. Affinites of various drugs to the binding site were determined by measuring their effect on the binding of ³H-clonidine. The affinity of adrenergic agonists decreased in the order clonidine = tramazoline > (–)-erythro--methylnoradrenaline > (–)-noradrenaline (–)-phenylephrine. (–)-Noradrenaline had about 20 times more affinity than the (+)-isomer. The affinity of -adrenoceptor antagonists decreased in the order phentolamine > rauwolscine = yohimbine > WB 4101 > pseudoyohimbine > prazosin = corynanthine. Yohimbine and rauwolscine had about 100 times more affinity than their stereoisomer corynanthine. Serotonin 10 M and metiamide 10 M did not affect the binding, and propranolol inhibited it only at high concentrations. — The results indicate that ³H-clonidine labels an ₂-adrenoceptor in guinea-pig ileum. The orders of affinity of -adrenoceptor agonists and antagonists agree well with their orders of potency in functional tests, namely as modulators of cholinergic transmission in the guinea-pig ileum and as modulators of noradrenaline release in the rabbit pulmonary artery. An -adrenoceptor should be classified as ₂ when the affinities of clonidine, tramazoline and -methylnoradrenaline greatly exceed the affinity of phenylephrine, and when the affinities of rauwolscine and yohimbine exceed those of prazosin and corynanthine.     

A Candidate for Cancer Gene Therapy: MIP-lα Gene Transfer to an Adenocarcinoma Cell Line Reduced T\imorigenicity and Induced Protective Immunity in Immunocompetent Mice

Purpose. To evaluate the possibility of cancer gene therapy by the gene delivery of chemokine, the effects of human macrophage inflammatory protein l (hu-MIP-l), murine-macrophage inflammatory protein l (mu-MIP-l), and human-interleukin 8 (hu-IL-8) on tumor progression and immunization were studied. Methods. Cachexia-inducing and highly tumorigenic adenocarcinoma cells (cell line colon 26, clone 20) were transfected with either a control plasmid, hu-MIP-l, mu-MIP-l, or hu-IL-8 expression vector. The production of hu-MIP-1 reached >1.5 ng/ml in vitro when transfectant cells were cultured at a cell density of 2 × 10⁵ cells in 7 ml for 3 days. Immunocompetent BALB/c mice were inoculated into the footpad with the tumor cells, and then primary tumor growth, morphological analyses, and tumor immunogenicity were studied. Results. The secretion of hu-MIP-l, mu-MIP-l, and hu-IL-8 did not affect the growth rate in vitro. Reduced tumorigenicities in vivo were observed in transfected cells with hu-MIP-l and mu-MIP-l. Morphologic observation of the site of inoculation of cells transfected with hu-MIP-l showed infiltration of macrophages and neutrophils on the 5th day after the inoculation. Mice that had rejected cells transfected with hu-MIP-l gene were immune to a subsequent challenge with the parental cells. Conclusions. The rejection of the cells depends on cytolysis and generates potent and long lasting antitumor immunity. These data suggest that tumor cells transfected with the MIP-l gene might be useful as an effective therapy for the treatment of certain tumors.     

Sympathomimetic inhibition of noradrenaline release: Mediated by prostaglandins?

Summary In isolated perfused rabbit hearts, the effec of oxymetazoline and phentolamine on the stimulation-induced overflow of noradrenaline was tested after the synthesis of prostaglandins had been blocked by indomethacin. Indomethacin changed neither the decrease of overflow caused by oxymetazoline nor the increase caused by phentolamine. It is concluded that prostaglandins are not involved to any major degree in the modulation of noradrenaline release by drugs with affinity to -receptors, or in the feed-back inhibition of transmitter release mediated by the effect of liberated noradrenaline on -receptive sites.     

Fenofibrate decreases asymmetric dimethylarginine level in cultured endothelial cells by inhibiting NF-κB activity

Previous investigations have demonstrated that endogenous inhibitors of nitric oxide synthase (NOS), such as asymmetric dimethylarginine (ADMA), contribute importantly to endothelial dysfunction, and that fenofibrate has a protective effect on the endothelium in rats treated with low-density lipoprotein (LDL) by reducing ADMA levels. In the present study, we explored further the possible mechanism underlying inhibition of ADMA generation by fenofibrate in cultured human umbilical vein endothelial cells (HUVECs). Endothelial injury was induced in cultured HUVECs by incubation with oxidative LDL (ox-LDL) and the levels of ADMA, lactate dehydrogenase (LDH), NO and tumour necrosis factor- (TNF-) in the conditioned medium were measured. Cell viability and the activity of dimethylarginine dimethylaminohydrolase (DDAH) and nuclear factor-B (NF-B) in the cultured HUVECs were also determined. Incubation of HUVECs with ox-LDL (100 g/ml) for 24 h markedly elevated ADMA, LDH and TNF- in the conditioned medium and significantly increased the activity of NF-B, concomitantly with a significant decrease in the activity of DDAH and the content of NO. Pretreatment with fenofibrate (3, 10 or 30 M) significantly inhibited the increases in ADMA, LDH and TNF-, attenuated the decreased levels of NO and the decreased activity of DDAH and prevented the activation of NF-B. Similar effects were observed in the presence of pyrrolidine dithiocarbamate (PDTC, 10 M), an antagonist of NF-B. The beneficial effects of fenofibrate on cultured endothelial cells were abolished by MK-886, a specific peroxisome proliferator-activated receptor- (PPAR) antagonist. The present results suggest that fenofibrate inhibits ox-LDL-induced endothelial cell damage by decreasing ADMA and increasing DDAH activity, and the protective effects of fenofibrate on endothelial cells may be related to reduction of NF-B activity by activation of the PPAR receptor.     

Supersensitivity to the central stimulant actions of adrenergic drugs following discontinuation of a chronic diet of α-Methyltyrosine

Summary Spontaneous and amphetamine-stimulated locomotor activity were determined before, during and after a two week period in which mice were fed a diet containing 0.3% dl--methyltyrosine. After one day of the -methyltyrosine diet both spontaneous and amphetamine-stimulated activity were depressed but tolerance to the depression developed by the thirteenth day of the diet. Both measures of activity were significantly higher than control (pre-diet) values one day after the diet was discontinued. Ephedrine-stimulated motor activity was also blocked after one day of the -methyltyrosine diet and enhanced one day after cessation of the diet whereas the stimulant effects of pipradrol and methylphenidate were not significantly altered at these times. The central stimulant actions of only those drugs which have been shown to release endogenous catecholamines (amphetamine and ephedrine) are blocked by -methyltyrosine and enhanced after a chronic diet of this drug is discontinued. It is proposed that the central excitation that follows cessation of chronic -methyltyrosine administration is the result of an increase in the sensitivity of central adrenergic receptors.Presented in part at the Fall Meeting of the American Society for Pharmacology and Experimental Therapeutics at Minneapolis, Minnesota, August 1968 (Pharmacologist 10, 181 (1968)]. Supported by USPHS Grant AM-11083 and MH-13174.Predoctoral fellow supported by USPHS Training Grant GM-01761-02.     

Characterisation of postjunctional α-adrenoceptors in isolated human femoral veins and arteries

Summary In order to characterise the pharmacological properties of postjunctional -adrenoceptors, both the contractile effects of -adrenoceptor agonists and the blocking potencies of selective -adrenoceptor antagonists were studied in isolated human femoral veins and arteries.The veins were more sensitive to noradrenaline than the arteries. Guanfacine had a higher intrinsic activity in veins than in arteries, whereas the reverse was true for phenylephrine.The antagonists rauwolscine and yohimbine were more potent against noradrenaline in the veins than in arteries, while corynanthine was equally potent in either tissue. They antagonised the noradrenaline response in a competitive manner. Prazosin proved to be the most potent competitive antagonist in arteries, while in veins it exerted weak and noncompetitive antagonism.The results suggest that the -adrenoceptor population at the postjunctional site differs between human femoral veins and arteries. The veins seem to contain more ₂- than ¹-adrenoceptors postjunctionally, whereas in the arteries the ₂-subtype prevails. The results indicate the possibility of influencing selectively adrenergic reactions in the capacitance and resistance vessels.