Responses of isolated human epigastric arteries to histamine.

Responses to histamine have been studied on ring preparations of epigastric artery obtained from normal and from pregnancy-induced hypertensive (PIH) women to characterize the mode of action of histamine in this vascular preparation. In non-contracted arterial rings, histamine elicited concentration-dependent H1 receptor-mediated contractions, competitively antagonised by mepyramine and cicletanine with pA2 values of 9.1 and 7.5, respectively. Arterial rings from pregnancy-induced hypertensive patients displayed greater sensitivity to histamine, but no change in maximal contractions, and were (at the EC30 histamine response) more susceptible to antagonism by mepyramine and cicletanine. Following precontraction by noradrenaline, histamine elicited relaxation responses only in the presence of H1 receptor antagonists. Endothelium removal or exposure to methylene blue significantly attenuated histamine-induced relaxation; the residual relaxations under these conditions appear to be due to a direct effect on H2 receptor on smooth muscle cells. The results show that, in human epigastric artery, histamine elicits H1 receptor-mediated contractions and that, following NA precontraction and in the presence of H1 receptor blockade, relaxations occur. Arterial rings from pregnancy-induced hypertensive patients showed modest but somewhat greater sensitivity to histamine, as well as being more susceptible to cicletanine and mepyramine.

In vitro vascular effects of cicletanine in pregnancy-induced hypertension.

1. The vascular effects of cicletanine have been studied in vitro on ring preparations of inferior epigastric arteries from normotensive human females and human females with pregnancy-induced hypertension (preeclampsia). 2. Cicletanine (10(-7)-10(-3) M) elicited concentration-dependent relaxation of vessels precontracted with 10(-7) M noradrenaline (NA) or 60 mM K+ but was more potent in the former. Relaxation was significantly greater in rings from preeclamptic patients and was uninfluenced by endothelium removal. 3. The intracellular Ca-dependent contractile responses to 10(-5) M NA in Ca-free medium as well as the subsequent extracellular Ca-dependent contractions (on restoration of external Ca) were significantly attenuated dose-dependently by cicletanine (10(-5) M, 3 x 10(-4) M) in arterial rings from both normotensive and preeclamptic patients. Cicletanine also relaxed rings precontracted by 25 mM K+ but was ineffective against 80 mM K(+)-induced contractions. 4. The inhibition of intracellular Ca-dependent contractions was significantly greater in rings from preeclamptic than from normotensive patients whereas extracellular Ca-dependent contractions were comparably inhibited in both groups. Nifedipine, on the other hand, had little effect on the intracellular Ca-dependent contractions but significantly depressed extracellular Ca-dependent contractions. 5. Cicletanine-induced relaxation was uninfluenced by pretreatment with propranolol, ouabain, tetraethylammonium, procaine, indomethacin, cimetidine or tetrodotoxin but was antagonized by glibenclamide. 6. The results show that cicletanine inhibits contractile responses of human isolated inferior epigastric arteries by a mechanism unrelated to endothelial factors but associated with inhibition of calcium metabolism. An action of cicletanine on glibenclamide-sensitive K+ channels is also suggested. Cicletanine-induced inhibition was significantly greater in arteries from preclamptic patients.

Effects of cicletanine on histamine-induced contractions of isolated rabbit mesenteric arteries.

The antagonism by cicletanine of contractile responses to histamine has been examined in vitro on ring preparations of rabbit mesenteric arteries. Cicletanine (10(-8)-10(-6) M) caused a parallel rightward shift of histamine concentration response curve, with a pA2 value of 7.48 (slope = 0.89 +/- 0.19, not significantly different from unity). Histamine-induced contractions were nifedipine-sensitive and associated with cicletanine-sensitive increased 45Ca uptake. Endothelium removal resulted in enhanced contractile responses to histamine, but did not significantly modify cicletanine-induced antagonism: KB (dissociation constant) values for cicletanine antagonism in the presence or absence of endothelium were: 3.7 (+/- 0.1) X 10(-8) M and 3.6 (+/- 0.3) X 10(-8) M, respectively. Cicletanine (greater than 10(-4) M) also significantly attenuated 10 mM caffeine-induced contractions in rings exposed to Ca-free 100 mM K+ depolarizing medium. The results suggest that cicletanine-induced antagonisms of histamine H1 receptor-mediated contractions of rabbit mesenteric arteries is associated with interference with calcium entry as well as at high concentrations, release from intracellular stores.

Histamine H1-receptors mediate phosphoinositide and calcium response in cultured smooth muscle cells--interaction with cicletanine (CIC).

Smooth muscle cells were cultured from guinea-pig aorta and labelled with 45Ca++ and 32Pi to investigate the possible effect of cicletanine, a new antihypertensive drug, on the release of intracellular Ca++ and the metabolism of phosphoinositide induced by histamine. In 45Ca++ labelled cells, histamine increased in a dose-dependent manner the 45Ca++ efflux in the first two minutes. Stimulation of 45Ca++ release was observed with H1-agonist [2-pyridylethylamine dihydrochloride (2-PEA)] but not with H2-agonist (dimaprit). In addition, histamine- or 2-PEA- induced 45Ca++ efflux was inhibited by the H1-antagonists (mepyramine and terfenadine) whereas the H2-antagonist (cimetidine) was without effect. Similar results were obtained in 32Pi labelled cells; both H1-agonists (histamine and 2-PEA) increased the labelling of phosphoinositides. This effect was completely blocked by mepyramine. These results demonstrate that the histamine-induced stimulation of 45Ca++ efflux and phosphoinositide metabolism are mediated through H1-receptors. In the above systems, cicletanine was as effective as the H1-antagonist (mepyramine) with an IC50 of 10(-6) M for both 45Ca++ efflux and phosphoinositide metabolism. Blockade of these systems by cicletanine may be part of the mechanism by which this drug produces relaxation of blood vessels and may account for its in vivo antihypertensive action.

The role of histamine in the cardiovascular system.

This article reviews briefly the role of histamine through its H1 and H2 receptors on the cardiovascular system and its action on calcium and catecholamines. The analogy between the adrenergic and the histaminergic systems is well demonstrated and there is evidence that histamine participates in myocardial damage and arrythmias, but the question of its exact role in the early stages of cardiovascular diseases, such as myocardial ischaemia and atherosclerosis, requires further study.

Competitive and stereoselective histamine H1 antagonistic effect of cicletanide in guinea-pig isolated ileum.

The histamine H1 antagonistic effects of the racemic form and the enantiomers of cicletanide, a new antihypertensive furopyridine derivative, were investigated in guinea-pig isolated ileum. Both the racemic and the (-) enantiomer behaved as competitive histamine antagonists (pA2 values of 6.8 and 7.2, respectively). The (+) enantiomer was at least 100 times less potent than the (-) enantiomer. The H1-blocking effect of cicletanide is the most potent and is the only stereoselective property so far reported for the drug.