title: Loss of M2 muscarinic receptor function inhibits development of hypoxic bradycardia and alters cardiac beta-adrenergic sensitivity in larval zebrafish (Danio rerio).
abstract: Fish exposed to hypoxia develop decreased heart rate, or bradycardia, the physiological significance of which remains unknown. The general muscarinic receptor antagonist atropine abolishes the development of this hypoxic bradycardia, suggesting the involvement of muscarinic receptors. In this study, we tested the hypothesis that the hypoxic bradycardia is mediated specifically by stimulation of the M(2) muscarinic receptor, the most abundant subtype in the vertebrate heart. Zebrafish (Danio rerio) were reared at two levels of hypoxia (30 and 40 Torr PO(2)) from the point of fertilization. In hypoxic fish, the heart rate was significantly lower than in normoxic controls from 2 to 10 days postfertilization (dpf). At the more severe level of hypoxia (30 Torr PO(2)), there were significant increases in the relative mRNA expression of M(2) and the cardiac type beta-adrenergic receptors (beta1AR, beta2aAR, and beta2bAR) at 4 dpf. The hypoxic bradycardia was abolished (at 40 Torr PO(2)) or significantly attenuated (at 30 Torr PO(2)) in larvae experiencing M(2) receptor knockdown (using morpholino antisense oligonucleotides). Sham-injected larvae exhibited typical hypoxic bradycardia in both hypoxic regimens. The expression of beta1AR, beta2aAR, beta2bAR, and M(2) mRNA was altered at various stages between 1 and 4 dpf in larvae experiencing M(2) receptor knockdown. Interestingly, M(2) receptor knockdown revealed a cardioinhibitory role for the beta(2)-adrenergic receptor. This is the first study to demonstrate a specific role of the M(2) muscarinic receptor in the initiation of hypoxic bradycardia in fish.
authors.affiliation: Univ. of Ottawa, Dept. of Biology, Ottawa, ON, Canada K1N 6N5.
authors.forename: Shelby L
authors.initials: SL
authors.lastname: Steele
authors.forename: Kwok Hong Andy
authors.initials: KH
authors.lastname: Lo
authors.forename: Vincent Wai Tsun
authors.initials: VW
authors.lastname: Li
authors.forename: Shuk Han
authors.initials: SH
authors.lastname: Cheng
authors.forename: Marc
authors.initials: M
authors.lastname: Ekker
authors.forename: Steve F
authors.initials: SF
authors.lastname: Perry
mesh_terms: D058671:Adrenergic beta-1 Receptor Antagonists; D058672:Adrenergic beta-2 Receptor Antagonists; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000825:Animal Structures / Q000378:metabolism; D000818:Animals; D001919:Bradycardia / Q000139:chemically induced / Q000209:etiology* / Q000378:metabolism / Q000503:physiopathology; D001921:Brain / Q000378:metabolism; D002217:Carbachol / Q000494:pharmacology; D004625:Embryo, Nonmammalian / Q000378:metabolism; D015870:Gene Expression / Q000235:genetics; D006321:Heart / Q000187:drug effects / Q000503:physiopathology*; D006339:Heart Rate / Q000502:physiology; D000860:Hypoxia / Q000150:complications* / Q000378:metabolism / Q000503:physiopathology; D007814:Larva / Q000187:drug effects / Q000378:metabolism; D009206:Myocardium / Q000378:metabolism; D009638:Norepinephrine / Q000494:pharmacology; D016376:Oligonucleotides, Antisense / Q000008:administration & dosage / Q000235:genetics; D043585:Receptor, Muscarinic M2 / Q000502:physiology*; D011943:Receptors, Adrenergic, beta / Q000235:genetics / Q000378:metabolism*; D018342:Receptors, Adrenergic, beta-1 / Q000235:genetics; D018343:Receptors, Adrenergic, beta-2 / Q000235:genetics; D015027:Zebrafish / Q000378:metabolism*
chemical_list: D058671:Adrenergic beta-1 Receptor Antagonists; D058672:Adrenergic beta-2 Receptor Antagonists; D000319:Adrenergic beta-Antagonists; D016376:Oligonucleotides, Antisense; D043585:Receptor, Muscarinic M2; D011943:Receptors, Adrenergic, beta; D018342:Receptors, Adrenergic, beta-1; D018343:Receptors, Adrenergic, beta-2; D002217:Carbachol; D009638:Norepinephrine