Alpha 2-adrenergic inhibition of Cl- transport by opercular epithelium is mediated by intracellular Ca2+.
We isolated the opercular epithelium of sea-water killifish (Fundulus heteroclitus) to study the mediation of catecholamine inhibition of Cl- secretion. The receptors are alpha 2-adrenergic, as they have a high affinity for the alpha 2-adrenergic agonist clonidine over phenylephrine and clonidine action is blocked by yohimbine. Pertussis toxin and indomethacin did not block the clonidine effect; hence inhibitory guanine nucleotide-binding proteins (Gi proteins) and prostaglandins (respectively) are not involved. Intracellular pH (pHi) of single chloride cells was measured microspectrofluorometrically and resting pHi was 7.22 +/- 0.03. However, pHi was unaffected by clonidine; hence pHi and Na+/H+ exchange are not involved. The lipoxygenase inhibitors nordihydroguaiaretic acid and baicalein and the lipoxygenase products (12S)- and (12R)-12-hydroxyeicosatetraenoic acid stimulated Cl- secretion. Protein kinase C is an unlikely site of action because the diacylglycerol kinase inhibitor R59022 had no effect alone and did not block the clonidine effect. Ionomycin (1 microM) in normal but not low-Ca2+ solutions mimicked the action of clonidine and both inhibitions were reversible by isoproterenol. Thapsigargin, a releaser of intracellular Ca2+, inhibited Cl- secretion and this effect was reduced in low-Ca2+ solutions. Low-Ca2+ solutions also blunted but did not block entirely the clonidine response, indicating that the primary Ca2+ release was from intracellular stores. Whereas alpha 1-adrenergic receptors commonly act via the Ca2+/inositol trisphosphate pathway, to our knowledge this is the first report of a Ca(2+)-mediated alpha 2-adrenergic response in a nonmammalian vertebrate.
Marshall
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Department of Biology, St. Francis Xavier University, Antigonish, NS, Canada.
Bryson
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Garg
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D019377:12-Hydroxy-5,8,10,14-eicosatetraenoic Acid; D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002712:Chlorides / Q000378:metabolism*; D003000:Clonidine / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004848:Epithelium / Q000187:drug effects / Q000502:physiology; D044950:Flavanones; D005419:Flavonoids / Q000494:pharmacology; D005880:Gills / Q000502:physiology*; D006863:Hydrogen-Ion Concentration; D006893:Hydroxyeicosatetraenoic Acids / Q000494:pharmacology; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology*; D007695:Killifishes; D007700:Kinetics; D009637:Masoprocol / Q000494:pharmacology; D037342:Pertussis Toxin; D010656:Phenylephrine / Q000494:pharmacology*; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D013729:Terpenes / Q000494:pharmacology; D019284:Thapsigargin; D010566:Virulence Factors, Bordetella / Q000494:pharmacology; D015016:Yohimbine / Q000494:pharmacology
D002712:Chlorides; D044950:Flavanones; D005419:Flavonoids; D006893:Hydroxyeicosatetraenoic Acids; D011942:Receptors, Adrenergic, alpha; D013729:Terpenes; D010566:Virulence Factors, Bordetella; D010656:Phenylephrine; D015016:Yohimbine; C006680:baicalein; D019377:12-Hydroxy-5,8,10,14-eicosatetraenoic Acid; D019284:Thapsigargin; D009637:Masoprocol; D037342:Pertussis Toxin; D000252:Calcium-Transporting ATPases; D007545:Isoproterenol; D003000:Clonidine; D002118:Calcium