Effects of some antihypertensive drugs on cutaneous blood flow and inflammatory skin responses following allergen challenge in guinea pigs

The effects of the antihypertensive drugs clonidine, prazosin, and MK 422 (the active parent diacid of enalapril) were studied on the cutaneous blood flow and allergen evoked inflammatory skin reactions in ovalbumin sensitized guinea pigs. The hypotensive effect of the drugs did not significantly change the basal cutaneous blood flow at the time of allergen challenge. MK 422 (0.02 mg/kg) markedly enhanced the wheal and flare reaction following allergen challenge, whereas clonidine (0.005 and 0.05 mg/kg) inhibited the inflammatory response. Prazosin (0.03 mg/kg) did not significantly influence the wheal and flare reaction. Our results indicate that some antihypertensive drugs (clonidine) could be beneficial to antihypertensive patients with inflammatory diseases, while others (ACE-inhibitors) may enhance their inflammatory disorders.

Comparison of the effects of clonidine and guanfacine on the histamine liberation from human mast cells and basophils and on the human bronchial smooth muscle activity

The inhibitory effects of clonidine and guanfacine on antigen-induced histamine release from human basophils and mast cells and on bronchial muscle tension were compared. Clonidine, but not guanfacine inhibited the antigen-induced histamine release from human basophils and mast cell preparations. This effect of clonidine was prevented by an H2-antagonist, cimetidine. Clonidine and guanfacine, like norepinephrine, inhibited the twitch contraction of electrically stimulated human segmental bronchi by stimulation of alpha 2-adrenoceptors. Clonidine and guanfacine had no effects on the basal bronchial muscle tone, but in very high concentrations they could reduce a carbacholine-induced bronchial tone. It is suggested that the inhibitory effects of clonidine on allergic reactions and on exicitatory nerve transmission in human airways may be useful in the treatment of asthmatic patients.

Effects of enprofylline, a new xanthine derivate, on human pregnant myometrium

The recently developed xanthine derivate, enprofylline, was studied in an in vitro system with human pregnant myometrium, in particular its effect on spontaneous myometrial contraction, cyclic adenosine monophosphate content, cyclic adenosine monophosphate protein kinase, and cyclic adenosine monophosphate-and cyclic guanosine monophosphate-dependent phosphodiesterase activity. Enprofylline was shown to be a rather potent smooth muscle relaxant [inhibitory concentration that decreases response by 50% (IC50) = 3 X 10(-5) mol/L] and an almost equally potent cyclic adenosine monophosphate-dependent phosphodiesterase inhibitor (IC50 = 10(-4) mol/L), whereas it was a less potent cyclic guanosine monophosphate-dependent phosphodiesterase inhibitor. Enzyme kinetic studies revealed that enprofylline is a competitive cyclic adenosine monophosphate phosphodiesterase inhibitor. Enprofylline increased the cyclic adenosine monophosphate content in a dose-dependent way with subsequent increased activity of cyclic adenosine monophosphate-dependent protein kinase. It is suggested that enprofylline relaxes myometrial smooth muscles and that this is at least partly the result of interference with the cyclic adenosine monophosphate system.

Potentiation of inflammatory reactions in guinea-pig skin by an angiotensin converting enzyme inhibitor (MK 422)

There have recently been reports of persistent cough and increased broncho-obstruction likely to have been induced by ACE inhibitors. In order to study the effect of MK 422 (the active parent diacid of enalapril) on the inflammatory response, ovalbumin-sensitized guinea-pigs were tested intradermally with ovalbumin, capsaicin and bradykinin. All inflammatory responses were enhanced by treatment with MK 422 for 2 days prior to testing as compared to the responses of control animals. Infiltration of neutrophils, eosinophils, basophils and monocytes was increased following ovalbumin challenge in the MK 422-treated animals. We suggest that skin reactions and airway symptoms noticed during ACE inhibitor therapy might have been due to induced or potentiated inflammatory reactions in the skin or in the bronchial wall.

Inhibitory effects of clonidine on allergic reactions in guinea-pig skin

An experimental guinea-pig model was used to show that clonidine inhibited dose dependently the wheal and flare reaction to i.d. ovalbumin in sensitized animals. The effect of clonidine was counteracted by an alpha 2-adrenoceptor antagonist, yohimbine, but not by an alpha 1-adrenoceptor antagonist, prazosin. An H1-receptor antagonist, clemastine, initially reduced the wheal and flare reaction but did not influence the effect of clonidine. Cimetidine, an H2-receptor antagonist slightly reduced the effect of clonidine on the wheal and flare reaction. It is suggested that clonidine reduces the wheal and flare reaction by stimulating alpha 2-adrenoceptors, which inhibits the axon reflex of the response.

Effects of clonidine on the dermal inflammatory cell response of experimental toxic and allergic contact reactions and intradermal hypersensitivity

In previous studies, the alpha 2-adrenoceptor agonist clonidine has been shown to suppress the wheal and flare reaction in guinea pigs sensitized to ovalbumin. This phenomenon has been further studied with special reference to effects on the dermal inflammatory cell infiltrate and mast cells. Clonidine lessens the degranulation of mast cells seen in control untreated immediate hypersensitivity reactions. Less neutrophils and eosinophils arrive to the treated reactions. Basophils and mononuclear cells (chiefly lymphocytes) which characterize the late phase of the wheal and flare reaction were not influenced by clonidine. Clonidine had a possible minimal effect on allergic contact (delayed hypersensitivity) reactions. The toxic contact reaction to croton oil (nonspecific cutaneous inflammation) was not affected.

Comparative studies on nerve- and noradrenaline-induced melanosome aggregation within different species of fish

1. The aggregation of melanosomes within melanophores of the cuckoo wrasse (Labrus ossifagus; belonging to the family Labridae) has, on pharmacological grounds, been shown to be mediated by postsynaptic alpha 2-adrenoceptors which in turn act via an inhibitory control of adenylate cyclase. 2. In the present paper we have investigated some American species belonging to the Labridae, Haemulidae, Embiotocidae, Clinidae and Pleuronectidae. 3. In all instances, except in the case of sargo (Haemulidae), we could demonstrate that melanosome aggregation probably was mediated by postsynaptic alpha 2-adrenoceptors which mediate their effect by inhibiting the adenylate cyclase of the melanophores. 4. Although these receptors apparently, on pharmacological grounds, may be classified as alpha 2-adrenoceptors it was also concluded that there is a phylogenetic divergence among these receptors.

Innervation of airway smooth muscle. Efferent mechanisms

The classical view, with one excitatory (cholinergic) and one inhibitory (noradrenergic) component, of the innervation of airway smooth muscle is incomplete and at least two other, possibly peptidergic, types of innervation must be included when the innervation of airways is considered. A summary of these neuronal components is given in Fig. 1 and their possible origin is outlined. Besides the inhibitory noradrenergic innervation of the airways observed in some species, an inhibitory NANC (i-NANC) innervation has been demonstrated. The polypeptide, VIP, seems to be the most likely candidate for the neurotransmitter in the i-NANC innervation of the airways. The excitatory cholinergic innervation is present in the airways from the trachea down to the peripheral bronchi. In the guinea-pig bronchi an excitatory NANC (e-NANC) innervation has been demonstrated as well. The e-NANC nerves may correspond to chemosensitive primary afferent nerves with substance P or a related tachykinin as transmitter. When the innervation of airway smooth muscle of different mammalian species is compared it is evident that all nerve components except the cholinergic, show a considerable variability among species. The cholinergic innervation seems to be present in all mammalian species whereas the other components may be completely absent from some species. Distinct regional variations in the innervation of the airways may occur, which is exemplified by the distribution of the autonomic innervation in the guinea-pig tracheo-bronchial tree. Cholinergic neurotransmission in for example the guinea-pig and human airways can be modulated by NA via prejunctional inhibitory alpha 2-adrenoceptors. Furthermore, the e-NANC neurotransmission in the guinea-pig airways may be modulated by NA or by selective alpha 2-adrenoceptor agonists, acting via prejunctional inhibitory alpha 2-adrenoceptors. The clinical importance of the NANC innervation in relation to asthma is discussed. The i-NANC nerves may exert a modulating effect on bronchoconstriction, and a functional defect would presumably lead to an exaggerated response to constrictor stimuli. The e-NANC nerves in the airways may also be clinically relevant since the transmitter (tachykinins) from these nerves can produce bronchoconstriction and promote inflammation of the airway epithelium, either by direct mechanisms or indirectly by activation of mast cells, and thus contribute to the features of asthma.(ABSTRACT TRUNCATED AT 400 WORDS)

The effect of inhaled clonidine in patients with asthma

To study the effect of inhaled clonidine on broncho-obstruction in asthmatics, 10 patients with extrinsic asthma were broncho-provoked twice with an allergen that had been shown to give positive skin prick test results. Before the provocations, saline or clonidine (75 micrograms) was inhaled via a DeVilbiss no. 40 nebulizer. Clonidine slightly improved the basal respiratory function without significantly influencing the blood pressure. There was a reduction in the allergen-evoked broncho-obstruction by 42 to 65% after pretreatment with clonidine. The eosinophilic leukocytes in peripheral blood had a slight tendency to leave the circulatory system during the provocation preceded by the clonidine treatment. These data add further evidence to the finding that clonidine may be of interest in the treatment of broncho-obstructive diseases.

Effects of mepacrine and p-bromophenacyl bromide on anti-IgE and phospholipase A2-induced histamine release from human basophils

Exposure of human basophils to purified phospholipase A2 caused a release of histamine, the process could be divided in one Ca2+-independent and one Ca2+-dependent stage. Low concentrations of mepacrine and p-bromophenacyl bromide (BPB) inhibited both phospholipase A2- and anti-IgE-induced histamine release. Mepacrine was more potent than BPB when the two-stage-method was used. The inhibition of mepacrine was most effective when the drug was added in the second Ca2+-dependent stage. The effect of mepacrine in the whole reaction of the anti-IgE-induced histamine release was biphasic and mepacrine was less effective than in the inhibition of the separated stages. The effect of BPB on the whole reaction was rather similar to mepacrine, although it was not biphasic. The results presented in this work confirm a previous hypothesis suggesting that activation of phospholipase A2 is an important step in the IgE-mediated histamine release process. The results also suggest that inhibition of histamine release due to inhibition of phospholipase A2 might be of therapeutical value as the system can be inhibited at very low drug concentrations.

Development of tolerance to glyceryl trinitrate in an isolated human peripheral vein and its relation to cyclic GMP metabolism

Human peripheral vein (v. saphena magna) was exposed in vitro to glyceryl trinitrate (GTN) 0.1 mM for 1 h. The subsequent relaxant effect of GTN (10nM-0.1 mM) on vessels contracted by serotonin (0.25 microM) was significantly reduced on vessels preexposed to GTN as compared to control vessels, indicating a development of partial tolerance as far as the vascular smooth muscle relaxant effect is concerned. The impaired relaxant effect was paralleled by a reduced increment of intracellular cyclic GMP. This is, in turn, probably a consequence of both a diminished guanylate cyclase activity and an elevated phosphodiesterase activity in the GTN-tolerant vessels. The relaxant activity as well as the level of intracellular cyclic GMP were restored in GTN-tolerant vessels by dipyridamole (5 microM), an agent with phosphodiesterase inhibiting properties.

Inhibitory effects of clonidine on bronchospasm induced in guinea-pigs by vagal stimulation or antigen challenge

The effects of clonidine on the bronchospastic responses induced by vagal stimulation or antigen challenge were studied in anaesthetized guinea-pigs. Electrical stimulation of the vagus nerves by 2-4 Hz induced a vigorous, mainly atropine-sensitive bronchoconstriction, which was strongly inhibited by clonidine (0.05 mg/kg i.v.). The inhibitory effect of clonidine was significantly reduced by the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg i.v.). Another series of experiments was done with ovalbumin-sensitized guinea-pigs. Respiratory anaphylaxis was induced by antigen inhalation resulting in an increase of pulmonary resistance from 100% (baseline) to about 190% in the control group. Animals pretreated with a clonidine aerosol (0.03%) showed a marked inhibition of the bronchospastic response. It is suggested that the inhibition of the bronchospastic responses induced by clonidine may be mediated by a stimulation of alpha 2-adrenoceptors, which exerts an inhibitory control of the excitatory vagal activity in the guinea-pig airways.

Alpha-adrenergic receptors in human myometrium during pregnancy

The distribution and mechanisms of alpha-adrenergic receptors have been studied in myometrial preparations from women delivered by cesarean section at term. With the use of the radioligand tritiated dihydroergocryptine, the number of alpha-adrenergic receptors was 210 fmol/mg of protein in the uterine fundus and 195 fmol/mg of protein in the lower uterine segment. Competition experiments showed that 60% of the alpha-adrenergic receptors had properties as alpha 1-adrenergic receptors and 40% as alpha 2-adrenergic receptors. In vitro tension studies verified the existence of physiologically active alpha 1- and alpha 2-adrenergic receptors in the myometrial preparations. alpha 2-Adrenergic receptor stimulation resulted in lowered levels of intracellular cyclic adenosine monophosphate. The intracellular cyclic adenosine monophosphate was further reduced by additional alpha 1-adrenergic receptor stimulation, probably secondary to an activation of calcium-calmodulin-dependent phosphodiesterase.

The relaxant effect of glyceryltrinitrate on isolated human peripheral vein and its relation to cyclic GMP metabolism

The relaxant effect of glyceryltrinitrate (GTN) on human vena saphena magna was studied in vitro. Vessels contracted by serotonin (0.25 microM) and phenylephrine (0.1 mM) were relaxed to the same extent (EC50 = 10 microM) by GTN, whereas in 100 mM K+-depolarized vessels the relaxation was significantly lower. The relaxant effect produced by GTN was preceded by an elevation of cyclic guanosine-3',5'-monophosphate (cGMP). For 0.1 mM GTN there was a 3-fold increase in cGMP after 3 min. A correlation between relaxation and increase in cGMP was established. When GTN was combined with dipyridamole (5 microM) the relaxant effect of GTN was significantly greater (EC50 = 0.1 microM). Phosphodiesterase inhibition, as a possible mechanism behind the observed better relaxation for the combination (GTN+dipyridamole), is briefly discussed. In conclusion, the relaxant effect of GTN on isolated human vena saphena magna seems to be dependent on the contractile stimuli used, increased by the addition of DIP and to be mediated via cGMP.

Inhibition of the cholinergic neurotransmission in human airways via prejunctional alpha-2-adrenoceptors

Previous investigations on guinea-pig airways indicate that the excitatory, cholinergic neurotransmission can be inhibited via prejunctional alpha-2-adrenoceptors. The aim of this study was to investigate whether a similar inhibition of the cholinergic neurotransmission is present in human airways. Ring preparations of human bronchi were mounted for recording of isometric tension and immersed in oxygenated Krebs solution. Electrical field stimulation of the preparations elicited atropine-sensitive twitch contractions. Exogenous noradrenaline (in the presence of cocaine and propranolol) inhibited the electrically evoked contractions. The noradrenaline-induced inhibition could be antagonized by yohimbine whereas prazosin was ineffective, indicating that the inhibition was mediated by alpha-2-adrenoceptors. Contractions evoked by exogenous acetylcholine (in the presence of cocaine and propranolol) was unaffected by the addition of noradrenaline, which suggests that the alpha-2-adrenoceptors have a prejunctional localization. In conclusion, this report gives evidence that the human bronchial, cholinergic neurotransmission can be inhibited by stimulation of prejunctional alpha-2-adrenoceptors.

The control of granule movement in fish melanophores

The effects of electrical field stimulation, alpha-2-adrenoceptor activation and manipulation of the intracellular machinery on aggregation and dispersion of melanosomes within melanophores of Labrus ossifagus were studied. Neither melatonin nor MSH exerted any direct effects on the melanophores, and furthermore MSH alone did not affect the responses elicited by electrical stimulation or by noradrenaline. Omitting extracellular Ca2+ blocked the nerve-mediated aggregation but was without effect on noradrenaline-induced aggregation. Colchicine and demecolcine inhibited the melanosome aggregation induced by noradrenaline. The characteristics of aggregation and dispersion were found to diverge in several ways. In melanophores made permeable by dispersion were found to diverge in several ways. In melanophores made permeable by using saponin, cyclic AMP caused dispersion. The dynein-ATP-ase inhibitor vanadate prevented the melanosome aggregation within saponin-treated melanophores, while vandate was without effect on the cyclic AMP-induced dispersion. The melanosome transport in saponin-treated melanophores could be activated even in the absence of Ca2+, indicating that the melanosome movements were completely Ca2+-independent.

The effect of pertussis toxin on alpha-2-adrenoceptor-mediated pigment migration in fish melanophores

The aggregation of melanin-granules within fish pigment cells (melanophores) can be elicited either by electrical stimulation of intrinsic nerves or by the addition of adrenergic agonists. The pigment aggregation seems to be mediated by alpha-2-adrenoceptors. In this investigation we have used various agonists and antagonists (noradrenaline, (+)- and (-)-adrenaline, isoprenaline, yohimbine and prazosin) to further characterize the pigment-aggregating receptor of Labrus ossifagus. All the results obtained support the notion of alpha-2-adrenoceptor-mediated pigment aggregation. The pertussis toxin, islet-activating protein (IAP), is known to inhibit the alpha-2-adrenoceptor-mediated signal transduction in mammals. We have used IAP to investigated whether fish melanophore alpha-2-adrenoceptors are also inhibited by this toxin. We found that IAP inactivated the alpha-2-adrenoceptor-mediated pigment aggregation in a dose-dependent manner. The inhibitory IAP-effect had a remarkably short onset-time in the melanophores (maximal effect was obtained within 10 min of incubation). Interestingly, binding of an agonist (noradrenaline) to the receptors prevented IAP from exerting its inhibitory action, whereas binding of an antagonist (yohimbine) gave no protection against the IAP-inactivation. In conclusion, the pigment-aggregating receptors of melanophores of L. ossifagus are very similar to the mammalian alpha-2-adrenoceptors. It is possible to inactivate the melanophore receptor system with IAP and this inactivation has a remarkably short onset-time. Stimulation of the alpha-2-adrenoceptors prevents IAP from inactivating the receptor system.

Effects of dipyridamole on the glyceryl-trinitrate-induced inhibition of coronary artery muscle tone and platelet aggregation in relation to cyclic nucleotide metabolism

The effects of glyceryl-trinitrate (GTN) and dipyridamole (DIP) on relaxation of bovine coronary arteries and on inhibition of aggregation of human platelets have been studied in vitro with special reference to the cyclic GMP (cGMP) system. GTN had a dose-dependent relaxant effect on bovine coronary arteries, and at a high concentration (10(-5) M) it had an inhibiting effect on platelet aggregation. The effects were associated with an increase in the cGMP levels of the tissues. DIP (5 X 10(-7) M respectively 5 X 10(-6) M) potentiated the coronary artery relaxation induced by GTN (10(-8) M) and the inhibition of platelet aggregation caused by GTN in the concentrations 10(-7)-10(-4) M. The potentiation was associated with higher levels of cGMP than those produced by GTN alone, at least in bovine coronary arteries. However, at a concentration of 10(-4) M, GTN, in combination with DIP, caused a significant fall in the cGMP level compared to GTN alone. GTN and DIP were not found to significantly increase the cAMP levels in the concentrations tested. DIP was shown to inhibit phosphodiesterase (PDE) from both platelets and bovine coronary arteries. This might be one of the possible mechanisms that can explain the above mentioned potentiation. It is suggested that the combination of DIP + GTN may be of some clinical importance since the potentiating effects were seen at concentrations comparable to the therapeutic plasma concentration for the respective drugs.

Contraction and cyclic AMP-related relaxation of the intimal and medial smooth muscle layers of pig thoracic aorta

The contractile responses of an alpha-adrenoceptor agonist, phenylephrine, and of histamine were compared in the intimal and medial smooth muscle layers of the pig aortic arch. Further, the relaxant effects evoked by some compounds influencing the cyclic AMP system were compared in the two muscle layers, as well as their effects on the cyclic AMP content and phosphodiesterase activity. Phenylephrine and histamine induced contraction of the smooth muscle layers. The increase in tension was faster in the intimal than in the medial layer. The alpha-adrenoceptor agonist phenylephrine was a more potent contractile agent in the intimal than in the medial smooth muscle. With histamine, no significant difference in the dose-response curves between the two muscle layers was found. Histamine-contracted muscle preparations were relaxed in a dose-dependent manner by the phosphodiesterase-inhibiting compound 3-isobutyl-1-methylaxanthine (MIX) and by 8-bromo-cyclic AMP. The two substances were more potent relaxants in the medial than in the intimal smooth muscle layer. The content of cyclic AMP in the intimal and the medial smooth muscle was increased by MIX. Isoprenaline had no relaxing effect on the muscle preparations and did not change the content of cyclic AMP. There were no differences in the basal levels of cyclic AMP in the intima and media. Vmax of phosphodiesterase activities differed, however, between the two preparations. This study demonstrates that the intimal layer is characterized by a larger contractile responsiveness to phenylephrine and a lower relaxant response to compounds influencing the cyclic AMP-system than those of the medial layer.

Beta-adrenergic receptors in human myometrium during pregnancy: changes in the number of receptors after beta-mimetic treatment

The concentration of available beta-adrenoceptors has been studied in the myometrium of women treated with terbutaline for premature uterine contractions and in an untreated control group. Myometrial strips were taken at cesarean section from the lower uterine segment and the uterine fundus. The concentration of beta-adrenoceptors was determined with a radioligand binding assay. In untreated women we found no difference in the concentration of beta-adrenoceptors in the uterine fundus compared to that in the lower uterine segment. The cyclic adenosine monophosphate production after beta-adrenoceptor agonist stimulation in vitro was equal in both locations. In the terbutaline-treated women, the binding site concentrations in both the fundus and lower uterine segment were lower compared to those in the control group. The decrease was most pronounced in the fundus where receptor concentration was only half that found in the control group. The results suggest that treatment with beta-mimetics causes a desensitization in the beta-adrenoceptor system of human myometrium during pregnancy. This desensitization may partly explain the limited duration of the relaxant effect of beta-mimetics often seen in the treatment of preterm labor.

In vivo demonstration of alpha-2-adrenoceptor-mediated inhibition of the excitatory non-cholinergic neurotransmission in guinea pig airways

In anaesthetized guinea pigs, vagal nerve stimulation caused an atropine- and hexamethonium-resistant increase in the respiratory insufflation pressure. Clonidine (0.05 mg/kg i.v.) inhibited the increase in insufflation pressure caused by vagal stimulation. This effect of clonidine was antagonized by the alpha-2-antagonist yohimbine (0.5 mg/kg i.v.), but not by the alpha-1-antagonist prazosin (0.03 mg/kg i.v.). In conclusion, the increase in the respiratory insufflation pressure, caused by stimulation of noncholinergic nerves, seemed to be controlled by inhibitory alpha-2-adrenoceptors in guinea pig airways.

Effects of enprofylline and theophylline on purified human basophils

Extracts from purified human basophils revealed activity of cAMP-phosphodiesterase with a km-value of 0.59 microM. The enzyme was not activated by Ca-ions. Enprofylline and theophylline inhibited the enzyme in a competitive manner. Enprofylline was more potent than theophylline. These drugs did also inhibit the anti-IgE-induced histamine release from the basophils. These results favour the hypothesis that inhibition of histamine release of enprofylline is caused by an inhibition of phosphodiesterase. Although accumulating data have indicated that theophylline, at therapeutic concentrations, is a weak inhibitor of cAMP-phosphodiesterase activity, there is reason to believe that enprofylline, at therapeutic concentrations, may act at least partly as a phosphodiesterase inhibitor.

Potent bronchodilating effects of enprofylline and theophylline on contractions induced by egg albumin or by slow reacting substance (SRS)

Isolated sensitized guinea pig tracheal smooth muscle tone was induced by use of egg albumin or SRS-A (slow reacting substance of anaphylaxis). The dose-response relationships of theophylline and enprofylline were studied on these preparations. Enprofylline was more potent than theophylline in relaxing the egg albumin- or SRS-A-induced tracheal muscle tone. The theophylline relaxation-curve was significantly shifted to the left after addition of adenosine-deaminase to the egg albumin-contracted trachea, but this was not observed with the enprofylline relaxation-curve. In SRS-A-contracted tracheas, the addition of adenosine-deaminase did not significantly alter the relaxation curves of theophylline or enprofylline. It is therefore suggested that the relaxing effects of theophylline and enprofylline on SRS-A-induced contractions, at the therapeutically relevant concentrations demonstrated in this study, might be of importance for the anti-asthmatic effects of xanthines.

Adrenergic nerves and the alpha 2-adrenoceptor system regulating melanosome aggregation within fish melanophores

Nervous control of melanosome aggregation of melanophores of Labrus ossifagus was studied by electrical field stimulation of isolated scales. Field stimulation elicited a rapid melanosome aggregation which was reversible upon interruption of the stimuli. The response was blocked by guanethidine indicating that adrenergic nerves were stimulated. The melanophore receptors which mediate the nerve-controlled pigment aggregation were characterized to be of the alpha 2-adrenoceptor subtype, since they were inhibited by yohimbine but not by prazosin. Stimulation of the alpha 2-adrenoceptors by noradrenaline was associated with a significant reduction of the cyclic AMP content of the melanophores. It is suggested that adrenergic nerves control melanosome aggregation via alpha 2-adrenoceptors and that these receptors are coupled to the adenylate cyclase-cyclic AMP system of the melanophores.