The inhibitory effect of clonidine on the oestrogen-primed rat isolated uterus

The investigation was undertaken to study the mechanism of the inhibitory effects of clonidine on the oestrogen-primed rat isolated uterus. Clonidine produced dose-dependent relaxations of the rat isolated uterus which were competitively blocked by yohimbine but not by prazosin. Metiamide, a specific H2-histamine receptor antagonist, inhibited clonidine-induced responses only at a high concentration (1.0 X 10(-6)M). Propranolol, a beta-adrenoreceptor antagonist produced competitive antagonism of the clonidine responses. Like clonidine, noradrenaline also produced dose dependent relaxations of the rat isolated uterus. The responses to noradrenaline were not antagonized by either prazosin or yohimbine but were competitively antagonized by propranolol. The responses to clonidine were significantly inhibited by reserpine pretreatment (5 mg/kg; i.p., 24 h before) whilst responses to noradrenaline were significantly increased by this treatment. It is concluded that clonidine inhibits the uterus via an action on alpha 2-adrenoreceptors and possibly also to a lesser extent on H2-histamine receptors, leading ultimately to release of noradrenaline from endogenous stores which causes relaxation by acting on beta-adrenoreceptors.

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.

Cyclic AMP-dependent and independent positive inotropic effects of phenylephrine

To summarize, a large number of studies indicate that the effects of phenylephrine on cardiac contractility are mediated through alpha-adrenoceptors, although not exclusively. There are an equally large number of reports indicating some beta-adrenoceptor stimulant effects of phenylephrine. Of the latter, some are based on the finding of antagonism by beta-receptor antagonists and some on the fact that phenylephrine is capable of elevating cyclic AMP levels, which is primarily a beta-, and not alpha-adrenoceptor effect. Although it is now widely accepted that phenylephrine elevates cyclic AMP levels, the contribution and significance of the elevated nucleotide levels to the overall positive inotropy of phenylephrine appears to be small. Alpha-Adrenoceptors may be involved with calcium, although the precise interaction between calcium and the alpha-adrenoceptor is not clear. Finally, the adrenergic effects seem to depend on the experimental and physiological conditions including calcium ion concentration, thyroid state and driving frequency.

Comparative studies of the effects of 5-hydroxytryptamine and noradrenaline on the rat anococcygeus muscle

The effects of 5-hydroxytryptamine (5HT) and noradrenaline (NA) have been studied on rat anococcygeus muscle. 1. 5HT and NA produced a dose-dependent contraction of rat anococcygeus muscle. Cyproheptadine (1.0 X 10(-6) M), a specific 5HT receptor blocker, failed to inhibit the responses to either 5HT or NA. 2. However, phentolamine, a specific alpha receptor antagonist competitively blocked the responses to 5HT and NA. 3. The responses to 5HT were inhibited in the reserpinized (5 mg/kg i.p. 24 h) and 6-hydroxydopamine (6-OHDA) pre-treated preparations. 6-OHDA produced a leftward shift of the dose-response curve of NA. Reserpine pre-treatment potentiated lower doses of NA and the threshold dose of NA was significantly decreased. 4. Nialamide (2.2 x 10(-6) M), the mono-amine oxidase inhibitor produced a significant leftward shift of the dose-response curve of both 5HT and NA. Pyrogallol (2.3 x 10(-5) M), the catechol-o-methyl transferase inhibitor also potentiated the responses to both 5HT and NA, but the potentiation was significant at lower doses of 5HT and NA. 5. Our data suggest that 5HT- and NA-induced contractions in rat anococcygeus muscle are mediated through common alpha adrenoceptors. 5HT actions are probably indirect, mediated through the release of NA.

Evidence for histamine and 5-hydroxytryptamine like effects of MK-212 on guinea pig ileum, taenia coli and rat fundus strip

MK-212 (1 x 10(-7)M -- 1 x 10(-5)M) produced dose-dependent contractions of guinea pig ileum, taenia coil and rat fundus strip. The responses to MK-212 in all three preparations were blocked competitively by cyproheptadine (1 x 10(-8)M) a 5-HT receptor antagonist. Mepyramine (1 x 10(-8)M)-H1 receptor antagonist also inhibited competitively the responses of guinea pig ileum and taenia coli to MK-212. However, it failed to block significantly the responses of rat fundus strip to MK-212. Metiamide (1 x 10(-6)M), propranolol (1 x 10(-6)M) or atropine (1 x 10(-6)M) did not produce any significant effects on MK-212 induced contractile responses of guinea pig ileum, taenia coli and rat fundus strip. Our findings suggest that MK-212 produces both 5-HT as well as histamine like effects on the guinea-pig ileum, taenia coli and rat fundus strip.

Identification and specific blockade of histaminergic receptors in guinea pig vasa deferentia

Histamine produces contractions of the guinea pig vas deferens. The present investigation was undertaken to characterize the nature of histaminergic receptors in this tissue. Histamine (1.6 X 10(-6) M to 3.2 X 10(-5) M) produced dose-related contractions of guinea pig vas deferens (GPVD). Mepyramine (5.3 X 10(-8) M and 1 X 10(-9) M) blocked the responses to histamine competitively. Metiamide (1.23 X 10(-5) M) did not block the responses to histamine significantly. Specific H1 and H2 receptor agonists, namely 2-(2'-pyridyl)ethylamine (PEA) (2.55 X 10(-6) M to 3.0 X 10(-5) M) and 4-methylhistamine (4-MH) (2.52 X 10(-5) M to 3.0 X 10(-4) M), respectively, produced dose-related contractions of GPVD. The responses to PEA were blocked competitively by mepyramine, whereas the responses to 4-MH were blocked by metiamide. Reserpine pretreatment (5 mg/kg, i.p., 24 h) did not alter the responses to histamine and PEA. Our data suggest the presence of both H1 and H2 receptors in the GPVD which are excitatory in nature.

Pharmacological investigations into the effects of histamine and histamine analogs on guinea pig and rat uterus

The effects of histamine and its analogs 2-(2-pyridyl) ethylamine (PEA) and 4-methylhistamine (4 MH) have been studied on uterine preparations obtained from estrogen-primed guinea pigs and rats. (1) Histamine and 4 MH, a specific H2-receptor agonist produced relaxation in estrogen-primed rat uterus, whereas these agonists produced contraction in the estrogen-primed guinea pig uterus. (2) PEA, a specific H1-receptor agonist produced contraction in the guinea pig uterus but had no effect on the rat uterus. (3) Metiamide blocked responses to histamine and 4 MH in the rat uterus, whereas mepyramine blocked responses to histamine and PEA in the guinea pig uterus. (4) Propranolol produced competitive antagonism with histamine in the rat uterus, whereas it had no significant effect on the histamine or PEA responses in the guinea pig uterus. (5) Reserpine pretreatment completely abolished the responses to histamine and 4 MH in the rat uterus but did not alter the response in the guinea pig uterus. (6) Our data suggest that in rat uterus only H2-receptors are present and they are indirectly through the release of noradrenaline. In the guinea pig uterus both H1- and H2-receptors are present and are excitatory and directly acting.

The direct excitatory effects of cimetidine on the smooth muscle of guinea pig ileum

1. Cimetidine produced dose-dependent contractions in isolated guinea pig ileum and these responses were not blocked by mepyramine the H1-receptor antagonist. 2. Atropine competitively inhibited the cimetidine-induced contractions in the guinea pig ileum. 3. Cimetidine-induced responses were potentiated in the presence of eserine. Magnesium ions non-competitively inhibited the contractions due to cimetidine. Our findings suggest that cimetidine excites the guinea pig ileum through muscarinic receptors by releasing acetylcholine.

Histaminergic H1 and H2 excitatory receptors in the guinea pig uterus and taenia coli

Histamine, 2-(2-pyridyl)ethylamine (PEA), a specific H1 receptor agonist, and 4-methylhistamine (4-MH), a specific H2 receptor agonist, produced a dose-dependent contraction of estrogen-primed guinea pig uterus and taenia coli. Mepyramine (10(-7) M), a specific H1 receptor antagonist, competitively blocked the responses to histamine and PEA in both the preparations. Metiamide (10(-6) M), a specific H2 receptor antagonist, inhibited the responses to histamine and 4-MH in the guinea pig uterus and taenia coli. Our data suggest the presence of both H1 and H2 receptors in the guinea pig uterus and taenia coli. Both receptors appear to be excitatory in nature.

The cardiac inotropic responses to insulin in the rat heart

Insulin (5 to 40 I.U.) produced dose-dependent positive inotropic effect in the isolated rat heart. The responses to insulin were markedly inhibited in the presence of propranolol (1 . 1X10(-6) M). Insulin responses were markedly reduced in reserpine pretreated (5 mg/kg, i.p.) rats. Theophylline (4.4 mM), the phosphodiesterase inhibitor, potentiated the responses to insulin, whereas imidazole (20 mM), the phosphodiesterase stimulator inhibited the responses to insulin. The data suggest that the positive inotropic effects of insulin in rat heart is mediated through the release of cardiac catecholamines which stimulates beta-adrenoceptors. The final mediator of cardiac action seems to be cyclic-AMP.

Blockade by burimamide of the effects of clonidine on cardiac contractility phosphorylase activation and cyclic adenosine monophosphate in isolated guinea pig heart

Clonidine in a dose-range of 2.5 microgram to 80 microgram caused positive inotropic effect, which was accompanied by increase in the cyclic AMP levels and phosphorylase-activation of the isolated perfused guinea pig heart. Clonidine-induced biochemical and mechanical effects were blocked by burimamide, an H2-receptor antagonist Propranolol (1 x 10(-6)M), phentolamine (1 x 10(-6)M) or reserpine pretreatment, did not affect the clonidine responses on the perfused guinea pig heart. Clonidine reduced the 4-methyl-histamine (H2-agonist) responses of guinea pig heart. Our data suggest that the cardiac effects of clonidine may be due to stimulation of H2-type of receptors.

Investigations into the cardiac effects of tolazoline in guinea pig atria and ventricular strips

The positive inotropic, chronotropic and cyclic AMP producing effects of tolazoline were studied on atrial and ventricular preparations obtained from guinea pig heart. (1) The direct positive inotropic effects of tolazoline on the paced left atrial preparation from the guinea pig hearts was blocked by promethazine, but not by burimamide. Tolazoline did not elevate cyclic AMP levels in this preparation. (2) Tolazoline produced a positive chronotropic effect which was blocked by burimamide and not by promethazine and caused a 2-3 fold elevation of cyclic AMP in spontaneously beating right atria. (3) Burimamide antagonized the inotropic and cyclic AMP increasing effects of tolazoline on electrically driven ventricular strips. (4) The effects of tolazoline were unchanged by reserpine pretreatment of the guinea pigs or by prior exposure to phentolamine or propranolol. (5) These results suggest that tolazoline can activate both H1 and H2 receptors in the guinea pig heart. Furthermore the data suggests that H2 receptors are present in right atria and ventricle and that such receptors are associated with cyclic AMP. H1 receptors are present in the left atria and are not associated with the cyclic nucleotide.

H2-histaminergic activity of clonidine in the guinea pig heart

Clonidine perfusion (1 muM) or injection (9.4 to 150.4 nmoles) into the isolated perfused guinea pig heart caused an increase in contractile force, phosphorylase a and cyclic adenosine monophosphate (cyclic AMP) levels. The effects of clonidine were blocked by burimamide (30 muM). Proranolol (1 muM), phentolamine (1 muM) or reserpine treatment (5 mg/kg i.p., 24 hours prior to the experiment) did not influence the cardiac effects of clonidine. Clonidine (0.1 - 100 muM) also produced a dose dependent positive inotropic effect on right ventricle strips but had a negative chronotropic effect on the spontaneously beating right atria at higher concentration (0.1 to 1.0 M). Burimamide did not block the clonidine-induced negative chronotropic effect. Clonidine did not increase contractile force in guinea pig left atria. It also did not influence the isoproterenol-and phenylephrine-induced increases in cardiac force of contraction but did antagonize the positive inotropic effect of 4-methylhistamine in the right ventricle. This observation suggests that clonidine may compete with 4-methylhistamine for the same receptor sites. The results indicate that the cardiac receptors for both biochemical and mechanical effects of clonidine are histamine receptors of the H2-type.

Cardiac histamine receptors: differences between left and right atria and right ventricle

Histamine can stiumulate the heart by directly interacting with cardiac histamine receptors. In the present study we have investigated the cardiac effects of histamine, 4-methylhistamine (a specific H2-receptor agonist) and 2-pyridylethylamine PEA, a specific H1-receptor agonist] on spontaneously beating right atria and electrically driven left atria and right ventricle strips of the guinea pig. Left atria were driven at 1 Hz and right ventricle strips at 2.5 Hz and at twice the threshold voltage. Histamine and PEA produced a dose-dependent positive inotropic effect on the left atria. The dose-response curves were shifted to the right in a parallel fashion by promethazine (3 x 10(-6)M)+... Burimamide did not affect either dose-response curve. Histamine and 4-methylhistamine had a positive chromnotropic effect on right atria and a positive inotropic effect on right ventricle strips.

Isoproterenol-induced relaxation, phosphorylase activation and cylic adenosine monophosphate levels in the polarized and depolarized rat uterus

There is some evidence in the literature that catecholamines relax uterine and other types of smooth muscle by increasing tissue levels of cyclic adenosine monophosphate (cyclic AMP). In the present study, isoproterenol completely relaxed uterine strips obtained from estrogen-primed rats and also increased tissue levels of cyclic AMP and phosphorylase a. In uterine strips depolarized and put into contracture for 15 minutes by 127 mM K+, isoproterenol did not increase phosphorylase a or cyclic AMP but was still capable of producing relaxation. When uterine strips were exposed to the methoxy derivative of verapamil, D-600, a compound known to prevent the influx of calcium, the uterus relaxed completely without an increase in cyclic AMP. The addition of isoproterenol at this stage resulted in an increase in cyclic AMP similar to that noted in nondepolarized uterine strips. The addition of 127 mM K+ also resulted in time-dependent biochemical changes as well as contracture. Cyclic AMP was increased 3-fold after 2 minutes of K+ depolarization and phosphorylase a was increased as well. The increase in cyclic AMP was prevented by propranolol but propranolol did not affect the contracture response to K+. D-600 prevented contracture but did not affect the K+-induced increase in cyclic AMP. The data suggest that an increase in whole tissue levels of cyclic AMP are not necessary in order for isoproterenol to relax depolarized rat uterine strips. The data also suggest that intracellular calcium levels can affect the level of cyclic AMP in the rat uterus.

Actions and interactions of theophylline and imidazole on cardiac contractility, phosphorylase activation and cyclic AMP

Injection of 1.0 mg theophylline and 1.6 mg imidazole into guinea-pig hearts perfused by the Langendorff technique increased cardiac contractile force by 20.0 +/- 1.0% and 23.3 +/- 1.2% over control and activated phosphorylase without producing significant changes in cyclic AMP levels. When guinea-pig hearts were perfused with theophylline (10(-3) M) or imidazole (20 mM), injection of either imidazole or theophylline did not further increase cardiac contractility or phosphorylase a levels. In all cases there was no change in cycylic AMP levels. Data in the literature indicate that calcium plays a role in the cardiac biochemical and mechanical effects of theophylline and imidazole. Both drugs may thus produce their cardiac effects by a calcium rather than a cyclic AMP mediated mechanism.

Biochemical and mechanical effects of phenylephrine on the heart

Injections of phenylephrine into isolated perfused guinea pig hearts increased cyclic AMP and phosphorylase a. Reserpine pretreatment or propranolol decreased or abolished the phenylephrine-induced biochemical changes without affecting the contractile response. Phentolamine, on the other hand, shifted the phenylephrine dose-response curve for contractility to the right without affecting the other parameters. The biochemical effects of phenylephrine are apparently due to the release of noradrenaline while the contractile effect is mediated directly through stimulation of alpha-adrenoceptors.

The effect of histamine, isoproterenol and tyramine on rat uterine cyclic AMP

Isoproterenol (10(-4) M), histamine (10(-3) M) and tyramine (10(-3) M) elevated cyclic AMP levels in incubated rat urerine segments. Promethazine (10(-6) M) an H1-receptor antagonist was ineffective in blocking the increase in cyclic AMP due to isoproterenol, histamine and tyramine. Buriamide (10(-6) M), an H2 receptor antagonist, was effective in almost abolishing the cyclic AMP elevation due to histamine, whereas burimamide did not affect the cyclic AMP levels elevated by tyramine and only slightly reduced the isoproterenol response. Propranolol was effective in reducing or abolishing the cyclic AMP accumulation by all three agonists. The respective blockade by burimamide or propranolol was overcome by increasing the concentration of the agonists. Histamine (10(-3) M) and tyramine (10(-3) M) failed to elevate cyclic AMP in uterine segments obtained from reserpine treated rats. The data supports the hypothesis that histamine, like tyramine, has an indirect effect on the rat uterus mediated through the release of endogenous norepinephrine.

Histamine2 receptors in rat uterus

Histamine and three histamine analogs (4-methylhistamine, 3-beta-aminoethyl 1,2,4 triazole (TD) and betazole) all produced relaxation in depolarized rat uterine strips. The rank order obtained was the same as that noted previously in the heart and gastric mucosa and the effects of the agonists were blocked by burimamide. The uterine histamine receptor thus appears to be of the H2-type. Adenylate cyclase prepared from the uterus was not stimulated by histamine. Uterine H2-receptors thus do not appear to be associated with adenylate cyclase.

Lack of interaction between norepinephrine or histamine and theophylline on cardiac cyclic AMP

The hypothesis that theophylline can enhance the cardiac effects of norepinephrine and histamine by inhibiting phosphodiesterase was tested in isolated perfused hearts of rats and guinea pigs. Theophylline did enhance the inotropic and phosphorylase-activating effect of both amines. Cyclic AMP, increased by the injection of the amines, was not further increased when the hearts were perfused concomitantly with theophylline. When theophylline was injected into the heart, an increase in contractility and phosphorylase a was noted but cyclic AMP values were not affected. The data do not support phosphodiesterase inhibition as the mechanism by which theophylline produces its cardiac effects.

Stimulation of rat gastric adenylate cyclase by histamine and histamine analogues and blockade by burimamide

Histamine, 4-methylhistamine, 3-(beta-aminoethyl)-1,2,4 triazole and betazole, in that order, stimulated adenylate cyclase prepared from rat gastric tissue in a dose-dependent manner. Burimamide, an H(2)-receptor blocking agent, in concentrations of 1-5 x 10(-6) M antagonized this effect. The data lend some support to the hypothesis that elevated levels of cyclic adenosine 3',5'-monophosphate may be involved in histamine-stimulated gastric secretions and that H(2)-receptors are associated with adenylate cyclase.

Blockade of cardiac histamine receptors by promethazine

The inotropic and chronotropic effects of histamine on the isolated perfused guinea pig heart were antagonized by promethazine over a concentration range 4 × 10−6 – 16 × 10−6 M. Promethazine (4 × 10−6 M) decreased the ability of histamine (1 μg) to elevate cardiac cyclic AMP. A higher dose of histamine could not overcome the promethazine blockade. Promethazine (4 × 10− 6 M) did not block the inotropic effect of noradrenaline. Higher concentrations of promethazine, particularly 16 × 10−6 M did decrease the noradrenaline response. The data indicate that promethazine can interact with cardiac histamine receptors but the interaction is either noncompetitive or competitive non-equilibrium in nature.

Problems in selection of topical anti-inflammatory corticosteroids

Desonide, a non-fluorinated topical corticosteroid (16-alpha-hydroxyprednisolone acetonide), is compared with its fluorinated analogue, triamcinolone acetonide, as to vasoconstriction ability and epidermal penetration rate. Clinical effectiveness of desonide is further assessed by means of a double-blind paired comparison with an accepted potent fluorinated steroid, betamethasone 17-valerate. It is demonstrated that fluorination of the steroid molecule is not necessary to achieve potent topical anti-inflammatory effect. Vasoconstriction and epidermal penetration are not enhanced by fluorination in the drugs studied.