title: H2-histaminergic activity of clonidine in the guinea pig heart.
abstract: Clonidine perfusion (1 muM) or injection (9.4 to 150.4 nmoles) into the isolated perfused guinea pig heart caused an increase in contractile force, phosphorylase a and cyclic adenosine monophosphate (cyclic AMP)  levels. The effects of clonidine were blocked by burimamide (30 muM). Proranolol (1 muM), phentolamine (1 muM) or reserpine treatment (5 mg/kg i.p., 24 hours prior to the experiment) did not influence the cardiac effects of clonidine. Clonidine (0.1 - 100 muM) also produced a dose dependent positive inotropic effect on right ventricle strips but had a negative chronotropic effect on the spontaneously beating right atria at higher concentration (0.1 to 1.0 M). Burimamide did not block the clonidine-induced negative chronotropic effect. Clonidine did not increase contractile force in guinea pig left atria. It also did not influence the isoproterenol-and phenylephrine-induced increases in cardiac force of contraction but did antagonize the positive inotropic effect of 4-methylhistamine in the right ventricle. This observation suggests that clonidine may compete with 4-methylhistamine for the same receptor sites. The results indicate that the cardiac receptors for both biochemical and mechanical effects of clonidine are histamine receptors of the H2-type.
mesh_terms: D000818:Animals; D002049:Burimamide / Q000494:pharmacology; D003000:Clonidine / Q000494:pharmacology*; D000242:Cyclic AMP / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005260:Female; D006168:Guinea Pigs; D006321:Heart / Q000187:drug effects*; D006339:Heart Rate / Q000187:drug effects; D008297:Male; D008761:Methylhistamines / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D010646:Phentolamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D006005:Phosphorylases / Q000378:metabolism; D011433:Propranolol / Q000494:pharmacology; D012110:Reserpine / Q000494:pharmacology
chemical_list: D008761:Methylhistamines; D010656:Phenylephrine; D012110:Reserpine; D011433:Propranolol; D000242:Cyclic AMP; D006005:Phosphorylases; D003000:Clonidine; D002049:Burimamide; D010646:Phentolamine