α2-Adrenergic Inhibition of Cl-Transport by Opercular Epithelium is Mediated by Intracellular Ca2

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 90; no. 12; pp. 5504 - 5508
• Main Authors: Marshall, William S., Bryson, Sharon E., Garg, Dipti
• Format: Journal Article
• Language: English
• Published: Washington, DC National Academy of Sciences of the United States of America 15.06.1993
• Subjects: Acidosis; Agonists; Biological and medical sciences; Chlorides; Diglycerides; Epithelial cells; Epithelium; Fundamental and applied biological sciences. Psychology; Metabolisms and neurohumoral controls; Receptors; Secretion; Toxins; Vertebrates: anatomy and physiology, studies on body, several organs or systems; Water and mineral metabolism. Osmoregulation. Acidobasic balance; Whooping cough; Marine environment; α2-Adrenergic receptor; Signal transduction; Vertebrata; Calcium; Secretion; Pisces; Ion transport; Chloride cell; Epithelium; Operculum; Fundulus heteroclitus
• ISSN: 0027-8424; 1091-6490

We isolated the opercular epithelium of seawater killifish (Fundulus heteroclitus) to study the mediation of catecholamine inhibition of Cl-secretion. The receptors are α2-adrenergic, as they have a high affinity for the α2-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 (Giproteins) and prostaglandins (respectively) are not involved. Intracellular pH (pHi) of single chloride cells was measured microspectrofluorometrically and resting pHiwas 7.22 ± 0.03. However, pHiwas unaffected by clonidine; hence pHiand 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 μM) 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 α1-adrenergic receptors commonly act via the Ca2+/inositol trisphosphate pathway, to our knowledge this is the first report of a Ca2+-mediated α2-adrenergic response in a nonmammalian vertebrate.