Effect of some new Histamine H2-receptor antagonists on the guinea-pig papillary muscle

[3-(1H-imidazol-4-yl)propyl]guanidines containing furoxan moieties: a new class of H3-antagonists endowed with NO-donor properties

Synthesis and pharmacological characterisation of a series of products obtained by coupling the H(3)-antagonist SKF 91486 through appropriate spacers with the NO-donor 3-phenylfuroxan-4-yloxy and 3-benzenesulfonylfuroxan-4-yloxy moieties, as well as with the corresponding furazan substructures, devoid of NO-donating properties, are reported. All the products were tested for their H(3)-antagonistic and H(2)-agonistic properties on electrically-stimulated guinea-pig ileum segments and guinea-pig papillary muscle, respectively. The whole series of compounds displayed good H(3)-antagonist behaviour and feeble partial H(2)-agonist activity. Among furoxan derivatives, the benzenesulfonyl hybrid 28, a good NO-donor, triggered a dual NO-dependent muscle relaxation and H(3)-antagonistic effect on guinea-pig intestine.

Clinical aspects of cardiovascular effects of H2-receptor antagonists

In vitro studies and animal experiments as well as clinical observations in humans concerning cardiovascular effects of H2-receptor antagonists have been published shortly after the development. Thus, clinical studies were performed to investigate these effects. The following review summarizes the results from in vitro studies up to the clinical investigations performed.

Histamine receptors mediating a positive inotropic effect in guinea pig and rabbit ventricular myocardium: distribution of the receptors and their possible intracellular coupling processes

The difference in histamine receptor subtypes that are involved in the positive inotropic effect of histamine in guinea pig and rabbit ventricular myocardium was analytically characterized. In guinea pig papillary muscles, the positive inotropic effect of histamine was antagonized by cimetidine but not by mepyramine. The converse was true in rabbit papillary muscles. However, histamine evoked a positive inotropic effect through H1- and H2-receptors after blockade of H2- and H1-receptors in guinea pig and rabbit papillary muscles, respectively. Adenylate cyclase was significantly activated by histamine via H2-receptors in guinea pig but not in rabbit myocardial ventricular membranes. Accumulation of [3H]inositol monophosphate in ventricular strips prelabeled with myo-[3H]inositol was increased by histamine via H1-receptors to a similar extent in rabbits and guinea pigs. Radioligand binding experiments with [3H]mepyramine and [3H]tiotidine showed an increased number of H1-receptors and a decreased number of H2-receptors in guinea pig compared with rabbit ventricular myocardium. These results suggest that the positive inotropic effects of histamine are dominated by an H1-receptor-mediated effect in rabbits and by an H2-receptor-mediated one in guinea pig ventricular myocardium, and the positive inotropic effect manifested by one subtype apparently restricts the expression of the positive inotropic effect mediated by the other subtype. This species difference is not due to a difference in densities of the receptor subtypes, but may be partly related to a difference in the extents of coupling of H2-receptors to adenylate cyclase.

In vitro cardiac pharmacology of the new histamine H2-receptor agonist amthamine: comparisons with histamine and dimaprit

The cardiac activity of the novel histamine H2-receptor agonist amthamine was investigated in a variety of isolated heart preparations from guinea pigs and humans and in the isolated rabbit aorta. Amthamine caused an increase in the sinus rate of spontaneously beating guinea-pig atria (pD2 = 6.72) and in the contractility of the electrically driven guinea-pig papillary muscle (pD2 = 6.17) and of the human atrium (pD2 = 5.38). In all these systems, amthamine behaved as a full agonist with a potency comparable to or slightly higher than that of histamine and 10 times higher than that of dimaprit. The positive effects of amthamine were competitively antagonized by ranitidine which had pA2 values (6.46 and 6.25 in the guinea-pig atria and papillary muscle, respectively) comparable with those calculated against histamine and dimaprit. In the isolated rabbit aorta amthamine was devoid of H1-mediated activities up to 3 x 10(-4) M. These results indicate that amthamine is a potent and selective histamine H2-receptor agonist which can be considered a valuable tool for investigating H2-receptor mediated effects in cardiac tissues.

Characterization of histamine receptors modulating inotropic and biochemical activities in rabbit left atria

The experiments were performed to identify histamine H1- and H2-receptors in rabbit left atrium and to characterize the pharmacological properties mediated by the respective subtypes of histamine receptors. High-affinity saturable binding to the left atrial membranes was obtained for [3H]mepyramine, yielding a maximum binding capacity (Bmax) of 96 fmol/mg of protein and an equilibrium dissociation constant (KD) of 3.8 nM and also for [3H]tiotidine, yielding a Bmax of 126 fmol/mg of protein and a KD of 14.7 nM. In isolated left atrium, histamine produced a concentration-dependent positive inotropic effect, an effect which was competitively antagonized by cimetidine but not altered by chlorpheniramine. Schild analysis showed that the pA2 value for cimetidine was 6.55 and the slope was not significantly different from unity. An excellent correlation was found between the increase in force of contraction and cyclic AMP in the presence of histamine, suggesting that the positive inotropic effect of histamine in rabbit left atrium is dependent on an increased level of intracellular cyclic AMP through stimulation of histamine H2-receptors. Histamine also produced concentration-dependent stimulation of phosphoinositide hydrolysis as measured by [3H]inositol monophosphate accumulation. The phosphoinositide response to histamine was blocked by chlorpheniramine and mepyramine but not by cimetidine. The data indicate that histamine H1-receptors, in addition to histamine H2-receptors, are present in the rabbit left atrium. Although this tissue lacks an inotropic response to histamine H1-receptor stimulation, the histamine H1-receptors interact with histamine to mediate the stimulation of phosphoinositide hydrolysis.

Mechanism of action of H2-antagonists on histamine- or dimaprit-stimulated H2-receptors of spontaneously beating guinea-pig atrium

Cimetidine, ranitidine and famotidine are antagonists of the histamine H2-receptors on the spontaneously beating right atrium of the guinea pig. When analyzed by the classical Schild method their pA2-values are respectively: 6.3, 6.8 and 7.7 with dimaprit as agonist and 5.8, 6.5 and 7.7 with histamine as agonist. Radioligand-displacement studies with [3H]-tiotidine as radioligand resulted in pKd values for cimetidine, ranitidine and famotidine of 6.3; 6.9 and 8.2 respectively. In dimaprit-stimulated atria all antagonists added at concentrations above their Kd values depressed the maximal increase in frequency. In the presence of histamine this effect was much less pronounced and only visible at concentrations of ranitidine and famotidine around 10.Kd. The rightward shift of the curves as well as the decrease in Emax are reversible but the dissociation constants of the antagonists are small (less than 10(-3) s-1). The spontaneously beating right atrium showed receptor reserve for histamine and virtually no receptor reserve for dimaprit. The results have been interpreted in a model in which H2-antagonists act mainly by competing with the agonist for the histamine receptor site but have in addition a distinct affinity for a secondary site on the receptor. Occupation of this site by the antagonist prevents building up of the stimulus elicited by the agonist and thus decreases the Emax. In systems with receptor reserve (histamine) the effect of antagonist binding to the secondary binding site is seen only at high concentration of antagonist while in absence of receptor reserve (dimaprit) the depression of Emax is directly visible. Simulations of the model show that the affinity of this secondary binding site is 50-(famotidine) to 100-(cimetidine and ranitidine) fold lower than for the agonist binding site.

Changes in the ionic environment may alter the kind of antagonism of some histamine H2-receptor blockers in the guinea pig papillary muscle

The effect of changes in the composition of the bathing medium on the effect of histamine and histamine H2-receptor antagonists was investigated in the isolated guinea pig papillary muscle. Ringer or Krebs-Henseleit solutions were used as nutrient fluids. They mainly differed with respect to pH and to Mg2+ and H2 PO4- content. Whereas the effect of histamine was not altered by ionic changes, the antagonism by some H2 blockers was different in the two nutrient solutions. The insurmountable antagonism elicited by high concentrations (greater than or equal to 10(-6) M) of famotidine, oxmetidine and mifentidine in Ringer solution was converted to surmountable when these drugs were tested in Krebs-Henseleit solution. Conversely, the antagonism induced by ranitidine was surmountable in both solutions, and that induced by high amounts of Ioxtidine was insurmountable in both nutrient fluids. Results obtained in Ringer solution were not modified by pH adjustments or by the addition of ions present in Krebs-Henseleit medium. These results suggest that the interaction of histamine with H2 receptors in the guinea pig papillary muscle was not influenced by alterations in the ionic composition of the nutrient fluid, whereas the antagonism may be critically dependent on the ionic environment.

Action of the new H2-receptor antagonist oxmetidine on the duodenum of different species

The new H2-receptor antagonist, oxmetidine was studied for its effects on the motility of the duodenum of different species. Oxmetidine did not modify basal motility of the rat and guinea pig duodenum but it reduced or abolished the contractions elicited by different spasmogenic compounds (acetylcholine, BaCl2, KCl, eledoisin). In the rabbit duodenum, oxmetidine reduced in a dose-dependent fashion the basal motility. The effect of this compound appears to be indirect on the smooth muscle and not mediated through specific receptors. Experiments performed in Ca++-free and K+-depolarizing solution suggest that oxmetidine acts through interference with the transport and/or utilization of intracellular Ca++ions.

Differential effects of histamine mediated by histamine H1- and H2-receptors on contractility, spontaneous rate and cyclic nucleotides in the rabbit heart

The effects of histamine on the contractile force, spontaneous rate of contraction, and cyclic AMP and cyclic GMP content were investigated in isolated rabbit cardiac preparations. Histamine had a positive inotropic effect in the left atrium and papillary muscle, and a positive chronotropic effect in the right atrium. Both effects were produced in a concentration-dependent manner. Impromidine also induced the same effect in the left and right atrium as histamine did. The effects produced by histamine and impromidine were antagonized by cimetidine and tiotidine. On the other hand, the positive inotropic response of papillary muscle to histamine was antagonized by mepyramine and chlorpheniramine and was mimicked by 2-(2-pyridyl)ethylamine. Impromidine at a high concentration induced a small increase in the contractile force, an effect which was antagonized by cimetidine. Histamine significantly increased the cyclic AMP levels in both atria but not in papillary muscles. The increase in cyclic AMP was abolished by cimetidine. Histamine also increased cyclic GMP levels in all of the preparations. The increase in cyclic GMP was abolished by chlorpheniramine. The results suggest that both H1- and H2-receptors exist in all parts of the rabbit heart. However, the positive inotropic and chronotropic effects induced by histamine in left and right atrium are mediated predominantly via H2-receptors, whereas the positive inotropic effect in papillary muscle is predominantly mediated via H1 receptors.

Antagonistic activity of tiotidine and ranitidine on guinea-pig and rabbit atria

Isolated, spontaneously beating atrial pairs from rabbits and guinea-pigs were used to determine and to compare the activity of ranitidine and tiotidine as antagonists of histamine stimulated chronotropic activity. Ranitidine produced a classical competitive, reversible antagonism of histamine effects with a pA2 in rabbit atria of 8.2. In contrast, tiotidine produced both a dextral shift of the log dose-response curve, as well as a previously unreported suppression in the maximal response produced by histamine. In accordance with receptor theory, this type of activity represents a dualistic antagonism of histamine chronotropic responses.

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.

Rat gastric secretion "in vitro": interaction between histamine and various antisecretagogues acting by different mechanisms

The interaction between histamine and various antisecretagogues acting by different mechanisms has been investigated in the isolated fundus from the rat stomach. Histamine evoked a concentration-dependent stimulatory effect which was competitively antagonized by the H2-receptor antagonist, ranitidine and non competitively by the H+/K+-ATPase inhibitor, omeprazole. The histamine induced secretion was highly resistant to the action of the calcium entry blocker verapamil, somatostatin and KSCN, but some inhibition was obtained with the calmodulin antagonist, trifluoperazine. Removal of calcium ions from the bathing media (both mucosal and serosal) greatly enhanced histamine-induced gastric secretion. The results suggest that the relationship between receptor stimulation and the intracellular events leading to acid secretion is far from being elucidated.

Action of histamine and of some H2-antagonists on gastric secretion 'in vitro'

The effect of histamine and of some H2-antagonists on isolated gastric mucosal preparation from immature (14-18 days) rats, was investigated. Basal secretion varied, in our experimental conditions, between 1.06 and 3.54 mumol cm-2 h-1, reaching higher values (approximately 4.6 mumol cm-2 h-1) only in a small percentage of animals (10%). Histamine exerted a concentration-dependent stimulation of acid secretion in concentrations varying between 2 X 10(-6) and 1.6 X 10(-4) M. The response to histamine was competitively antagonized by ranitidine (pA2 value = 6.78) and by 4(5)-(4- isopropylaminomethyleniminophenyl ) imidazole (compound marked DA 4577) (pA2 value = 7.37). Oxmetidine acted as a competitive antagonist only for concentrations as low as 10(-8) M; higher concentrations (10(-7) and 10(-6) M) determined a non-competitive inhibition. Ranitidine and compound marked DA 4577 did not affect basal secretion up to concentrations of 3 X 10(-4) M. On the contrary oxmetidine exerted a concentration-dependent inhibition starting from 10(-5) M. Since in our experimental conditions the role of calcium ions in the regulation of basal secretion could not be established, the mechanism of action of oxmetidine was not completely clarified, even if an interference in the utilization of calcium ions may be suggested. In any case it is deemed of interest that this H2-antagonist was the only compound capable of inducing a reversible complete inhibition of basal acid secretion (only KSCN, in very high concentrations, had a similar behaviour).

Operational models of pharmacological agonism

The traditional receptor-stimulus model of agonism began with a description of drug action based on the law of mass action and has developed by a series of modifications, each accounting for new experimental evidence. By contrast, in this paper an approach to modelling agonism is taken that begins with the observation that experimental agonist-concentration effect, E/[A], curves are commonly hyperbolic and develops using the deduction that the relation between occupancy and effect must be hyperbolic if the law of mass action applies at the agonist-receptor level. The result is a general model that explicitly describes agonism by three parameters: an agonist-receptor dissociation constant, KA; the total receptor concentration, [R0]; and a parameter, KE, defining the transduction of agonist-receptor complex, AR, into pharmacological effect. The ratio, [R0]/KE, described here as the 'transducer ratio', tau, is a logical definition for the efficacy of an agonist in a system. The model may be extended to account for non-hyperbolic E/[A] curves with no loss of meaning. Analysis shows that an explicit formulation of the traditional receptor-stimulus model is one particular form of the general model but that it is not the simplest. An alternative model is proposed, representing the cognitive and transducer functions of a receptor, that describes agonist action with one fewer parameter than the traditional model. In addition, this model provides a chemical definition of intrinsic efficacy making this parameter experimentally accessible in principle. The alternative models are compared and contrasted with regard to their practical and conceptual utilities in experimental pharmacology.

Negative inotropic effect of some H2-receptor antagonists on the isolated human atria

H2-Receptor antagonists were found to possess in various degrees a negative inotropic effect on human atria in vitro. This effect seemed to be independent of H2-receptor blockade and, at least in the case of oxmetidine, seemed to involve calcium ion transport and/or utilization.

Cardiac effects of the new H2-receptor antagonists

A series of new H2-receptor antagonists were tested for their effects on different isolated heart preparations. In the guinea-pig atria and papillary muscle the inhibitory effect on histamine H2-receptors was evaluated. In the perfused rabbit heart and in strips of human atria the effect of the H2-antagonists on the spontaneous or electrically-stimulated contractions was evaluated. In the first two preparations some main quantitative differences were pointed out, tiotidine and compound SKF 93479 being the most potent antagonists, cimetidine, metiamide and ranitidine the less effective. In the rabbit heart and in human atria results were quite different: cimetidine and ranitidine were virtually ineffective up to the maximum concentration tested (3 x 10(-3) M), oxmetidine and compound SKF 93479 had a negative inotropic and chronotropic effect starting from concentrations of 3 x 10(-6)-10(-5) M. On the basis of the behaviour of other compounds endowed with negative cardiac effects (propranolol, anaesthetic-like compounds, verapamil) and of that of compounds capable of counteracting the effect of oxmetidine (increased concentration of calcium ions and isoproterenol) it was hypothesized that oxmetidine may interfere in the transport of calcium ions. Our data emphasize the importance of the different structure of the H2-antagonists in determining non-specific effects absolutely independent of the primary action that is the H2-receptor blockade.

Cholinergic-like effects of the new histamine H2-receptor antagonist ranitidine

The new H2-receptor blocker ranitidine, together with the effect on histamine H2-receptors, possesses a series of cholinergic-like actions: it provokes atropine-sensitive contractions of several isolated smooth muscle preparations from different animal species and it potentiates the stimulant effect of acetylcholine. Moreover it contracts human lower esophageal sphincter in vivo, an effect which is completely prevented by small doses of atropine. Finally, ranitidine potentiates the stimulant effect of bethanechol and of carbachol on salivary glands of the rat while leaving unaffected the secretagogue effect of physalaemin which is known to be completely independent of the cholinergic system. In the in vivo experiments the doses of ranitidine capable of eliciting cholinergic-like effects were of the same order of magnitude as those necessary to cause the H2-receptor blockade.