Biogenic amines and control of melanophore stimulating hormone release.

Release of melanophore stimulating hormone (MSH) from the vertebrate pars intermedia is under inhibitory control by the hypothalamus. Removal of the rat pituitary or the neurointermediate lobe of the frog (Rana pipiens) to in vitro incubation medium results in rapid uninhibited release of MSH. This secretion is inhibited by norepinephrine, epinephrine, phenylephrine, and dopamine, and the inhibition is antagonized by alpha-adrenergic receptor blocking agents. Isoproterenol stimulation of MSH secretion from isolated glands is blocked by pro-pranolol, a beta-adrenergic receptor antagonist. These results implicate dopaminergic or classical alpha-adrenergic receptors (or both) in inhibition of MSH release by catecholamines, and implicate beta-adrenergic receptors in stimulation of MSH release by the bioamines.

Peculiarities of neural regulation of the thyroid, adrenocortical and testicular functions in old age.

The age changes of neural control over the function of the thyroid, adrenal cortex and testicles were examined in adult (6 months) and old (28 months) male rats. In old age, there was a weakening of adrenergic control of thyroidogenesis, alpha-adrenergic and M-cholinergic control of glucocorticoid function of the adrenal cortex and reduction of adrenergic and M-cholinergic influences in the regulation of steroidogenic function of the testicles.

Synthesis and adrenoreceptor blocking action of aziridinium ions derived from phenoxybenzamine and dibenamine.

Crystalline perchlorate salts of aziridinium ions derived from phenoxybenzamine and dibenamine were prepared. Both aziridinium ions were tested on the rat vas deferens and found to possess alpha-adrenergic potencies which were nearly identical with those of the parent compounds. The hydrolysis rates of phenoxybenzamine and dibenamine aziridinium ions (2a,b) in physiological medium were found to be 6.0 4 x 10(-4) and 8.35 x 10(-4) sec-1, respectively. The rates of cyclization of the parent amines to 2a and 2b in aqueous medium were 1.9 x 10(-2) and 7.2 x 10(-3) sec-1, respectively. The potencies and kinetic profiles indicate that the aziridinium ion is the only active species in alpha-adrenergic blockade. Moreover, differences in potency between phenoxybenzamine and dibenamine appear to be exclusively to a difference in receptor affinity rather than to a difference in intrinsic alkylating ability.

Temperature modulation of alpha- and beta-adrenoceptors in the isolated frog heart.

1. The effects of adrenaline on the isolated frog's heart at 27 degrees C are not antagonized by phentolamine (1.5 x 10(-6)M) but are abolished at 7 degrees C.2. At 27 degrees C isoprenaline was more potent than noradrenaline, but at 7 degrees C noradrenaline was more potent than isoprenaline.3. Phenoxybenzamine (1.5 x 10(-5)M) or dibenamine (1.5 x 10(-5)M) at 7 degrees C abolished the work output induced by adrenaline. When the temperature was raised to 24 degrees C, adrenaline caused an increase in work output.4. It is concluded that in the isolated frog heart there are at least two pools of adrenoceptors, the availability of which can be governed by temperature.

Reversal by pronethalol of dibenamine blockade: a study on the seminal vesicle of the guinea-pig.

1. The guinea-pig seminal vesicle has been shown to be a very suitable test object for the study of mechanisms involving alpha-adrenoceptive receptors, because no beta-receptors were found in this preparation.2. Adrenaline, noradrenaline and phenylephrine were directly acting agonists, their ED50 values being 7.1 x 10(-6)M, 1.5 x 10(-5)M and 2.7 x 10(-5)M, respectively.3. Pretreatment with reserpine had no influence on the contractions caused by adrenaline, noradrenaline and phenylephrine but abolished or greatly reduced the contractions caused by dopamine. Cocaine enhanced the effects of adrenaline, noradrenaline and phenylephrine and reduced those of dopamine.4. Pronethalol (6.8 x 10(-5)M) reversed the alpha-receptor blockade by dibenamine, ergotamine and phentolamine of responses to adrenaline, noradrenaline and phenylephrine; it did not affect the blockade by dibenamine of responses to histamine.5. Reversal of the blockade by dibenamine was observed only when its concentration was such that it caused a parallel shift of the dose-effect curves of the agonists to the right; higher concentrations, which caused an unsurmountable depression of the maximal contraction, were not antagonized by pronethalol.6. It is assumed that the reversal is dependent on a direct action on alpha-receptors, "spare receptors" being probably involved.

Investigations on the mode of action of ergotamine in the isolated femoral vein of the dog.

1 Experiments on spiral strips cut from the femoral vein of dogs suspended in Krebs-Henseleit solution were carried out.2 Ergotamine caused stimulation in concentrations about 350 times lower than noradrenaline (ED(50) of ergotamine = 2.2 x 10(-9) M; ED(50) of noradrenaline = 7.6 x 10(-7) M), but the maximal responses to ergotamine were only about one third those to noradrenaline.3 The pA(2) value of ergotamine against noradrenaline was 8.8.4 The effects of ergotamine can be blocked by prior administration of phentolamine. The pA(2) value for phentolamine against ergotamine was 6.8 and the pA(2) value for phentolamine against noradrenaline was 7.5.5 It is concluded that the stimulant action of ergotamine on smooth vascular muscle probably is mediated mainly via alpha-adrenoceptors.

Inhibition of catecholamine uptake in the isolated rat heart by haloalkylamines related to phenoxybenzamine.

1. Twenty-one haloalkylamine derivatives were tested as inhibitors of both the neuronal uptake of (3)H-noradrenaline (NA) by the Uptake(1) mechanism and the extraneuronal uptake of (3)H-NA by the Uptake(2) mechanism in the isolated rat heart.2. At a concentration of 50 muM most of the compounds tested caused a significant inhibition of both uptake processes, although there were wide differences in the relative effects on Uptake(1) and Uptake(2). Some tentative structure activity relationships for uptake inhibition were formulated from these results.3. Phenoxybenzamine was confirmed to be a potent inhibitor of both the Uptake(2) and Uptake(1) mechanisms, with IC50 values for these two systems of 2.8 muM and 0.9 muM respectively.4. The substances N-(9-fluorenyl)-N-methyl-beta-chloroethylamine (SKF 550), N-(3,4-dimethoxyphenylisopropyl)-N-benzyl-beta-chloroethylamin (SKF 625A) and N-(4-methoxyphenoxyisopropyl)-N-benzyl-beta-chloroethylamine (SKF 784A) were significantly more potent than phenoxybenzamine as Uptake(2) inhibitors, and were all less potent than phenoxybenzamine as Uptake(1) inhibitors. The compound SKF 550 is the most potent and selective inhibitor of Uptake(2) so far described. It has an IC50 for Uptake(2) of 0.08 muM, and an IC50 for Uptake(1) of approximately 40.0 muM.5. Comparison of the present results with the known activities of these blocking agents suggests that no correlation exists between adrenoceptor blocking activity and ability of the substances to act as inhibitors of Uptake(2) or Uptake(1).

Adrenoceptors in the cat choledochoduodenal junction studied in situ.

1. The effects of adrenaline, noradrenaline, isoprenaline and terbutaline (PINN), 1-(3,5-dihydroxyphenyl)-2(t-butylamino)-ethanol, on the sphincter of Oddi were studied in anaesthetized cats. Both adrenaline and noradrenaline (1-4 mug/kg) increased resistance to flow through the sphincter. This effect was blocked by dibenamine (0.5-2.5 mg/kg) and by phenoxybenzamine (0.5-1.0 mg/kg).2. Isoprenaline (0.5-5.0 mug/kg) as well as terbutaline (0.5-10.0 mug/kg) inhibited the spontaneous sphincter activity and decreased flow resistance. In these doses, isoprenaline provoked marked tachycardia and relaxation of the duodenum, whereas the cardial and intestinal effects of terbutaline were minimal. The effects were blocked by propranolol (0.5-1.0 mg/kg).3. The results confirm earlier observations of the presence of alpha-adrenoceptors active in contraction of the sphincter musculature and of beta-adrenoceptors active in its relaxation. It is suggested that the beta-adrenoceptors belong to the beta(2) type according to Lands' classification.

Evidence for the presence of -adrenoceptors in the central thermoregulatory mechanism of rabbits.

1. The effects of intracerebroventricular administration of some alpha- and beta-adrenoceptor stimulants and antagonists on the body temperature of rabbits were investigated.2. Noradrenaline produced a dose dependent rise in body temperature. Other catecholamines were less active.3. The noradrenaline response was blocked by alpha-adrenoceptor blocking agents while beta-adrenoceptor antagonists had no effect.4. alpha-Methyl-noradrenaline and metaraminol had some hyperthermic effect, but significantly reduced the response of noradrenaline.5. The possible presence of alpha-adrenoceptors in the central thermoregulatory mechanisms is suggested.

Selectivity of 4-methylhistamine at H1- and H2-receptors in the guinea-pig isolated ileum.

The selectivity of 4-methylhistamine (4-MH) as an agonist at histamine H1- and H2-receptors has been evaluated in the guinea-pig isolated ileum. The EC50 values of 4-MH on H1- and H2-receptors that mediate contractile responses were determined. The EC50 at H1-receptors was estimated after selective blockade of H2-receptors by tiotidine and the EC50 at H2-receptors estimated after selective blockade of H1-receptors by mepyramine. The -log EC50 values at H1- and at H2-receptors were 4.57 and 5.23, respectively. The dissociation constants for the interaction of 4-MH with H1- and H2-receptors were determined. The -log KD values at H1- and H2-receptors were 3.55 and 4.27, respectively. These results suggest that 4-MH is only about 5 times as potent at H2- as it is at H1-receptors in the guinea-pig ileum and that 4-MH should be used with caution to discriminate between H1- and H2-receptors.

Do both adrenaline and noradrenaline stimulate cardiac alpha-adrenoceptors to induce positive inotropy of rat atria?

1. The positive inotropic responses of rat paced left atria to adrenaline and noradrenaline were recorded. Desmethylimipramine (DMI, 1 microM) and metanephrine (10 microM) were initially present throughout. 2. The positive chronotropic responses of spontaneously beating right atria to adrenaline were used as a reference. In these, pindolol, in increasing concentrations, caused progressive shift of the concentration-response curves to the right, which yielded a pA2 value (8.15) compatible with antagonism of beta-adrenoceptors. 3. The left atrial tension responses to adrenaline showed an initial progressive displacement by pindolol (up to 3 microM) which gave an unexpectedly low pA2 value (6.48). However, with further increases in pindolol concentration there was no additional shift of the curve. In the presence of pindolol (3 microM), prazosin (0.1 microM) displaced the curve to the right but the pA2 value derived from this shift (7.75) was less than expected for alpha 1-adrenoceptor antagonism. 4. When the experiments in the presence of pindolol (3 microM) were repeated in the absence of DMI, prazosin displaced the concentration-response curves for adrenaline-induced left atrial tension to a greater extent and the pA2 value (8.76) was now compatible with adrenaline stimulating typical alpha 1-adrenoceptors. 5. The concentration-response curves for noradrenaline-induced left atrial tension were also progressively displaced to the right by pindolol (0.1, 0.3 and 1.0 microM). These concentrations yielded a Schild plot of unity slope and a pA2 value of 7.94 +/- 0.04. This was not significantly different from the pA2 value of 8.02 +/- 0.07 determined for pindolol against isoprenaline in the left atria, which indicates a normal interaction of noradrenaline with beta-adrenoceptors in the absence and presence of low concentrations of pindolol. 6. A further increase in the concentration of pindolol to 3 microM failed to induce an additional shift of the noradrenaline curves, whether a 'before and after' antagonist or a 'naïve tissue' design was adopted. Similarly, the rightwards shift of the concentration-response curves by timolol reached a limit as the concentration was increased. In all cases the limit of shift occurred at a noradrenaline EC50 value of 5-10 microM. 7. At the limit of beta-adrenoceptor antagonism, prazosin and dibenamine did not displace the noradrenaline curves further. The residual inotropic response to noradrenaline therefore appeared to be mediated via neither alpha- nor beta-adrenoceptors. 8. DMI, in the absence of beta-blockade, produced the potentiation of adrenaline and noradrenaline expected of a neuronal uptake inhibitor. However, in the presence of pindolol, there was no potentiation of the right atrial rate response to adrenaline while its left atrial tension responses were antagonized. This suggested that DMI was acting as an alpha-adrenoceptor antagonist. It also explained the less-than-expected shift by prazosin of the adrenaline responses in the presence of both pindolol and DMI, the latter drug already exerting some alpha-blocking activity. In contrast, the left atrial tension responses to noradrenaline in the presence of pindolol (1 microM) were neither potentiated nor antagonized by DMI. 9. When the effects of prazosin upon left atrial tension responses to noradrenaline in the presence of pindolol (10 microM) were examined in the presence of a lower concentration of DMI (O.1 microM) or cocaine (1O microM), again there was no further shift of the curve. However, when the effect of prazosin) The Macmillan Press Ltd 1989 598 K.L. WILLIAMSON & K.J. BROADLEY was examined in the absence of DMI, but in the presence of pindolol (1 and 1O microM) or timolol (3 microM), there was a small shift of the curves by prazosin (0.1 microM). This yielded pA2 values of 7.19, 7.34 +/- 0.1 and 7.66 +/- 0.09, which were at least one order of magnitude less than literature values and that obtained with adrenaline (8.76 +/- 0.18), and are not consistent with noradrenaline stimulating an alpha 1-adrenoreceptor in the presence of beta-adrenoceptor blockade, the increase in left atrial tension by noradrenaline does not appear to be mediated by beta l- or typical alpha-adrenoceptors. This is in contrast to adrenaline which in these conditions stimulates typical alpha 1-adrenoceptors.

Interactions between responses mediated by activation of adenosine A2 receptors and alpha 1-adrenoceptors in the rabbit isolated aorta.

1. This paper describes aspects of the functional antagonism between the responses mediated by activated alpha 1-adrenoceptors and adenosine A2 receptors in the adventitia- and endothelium-denuded aorta of the rabbit. 2. Adenosine A2 receptor agonists relaxed aortic rings pre-contracted with phenylephrine. The relaxation response was agonist concentration-dependent and saturable. The respective contractile and relaxation responses were stable, reproducible, and reversible. 3. Increasing the phenylephrine concentration caused a progressive attenuation of the action of adenosine A2 receptor agonists, consisting of a decreased maximal response and a dextral shift of the adenosine agonist concentration-response curve. This functional antagonism could be completely reversed upon removal of adenosine by either the addition of adenosine deaminase or by wash-out of the adenosine agonist from the tissue. The relaxation response to the adenosine A2 receptor partial agonists, N6-cyclohexyladenosine and R-(-)-N6-(2-phenylisopropyl)adenosine, was abolished at higher phenylephrine concentrations (e.g. 30 EC50). 4. A 1000 fold increase in the adenosine concentration was required to shift the value of the EC50 of phenylephrine six fold, while a similar increase in the value of the EC50 of adenosine could be elicited by only a 32 fold increase in the phenylephrine concentration. A 30 fold increase in the phenylephrine concentration shifted the value of the EC50 of 5'-N-ethylcarboxamidoadenosine four fold. 5. Analysis of the functional antagonism between the responses mediated by these receptors using the Black & Leff (1983) operational model of agonism allowed for the estimation of the agonist dissociation constant, KA, and the apparent efficacy, tau, for both phenylephrine and adenosine A2 receptor agonists. Increasing the concentration of phenylephrine reduced the value of tau for adenosine agonists in a concentration-dependent and saturable manner. Similarly, increasing the concentration of adenosine reduced the value of tau for phenylephrine in a concentration-dependent and saturable manner. The phenylephrine KA value obtained by the method of functional antagonism (1.9 microM) was similar to that obtained by the receptor inactivation method (2.1 microM). 6. Partial occlusion of the alpha 1-adrenoceptor by the alkylating agent, dibenamine, demonstrated that the magnitude of the adenosine A2 receptor-mediated relaxation was inversely proportional to the number of functional alpha 1-adrenoceptors. 7. It is concluded that the magnitude of functional antagonism is proportional to the stimulus elicited through either receptor. We propose that this tissue preparation and pair of receptors is a good model to study quantitative aspects of functional antagonism between activated receptors.

Discrimination by SZL49 between contractions evoked by noradrenaline in longitudinal and circular muscle of human vas deferens.

The effects of irreversible alpha1-adrenoceptor antagonists, SZL-49 (an alkylating analogue of prazosin), dibenamine and benextramine on contractions to noradrenaline (NA) in longitudinal and circular muscle of human epididymal vas deferens were investigated. Competitive alpha1-adrenoceptor antagonists were also used to further characterize the alpha1-adrenoceptor subtype stimulated by NA in longitudinal and circular muscle. NA evoked concentration-dependent contractions of both muscle types (pD2; 5.4 and 5.2 respectively). The contraction of circular muscle was comparatively more sensitive than that of longitudinal muscle to pretreatment with SZL-49. In contrast, dibenamine or benextramine produced comparable effects in both muscle types. The relationship between receptor occupancy and contraction in either longitudinal or circular muscle was nonlinear, with half-maximal response requiring similar receptor occupancy (longitudinal muscle 14%, circular muscle 16%). Maximal response in both muscle types occurred with little or no receptor reserve (<10%). The competitive alpha1-adrenoceptor antagonists produced dextral shifts of the dose-response curves to NA in longitudinal and circular muscle. The inhibitory potencies, estimated from the apparent pKB values were significantly different in longitudinal and circular muscle respectively for either WB 4101 (pKB, 8.6 and 9.5) or RS-17053 (pKB, 7.1 and 9.0) but not for Rec 15/2739 (pKB, 9.2 and 9.8) or HV 723 (pKB, 8.3 and 8.4). In conclusion, the potency profile of the competitive alpha1-adrenoceptor antagonists and the lack of different receptor reserves for NA in the muscle types suggest that the discriminatory effects of SZL-49 is primarily due to a predominance of the alpha1L-adrenoceptor subtype in longitudinal muscle and alpha1A-subtype in circular muscle.

Therapeutic trials in hamster dystrophy.

Verapamil and prenylamine, which antagonize calcium influx into heart muscle cells, Dibenamine and propanolol, alpha and beta adrenergic bockers, respectively, and prostaglandin E1, which acts on permeability of cell membranes and on adrenergic neurotransmission, were all shown to markedly reduce the severity of heart lesions in UM-X7.1 cardiomyopathic hamsters. The beneficial effects of these compounds seen essentially preventive, in that they do not afford protection for fully developed skeletal muscle lesions. The occurrence of the pathologic changes in the myocardium coincides with an increased adrenergic nerve activity, and it is believed that these drugs function mainly by decreasing calcium conductivity across the sarcolemmal membranes of cardiocytes.

Chronic Chagas' disease in the mouse: contractile response of the isolated myocardium.

The effects of different pharmacological agents on the isometric developed tension (IDT) of right ventricles isolated from chronic chagasic C3HS mice were studied. The IDT in ventricles of chagasic animals was lower than in ventricles of normal mice (P less than 0.01). Chagasic ventricles exhibited hyperreactivity to norepinephrine but not to epinephrine. This hyperreactivity was masked by propranolol and not by dibenamine. Acetylcholine produced a reduction of IDT to similar values in normal and chagasic myocardia. The possibility that the effect of the drugs tested might not result from specially beta adrenoceptor-mediated reactions, but from alterations at other cellular sites is discussed.

Action of sauvagine on the mesenteric vascular bed of the dog.

Sauvagine, a linear peptide of 40 amino acids, produced hypotension when administered intravenously to anesthetized dogs. Diastolic pressure was always more affected than systolic pressure. Aortic blood flow and venous return both increased to the same extent. The mechanism of the hypotensive response was mainly, if not exclusively, due to dilatation of the superior and inferior mesenteric arteries. Intravenous infusion of sauvagine in doses ranging from 3 to 10 ng . kg-1 . min-1 produced a dose-related increase of mesenteric blood flow up to 400% control values. Mucosal-submucosal blood flow of ileum and colon was increased, while blood flow in muscle was unaffected or slight decreased. The mesenteric vasodilator response was not prevented by adrenergic or muscarinic receptor blockade. The hypotensive response was more marked and sustained in dibenamine-propranolol treated dogs.

Dibenamine blockade on gastric acid secretion in vitro and its protection by histamine antagonists.

Experiments to protect the histamine receptor against dibenamine blockade were carried out to elucidate the pharmacological characteristics of the H2-histamine receptor system for gastric acid secretion in isolated bullfrog gastric mucosa. Dibenamine alone (5 times 10-minus 5 g/ml) irreversibly blocked both basal and histamine-stimulated acid secretion. However, when the preparation was treated with dibenamine in combination with histamine (1 times 10-minus 5 g/ml) irreversibly blocked both basal and histamine-stimulated acid secretion. However, when the preparation was treated with dibenamine in combination with histamine (1 times 10-minus 5 g/ml), the acid secretory response to histamine was restored after washing out dibenamine. Burimamide, an H2-receptor antagonist, also protected the histamine sensitivity against dibenamine blockade in the concentration of 1 times 10-minus 4 g/ml. Diphenhydramine and pyribenzamine were also protected with histamine receptor against dibenamine blockade. The acid secretion induced by the action of histamine on the diphenhydramine-protected receptor was antagonized by diphenhydramine as well as burimamide.

The effect of strontium on the drug-receptor interaction on cholinergic drugs.

The effect of replacing calcium with strontium in the perfusion fluid was qualitatively and quantitatively studied in the isolated longitudinal muscle of the guinea-pig ileum. In the presence of strontium hyoscine could be considered a competitive antagonist of acetylcholine for the acetylcholine receptor of the longitudinal muscle of the guinea-pig ileum; dibenamine still blocked this receptor in an irreversible way. The equilibrium constants for acetylcholine (KA) and hyoscine (KI) were obtained in the presence of calcium (KA = 3.16 +/- 0.63 microM; KI = 0.38 +/- 0.07 nM), and strontium (KA = 7.00 +/- 0.89 microM; KI = 0.93 +/- 0.16 nM). The results show a decrease in the affinity of both drugs for the muscarinic receptor in the presence of strontium.

Discrepancy between alpha 1-adrenoceptor-mediated contraction and the occupation theory in rat vas deferens--possible existence of a 'silent' receptor.

The relation of the amount of alpha 1-adrenoceptors (alpha 1-R) with contraction to norepinephrine (NE) through alpha 1-R in rat vas deferens was examined by means of radiobinding assays. Treatment with dibenamine decreased the maximal contraction to NE with a decrease in the amount of alpha 1-R but the relation was not linear. The contractile response disappeared completely when 20% of the alpha 1-R still remained. Moreover, culture of dibenamine-pretreated muscle restored the contraction to NE without a significant increase in the amount of alpha 1-R in the muscle. These findings suggest that some alpha 1-R are 'silent' in the contraction of rat vas deferens in response to NE under physiological conditions.

Irreversible blockade of responses to a partial agonist acting at the histamine H1-receptor.

N,N-Diethyl-2-(1-pyridyl)-ethylamine (E-2-P) has been shown previously to behave as a simple partial agonist at the histamine H1-receptor of guinea-pig ileum. When isolated longitudinal muscle strips from this preparation were tested with E-2-P before and after blockade with 2-haloalkylamines, it was found that these agents produced an irreversible shift to the right in the dose-response curve without significant depression of the maximum response even at very high antagonist concentrations. Under these circumstances the maximum response to the partial agonist may exceed the maximum response to histamine itself since the latter shows a much diminished maximum response at a very high concentrations of antagonist. These findings are not readily explicable in terms the usual "receptor-reserve' model of the histamine receptor system in ileum. A tentative explanation is provided, involving interaction with the antagonist at more than one site.