Action of histamine receptor agonists and antagonists on the rat uterus

Relaxant Effects of the Aqueous Extract of Excoecaria grahamii (Euphorbiaceae) Leaves on Uterine Horn Contractility in Wistar Rats

In uterine smooth muscle, the effects of Excoecaria grahamii are not yet documented. To fill this gap, we investigated the pharmacological effect of Excoecaria grahamii on the contraction of the rat isolated uterine horns. The isolated segments were exposed to different concentrations of the aqueous extract of Excoecaria grahamii leaves and pharmacological drugs. The results showed that Excoecaria grahamii aqueous extract decreased the amplitude and frequency by concentration-related manner. IC50 values were 2.4 and 2.6, respectively, for amplitude and frequency. Our study revealed that the extract did not act through histamine H2-receptors or the nitric oxide pathway. It also inhibited uterine contractions induced by oxytocin and potassium chloride (KCl). These data suggest that Excoecaria grahamii active compound can be used for calming uterine contractions. The action of Excoecaria grahamii showed that it can be useful to fight against diseases which caused uterotonic effects. It can be useful to prevent preterm birth and pains caused by menstruations but further investigation is needed to clarify the mechanism action.

Spasmogenic effects of Sclerocarya birrea stem bark aqueous extract on rat isolated uterine horns

ETHNOPHARMACOLOGICAL RELEVANCE

Sclerocarya birrea (Anacardiaceae), popularly known as "marula", is used as a traditional remedy for allegedly treating dysmenorrhoea and a host of other ailments such as malaria, fever, diarrhoea, dysentery, stomach disorders, headaches and to facilitate childbirth. This study investigated the pharmacological effects of S. birrea extract (SBE) on isolated, spontaneously-contracting uterine horns of healthy, young adult, female Wistar rats.

MATERIALS AND METHODS

One kilogramme of S. birrea fresh stem bark was identified and authenticated. The bark was air-dried at room temperature (26 ± 1°C) for two weeks. The dried stem bark was milled and macerated in 2.5l of distilled water for 48 h and filtered. A rotary evaporator was used to concentrate the aqueous extract by drying it at 60 ± 1°C. Freeze-drying gave 61.3g/kg (i.e., 6.13% yield) of a dark-brown, powdery, crude extract. Rat isolated uterine horns were mounted in 25-ml Iworx tissue organ-baths containing De Jalon׳s physiological solution, and exposed to graded concentrations of SBE (25, 50, 100, 200, 300, 400mg/ml/kg). The effects of SBE and atropine, oxytocin, verapamil, indomethacin, acetylcholine, serotonin, cimetidine and histamine on the isolated uterine muscles, were recorded using LabScribe2 software.

RESULTS

The aqueous stem bark extract of S. birrea produced significant (p<0.05) concentration-dependent contractions of the uterine horn muscle preparation reaching a maximum at the 300 mg/ml dose. SBE mimicked and potentiated uterine muscle contractile effects of oxytocin (0.5-5 µU/ml) and acetylcholine (0.1-3 µg/ml). Pre-incubation of tissues with atropine (1-3 µg/ml), non-significantly (p>0.05) inhibited SBE-induced contractions on uterine muscles. Verapamil (2 µg/ml), indomethacin and -p-tosyl-l-phenylalanine-chloromethyl-ketone (TPCK) inhibited the contractile effects of SBE (25-400mg/ml/kg), suggesting possible calcium-mediated mechanism of action for SBE and possible COX-enzyme inhibition. Pre-incubating tissues with histamine (10(-8)-10(-5)M) resulted in relaxation of the uterus, while cimetidine potentiated the contractile effects of SBE. Serotonin potentiated the contractile effects of SBE.

CONCLUSION

These results indicate that SBE causes contraction on uterine smooth muscles possibly through its effects on oxytocin, acetylcholine and serotonin receptors. As a result SBE should not be used by patients suffering from dysmenorrhoea as it can worsen it or pregnant patients as it can result in miscarriage/abortion.

Gastrointestinal stimulatory and uterotonic activities of dietary radish leaves extract are mediated through multiple pathways

Raphanus sativus, commonly known as radish, is a food plant known worldwide for its culinary and medicinal properties especially as a laxative and abortifacient. This study reports the gastrointestinal and uterine tone modulatory activities of the crude extract (Rl.Cr) of radish leaves. Rl.Cr, showing the presence of saponins and alkaloids, exhibited a spasmogenic effect (0.03-10 mg/mL) in isolated rabbit jejunum, rat stomach fundus and uterus which was partially blocked by atropine. In contrast, Rl.Cr was found to be devoid of any stimulatory effect in rat ileum, instead showed an inhibitory effect (0.1 mg/mL) on the ACh dose-response curves. A mild relaxant effect was also observed in rabbit jejunum at the lower doses (0.1-0.3 mg/mL) but not against K(+)-induced contractions, ruling out a calcium channel blocking effect. In guinea-pig ileum, Rl.Cr exhibited a stimulant effect resistant to atropine while sensitive to pyrilamine pretreatment. The aqueous fraction, showing a strong presence of saponins, was found to be more efficacious than the non-polar fractions in its spasmogenic effect. This study shows the presence of species-dependent gastrointestinal effects of radish mediated partially through cholinergic receptors in rabbit and rat tissues, but through histaminergic activation in the guinea-pig, providing a scientific basis for its use in gut and uterine affections while also giving a wider picture of the activity profile of radish by using different species of animals.

Histamine mediates the muscle layer-specific responses in the isolated swine myometrium

To clarify the role of histamine in uterine contractility, the effect of this biogenic amine on the myometrium of cyclic mature gilts was investigated by an isometric tension recording study in vitro. In addition, using crude membrane preparations isolated from the longitudinal (LM) and circular muscle (CM), the distribution of H1 histamine receptors was characterized by 3H-pyrilamine binding assay. Histamine caused a tetrodotoxin-resistant contractile response of LM and CM in Krebs solution, but LM (-logEC50 = 6.34) was more sensitive than CM (-logEC50 = 5.4). Pyrilamine decreased the excitatory response of histamine in both muscle layers. In pyrilamine-treated LM, a high concentration of histamine (1-30 microM) caused a slight inhibition of spontaneous contraction, and this inhibition was abolished by ranitidine. On the other hand, histamine did not cause any inhibition in the pyrilamine-treated CM preparations. Dimaprit (10-300 microM) concentration-dependently inhibited the spontaneous contraction of LM but not of CM. In the presence of pyrilamine and ranitidine, N alpha-methylhistamine, even at 10 microM, did not affect the spontaneous and electrical field stimulation (5Hz)-induced contraction of LM and CM layers. Specific 3H-pyrilamine binding sites were distributed heterogeneously in the swine myometrium. The maximum number of binding sites in LM (132.5 +/- 9.9 fmol/mg protein, n = 10) was 2.5 times higher than that in CM (52.2 +/- 3.2 fmol/mg protein, n = 6). These results indicate that there is a muscle layer-dependent difference of histamine-induced response in the swine myometrium. In the LM layer, histamine acts on both H1 and H2 histamine receptors, and causes contraction (via H1 receptors at a low concentration) or relaxation (via H2 receptors at a high concentration in the presence of pyrilamine). However, histamine causes only a contraction in the CM layer, likely the result of the absence of H2 histamine receptors. Histamine-induced contraction is conspicuous in the LM layer, because of the heterogeneous distribution of H1-receptors between LM and CM.

Histamine N-methyltransferase inhibitor potentiates histamine- and antigen-induced airway microvascular leakage in guinea pigs

BACKGROUND

Histamine N-methyltransferase (HMT) modulates histamine- and antigen-induced bronchoconstriction. However, it is unclear whether vascular permeability evoked by an allergic reaction can be exaggerated by inhibition of HMT activity.

METHODS

We studied the effects of intravenously injected SKF 91488, a specific HMT inhibitor, on increases in plasma extravasation induced by intravenously injected histamine in unsensitized guinea pigs and by intravenously injected ovalbumin antigen in guinea pigs sensitized to ovalbumin in vivo with Evans blue dye as a marker.

RESULTS

Pretreatment with SKF 91488 shifted, in a dose-dependent fashion, the dose-response curves of the leakage of dye to histamine to lower concentrations in the trachea, main bronchi, and nasal mucosa. Likewise, pretreatment with SKF 91488 (20 mg/kg intravenously) significantly increased the leakage of dye induced by ovalbumin antigen (200 micrograms/kg intravenously) in three parts of the airway (p < 0.05). In contrast to SKF 91488, intravenously injected aminoguanidine, a specific inhibitor of diamine oxidase (16 mg/kg intravenously), did not alter the leakage of dye induced by histamine (from 0.001 microgram/kg to 10 micrograms/kg intravenously) (p < 0.20). HMT activities were observed in the nasal mucosa, as well as in the trachea and main bronchi, as shown in a previous study.

CONCLUSION

These findings suggest that HMT modulates the effects of exogenous histamine and endogenously released histamine induced by antigen challenge on plasma extravasation in the airway in guinea pigs in vivo.

HMT regulates histamine-induced Cl- secretion across the canine tracheal epithelium

Although histamine N-methyltransferase (HMT), the primary enzyme responsible for the inactivation of histamine, has been shown to exist in the airway epithelium, it is still unknown whether this enzyme regulates ion transport across the airway epithelium. Using an Ussing chamber, we examined the effect of a HMT inhibitor, SKF 91488, on potential difference (PD) and short circuit current (SCC) in epithelial membranes from the posterior portion of canine trachea. SKF 91488 itself did not significantly alter PD or SCC values. Pretreatment with SKF 91488 significantly augmented PD and SCC induced by histamine. Amiloride did not significantly alter the augmentation by SKF 91488 in histamine-induced PD and SCC rises. These findings indicate that HMT regulates Cl- secretion across airway epithelium.

Histamine N-methyltransferase modulates histamine- and antigen-induced bronchoconstriction in guinea pigs in vivo

To examine whether histamine N-methyltransferase (HMT; EC 2.1.1.8) modulates the effects of allergic reaction in vivo, we studied the effects of aerosolized SKF 91488, a specific HMT inhibitor, on the responses to aerosolized histamine in unsensitized guinea pigs and to ovalbumin (OA) antigen inhalation in guinea pigs sensitized to OA. Airway responsiveness was assessed by determining provocation concentrations of histamine and OA aerosols that increased pulmonary resistance to twice the baseline values. SKF 94188 shifted, in a dose-dependent fashion, the dose-response curves to histamine and OA antigen to lower concentrations, and it significantly decreased provocation concentrations of both histamine and OA antigen (p < 0.01). In contrast of SKF 91488, aerosolized aminoguanidine, a specific inhibitor of diamine oxidase (10(-2) M, 90 breaths), did not alter the provocation concentration of histamine (p > 0.20). SKF 91488 (10(-2) M, 90 breaths) caused no significant changes in response to acetylcholine (p > 0.30). HMT activities were observed in the entire airways of the trachea, main bronchi, segmental bronchi and bronchioles, and parenchymal tissues. These findings suggest that HMT modulates the effects of exogenous histamine and endogenously released histamine by antigen challenge on bronchoconstrictor responses in guinea pigs in vivo.

Influence of hormonal treatment on the response of the rat isolated uterus to histamine and histamine receptor agonists

The response of the isolated uterus to histamine and histamine agonists was investigated in progesterone- and oestrogen-treated rats. The uterine inhibitory responses to histamine and 4-methylhistamine (a histamine H2 receptor agonist) were similar in KCl-contracted uteri from progesterone- and oestrogen-treated rats. The histamine H1 receptor agonist, 2-pyridyl-ethylamine, produced a relaxant response only in progesterone dominant uterus. This was inhibited by the histamine H1 receptor antagonist. In the rat isolated uterus which was not preconstricted by KCl, neither histamine, 4-methylhistamine, nor 2-pyridyl-ethylamine produced any effect in the presence or absence of ranitidine. Ranitidine competitively antagonized the histamine-relaxant uterine response in oestrogen-treated rats (pA2 = 7.21 (6.83-7.58)), but not in progesterone-treated rats, except in the presence of clemizole (10(-7) M) when the pA2 value of ranitidine against histamine was similar to that obtained in oestrogen-treated rats (pA2 = 6.74 (6.64-6.85)). These results indicate that treatment with ovarian steroids influences responses mediated by the histamine receptors of the isolated rat uterus. Both histamine H2 and H1 receptors contribute to the uterine inhibitory effect of histamine in progesterone-treated rats.

Naloxone-insensitive inhibitory and excitatory effects of opioid agonists in the rat isolated uterus

Morphine (3 x 10(-6)-10(-3) M) produced a concentration-dependent inhibition of both spontaneous rhythmic contractions and tonic contraction induced by potassium chloride (KCl) (50 mM) in rat isolated uterus. Uteri at the metoestrus phase were the most sensitive to the inhibitory effect of morphine. Morphine-induced relaxation was characterized by a post-wash tonic contraction. The mu-specific opioid agonist, D-alaglymepheglycol (DAGO) (4 x 10(-8)-1.2 x 10(-5) M) did not affect spontaneous rhythmic contraction but produced a partial inhibition of the KCl depolarized uterus. There was no post-wash contraction following DAGO. In contrast to the inhibition produced by both morphine and DAGO, methionine enkephalin (8 x 10(-7)-2.4 x 10(-5) M) produced a concentration-dependent contraction of the KCl depolarized uterus. Naloxone up to 100 microM, propranolol (10 microM), flurbiprofen (50 microM) and metiamide (10 microM) did not affect either the relaxation produced by morphine and DAGO or the contraction produced by methionine enkephalin. The results showed that the opioid agonists may have non-receptor mediated direct effects on the rat uterus.

Histamine alters prostaglandin output from diestrous rat uteri. Involvement of H2-receptors and 9-keto-reductase

The effects of exogenous histamine (H) on prostaglandin (PG) generation and release in uteri isolated from diestrous rats and the influences of H2-receptors blockers (cimetidine and metiamide) on the output of uterine PGs, were explored. Moreover, the action of H on the uterine 9-keto-reductase, was also studied. Histamine (10(-4) M) failed to alter the basal output of PGE1 but reduced significantly the generation and release of PGE2 and augmented the output of PGF2 alpha. On the other hand, cimetidine (10(-5) M) enhanced the basal release of PGE2 but had no action on the outputs of PGs E1 or F2 alpha. The enhancing effect of H on the production and release of PGF2 alpha was abolished in the presence of cimetidine. Also, the antagonist reversed the influence of H on the output of PGE2. Metiamide, another H2-receptor antagonist, did not alter the basal control generation and release of uterine PGs, but antagonized the augmenting influence of H on PGF2 alpha uterine output, as much as cimetidine did, and prevented the depressive action of H on the release of PGE2 from uteri. Histamine (10(-4) M) significantly stimulated uterine formation of cyclic-adenosine monophosphate, an action which was antagonized by the presence of cimetidine (10(-5) M), a blocker of H2 receptors. Also, histamine (10(-5) M) and dibutyrylcyclic-adenosine monophosphate (DB-cAMP) at 10(-3) M, enhanced significantly the formation 3H-PGF2 alpha from 3H-PGE2. Results presented herein demonstrate that H is able to diminish the generation of PGE2 in uteri from rats at diestrus augmenting the synthesis of PGF2 alpha, apparently via the activation of H2-receptors, enhancing adenylate-cyclase. These effects appear to increase uterine 9-keto-reductase activity which transforms PGE2 into PGF2 alpha. Relationships between the foregoing results and those evoked by estradiol, are also discussed.

Is there a prostaglandin involvement in the positive inotropic action of histamine in isolated pregnant rat uterus, apparently mediated via H1-receptors activation?

Cumulative dose-response curves for histamine induced responses in mesometrial (ME) and antimesometrial (AME) regions of uterine horns isolated from rats at 7th, 16th and 22nd days of pregnancy, were constructed. Histamine inhibited, in dose-related fashion, the isometric developed tension in ME and AME strips obtained at the 7th day of pregnancy, an action antagonized by cimetidine (10(-4)-10(-5) M). On the contrary, at the 16th and 22nd days, histamine (10(-5)-10(-3) M), stimulated spontaneous contractions in the ME region but had no effect in the AME segment. Although histamine and SKF-71481-A2,aH1-receptor agonist, both at 10(-4) M, enhanced similarly ME inotropism at the 16th and at 22nd days of pregnancy, the positive contractile action of histamine was greater at the 16th than at the 22nd day. Moreover, cumulative dose-response curves for histamine and SKF-71481-A2 in the ME region of uteri isolated at the 16th day of gestation, showed that both agonists have approximately the same inotropic potency and efficacy. On the other hand, pyrilamine (at 10(-4) M, but not at 10(-5) M aH1-receptor antagonist, shifted to the right the dose-response curve for histamine in ME strips from uteri at the 16th day of pregnancy and attenuated significantly, the magnitude of the positive inotropism evoked by the amine. Similar findings were observed in the presence of chlorpheniramine (at 10(-6) M), another H1-receptor blocker. In addition, the positive uterine inotropism evoked by histamine in the ME region of preparations isolated at the 16th day of pregnancy, was significantly reduced by an antagonist of phospholipase A2 (mepacrine, 10(-4) M) as well as by acetylsalicylic acid (ASA at 10(-4) M), an inhibitor of cyclooxygenase. Results also indicate that the excitatory uterine inotropism elicited by the agonistic amine in ME strips isolated from rats at the 16th day of pregnancy, was coincident with an enhanced release of prostaglandins (PGs) E2 and F2 alpha, but not of PGE1 and that both augmenting actions of histamine were antagonized by histamine H1 receptor-blockers, namely pyrilamine (mepyramine or chlorpheniramine. Results suggest that histamine at early pregnancy diminished myometrial inotropism via its interaction with H2-receptors, whereas from mid pregnancy up to the moment of parturition it evokes contractile stimulation, most likely due to the activation of H1-receptor located at the mesometrial region of rat uterine horns.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacology of the novel H2 antagonist famotidine: in vitro studies

The novel antiulcer drug famotidine was found to be a potent and selective inhibitor of histamine H2 receptors. Its activity on different parameters involving H2 receptors was higher than that of other compounds of the family: pA2 values were 8.33, 7.86 and 7.83 in the guinea pig atria, guinea pig papillary muscle and isolated rat gastric secretion, respectively. Apart from quantitative differences, famotidine differed from the other compounds, since it caused a competitive antagonism only at low concentrations and an unsurmountable antagonism at higher concentrations. The duration of the inhibitory action on the "in vitro" gastric secretion resembled that of cimetidine and ranitidine. Famotidine was highly effective (approximately 10 times as potent as ranitidine) also on the rat uterus (unsurmountable antagonism) and on the guinea pig gallbladder (pA2 value = 7.71). Famotidine was apparently devoid of non-specific effects concerning the gastrointestinal motility even at very high concentrations (10(-4) M). In this respect, famotidine appeared to be more selective than cimetidine and ranitidine at the H2 receptor level. The high potency, the peculiarity of the antagonism and the lack of side-effects on a number of isolated preparations, indicate this H2 antagonist as a very peculiar member of the group.

The zoapatle. X. The in vitro effect of zoapatle aqueous crude extract (ZACE) and histamine upon rat and guinea pig uterine strips

In vitro histamine response in rat and guinea pig uterine strips was similar to the one observed with zoapatle aqueous crude extract (ZACE). The relaxing effect elicited by ZACE/tomentosa on the in vitro rat uterine strips was not mediated by activation of H-2 histamine receptors; moreover, the increase on uterine contractility elicited by ZACE/tomentosa on the in vitro guinea pig uterine strip was not mediated through the activation of H-1 histamine receptors.

The zoapatle. XI. Effects elicited by Montanoa tomentosa and Montanoa frutescens on rat uterine strips

Zoapatle aqueous crude extract (ZACE) obtained from Montanoa tomentosa (M.t.) inhibits the in vitro spontaneous contractility pattern of rat uterine tissue. The opposite effect was observed with ZACE from Montanoa frutescens (M.f.) in the same preparation. Both plant extracts, M.t. and M.f., increased the in vitro spontaneous contractility pattern in the uterine guinea pig assay. In depolarized uterine tissue, propranolol (beta-blocker) inhibited the relaxing effect induced by M.t.. Atropine (cholinergic antagonist) abolished the increase in uterus contractility produced by the presence of M.f.

Prostaglandins and ovarian factors as modulators of the negative inotropic action of histamine in the isolated rat uterus

The present study reports findings on: a) the action of histamine (H) on the spontaneous motility of the uterus isolated from rats during four different stages of the sex cycle; b) the relevance of endogenous and exogenous prostaglandins (PGs) for the inotropic effect of H and c) the relationships between these factors and the generation and release of PGs in rat uterine horns obtained during proestrus, estrus, metestrus and diestrus. Cumulative dose-response curves for H, in strips isolated from proestrous rats, showed that the agonist inhibited, at all the concentrations tested, spontaneous myometrial contractions, evoking a maximum effect (100% inhibition) at 10(-5)M. Incubation with acetylsalicylic acid (ASA; 10(-4)M) did not modify the dose-response curve for H. In strips from estrous rats, the maximum effect of H was obtained at 10(-3)M and, after incubating with ASA, the control dose-response curve for H was shifted to the left. A subthreshold dose of PGE2 (10(-9)M), reversed completely the action of ASA on H uterine responses, whereas PGF2 alpha was not effective. Dose-response curves for H, in uterine strips isolated from metestrous rats, showed the maximum effect (85% of inhibition) at 10(-3)M. Incubation with ASA, shifted the curve to the left and, the maximum effect increased up to 100% inhibition. A subthreshold dose (10(-9)M) of PGE2 or of PGF2 alpha shifted to the right the dose-response curve for the amine. In uterine strips isolated from diestrous rats the maximum inotropic action of H evoked only 75% of contractile inhibition and, in the presence the ASA, this influence reached 100%; the dose-response for the agonist being shifted to the left. Both PGE2 or PGF2 alpha, added in the presence of ASA, abolished the displacement evoked by ASA on the control dose-response curve for H. A significant correlation between the tissue output of "PGE-like material" into the bathing solution and the ED50 for the contractile influence of H, was also detected. On the contrary the release of "PGF2 alpha -like material" did not correlate with the ED50 for H. The foregoing results suggest that some endogenous uterine prostaglandins of the 2 series, modulate the depressive inotropic action of H in isolated uterine strips, depending on the ovarian hormonal status of the animals.

Action of mifentidine and ranitidine on the isolated rat uterus

The new H2-antagonist mifentidine (compound marked DA 4577) was tested for its inhibitory effect on the relaxation induced by histamine on the rat uterus and was compared with the well known H2-blocker ranitidine. Mifentidine was shown to be more effective than ranitidine (about 10 times). However whereas ranitidine behaved as a "classical" competitive antagonist, mifentidine at concentrations of 10(-7) M, caused a remarkable depression of the maximum response to histamine. This "unsurmountable" antagonism may connected with a tight binding of the compound to the receptor with a consequent low degree of dissociation. Ranitidine, but not mifentidine, at concentrations of 10(-5) M was able to potentiate the stimulatory effect of acetylcholine thus confirming also in the uterus its cholinergic-like effects so far observed mainly in the gastrointestinal tract.

The inhibitory effect of histamine on the motility of rat uterus in vivo

The experiments concerned the effects of histamine and its analogs 4-methylhistamine and 2-pyridyl-ethylamine on the spontaneous activity of the estrogenized rat uterus in vivo. These agonists given intravenously inhibited uterine activity. Histamine and 4-methylhistamine, but not 2-pyridyl ethylamine, produced this effect in a dose-dependent manner. The ability of histamine to reduce uterine activity was lowered by pretreatment with reserpine, adrenalectomy or 6-hydroxydopamine. The effect of 4-methylhistamine also was reduced by reserpine or adrenalectomy. The inhibitory effect of 2-(2-pyridyl) ethylamine was completely abolished by reserpine or adrenalectomy. Cimetidine attenuated the residual inhibitory effect of histamine in reserpinized rats. These results suggest that the uterine inhibitory action of histamine and 4-methylhistamine is mediated through catecholamine release and direct stimulation of the H2-receptors. The effect of 2-(2-pyridyl) ethylamine is only mediated by release catecholamine.

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(-8) M), a specific H2-receptor antagonist. Propranolol, a selective beta-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 X 10(-6) M), the monoamine oxidase inhibitor, and cocaine (4.3 X 10(-6) M), the selective noradrenaline uptake blocker, potentiated the responses to histamine. However bretylium (2.4 X 10(-5) M), and 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 beta-receptors and produces relaxation of the rat uterus.

Effect of impromidine (SK&F 92676) on the isolated papillary muscle of the guinea-pig

Impromidine was found to have a positive inotropic effect on the isolated papillary muscle from the guinea-pig. The dose-response curve to impromidine was shifted to the right by cimetidine and ranitidine. Impromidine was 35 times more potent than histamine but with a maximal response of only 81% that obtained with histamine. This difference, which was statistically significant (P < 0.005), suggested that impromidine acts as a partial agonist at the histamine H2-receptors of the papillary muscle as has been observed in other tissues.