Inhibition by (−)-Deprenyl of Agonist-Evoked Contractions in Rabbit Aorta

Abstract: The effect of (?)-deprenyl, a relatively selective MAO-B inhibitor, was examined for its ability to inhibit the contractions of rabbit isolated aorta evoked by various agonists and potassium. (?)-Deprenyl (10^?5?3 × 10^?4 M) antagonized the contractions evoked by noradrenaline (10^?8?3 × 10^?4 M); pA₂: 5.10. The antagonism was reversible. It was attenuated by cocaine (3x M); pA₂: 4.38, unchanged by corticosterone (4 × 10^?5 M); pA₂ 4.79 and enhanced by cocaine (3 × 10^?5 M) plus corticosterone (4 × 10^?5 M); pA₂: 5.48. (+)-Deprenyl (10^?6?10^?4 M) did not alter the contractions evoked by noradrenaline (3 × 10^?9?10^?4 M). Clorgyline (10^?5 and 10^?4 M) antagonized the noradrenaline-evoked contractions. Pargyline (10^?4 and 3 × 10^?4 M) had no effect. (?)-Deprenyl (10^?5?3 × 10^?4 M) antagonized the contractions evoked by phenylephrine (10^?8?10^?4 M); pA₂: 5.10. Removal of the endothelium did not alter the antagonism; pA₂: 5.35. (?)-Deprenyl (10^?5?3 × 10^?4 M) antagonized the contractions evoked by either 5-hydroxytryptamine (3 × 10^?8?3 × 10^?4 M); pA₂: 4.61 or by histamine (10^?6?3 × 10^?2 M); pA₂: 4.84. (?)-Deprenyl (3 × 10^?4 M) caused a non-competitive antagonism of the contractions evoked by potassium (1.5-5.5 × 10^?2 M). It is concluded that (?)-deprenyl is a weak inhibitor of postjunctional α-adrenoceptors, 5-hydroxytryptamine (5-HT₂) receptors, and histamine (H₁) receptors.     

Effect of Dopexamine Hydrochloride on Sympathetic Neuroeffector Transmission in Rabbit Isolated Pulmonary Artery

Abstract: The effects of dopexamine hydrochloride on sympathetic neuroeffector transmission were studied in rabbit isolated pulmonary artery. Short-term exposure of dopexamine (10-⁸x10^-7 M) and cocaine (10^-6-3x10^-5 M), but not desipramine (3x10^-9-3x10^-7 M), to the artery enhanced the contractions evoked by electrical-field stimulation. Corticosterone (4x10^-5 M), corticosterone (4x10^-5 M) plus cocaine (3x10^-8 M), but not cocaine (3x10^-5 M), attenuated the enhancement seen with dopexamine. High concentrations of dopexamine (10^-5-3x10^-5 M), cocaine (10^-4 M), and desipramine (10^-6-10^-5M) decreased the stimulation-evoked contractions. Dopexamine (10^-7-3x10^-5 M), but neither cocaine nor desipramine, caused an increase in resting tension that waned with time. Corticosterone (4x10^-5 M), but not cocaine (3x10^-5 M), attenuated the increase in resting tension. Propranolol (10^-6 M) did not alter the enhancing and inhibitory effects of dopexamine. A single concentration (10^-7 and 10^-6 M) of either dopexamine or desipramine caused a time-dependent biphasic response as regards the repetitive stimulation-evoked contractions of pulmonary artery: initial enhancement followed by inhibition. The inhibitory effect of dopexamine (10^-6 M) and desipramine (3x10^-6 M) seen after prolonged exposure was almost irreversible and partially reversible, respectively, by washing the preparations with drug-free salt solution. Cocaine caused a monophasic steady-state response: either enhancement (10^-5 M) or inhibition (2x10^-4 M). In both cases, the onset was rapid. The reduction caused by cocaine (2x10^-4 M) and by prazosin (10^-9 M) was fully reversed. Dopexamine (10^-5 M) antagonized competitively the contractions evoked by noradrenaline (3x10^-9-10^-4 M). It is concluded that (1) the dopexamine-induced enhancement of neurogenic contractions is not due to either inhibition of neuronal and extraneuronal uptake of noradrenaline or an agonist action on prejunctional β₂-adrenoceptors; (2) that the dopexamine-induced inhibition of stimulation-evoked contraction is due to an inhibition of postjunctional α₁-adrenoceptors; and (3) that the dopexamine-induced increase in resting tension is due to its metabolite methyldopexamine.     

Prejunctional Muscarinic Receptor Modulation of Noradrenaline Release from Sympathetic Neurones in Rabbit Aorta *

The prejunctional muscarinic modulation of stimulation‐evoked release of ³H‐noradrenaline from sympathetic neurones in rabbit aorta was examined. The role of transmitter uptake, α‐adrenoceptor blockade, stimulation frequency and endothelium on the modulation was investigated. Rings of aorta were incubated with (‐)‐³H‐noradrenaline and subsequently subjected to electrical‐field stimulation. Fractional ³H‐overflow was determined by liquid scintillation counting. Acetylcholine (10^?8–3×10^?6 M) added cumulatively, reduced the stimulation‐evoked ³H‐overflow up to 80%. The effect of acetylcholine was the same in intact and endothelium‐free aorta. The inhibitory effect of acetylcholine was inversely related to the frequency of stimulation (1–10 Hz). The maximal inhibition (%) was 80 (1 Hz), 53 (3 Hz) and 14 (10 Hz). The inhibitory effect of acetylcholine (10^?6 M) and carbachol (10^?5 M) reached a maximum 15 min. after addition and then remained almost constant. Cocaine (3×10^?5 M) did not alter the effect of acetylcholine. Desipramine (10^?6 M) and corticosterone (4×10^?5 M) attenuated the inhibition seen with low concentrations (10^?8–10^?7 M) of acetylcholine. The acetylcholine‐induced inhibition was antagonized by desipramine. Cocaine plus corticosterone attenuated the inhibition seen with high concentrations (10^?6–3×10^?6 M) of acetylcholine. Rauwolscine (10^?6 M) enhanced the maximal inhibitory effect of acetylcholine. We conclude that the inhibitory effect of acetylcholine on ³H‐overflow from rabbit aorta preloaded with ³H‐noradrenaline is (1) inversely related to stimulation frequency; (2) independent of endothelium; (3) unaffected by neuronal and extraneuronal transmitter uptake; (4) that cocaine is not a prejunctional muscarinic antagonist; (5) that cocaine, but not desipramine, is suited as a neuronal uptake inhibitor in studies of prejunctional muscarinic receptor subtypes; and (6) and that there is an inverse interaction between prejunctional α₂‐adrenoceptors and muscarinic receptors.     

Mechanism of action of relaxant effect of Agastache mexicana ssp.mexicana essential oil in guinea-pig trachea smooth muscle

Context: Agastache mexicana ssp. mexicana (Kunth) Lint &; Epling (Lamiaceae), popularly known as ‘toronjil morado’, is used in Mexican traditional medicine for the treatment of several diseases such as hypertension, anxiety and respiratory disorders.

Objective: This study investigates the relaxant action mechanism of A. mexicana ssp. mexicana essential oil (AMEO) in guinea-pig isolated trachea model.

Materials and method: AMEO was analyzed by GC/MS. The relaxant effect of AMEO (5–50?μg/mL) was tested in guinea-pig trachea pre-contracted with carbachol (3?×?10?^??⁶?M) or histamine (3?×?10?^??⁵?M) in the presence or absence of glibenclamide (10?^??⁵?M), propranolol (3?×?10?^??⁶?M) or 2′,5′-dideoxyadenosine (10?^??⁵?M). The antagonist effect of AMEO (10–300?μg/mL) against contractions elicited by carbachol (10?^??¹⁵–10?^??³?M), histamine (10?^??¹⁵–10?^??³?M) or calcium (10–300?μg/mL) was evaluated.

Results: Essential oil composition was estragole, d-limonene and linalyl anthranilate. AMEO relaxed the carbachol (EC_50?=_?18.25?±?1.03?μg/mL) and histamine (EC_50?=_?13.3?±?1.02?μg/mL)-induced contractions. The relaxant effect of AMEO was not modified by the presence of propranolol, glibenclamide or 2′,5′-dideoxyadenosine, suggesting that effect of AMEO is not related to β₂-adrenergic receptors, ATP-sensitive potassium channels or adenylate cyclase activation. AMEO was more potent to antagonize histamine (pA₂′?=??1.507?±?0.122) than carbachol (pA₂′?=??2.180?±?0.357). Also, AMEO antagonized the calcium chloride-induced contractions.

Conclusion: The results suggest that relaxant effect of AMEO might be due to blockade of calcium influx in guinea-pig trachea smooth muscle. It is possible that estragole and d-limonene could contribute majority in the relaxant effect of AMEO.     

Inhibition of 3H-noradrenaline accumulation by dopexamine hydrochloride in the isolated aorta of the rabbit

Summary The aim of the present work was to study the ability of dopexamine hydrochloride to interfere with the neuronal and extraneuronal uptake mechanisms by investigating the effect of dopexamine hydrochloride on ³H-noradrenaline accumulation by rabbit isolated aorta. Dopexamine hydrochloride (3 × 10^–9 – 10^–5 mol/l) reduced the accumulation of tritium by aorta incubated with ³H-noradrenaline (10^–8 mol/l). The effect of dopexamine was compared to cocaine, dopamine, dobutamine, ADTN [(+-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene], ouabain and isoprenaline. Dopexamine hydrochloride (3 × 10^–9 – 10^–7 mol/l) caused the same degree of inhibition irrespective of whether corticosterone (4 × 10^–5 mol/l) was present or not. The order of inhibitory potency was: desipramine > dopexamine hydrochloride > dopamine > ADTN cocaine > dobutamine > ouabain > isoprenaline. In the presence of desipramine (10^–6 mol/l), corticosterone (10^–6 – 10^–4 mol/l), but not dopexamine hydrochloride (10^–6 – 10^–4 mol/l), reduced the ³H-accumulation. It is concluded that dopexamine hydrochloride is a potent inhibitor of uptake-1 in rabbit isolated aorta. Dopexamine hydrochloride has no affinity for the uptake-2 mechanism in this tissue. Send offprint requests to O. A. Nedergaard at the above address     

Mechanism of action of histamine in the estrogen-primed rat uterus

Histamine produced a dose-dependent relaxation of uterine strips obtained from the estrogen-primed rat uterus. The responses to histamine were blocked competitively by metiamide (10^?8 ?10^?8M), a specific H₂-receptor antagonist. Propranolol, a selective β-receptor blocker also produced competitive antagonism of the responses to histamine in the same dose range (10^?8?10^?6 M). The pA₂ value obtained for metiamide (8.9) was not significantly different from that obtained for propranolol (8.6). Nialamide (2.2 × 10^?6 M), the monoamine oxidase inhibitor, and cocaine (4.3 × 10^?6 M), the selective noradrenaline uptake blocker, potentiated the responses to histamine. However bretylium (2.4 × 10^?5 M), an adrenergic neuron blocker inhibited the responses to histamine. The combined effect of tyramine and histamine was found to be additive. Our data suggest that the histamine-induced relaxation of rat uterus may be produced through the stimulation of presynaptic H₂-receptors which causes the release of noradrenaline. The released noradrenaline acts on the postsynaptic β-receptors and produces relaxation of the rat uterus.     

Dual Effect of the Muscarinic Agonist McN-A-343 on Vascular Neuroeffector Transmission

Abstract: The site and mechanism of action of McN-A-343 (4-m-chlorophenylcarbamoyloxy)-2-butynyltrime-thylammonium chloride) on sympathetic neuroeffector transmission in the rabbit isolated pulmonary artery was studied. Low concentrations (10^?6 — 3 × 10^?5 M) of McN-A-343 and cocaine enhanced (up to 210 and 236%, respectively) the contractions evoked by electrical-field stimulation, while higher concentrations (10^?4 — 3 × 10^?4 M) inhibited them. McN-A-343 (10^?4 M) caused an initial transitory potentiation (222% of control) of the stimulation-evoked contractions followed by an inhibition. In the presence of cocaine (3 × 10^?5 M), the potentiation caused by McN-A-343 was prolonged and the secondary inhibitory phase was thus abolished. Physostigmine (10^?5 — 10^?4 M), hexamethonium (10^?5 M), atropine (3 × 10^?7 M), suprofen (10^?5 M), and 4-aminopyridine did not alter the effect of McN-A-343 (10^?4 M). Cocaine (3 × 10^?5 M)and(+)-amphetamine (10^?5 M) reversed the McN-A-343-evoked block, while they did not alter the inhibition caused by tetracaine (3.2 × 10^?5 M). Atropine (3 × 10^?7 M) had no effect on the McN-A-343-induced block, while 4-aminopyridine (10^?4 M) caused a partial and transitory reversal. On the aorta McN-A-343 (10^?4 M) did not alter the contractile concentration-response curve of (—)-noradrenaline (10^?9 — 3 × 10^?4 M), while that of serotonin (10^?8 — 3 × 10^?5 M) was moved to the right in a competitive manner. McN-A-343 (10^?4 M) did not alter the contractions evoked by noradrenaline (10^?7 M) during the period corresponding to the stimulation-evoked enhancement and subsequent inhibition. McN-A-343 (10^?4 M) slightly antagonized the contractions caused by tyramine (10^?6 — 10^?3 M) and carbachol (10^?6 — 10^?3 M). It is concluded that McN-A-343 enhances stimulation-evoked transmitter release by a presynaptic facilitatory action mediated via receptors localized on the outer surface of adrenergic neurones and to a lesser extent by inhibition of noradrenaline re-uptake. The enhancement does not involve presynaptic nicotine or muscarine receptors. Furthermore, McN-A-343 inhibits transmitter release by acting as an adrenergic neurone blocking agent at an intraneuronal site. The inhibition does not involve presynaptic muscarine inhibitory receptors and is prostaglandin-independent.     

Dual Inhibitory Action of ATP on Adrenergic Neuroeffector Transmission in Rabbit Pulmonary Artery

The aim of this study was to determine whether the inhibitory action of ATP on sympathetic neuroeffector transmission in the isolated pulmonary artery is due to ATP itself or one of its dephosphorylated breakdown products, ADP, AMP or adenosine. Furthermore, the mechanism of the inhibitory action was investigated. ATP (10^?6?3 × 10^?4 M), the degradation-resistant ATP-analogue, β, γ-methylene-5′-triphosphate (10^?5?3 × 10^?4 M), ADP (10^?6?3 × 10^?4 M), AMP (10^?5?3 × 10^?4 M), adenosine (10^?5?3 × 10^?4 M) and 2-chloroadenosine (10^?7?3 × 10^?4 M) reduced the contractions evoked by field-stimulation. This was also the case for prostaglandin E₂ (3 × 10^?9?3 × 10^?7 M), while prostaglandin F_2α(1.4 × 10^?8 M) slightly augmented the neurogenic response. The time course of the inhibitory effect of purinergic compounds on the stimulation evoked contractions was studied. In the case of ATP and ADP the inhibition was biphasic: an initial marked block (1 min. after drug addition) which in the continued presence of either compound recovered partially 10 min. later and then remained almost constant for another 90 min. The other purinergic agents caused a monophasic reduction. In the presence of indomethacin (5 × 10^?5 M), ATP and ADP also reduced the neurogenic contractions in a monophasic manner. Indomethacin did not alter the β,γ-methylene-5′-triphosphate-induced inhibition. Dilazep (3 × 10^?6 M) plus deoxycoformycin (3.6 × 10^?6 M), augmented the inhibitory effect of ATP. In contrast, theophylline (5 × 10^?5 M) did not alter the effect of ATP. The inhibitory effect of ATP (10^?4 M) on stimulation-evoked contractions was inversely proportional to the extracellular Ca²⁺ concentration (0.3–5.2 mM) and to frequency of stimulation (3–15 Hz). These results suggest that ATP initially causes a presynaptic inhibition of noradrenaline release evoked by field-stimulation. This phase I block is probably mainly due to an ADP-mediated short-lasting release of prostaglandins of the E type. The continuous inhibition (phase II) is probably due to ATP and its metabolites, possibly mainly adenosine. The phase II inhibition may possibly involve a decreased entry of Ca²⁺ into adrenergic nerve terminals during depolarization.     

Effect of Cocaine and Corticosterone on the Metabolism of 3H-Noradrenaline Released from Rabbit Isolated Aorta and Adventitia

Abstract: The effect of cocaine and corticosterone (neuronal and extraneuronal uptake inhibitors, respectively) on the metabolism of ³H-noradrenaline (³H-NA) released spontaneuosly or by electrical-field stimulation was studied in the rabbit isolated aorta and tunica adventitia preloaded with ³H-NA. The spontaneous outflow of tritium from untreated aorta consisted of ³H-NA (25%); ³H-3,4-dihydroxyphenylglycol (³H-DOPEG; 24%); ³H-3,4-dihydroxymandelic acid (³H-DOMA; 8%); ³H-O-methylated plus deaminated compounds (³H-OMDA; 45%); and ³H-normetanephrine (³H-NMN; 1%). The percentage distribution of ³H-NA and its ³H-metabolites in this outflow was not altered by either cocaine (3 × 10^?5 M), corticosterone (4 × 10^?5 M), or cocaine (3 × 10^?5 M) + corticosterone (4 × 10^?5 M). The spontaneous ³H-outflow from untreated tunica adventitia did not differ from that of aorta. Cocaine (3 × 10^?5 M) + corticosterone (4 × 10^?5 M) did not alter the composition of ³H-NA and its ³H-metabolites in the spontaneous outflow from adventitia. The stimulation-evoked ³H-overflow from untreated rabbit aorta preloaded with ³H-NA consisted of ³H-NA (40%); ³H-DOPEG (15%); ³H-DOMA (2%); ³H-OMDA (50%); and ³H-NMN (3%). Cocaine (3 × 10^?5 M) decreased the percentage recovered as ³H-DOPEG, increased ³H-NMN and had no effect on the other ³H-metabolites. Corticosterone (4 × 10^?5 M) had no effect on the percentage distribution of ³H-NA and its ³H-metabolites. Cocaine (3 × 10^?5 M) + corticosterone (4 × 10^?5 M) increased ³H-NA, decreased ³H-DOPEG and had no effect on the percentage distribution of the other ³H-metabolites. In the case of adventitia, cocaine (3 × 10^?5 M) + corticosterone (4 × 10^?5 M) only decreased the percentage recovered as ³H-DOPEG, without any significant effect on ³H-NA and its other ³H-metabolites. It is concluded that inhibition of neuronal plus extraneuronal uptake of H-NA by cocaine plus corticosterone, respectively does not fully prevent metabolism of ³H-NA released either spontaneously or by electrical-field stimulation.     

Frequency-Dependence of 3H-Noradrenaline Release from Rabbit Pulmonary Artery: Effect of α-Adrenoceptor Antagonists and Inhibitors of Transmitter Inactivation

Abstract: The aim of this study was to examine the modulating role of presynaptic α₂-adrenoceptors on transmitter release from vascular sympathetic neurones. This was done by examining the influence of removal of inactivation pathways on the effect of α-adrenoceptor antagonists on the release of transmitter from noradrenergic neurones. The rabbit main pulmonary artery preloaded with ³H-noradrenaline (³H-NA) was used. The artery was stimulated with 300 pulses at various frequencies (1, 3, 10 and 30 Hz). Pargyline (3 × 10^?4M) increased the stimulation-evoked ³H-overflow at 1 and 3 Hz and decreased it at 30 Hz. U-0521 (3′,4′-dihydroxy-2-methylpropiophenone; 3 × 10^?6M) enhanced the overflow at 1 Hz and had no effect at 3–30 Hz. Corticosterone (4 × 10^?5M) did not alter the stimulation-evoked ³H-overflow at 1–30 Hz. Cocaine (3 × 10^?6M) enhanced the ³H-overflow slightly at 1–30 Hz. At 3 × 10^?5M, cocaine enhanced ³H-overflow at 1 Hz and reduced it at 30 Hz. Neither corticosterone (4 × 10^?5M) nor propranolol (10^?7M) modified this effect of cocaine. Propranolol (10^?7M) alone decreased the ³H-overflow at 30 Hz and had no effect at 1–10 Hz. Phenoxybenzamine (10^?6M) and chlorpromazine (3 × 10^?6M) potentiated the stimulation-evoked ³H-overflow at 1–30 Hz. Phentolamine (10^?6M) decreased the ³H-overflow at 1 Hz and enhanced it at 3–30 Hz. Rauwolscine (10^?6M) enhanced the stimulation-evoked ³H-overflow maximally at 10 Hz and had no effect at 1 and 30 Hz. Cocaine (3 × 10^?5M), cocaine (3 × 10^?5M) + corticosterone (4 × 10^?5M), pargyline (3 × 10^?4M) and U-0521 (3 × 10^?6M), but not corticosterone (4 × 10^?5M), increased the rauwolscine-induced enhancement at 1 and 3 Hz, but had no influence at 10 and 30 Hz. It is concluded that at a low frequency (1 Hz) of nerve stimulation, inhibition of either neuronal uptake, extraneuronal uptake, monoamine oxidase (MAO) or catechol-O-methyltransferase (COMT) causes an enhancement in transmitter overflow that is due to removal of inactivation pathways. The enhancement is not mediated via facilitatory β-adrenoceptors. At higher frequencies (3–30 Hz) inhibition of either neuronal uptake or MAO causes a sufficient amount of transmitter in the neuromuscular gap to activate the presynaptic α-adrenoceptor mediated negative feed-back system causing a decrease in transmitter release. α- Adrenoceptor antagonists enhance the stimulation-evoked release of ³H-NA only when the junctional gap concentration of transmitter is sufficient to trigger the presynaptic α-adrenoceptor auto-inhibitory system.     

Properties of contractions induced by ouabain and K+-free solution in guinea pig taenia coli

The properties of contractions induced by ouabain and by K depletion were studied in comparison with those induced by other stimulants in guinea pig taenia coli. Ouabain 7.5 × 1o^?6 M and K depletion produced biphasic contractions with similar time courses. Both contractions were slightly inhibited by low (0.25 mM) and high (7.5 mM) Ca concentrations, while the high K contraction was strongly inhibited by low Ca and potentiated by high Ca. Contractions induced by ouabain and K depletion were highly sensitive to lowering of the temperature and abolished at 15°C, although contractions induced by carbachol 1.1 × 10^?7 M and histamine 5 × 10^?7 M were still observed below 15°C. In Cl deficient solution substituted by SO₄ or propionate, contractions induced by ouabain and K depletion were potentiated, while contractions induced by high K, carbachol and histamine were suppressed. These results indicate a striking similarity between contractions induced by ouabain and K depletion suggesting a common mechanism for both contractions.     

Fourth international symposium on quantitative mass spectrometry in life sciences

The compound 4(5)-[2-(4-azido-2-nitroanilino)ethyl]imidazole (AAH), a photoaffinity label analog of histamine, has been observed previously to produce a photolysis-dependent, specific and irreversible antagonism of histamine-induced contractile responses of the guinea-pig vas deferens. The pharmacological effects of AAH in isolated guinea-pig aorta (GPA) and dog trachealis (DT) were evaluated in the present study. Photolysis of 3 × 10^?5 M AAH for 5 min with visible light in organ baths containing the GPA resulted, after washout of AAH, in a nonequilibrium competitive antagonism of contractile responses to histamine; responses to norepinephrine and KCl were not affected. In contrast, photolyzed AAH (3 × 10^?5 M, one or two photoalysis treatments; 10^?4 M AAH, two treatments) did not antagonize contractile responses of the DT to histamine, nor were responses to acetylcholine or KCl affected. To test the possibility that AAH lacks affinity for histamine receptors in the DT, molar K_B values were obtained for nonphotolyzed AAH used as a conventional equilibrium competitive antagonist; K_B values for GPA and DT were not different (?log K_B were 5.48±0.18 and 5.04±0.23 for GPA and DT, respectively; P>0.05). pA₂ values for diphenhydramine, an H₁-histamine receptor antagonist, indicated a moderately greater potency in GPA compared to DT (pA₂ were 7.58±0.10 and 6.89±0.13 for GPA and DT, respectively; P<0.05); however the diphenhydramine: AAH potency ratio did not explain the resistance of the DT to photolyzed AAH. Cimetidine (10^?6?10^?4 M), an H₂-receptor antagonist, potentiated contractile responses of the GPA to histamine but was without an effect in the DT. No evidence for relaxation by histamine in preparations with induced tone was obtained; the resistance of the DT to photolyzed AAH therefore does not involve an inhibition of H₂-inhibitory receptors. Photolyzed AAH thus demonstrates tissue-selective antihistamine activity, perhaps as a result of differences in the characteristics of H₁-receptors in the GPA and DT.     

Characterization of 5-Hydroxytryptamine Uptake in Sliced Rat Lung

Abstract: Slices of rat lung were incubated with tritiated 5-hydroxytryptamine and the tissue uptake of tritium was studied after separating the free and bound radioactivity by filtration on glass fiber filters. At 37° a rapid uptake occured during the first 10 min. After that time the uptake was less marked but it was still present after 60 min. The uptake was moderately potentiated by the MAO inhibitor iproniazid (3 μM) after 30–60 min. incubation. The 5-hydroxytryptamine uptake was inhibited by some uptake inhibitors, their order of potency beeing: clomipramine > imipramine ≥ nortriptyline ≥ cocaine ≥ desipramine > maprotiline. At 0° the uptake of 5-hydroxytryptamine was negligible. Non-linear regression analysis of uptake data indicated that 5-hydroxytryptamine was taken up by two different mechanisms. One of the uptake processes was saturated by high concentrations of 5-hydroxytryptamine and showed an apparent K_m of 8×10^?7 M. The other uptake was linearly related to the 5-hydroxytryptamine concentration and could not be saturated even by concentration up to 5×10^?5 M of the amine.     

Investigation of Mechanisms Involved in (−)‐Borneol‐Induced Vasorelaxant Response on Rat Thoracic Aorta

Abstract: The monoterpene (?)‐borneol is present in essential oils of several medicinal plants. The aim of this study was to evaluate (?)‐borneol effects on rat thoracic aorta artery rings. The cumulative addition of (?)‐borneol (10^?9–3 × 10^?4 M) on a phenylephrine‐induced pre‐contraction (10^?6 M) promoted a vasorelaxant effect in a concentration‐dependent manner and independent of vascular endothelium. A similar effect was obtained on KCl‐induced pre‐contractions (80 mM). (?)‐Borneol (10^?5–3 × 10^?4 M) inhibited contractions induced by cumulative addition of CaCl₂ (10^?6–3 × 10^?2 M) in depolarizing medium without Ca²⁺ in a concentration‐dependent manner. On S‐(?) Bay K 8644‐induced pre‐contractions (10^?7 M), (?)‐borneol did not induce significant changes compared with KCl‐induced pre‐contractions. In a Ca²⁺‐free medium, (?)‐borneol (10^?5, 10^?4 or 10^?3 M) interfered in calcium mobilization from phenylephrine (10^?6 M)‐ or caffeine (20 mM)‐sensitive intracellular stores. The involvement of K⁺ channels was evaluated by tetraethylammonium (3 mM), 4‐aminopyridine (1 mM) and glibenclamide (10^?5 M) pre‐treatment, and (?)‐borneol‐induced vasorelaxation was markedly attenuated. Thus, this vasorelaxant effect can probably be attributed to calcium influx blockade through voltage‐operated calcium channels (Ca_VL), calcium mobilization from intracellular stores and potassium channels activation.     

Effect of a new potent H2-receptor antagonist 3[[[2-[(diaminomethylene)amino]-4-thiazolyl]methyl]thio]-N2-sulfamoylpropionamidine (YM-11170) on gastric mucosal histamine-sensitive adenylate cyclase from guinea pig

The effect of 3[[[2-[(diaminomethylene)amino]-4-thiazolyl]methyl]thio]-N²-sulfamoylpro-pionamidine (YM-11170), a new thiazole H₂-receptor antagonist bearing propionamidine at the terminus of a side chain, on histamine-sensitive adenylate cyclase [ATP pyrophosphate-lyase (cyclizing); EC 4.6.1.1] of gastric mucosa from the guinea pig was studied and compared with that of cimetidine. YM-11170 displaced the concentration-stimulation curve of histamine-sensitive adenylate cyclase to the right with a pA₂ of 7.65 (K_i, = 2.25 × 10^?8M). Stimulation of gastric adenylate cyclase by 0.1 mM histamine was competitively inhibited by YM-11170 and cimetidine in a dose-dependent manner, with ic₅₀ values of 5.9 × 10^?7M and 1.4 × 10^?5M respectively. Hippocampal histamine-sensitive adenylate cyclase in the presence of 0.1 mM histamine was also competitively inhibited by YM-11170 with an ic₅₀ of 1.1 × 10^?7 M. YM-11170 did not affect Gpp(NH)p-, NaF-, PGE₂-stimulated or basal activity of the gastric adenylate cyclase. These data, together with other results, indicate that YM-11170 is a highly selective and potent H₂-receptor antagonist which competes with histamine at the receptor site on the histamine-sensitive adenylate cyclase.     

Possible differences in α-adrenoceptors in rabbit ileum and spleen

In isolated tissues from reserpinized rabbits (5 mg kg^?1, i.m. 20 h before experiment) and in the presence of cocaine (3 × 10^?5 m ), corticosterone (2·8 × 10^?5 m ), tropolone (3 × 10^?5m ), propranolol (4 × 10^?6m ) and disodium EDTA (3 × 10^?5m ), the potency ratios (relative to (—)-noradrenaline) of (—)adrenaline, (—)-phenylephrine and (±)-methoxamine were (m ± s.e.) 203 ±0·13, 0·045 ± 0·003 and 0·0062 ± 0·0018 respectively in splenic strips and 1·77 ± 0·41, 0·093 ± 0·018 and 0·029 ± 0·004 respectively in isolated ileum. Although the pA₂ values for phentolamine and thymoxamine against (—)-noradrenaline in the two tissues were very similar there was a statistically significant difference when using yohimbine as the α-adrenoceptor blocking agent (pA₂ = 6·80 ± 0·30 in spleen; 5·60 ± 0·12 in ileum). These differences suggest that the α-adrenoceptor in the two tissues is not identical. The pA₂ value of phentolamine in rabbit ileum was not significantly different whether (—)-noradrenaline or (±)-methoxamine was used as agonist (7·91 ± 0·07 and 7·97 ± 0·06 respectively) while that of yohimbine was 5·56 ± 0·10 using (—)-noradrenaline and 6·19 ± 0·12 using (±)-methoxamine. In the light of this latter result and, considering the scatter of the experimentally determined values, there may be two α-adrenoceptors in rabbit ileum and either or both may not be identical in all respects to the α-adrenoceptor found in rabbit spleen.     

Effect of ω‐Conotoxin GVIA on Noradrenaline Release from Postganglionic Sympathetic Neurones in Rabbit Aorta

Abstract: The aim of the present work was to examine the effect of the selective N‐type calcium blocking agent ω‐conotoxin GVIA on stimulation‐evoked release of noradrenaline from sympathetic nerves in rabbit isolated aorta with regard to stimulation frequency, extracellular Ca²⁺ concentration, and transmitter uptake. Rings of rabbit isolated aorta were preloaded with (‐)‐³H‐noradrenaline and the fractional ³H‐overflow evoked by electrical‐field stimulation was determined by liquid scintillation spectrometry. ω‐Conotoxin GVIA (3×10^?10– 3×10^?8 M) did not alter the spontaneous ³H‐outflow. ω‐Conotoxin GVIA (3×10^?10– 3×10^?8 M) caused a slowly developing reduction of stimulation‐evoked ³H‐overflow at 1 and 30 Hz. The Emax for the ω‐conotoxin‐induced inhibition was less (70%) at 30 Hz than that (96%) seen at 1 Hz. Short‐term incubation with ω‐conotoxin GVIA caused a subsequent steady‐state inhibition. The inhibitory action of ω‐conotoxin GVIA (3×10^?10– 3×10^?9 M) was inversely related to the extracellular Ca²⁺ concentration (6.5×10^?4– 2.7×10^?3 M). Cocaine (3×10^?5 M) plus corticosterone (4×10^?5 M), neuronal and extraneuronal uptake inhibitors, respectively, did not alter the inhibitory effect of ω‐conotoxin GVIA (3×10^?9 M) on ³H‐overflow evoked by stimulation at a frequency of either 1 or 30 Hz. It is concluded that ω‐conotoxin GVIA acts on prejunctional N‐type calcium channels to inhibit stimulation‐evoked noradrenaline release from sympathetic neurone terminals in rabbit aorta. At a high frequency, another subtype calcium channel may possibly be involved. The action of ω‐conotoxin GVIA is independent of neuronal and extraneuronal uptake mechanisms for noradrenaline, but dependent on the amount of Ca²⁺ to be transported across the neurilemma from the extracellular space into the neurone.     

Comparative evaluation of the relaxant effects of MDL-257, its metabolits,and some related compounds on guinea pig tracheal cylindrical segments

The smooth muscle relaxant activities of MDL-257 [8-methyl-6-piperidino-s-triazolo (4,3-b)pyridazine], a new, nonsympathomimetic bronchodilator, its metabolites (in man and rats), and some related compounds were compared to that of aminophylline in guinea pig tracheal cylindrical segments. The IC₅₀ of MDL-257 against KCl-, histamine-, acetylcholine, and 5-hydroxytryptamine-induced contractions were 5.2 × 10^?5, 5.3 × 10^?5, 9.1 × 10^?5, and 1.4 × 10^?5M, respectively. The relaxant activity of aminophylline was equal to that of MDL-257 against all of the spasmogens studied. Dihydroxy substitution on the piperidine ring of MDL-257 reduced its relaxant activity against KCl, histamine, and 5-hydroxytryptamine 25--43-fold and its activity against acetylcholine was almost abolished. The monohydroxy and the amino alcohol metabolites of MDL-257 were two to 20 times less potent than the parent compound and their activities against acetylcholine-induced contractions were minimal. The acidic metabolites of MDL-257 exhibited very weak activity against all of the spasmogens tested.     

Studies on the action and mechanism of action of oxyfedrine on isolated tracheal chains

Oxyfedrine, a β-sympathomimetic drug, did not affect isolated rat and rabbit trachea in concentration from 2.86 × 10^?8 to 2.86 × 10^?4 M, but on the guinea-pig trachea, it caused a dose dependent relaxation of natural tone in lower concentrations (1.79 × 10^?7 to 2.86 × 10^?6 M. In higher concentrations (1.14 × 10^?5 to 2.86 × 10^?4 M), however, a contraction was observed, which was also dose dependent. This contraction was not affected by atropine, lysergic acid diethylamide or by pretreatment with reserpine but was blocked by antihistaminics (isothipendyl and clemastine). Adrenaline, noradrenaline, phenylephrine and isoprenaline did not contract the guinea-pig trachea, whereas contractions were observed after high concentrations of norephedrine, amphetamine, ephedrine and tyramine. These contractions were also unaffected by reserpine pretreatment.It is concluded that the contraction of the guinea-pig trachea by oxyfedrine is related to its structural relationship to the phenylethylamines and might be due to histamine release, an action on histamine receptors or a histamine-like action.     

Separate receptors mediating the positive inotropic and chronotropic effect of histamine in guinea-pig atria

The direct positive inotropic effect of histamine was studied on paced left atrial preparations from guinea pigs. Histamine (10^?8 to 10^?4 M) increased the maximum tension developed in left atria incubated at 35°C and driven at 2 Hz. The maximum increase in tension was 60% of that observed with norepinephrine. Metiamide (3 × 10^?5 M; a specific H₂-receptor antagonist) did not alter the inotropic response of left atria to histamine. However, tripelennamine (a typical H₁-receptor antagonist) competitively shifted the histamine inotropic dose—response curve to the right at concentrations from 10^?8 to 10^?7 M. Higher concentrations (3 × 10^?7 and 10^?6 M) caused little further additional shift to the right. The positive chronotropic effect of histamine on spontaneously beating atria was competitively antagonized by metiamide (10^?6 and 3 × 10^?6 M). These results demonstrate that in guinea-pig atria histamine increases myocardial contractility by an interaction with receptors closely related to classical H₁-receptors while its chronotropic effects is mediated by interaction with H₂-receptors.