Comparison of responses to histamine by constant-flow and constant-pressure perfusion methods in the rabbit isolated kidney preparation.

1. Reproducibility and sensitivity of responses of isolated perfused rabbit renal vascular bed to bolus administration of histamine was compared for the conditions of constant-flow and constant-pressure perfusion by Krebs-Henseleit solution. Two other vasoactive agents (noradrenaline and angiotensin II) were also tested in the same preparation for comparison with the effect of histamine. 2. In constant-pressure mode, different hydrostatic pressures were also employed for the analysis of time-effect phenomena. 3. Drug responses were recorded by computer and were evaluated as changes of perfusion pressure or flow, to correspond to drug activities at the resistance vessels and, also, change of organ weight, as effects on exchange and capacitance functions of the circulation. Both parameters were also recorded on a Grass polygraph. 4. Reproducibility and sensitivity of responses to vasoactive agents (histamine, noradrenaline, angiotensin II) were significantly low in the constant-flow conditions, compared to constant-pressure mode. 5. Responsivity also deviated for different perfusion pressures of constant-pressure mode and 80-100 mm H2O pressure was found to be the most convenient value. 6. It is concluded that constant-flow and constant-pressure conditions provide different hydrodynamic conditions. In constant-flow methodology, when the predetermined perfusion rate is not tolerated by the vascular bed, excessive flow in the circulation would lead to depression of vasoactivity, failing pharmacodynamic equilibria and loss of responsivity. Structural deviation of the vascular bed would also be expected due to differences of organ specimen and, probably, result in the variation of responsivity of constant-flow perfusion procedure.

Atypical Schild plots with histamine H1 receptor agonists and antagonists in the rabbit aorta.

Competitive antagonists of histamine H1 receptor were investigated for their effects on histamine-induced responses in the rabbit aorta. Antazoline-induced antagonism gave linear Schild plots with slope equal 1, while the other antagonists [+)-brompheniramine, (+/-)-chlorpheniramine, diphenhydramine and mepyramine) produced 'atypical' plots with slopes generally less than unity in the thoracic aorta. Schild plots obtained with these antagonists were evaluated using a two independent component model. The high affinity parameters thus estimated were compatible with those that have been reported for these antagonists. No such heterogeneity was observed in the abdominal aorta when diphenhydramine was investigated with different H1 agonists. The results suggest the presence of at least two components in H1-mediated responses in the thoracic aorta; these components are equally antagonized by antazoline, but differentially antagonized by the other antagonists used.

Regional histaminergic potencies in rabbit systemic circulation.

1. Regional activity of histamine was studied in various rabbit arteries, including segments of the aorta, aortic branches, branchout regions and arteries to fore and hind limbs by dose-response experiments. 2. Local histaminergic potencies were found to be heterogenceous. Circular and longitudinal smooth muscle responsivity in the aorta increased in the distal direction; aortic branches have their own histaminergic response characteristics beginning from their branchouts. Paired arteries (right and left subclavia, renal and iliac) possess matching histaminergic activities. 3. The observed characteristics of the responses suggest predetermined distribution of histaminergic activities along systemic circulation which may be regulatory in the distribution of blood.

Kinetics of antagonism at histamine-H1 receptors in isolated rabbit arteries.

Kinetics of antagonist-induced decrease of histamine-H1 receptor-mediated steady-state responses in isolated rabbit arteries were studied in the presence of histamine-H2 receptor antagonist famotidine. Data were fitted using a model which describes competition kinetics at the receptor level. Estimated rate and equilibrium constants were evaluated for their dependence on tissue, agonist and antagonist concentrations, using (+)-brompheniramine as antagonist. In large arteries (thoracic and arcus aorta), rate constants were observed to be modified by agonist and/or antagonist concentrations, suggesting a diffusion-controlled process. In relatively small (common carotid and iliac) arteries, estimated equilibrium constants (and consequently the rate constants) were found to diverge despite the invariance of equilibration times between arteries, leading us to include the effects of spare receptors in our evaluation. A model describing the effects of receptor reserve on the estimated equilibrium dissociation constant was developed and stimulated and the results then compared with those that had been experimentally estimated. The reserve hypothesis was experimentally verified in common iliac artery (where EC50 much less than KA) using the irreversible antagonist phenoxybenzamine. A rationalized rule for the optimization of experimental design for in-vitro disequilibrium-competition experiments was proposed. Common carotic artery was found to be favorable for the present design in view of its reserve properties. In addition, competition reaction seems to be the rate-determining step in this artery. Rate and equilibrium constants of mepyramine, (+)-brompheniramine, diphenhydramine and antazoline were therefore determined in the common carotid artery and were compared with those obtained from independent experiments. Results suggest that the estimated parameters reflect drug-receptor interaction.

Heterogeneity of histamine H2-receptor mediated responses in the rabbit aorta.

Distribution and properties of histamine H2-receptor mediated responses in segments of rabbit aorta was studied with histamine and H2-receptor stimulating drugs, dimaprit and impromidine. All agonists produced concentration-dependent tonus decreased in precontracted vascular strips, which were antagonised by selective H2-receptor antagonists, cimetidine and oxmetidine. Activities of the agonists were segment-dependent, and increased caudally along the aorta. A nonspecific smooth muscle relaxant, papaverine had homogeneous activity along the vessel, suggesting receptor specific nature of the observed heterogeneity.

A semiiterative method for derivation of concentration-response parameters.

A method for the analysis of concentration-response curves is described. The parameters of these curves, namely, maximal response, apparent affinity, and slope factor, are calculated by means of a semiiterative approach using a microcomputer. Considerably less computer processing time is required than with other iterative nonlinear curve fitting methods. The method is particularly helpful in cases in which accurate observation of maximal response is limited.

Segmental and age-dependent changes of histamine's affinity in the rabbit aorta.

Affinities of contractile responses to histamine and H1-receptor agonists thiazolylethylamine (TEA) and pyridylethylamine (PEA) were evaluated in three segments of rabbit aorta and increase of affinity in the caudal direction was found. alpha-adrenergic affinity of noradrenaline did not possess such differentiation. Evaluation of the data with histamine receptor agonists and antagonists suggested H1-receptor liability of the phenomena. Affinities of histamine and noradrenaline were also investigated in relation to body weight which reflects age. Histamine's affinity was found to be age dependent, decreasing with the increase of age, while no correlation was found for noradrenaline between two parameters. Results are suggesting the presence of histamine specific vascular function in the rabbit which is receded by the advance of age.

Dynamic properties of a retrograde superfusion system for isolated right atria including effects of histamine H2 receptor agonists.

A retrograde superfusion technique is described for spontaneously beating guinea pig right atrium where both force of contraction and rate are recorded simultaneously. Drugs are applied to the system in bolus injections and the concentrations attained in the organ bath after these injections are computed through an analog system by means of measuring injected dye dilutions in a spectrophotometer. Actions of histamine and histamine H2-receptor agonists 4-methyl histamine, dimaprit, and impromidine on the preparation for the mentioned parameters are investigated. By comparing responses to bolus injections with infusions of agonists, it is concluded that all four agonists tested are rapidly passing to the biophase, and the system can be used as an alternative for the classical organ bath. Dose-response characteristics of the tested agonists are satisfactory and affinity values found are in agreement with the values reported for bathed systems.

Histamine H2 receptor subtypes in the guinea pig right atrium.

Inotropic and chronotropic responses to histamine and to selective histamine H2 receptor agonists (4-methyl histamine, dimaprit, impromidine) and antagonists (metiamide and cimetidine) were studied in retrogradely superfused guinea pig right atrium preparations. All agonists tested were acting as chronotropic and, except impromidine, as inotropic agonists; impromidine had a dual inotropic action. Affinities of the antagonists varied, showing approximately 20-fold more activity on chronotropism. The findings suggest the existence of subtypes of histamine H2 receptors in the development of chronotropic and inotropic responses.

Evidence for the histamine-mediated myotropic effect of angiotensin II in the rabbit aorta.

Histamine H1-receptor antagonist mepyramine and H2-receptor antagonist metiamide, respectively diminished and potentiated angiotensin II(A II)-induced myotropic responses in the rabbit aortic strips. The responses of the octapeptide were also inhibited in the presence of histidine decarboxylase inhibitor, 2-hydroxy-5-carbomethoxybenzoxyamine and restored when subcontractile quantities of histamine are added to the inhibitor-containing medium. Inhibition of histamine degradation by aminoguanidine potentiates A II's responses. These results are taken as evidences indicating A II-induced histamine synthesis in the test preparation.

Possible involvement of endogenous histamine in the myotropic effect of clonidine on the isolated rabbit aorta.

Clonidine has a contractile effect in the isolated rabbit aorta which can be blocked by alpha-adrenergic antagonist, phentolamine. Histamine H1-receptor blocker, mepyramine, partly antagonizes its myotropic effect and histamine H2-blocker, metiamide, potentiates it, implying a histaminergic component in the response. Inhibition of histamine synthesis by histidine decarboxylase inhibitor, GYKI 11.121, reduces clonidine-induced contraction in this preparation, while diamine oxidase inhibition by aminoguanidine potentiates it. This is indirect evidence of the possibility of de novo histamine synthesis by clonidine, which may take part on the contractile effect of the drug in the rabbit aorta.