Further characterization of the guinea pig left atrial tension response to histamine by use of selective agonists

1. The responses of guinea pig left atrial tension and right atrial rate to histamine receptor agonists and histamine were compared.
2. Single doses of histamine and another non-selective agonist,N,N-dimethylhistamine, produced biophasic inotropic responses, with an initial increase in tension and a secondary sustained tension increase separated by a negative inotropic component.
3. Selective H₁-receptor agonists — 2-methylhistamine, 3-methylhistamine and 2-pyridylethylamine (2-PEA) — also exhibited biphasic inotropic responses. 2-PEA did so only after blockade with propranolol or in atria from reserpine-pretreated animals, indicating an additional release of endogenous catecholamines which masked the biphasic response.
4. Selective H₂-receptor agonists — 4-methylhistamine and dimaprit — failed to produce biphasic responses except at high concentrations only in the case of 4-methylhistamine.
5. The biphasic responses were converted to monophasic responses by mepyramine. This was a result of antagonism of the initial positive component and the secondary negative component which were therefore mediated via H₁-receptors. The production of a biphasic inotropic response therefore depends upon these two components which were exhibited preferentially by the H₁-receptor-selective agonists.

     

The effect of phosphodiesterase inhibitors on guinea-pig cardiac responses to histamine and isoprenaline

1. The effect of phosphodiesterase inhibitors was studied on right atrial rate, left atrial tension and left ventricular papillary muscle tension responses to histamine and isoprenaline.
2. Only responses mediated via β-adrenoceptors and H₂-receptors were potentiated by theophylline. This is proposed to be due to its phosphodiesterase inhibiting properties and therefore indicates the involvement of cAMP in these responses.
3. 3-Isobutyl-1-methylxanthine was approximately 30 times more potent than theophylline in producing leftward shifts of isoprenaline left atrial tension curves. The potentiating effect of papaverine was masked by an opposing depressant action.
4. Left ventricular papillary muscle tension responses to histamine were enhanced by the phosphodiesterase inhibitors confirming the reported involvement of cAMP in the right ventricle.
5. The left atrial tension dose-response curves to histamine were not potentiated. Single doses revealed a biphasic response consisting of an H₁-receptor mediated component and one resistant to both H₁- and H₂-receptor antagonists. Neither component was potentiated suggesting no involvement of cAMP.

     

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 antogonist.
2. Atropine competitively inhibited the cimetidine-induced contractions in the guinea pig ileum.
3. Cimetidine-induced reponses 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.     

Differentiation of H10and H2-receptors mediating positive chronoand inotropic responses to histamine on atrial preparations of the guinea-pig

The histamine blocking properties of burimamide and promethazine were determined on heart rate and contractility of the right and left atrium of the guinea-pig.

1. The positive chronotropic effect in spontaneously beating guinea-pig atria was competitively blocked by burimamide, but not by promethazine.
2. On the other hand, promethazine proved to be a potent antagonist against the positive inotropic response elicited by histamine on the electrical stimulated (2 Hz) left guinea-pig atrium, whereas burimamide was ineffective.
3. The positive inotropic response to histamine on the right atrium was affected by both antihistaminic drugs investigated. It should, however, be stressed that the blocking efficacy of both antagonists was less pronounced than that evoked by burimamide on the chronotropic and by promethazine on the inotropic effect on right and left atrium, respectively.
4. These results suggest that on guinea-pig heart the positive chronotropic effect elicited by histamine is mediated by ‘burimamide sensitive’ H₂-receptors, whereas the positive inotropic effect evoked by histamine is mediated by ‘promethazine sensitive’ H₁-receptors. The observation that the positive inotropic effect of histamine on the spontaneously beating right atrium could be antagonized by both types of antihistamines favours the view that part of the positive inotropic response to histamine on this preparation is due to a positive frequency-force relationship (Bowditch-Treppe).

     

Effects of the H1-antagonist promethazine and the H2-antagonist burimamide on chronotropic,inotropic and coronary vascular responses to histamine in isolated perfused guinea-pig hearts

On guinea-pig atria part of the inotropic response to histamine is attributable to a concomitant increase of the frequency [7]. Since the chronotropic effect of histamine is mediated by a stimulation of H₂-receptors a direct interaction of histamine with H₁-receptors mediating the inotropic response on heart may be overlooked. For this reason the ability of the H₁-antagonist promethazine and the H₂-antagonist burimamide to inhibit the positive chronotropic, inotropic and coronary vascular responses to histamine was determined in spontaneously beating and electrically driven perfused guinea-pig hearts.

1. Burimamide produced a competitive blockade of the positive chrono- and inotropic responses to histamine.
2. On the other hand, promethazine in concentrations that had no effect on cardiac function by itself, proved to be ineffective against the positive chrono- and inotropic responses produced by histamine on spontaneously beating and electrically driven heart preparations.
3. The predominant coronary vasodilation observed after infusion with histamine was competitively antagonized by promethazine and burimamide. This blockade was not attributable to an interaction with myocardial H₂-receptors mediating increases in heart rate and contractility and was, therefore, direct in nature.
4. Based upon the present study and former investigations [7] the following distribution of different histamine receptors in the guinea-pig heart does exist: H₁-receptors are present in the atrial muscle and the coronary vascular bed. H₂-receptors are located in the sinus node, the ventricular myocardium and the coronary vessels.

     

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.

     

Cardiovascular effects of burimamide and metiamide

In anaesthetized gastric fistula cats with a histamine infusion running the effect of the histamine H₂-receptor antagonists burimamide and metiamide on blood pressure and cardiac frequency was investigated. Burimamide causes a rise in blood pressure and an increase in cardiac frequency. Metiamide has almost no blood pressure effect, but it lowers cardiac frequency. In animals pretreated with mepyramine or phenoxybenzamine both burimamide and metiamide elevate the blood pressure lowered by histamine and has no or a lowering effect on cardiac frequency, respectively. It is concluded from the experiments that

1. burimamide like histamine releases catecholamines,
2. metiamide is free of catecholamine releasing properties, and
3. blood vessels responsible for the general blood pressure contain at least predominatly or exclusively histamine H₂-receptors.

     

H1- and H2-receptor mediated responses to histamine on contractility and cyclic AMP of atrial and papillary muscles from guinea-pig hearts

On guinea-pig heart we investigated whether cyclic AMP serves as a messenger for H₁- and/or H₂-mediated responses to histamine.

1. On papillary muscle histamine elicited positive inotropic responses which were antagonized by burimamide but not by promethazine. The stimulation of H₂-receptors was not only associated with an increase in contractility but also with an increase in cAMP. As shown by the time course of effects for 10^?5 M histamine, the maximal increase in cAMP preceded the maximum in contractility. The mechanical and biochemical responses to histamine were potentiated by the phosphodiesterase inhibitor papaverine, but antagonized by burimamide.
2. On the left guinea-pig atrium containing H₁-receptors the inotropic response to histamine (10^?5 M) was not accompanied by increases in cAMP at stimulation frequencies of 0.5 and 2 Hz, respectively. In addition, in the presence of papaverine (3×10^?5 M) no change in the cyclic AMP level occurred after application of histamine. Papaverine by itself, however, concomitantly increased contractility and cyclic AMP at a stimulation frequency of 0.5 Hz. In contrast, at 2 Hz papaverine increased only cAMP leaving the contractility unchanged. At this frequency the well-known Ca²⁺-antagonistic effect comes into prominence, thus masking the positive inotropic effect atributable to the inhibition of the phosphodiesterase.
3. On the right guinea-pig atrium the mediation of the positive charonotropic response to histamine by H₂-receptors which is partly involved in the inotropic effect via the frequency-force relationship does not lead to a concomitant increase in cAMP. Also, in the presence of papaverine, histamine had no influence on the cAMP. However, papaverine potentiated the cardioacceleration produced by histamine. Although it is very likely that the cAMP in the sinus node rises, we were not able to detect an increase in cAMP in the whole atrial tissue.

From the present results the conclusion can be drawn that the mediation of the inotropic effect due to stimulation of H₂-receptors by histamine is associated with an increase of cyclic AMP, whereas that of H₁-receptors is not. The view that cAMP may be the second messenger in the chronotropic action of histamine needs further elucidation by experiments on sino-atrial cells.     

The effects of histamine H2-receptor antagonists on PHA induced lymphocyte proliferation

Histamine H₂-receptor antagonists must be used with caution to define the pharmacology of histamine effects on lymphocyte mitogenesis induced by PHA

1. because they can enhance and/or suppress in their own right,
2. because these effects are similar to those of histamine itself
3. because mitogenic doses of PHA can release significant amounts of histamine from supposedly pure mononuclear cell preparations.

     

Histamine modification of spontaneous rate and rhythm in infarcted canine ventricle

1. Histamine (10^?3 M) increased the spontaneous rate similarly in isolated preparations of normal left ventricular tissue from control, i.e. normal and sham-operated, dogs (control preparations) and in preparations consisting of normaland contiguous infarcted left ventricular tissue from dogs with subacute, i.e. 24 hours after left coronary artery ligation, myocardial infarction (infarcted preparations).
2. Histamine (10^?3 M) markedly enhanced the irregular rhythm of infarcted preparations.
3. The H₁-receptor antagonist, chlorpheniramine (10^?4 M), and the H₂-receptor antagonist, cimetidine (10^?3 M), antagonized the effects of histamine (10^?3 M) on the spontaneous rate of both control and infarcted preparations.
4. The H₁-receptor agonist, 2-pyridyl ethylamine (PEA, 10^?4 M), increased the spontaneous rate of control and infarcted preparations; these effects were antagonized by chlorpheniramine (10^?4 M). The H₂-receptor agonist, dimaprit, had no effect.
5. Similar to histamine (10^?3 M), PEA (10^?4 M) enhanced the irregular rhythm of infarcted preparations; dimaprit had no effect.
6. High local concentrations of histamine may occur in poorly perfused ischemic tissue. The enhancement of irregular rhythm produced by histamine, and the specific H₁-receptor agonist, PEA, leads us to suggest its involvement in arrhythmias associated with subacute myocardial infarction.

     

Electrophysiological actions of histamien and H1-,H 2-receptor antagonists in cardiac tissue

Electrophysiological investigations of histamine in different cardiac tissues have led to the following results:

1. Histamine and the H₂-agonists dimaprit and impromidine show similar actions on electrophysiological parameters of ventricular myocardium (especially a decrease in action potential duration), which are completely blocked by cimetidine and enhanced by the phosphodiesterase inhibitor 1-methyl,3-isobutylxanthine (IBMX). These effects may be explained by an increase in cellular cAMP leading to an increase in slow inward current and outward currents as shown by voltage clamp experiments.
2. Histamine in contrast to IBMX increases action potential duration at 90% repolarization (APD₉₀) in atria. Histamine effects in atrial myocardium are completely reversed by the H₁-antagonist dimetindene. Stimulation of atrial H₁-receptors is suggested to directly cause an increase in Ca-channel conductance independent of intracellular cAMP content.
3. Histamine reduces AH-interval, increases \(\dot V_{max} \) of NH — cells and may induce AV — node arrhythmias (at concentrations ≥ 3 μmol/l). These effects remain unchanged by dimetindene, but are reversed by cimetidine. The results indicate that histamine increases AV — nodal conduction via H₂-receptors.
4. Unspecific membrane actions of cimetidine are not observed up to 100 μmol/l. Dimetindene increases action potential duration (APD) in left atria and decreases \(\dot V_{max} \) at concentrations ≥ 10 μmol/l. However, H₁-antagonistic actions of dimetindene are already observed at concentrations 1,000 to 10,000 times lower (pA₂—values 8.39–9.12) so that unspecific membrane actions are suggested not to occur on a therapeutic dose level.

     

An attempt to produce an antibody of histamine and histamine derivatives

Three conjugates of different histamine derivatives to bovine serum albumin were prepared, and an attempt was made to determine whether antibody against histamine and histamine derivatives, could be produced in rabbits by immunization with these conjugates.

1. Antibody produced by immunization with bovine serum albumin-succinylhistamine conjugate could not recognize the hapten moiety of the immunogen.
2. Immunization of rabbits with conjugate of bovine serum albumin andp-[2-(N ^α-trifluoroacetyl-histamine)azo]benzoic acid resulted in the production of antibody toN ^α-trifluoroacetylhistamine. The antibody had a specificity for histamine metabolites and their derivatives. However, the antibody also showed a small but not negligible affinity for trifluoroacetylated serotonin, norepinephrine and aniline, indicating that the antibody specificity was directed mainly to the trifluoroacetamide group.
3. After immunization of rabbits with bovine serum albumin andp-[2-(N ^α-propionylhistamine)azo]benzoic acid conjugate, antibody againstN ^α-propionylhistamine could be produced. However, the antibody had a very low affinity even forN ^α-propionylhistamine.

Specific antibody to histamine and histamine derivatives could thus not be produced by immunization with the present hapten-carrier conjugates.     

Problems of nomenclature III: ambiguous abbreviations and catchwords

To avoid ambiguity, the following emendations are advocated:

1. 2H, in place of H₂, to designate the second histamine receptor;
2. HN-2, in place of HN₂, to designate mechlorethamine (MBA);
3. ‘pre-drug’, in place of ‘pro-drug’, to describe molecular species which must undergo biotransformation before yielding an effective drug.

     

Potentiation of gastrin and histamine stimulated acid secretion in heidenhain pouches by distension — an atropine-resistant mechanism

1. Distension of the vagally denervated fundic gland area, although by itself a generally ineffective stimulus to acid secretion in this study, will markedly increase the acid secretory response to stimulation by endogenous gastrin, pentagastrin and histamine. Even maximal responses to histamine can be increased, thus demonstrating true potentiation between distension and humoral stimuli.
2. The magnitude of the distension potentiation does not appear to be influenced by the nature of the background stimulus or the background level of secretion.
3. The distension potentiation of the response to histamine is reduced but not abolished by intravenous atropine and hexamethonium. Thus, there appear to be two components to the local distension mechanism. One is cholinergic, the other, atropine-resistant component is possibly non-cholinergic. At least one of these components, possibly the cholinergic, involves a nicotinic synapse in its pathway.

     

In vitro actions of ranitidine,a new histamine H2-receptor antagonist

Ranitidine has been tested on isolated guinea-pig right atrium and rat uterine horn, tissues known to possess histamine H₂-receptors; and on isolated guinea-pig ileum a tissue containing histamine H₁-receptors. These experiments have shown ranitidine to be a potent competitive antagonist of histamine at H₂-receptor sites in vitro. This action is selective since high concentrations of ranitidine do not affect -adrenoceptor, histamine H₁-receptor and muscarinic receptor mediated responses.     

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 (1.0×10^?6 M), a specific 5HT receptor blocker, failed to inhibit the responses to either 5HT or NA.
2. However, phentolamine, a specificalpha 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 (2.2×10^?6 M), the mono-amine oxidase inhibitor produced a significant leftward shift of the dose-response curve of both 5HT and NA. Pyrogallol (2.3×10^?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 commonalpha adrenoceptors. 5HT actions are probably indirect, mediated through the release of NA.

     

Classification and biological distribution of histamine receptor sub-types

The distribution and classification of histamine receptors in mammalian and avian tissues have been summarized in Tables 1–4. It is evident that histamine receptors are present on a number of morphologically distinct cell types and the proportion of cells bearing H₁- and H₂-receptors varies not only with the species but also with the cell source. The pharmacological receptors mediating mepyramine-sensitive histamine responses have been defined as H₁-receptors. Receptors mediating mepyramine-resistant, but burimamide or metiamide-sensitive histamine responses have been classified as H₂-receptors. Histamine responses mediated via H₂-receptors seem to involve the adenylcyclase system resulting in elevation of intracellular cyclic-AMP level, which is susceptible to burimamide blockade but insensitive to -adrenergic blocking agents. This mode of action of histamine involving H₂-receptors and the adenyl cyclase system has been shown to stimulate the mammalian heart; promote gastric acid secretion; inhibit antigen-induced histamine release from leucocytes and inhibit lymphocyte-mediated cytotoxicity. It can further be concluded that both H₁- and H₂-receptors are widely distributed throughout the animal body in the gastro-intestinal, reproductive, respiratory and cardiovascular systems, nervous system and on mast cells and blood leucocytes. In these tissues, histamine receptors play an important role in physiological, immunological and immunopathological processes. Interaction of histamine with both H₁- and H₂-receptors in varying proportions modulates the overall manifestation of cardiovascular and respiratory syndromes during certain immunopathological conditions (e. g. inflammation, allergy and anaphylaxis). Histamine receptors also appear to play an important role in the development of immuno-competence and immunity.     

Actions and cross-reactivity of antiallergic agents and a calcium channel antagonist on rat peritoneal mast cells. Difference in the action mechanisms and cross-reactivity among the agents

The actions of the antiallergic agents, disodium chromoglycate (DSCG), tranilast and ketotifen, and of a calcium channel antagonist, nicardipine, and cross-reactivity among the agents were examined by observing the inhibition of⁴⁵Ca uptake and histamine release in rat mast cells stimulated by antigen and compound 48/80 (comp. 48/80).

1. All agents inhibited⁴⁵Ca uptake and histamine release in mast cells stimulated by antigen. The inhibition of⁴⁵Ca uptake by the antiallergic agents paralleled the inhibition of histamine release, while nicardipine inhibition of⁴⁵Ca uptake was stronger than its inhibition of histamine release.
2. The action of DSCG on⁴⁵Ca uptake and histamine release was significantly decreased in cells stimulated with antigen and phosphatidylserine (PS), while tranilast inhibition of histamine release was not affected by the addition of PS despite a significant decrease in the inhibition of⁴⁵Ca uptake.
3. The inhibitory effect of DSCG and tranilast was significantly lower in mast cells stimulated by comp. 48/80 than in the cells stimulated by antigen.
4. Tachyphylaxis was observed in cells re-exposed to DSCG and tranilast following previous exposure to the agents.
5. Cross-reactivity was found between DSCG and tranilast.

     

Observations on the staircase phenomenon in guinea pig atrium

The staircase phenomenon, occurring after a change in frequency was studied in isolated trabeculae from guinea pig atrium. The effects on tension, action potential form and function as well as ionic currents were investigated.

1. In the ascending part of the frequency-force relationship a sudden change to a new driving frequency resulted in a staircase response which consisted of two exponential phases (τ₁ = 1—2 s; τ₂ = 20 — 30 s).
2. The build-up or decline of twitch tension in response to either an increase or reduction of [Ca²⁺]_o followed a similar composed time course.
3. After a reduction in stimulation frequency the action potentials changed time dependently: In the first response the peak of the overshoot was reached faster and the plateau phase was shortened; afterwards these parameters remained constant while the repolarization phase continually shortened during the following 5–10 action potentials.
4. In voltage clamp experiments an analogous reduction in the frequency of depolarizing voltage clamp pulses induced an immediate increase of the slow inward current (I_si). In the following 10 pulses the increased I_si remained constant, while the late outward current continually increased.
5. The time course of recovery of the I_si-system was found to be slow in atrial trabeculae (τ=300–500ms at ?70mV). Thus the increase in I_si, observed after a reduction in frequency, could be explained by a more complete recovery of the I_si-system during the interval between two stimuli.
6. The increase in I_si during these recovery experiments was accompanied by an accelerated inactivation.
7. It is concluded that after a reduction in stimulation rate the faster development of the overshoot and the shortening of the plateau phase are due to an augmentation and a faster inactivation of the I_si, respectively. The shortening of the late repolarization phase which developed during successive action potentials is most probably related to the observed increase in late outward current.