Chronotropic and inotropic effects of histamine in developing chick heart: differential mechanisms before and after hatching

A role for histamine in cardiovascular regulation in late stage embryos of the red-footed tortoise, Chelonoidis carbonaria Spix, 1824

A chorioallantoic membrane artery in embryos of the red-footed tortoise, Chelonoidis carbonaria was occlusively cannulated for measurement of blood pressure and injection of drugs. Two age groups of embryos in the final 10 % of incubation were categorized by the ratio of embryonic body to yolk mass. All embryos first received cholinergic and β-adrenergic blockade. This revealed that β-adrenergic control was established in both groups whereas cholinergic control was only established in the older group immediately prior to hatching. The study then progressed as two series. Series one was conducted in a subset of embryos treated with histamine before or after injection of ranitidine, the antagonist of H2 receptors. Injection of histamine caused an initial phasic hypertension which recovered, followed by a longer lasting hypertensive response accompanied by a tachycardia. Injection of the H2 receptor antagonist ranitidine itself caused a hypotensive tachycardia with subsequent recovery of heart rate. Ranitidine also abolished the cardiac effects of histamine injection while leaving the initial hypertensive response intact. In series, two embryos were injected with histamine after injection of diphenhydramine, the antagonist to H1 receptors. This abolished the whole of the pressor response to histamine injection but left the tachycardic response intact. These data indicate that histamine acts as a non-adrenergic, non-cholinergic factor, regulating the cardiovascular system of developing reptilian embryos and that its overall effects are mediated via both H1 and H2 receptor types.

Inotropic effects of histamine on developing chick heart: release of transmitters from autonomic nerve terminals

Inotropic effects of histamine were examined in isolated ventricular preparations from late embryonic and hatched chick hearts. In 19 day-old embryonic preparations, histamine had little effect on the contractile force. In preparations from 1 to 2 day-old hatched chick, histamine produced a transient decrease in contractile force followed by a sustained increase. The negative and positive responses were antagonized by atropine and propranolol, respectively, but not by histamine antagonists terfenadine, cimetidine or thioperamide. Acetylcholine produced positive inotropic responses in the embryo while negative responses were observed after hatching. In myocardium of hatched chicks, compound 48/80, which releases histamine from mast cells, produced a transient decrease in contractile force followed by a sustained increase with a similar magnitude and time course to the case of exogenously applied histamine. The negative and positive responses were inhibited by atropine and propranolol, respectively, but not by terfenadine, cimetidine or thioperamide, which was similar to the case with the responses to histamine. The present results suggest that histamine, either applied exogenously or released from myocardial store sites, produces negative and positive inotropic responses in hatched chick myocardium which are due to release of acetylcholine and norepinephrine, respectively, from autonomic nerve terminals.