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.