C.A. Fontes Ribeiro

Mechanism of action of doxepin in the treatment of chronic urticaria

Summary— The present study examined 15 patients previously resistant to conventional antihistamines, in which doxepin at doses in the range of 50–75 mg/day was shown to be effective in treatment of chronic urticaria and without significant adverse side effects. However, some controversy remains about its mechanism of action in this particular disease. The aim of the present study was to examine the muscarinic, H1 and H2 blocking activity of doxepin. The following methods were used: a) gastric acid hypersecretion induced by histamine and carbachol in the pylorus‐ligated rat preparation; b) contractile dose‐response curves to histamine and carbachol in the guinea pig ileum; c) dimaprit‐stimulated guinea pig atrium in vitro. pA2 values were determined for atropine, mepyramine, cimetidine and doxepin. As regards histamine, doxepin (50 mg/kg, po) increased gastric pH and decreased secretion volume, gastric acid concentration and total acid output; with carbachol, doxepin weakly antagonized those values. In the ileum, doxepin competitively antagonized carbachol (pA2 = 7.08) and histamine (pA2 = 9.72); pA2 values for atropine and mepyramine against carbachol and histamine were 9.11 and 8.82, respectively. In the atria, the dose‐response curve to dimaprit was also competitively displaced by cimetidine (pA2 = 6.69) and doxepin (pA2 = 6.00). Doxepin displayed a very high affinity for H1 histaminic receptor, being 8‐fold more potent than mepyramine. Doxepin showed significant H2 blocking activity which was 5 times less potent than that of cimetidine. Doxepin competitively antagonized carbachol in the guinea pig ileum, and was 107 times less potent than atropine. The combined H1, H2 and muscarinic blocking activities of doxepin may contribute towards explaining its clinical efficacy in the treatment of chronic urticaria.

Experimental studies on the mechanisms of tiaprofenic acid photosensitization

Red blood cell lysis and histidine degradation, photosensitized by tiaprofenic acid (TIA), were investigated. Photohaemolysis was markedly enhanced in oxygenated solutions, but was also intense in the presence of nitrogen. Photohaemolysis was inhibited by butylated hydroxyanisole and reduced glutathione, but was unaffected by sodium azide, superoxide dismutase and mannitol. The TIA-induced photo-oxidation of histidine was greatly enhanced in the presence of oxygen and almost completely inhibited in solutions bubbled with nitrogen. Sodium azide, butylated hydroxyanisole and reduced glutathione inhibited the photodegradation of histidine. Phototoxicity to histidine was unaffected by mannitol and superoxide dismutase. The overall results suggest that molecular mechanisms involving free radicals and singlet oxygen are responsible for TIA-photosensitized reactions. These two in vitro models (photohaemolysis and histidine degradation) represent different mechanisms of phototoxicity, but complement one another in the investigation of potential phototoxic substances.

Catecholamine and MHPG plasma levels, platelet MAO activity, and3H-imipramine binding in heroin and cocaine addicts

AbstractThis work evaluated in a population of heroin and heroin plus cocaine human addicts:1.Norepinephrine (NE), epinephrine (Epi), and 3-methoxy-4-hydroxyphenylglycol (MHPG) (the principal metabolite of brain NE) plasma levels;2.Monoamine oxidase (MAO) activity; and3.3H-imipramine specific binding to the amine carrier in platelets. NE plasma levels were significantly lower in the short-term heroin user groups (1–3 and 4–6 yr), a finding not observed in both the long-term heroin user (>6 yr) and heroin plus cocaine user (>6 yr) groups. Epi levels changed in a similar manner, except that a significant increase was noted in heroin plus cocaine abusers. Conversely, dopamine and MHPG plasma levels increased with the duration of heroin use, and even more with cocaine abuse. Platelet MAO activity increased in all groups. Specific3H-imipramine binding sites showed an increase after 3 yr of heroin abuse and in all heroin plus cocaine addicts. In conclusion, short-term use of heroin decreases NE or Epi release, but with prolonged use, a slow adpatation occurs. In contrast, cocaine inhibits the neuronal Epi uptake, even in a situation of long duration of abuse. Probably the amine levels additionally regulate the amine carrier, resulting in changes that show a different pattern from major depression. These drugs of abuse may also influence directly or indirectly related enzymatic systems.

Dexamethasone Effect on Postoperative Pain and Tramadol Requirement after Thyroidectomy

Tramadol is a central-acting analgesic associated with nausea and vomiting. Clinical studies have demonstrated that glucocorticoids have analgesic and antiemetic effects when administered perioperatively. The aim of this study is to test the hypothesis that coadministration of tramadol and dexamethasone decreases both postoperative pain and tramadol requirement by patient-controlled analgesia (PCA). Forty female patients undergoing thyroidectomy under general anesthesia were enrolled in a double-blind randomized controlled study and allocated to receive dexamethasone 4 mg i.v. (dexamethasone group, n = 20) or saline (control group, n = 20). At 0, 1, 2, 4 and 22 h of PCA, tramadol consumption and pain were evaluated. Although pain (numerical rating scale 0–10) was significantly lower in the dexamethasone group compared to the control group (2.9 ± 1.4 vs. 3.8 ± 1.2, p = 0.02) at the beginning of PCA, tramadol demand was not significantly different. Although the results herein show a possible beneficial effect of a preoperative single low dose of dexamethasone on postoperative pain, the hypothesis that this corticosteroid decreases tramadol requirement is not supported.

Pharmacological characterization of the postsynaptic α-adrenoceptors in human uterine artery*

Prazosin and yohimbine were used to differentiate postjunctional α‐adrenoceptors in the human uterine artery in‐vitro. Two postjunctional α‐adrenoceptor subtypes were distinguished by the affinities of the receptor for yohimbine and prazosin. The pA2 for prazosin was 8.91 against phenylephrine with a slope not significantly different from unity (0.91), and the pA2 for yohimbine was 7.25 against naphazoline and 8.70 against clonidine, with slopes not significantly different from unity (1.11 and 1.18, respectively). Yohimbine was not very active against phenylephrine, while prazosin was very active against the mixed and selective α2‐adrenoceptor agonist noradrenaline and clonidine; the intercepts of the Schild plot were 8.80 and 8.82 but with slopes significantly less than unity (0.77 and 0.67, respectively). Prazosin competitively antagonized phenylephrine at the α1‐adrenoceptor, whereas yohimbine competitively antagonized naphazoline and clonidine at the α2‐adrenoceptor. It is concluded that both α1‐ and α2‐adrenoceptors are present in the human uterine artery.

Influence of 0.1 or 0.2% cholesterol-enriched diets on the induction of atherosclerosis and aorta reactivity in vitro

Current knowledge of atherogenesis is largely based on animal models of hypercholesterolemia, which rarely show changes similar to the lesions described in humans. We studied the influence of two low cholesterol-enriched diets on the development of anatomopathologic lesions and on the reactivity of the isolated aorta in rabbits. Compared with controls (rabbits fed a normal diet), a 0.1% cholesterol-enriched diet over a 6- or 9-month period produced increases of the 5-hydroxytryptamine (5-HT)-induced contractile responses, as well as a decreases in acetylcholine (ACh)-induced relaxing response (endothelium-dependent, through the production of NO). Noradrenaline (NA)-induced contractions and relaxations elicited by sodium nitroprusside (SNP; endothelium independent) were not significantly modified. Because at 6 months, significant anatomopathologic intimal early lesions were not found, functional endothelial changes can explain such findings. There was a defect in NO synthesis, release, or diffusion; 5 HT, but not NA, may be responsible for inducing NO production. In 0.2% cholesterol-fed rabbits at 4 and 12 weeks, increases of 5-HT- and NA-induced contractile responses were found. In both cases, there was a decrease of ACh-induced relaxing effect, whereas responses to SNP remained unchanged. Intimal early and advanced lesions were present at both the 4- and 12-week periods. These data suggest abnormalities of the NO system. The effects obtained with NA may be explained by a possible decrease of catechol-O-methyltransferase (COMT) or monoamine oxidase (MAO) activities or both or by decreased amine uptake. The extent to which NA may induce NO production is small, because changes in NA-induced contractions are verified only in the presence of significant alterations in the endothelium. The use of a 0.2% cholesterol diet for a short time may induce atherosclerotic lesions, whereas the 0.1% cholesterol diet for a 9-month period, besides being closer to the human diet, allows the detection of functional abnormalities before the evidence of structural lesions.

Pharmacological Characterization of the Postsynaptic Serotonergic Receptor in the Human Uterine Artery

Serotonergic receptors are involved in many vascular functions, but only a few studies have been made in human vessels. Thus, this study aimed to characterize these receptors in the human uterine artery without endothelium. The pD2 value of serotonin (5-HT) was 5.96, but the intrinsic activity was 59% of that of noradrenaline. Spiperone and ketanserin shifted the concentration-response curves of 5-HT to the right (pA2 = 8.56 and 9.76; slopes = 0.98 and 0.83, respectively). Propranolol, yohimbine, prazosin and atropine did not significantly shift the concentration-response curves to 5-HT. Phentolamine inhibited the 5-HT response (pA2 = 6.69), and previous treatment of the vascular strips with 6-hydroxydopamine only partially reduced such an effect. The results demonstrate the existence, in the human uterine artery, of 5-HT2 receptors which are blocked by high concentrations of phentolamine. In this tissue, 5-HT does not release noradrenaline from perivascular nerves.

Cyclosporin effect on noradrenaline release from the sympathetic nervous endings of rat aorta

Arterial hypertension is one of the main side effects of cyclosporin treatment and seems to be due to activation of the sympathetic nervous system. Some authors hypothesized that cyclosporin may act on the sympathetic nervous endings increasing catecholamine release, in agreement with our previous works which demonstrated an increase in rat plasma catecholamine levels after 30 mg/kg per day cyclosporin treatment for 7 weeks. Therefore, the aim of this work was to study the cyclosporin mechanism responsible for that increase in plasma catecholamines. Male Wistar rats were used. Noradrenaline release was performed in vitro experiments after loading rat aorta abdominal segments with 3H-noradrenaline (3H-NA). The release of 3H-NA was measured after electrical stimulation in the presence of 10(-6)M cyclosporin. In another set of experiments electrical stimulation was replaced by a pulse addition of cyclosporin (10(-6)M). Another group of rats was treated with 30 mg/kg per day cyclosporin for 7 weeks and catecholamine contents in aorta abdominal segments and adrenals were measured by high performance liquid chromatography system with electrochemical detection (HPLC-ECD). An increase in the 3H-NA release was observed in both types of in vitro experiments. Since cocaine abolished these cyclosporin effects, the obtained results suggest that cyclosporin may act on the catecholamine transporter across the membrane. In addition, after the 7 weeks of cyclosporin treatment, a significant decrease in catecholamine aorta contents was verified but in adrenals there was no difference regarding to controls. However, the dopamine synthesis/degradation ratio measured by the DA/DOPAC ratio suggests an increase in dopamine synthesis. These facts are in agreement with the enhanced plasma catecholamine levels and with the hypothesis of tissue catecholamine depletion. However, they do not explain the increase in plasma adrenaline levels, since adrenaline is a reflex of adrenal activity. The synthesized dopamine in adrenals seems to be unable to reach vesicles and to be metabolized in adrenaline. The observed decrease in HVA adrenal levels may be a consequence of extraneuronal uptake inhibition or inhibition by cyclosporin of the direct o-methylation of DOPAC. In conclusion, our results suggest that cyclosporin increases catecholamine release from the sympathetic nervous endings by a tyramine-like effect, i.e. by acting directly on the catecholamine transporter of the membrane.

Influence of Concurrent Heroin and Cocaine Abuse on the Adrenergic and Serotonergic Systems in Mana

ABSTRACT: Drugs of abuse interfere with the adrenergic activity at the periphery and the study of their effects In Vivo at this level may contribute to understand the central mechanisms of action. Free and sulfoconjugated catecholamines and serotonin (5‐hydroxytryptamine, 5‐HT) were measured by high‐performance liquid chromatography with electrochemical detection (HPLC‐ECD); plasma dopamine (DA)‐b‐hydroxylase (DBH) activity was determined by a HPLC technique. When compared with healthy subjects (n = 49), the results in drug addicts (n = 48) revealed: a) Significant increases in plasma DA (free and sulfoconjugated), epinephrine (Epi)‐ and norepinephrine (NE)‐sulfate levels; b) no significant differences in the levels of plasma free NE or Epi, serum and platelet 5‐HT. Concerning DBH activity, there was a tendency for an increase. In conclusion, since catecholamines are rapidly inactivated during blood circulation, the measurement of their sulfates may better reflect catecholamine turnover. The differences found may be interpreted as a reflection of the activation of adrenergic neurotransmission, principally as a consequence of catecholamine uptake blockade by cocaine. Finally, in our conditions the abuse of cocaine plus heroin does not significantly interfere with 5‐HT uptake by platelets.

Effects of neuropeptides on the sumatriptan-disturbed circulation in the optic nerve head of rabbits.

The purpose of this work was to study ‘in vivo’ the vascular responses of retinal vessels of New Zealand white rabbits to substance P (SP), neurokinin A (NKA), neurokinin B (NKB), senktide, capsaicin (CAPS), and calcitonin gene related peptide (CGRP) before and after selective antagonist administration. We examined the effects of these neuropeptides on the normal circulation in the optic nerve head of the rabbit. Drugs were injected via pars plana through a micropipette system. Ten minutes before perivascular injection of 10 nmol/l sumatriptan (to contract the vessel), a selective antagonist or its solvent was administered. Then, cumulative injection of the agonist was performed. The other eye was used as control. Direct measurement of retinal arteriole diameters was performed using digital angiography. The quantification of the relaxing effect is expressed as percentage related to the precontracted vascular diameter. Microinjection of SP (NK1 receptor agonist) up to 10 nmol/l induced a dose-dependent arteriolar dilating effect [Emax (mean ± SEM) 21.3 ± 2.3%]. After the perivascular preinjection of 1 nmol/l L-668,169 or 1 nmol/l L-733,060 (NK1 receptor antagonists), the SP dose-response curve was shifted to the right. The same results were obtained with NKA (NK2 receptor agonist) which induced the most potent effect of all neuropeptides (Emax 53.3±2.5%). The NK2 receptor antagonists L-659,877 and GR 159897 (1 nmol/l) strongly inhibited this arteriolar vasodilation. As for CGRP, doses up to 10 nmol/l induced a marked vasodilation (Emax 41.1±0.4%) which decreased after microinjection of the selective antagonist CGRP8-37. The NK3 receptor agonists (senktide and NKB) showed a minor vasodilating effect (Emax 5.1±1.2 and 8.0±0.9%, respectively). On the contrary, CAPS showed a marked dose-dependent vasodilating effect (Emax 43.2±2.9%), antagonized by the tachykinin receptor antagonists and CGRP8-37. These results suggest, for the first time, the presence of NK1, NK2, and CGRP receptors on the retinal arteriolar wall of the rabbit.