title: The unusual adrenergic-like excitatory action of acetylcholine on the ventricular cardiac muscle of the horned shark, Heterodontus portusjacksoni.
abstract: The atypical excitatory effect of acetylcholine on cardiac ventricular muscle was investigated in the horned shark, Heterodontus portusjacksoni. Electrically paced ventricular strips produced a massive 391.45% (+/-26.39%) increase in basal force of contraction in response to exogenously applied acetylcholine. The response was similar in nature to that produced by applied adrenaline, which caused a 382.52% (+/-72.47%) increase. The response to acetylcholine was blocked by the muscarinic cholinoceptor antagonist atropine and the competitive beta-adrenoceptor antagonist propranolol and was reduced by bretylium, an agent known to inhibit the release of catecholamines from adrenergic nerves. These findings strongly suggest that acetylcholine mediates a localised release of a catecholamine via muscarinic cholinoceptors in shark heart. A cholinergically controlled catecholamine store has been proposed (cholinergic-adreno complex), implying that elasmobranchs may be capable of finer control of cardiac output than has previously been suspected. This complex may represent a transitional adrenergic state between humoral and neuronal regulation. The spontaneously beating atrium showed no evidence of such an excitatory response to applied acetylcholine but produced an atropine-sensitive slowing, a response typical of other vertebrates.
authors.affiliation: Department of Zoology, University of Western Australia, Nedlands, Australia.
authors.forename: A P
authors.initials: AP
authors.lastname: Thompson
authors.forename: J E
authors.initials: JE
authors.lastname: O'Shea
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology*; D018663:Adrenergic Agents / Q000494:pharmacology*; D000818:Animals; D016275:Atrial Function / Q000187:drug effects; D001285:Atropine / Q000494:pharmacology; D001949:Bretylium Compounds / Q000494:pharmacology; D002304:Cardiac Pacing, Artificial; D002909:Chronobiology Phenomena; D004837:Epinephrine / Q000494:pharmacology; D005260:Female; D008297:Male; D018727:Muscarinic Antagonists / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects; D011433:Propranolol / Q000494:pharmacology; D012754:Sharks / Q000502:physiology*; D016276:Ventricular Function / Q000187:drug effects*
chemical_list: D018663:Adrenergic Agents; D001949:Bretylium Compounds; D018727:Muscarinic Antagonists; D001285:Atropine; D011433:Propranolol; D000109:Acetylcholine; C045166:bretylium; D004837:Epinephrine