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.