Ca super(2+) transport by opercular epithelium of the fresh water adapted euryhaline teleost, Fundulus heteroclitus

• Published in: Journal of comparative physiology. B, Biochemical, systemic, and environmental physiology Biochemical, systemic, and environmental physiology, 1995-01, Vol.165 (4), p.268-277
• Main Authors: Marshall, W S, Bryson, SE, Burghardt, J S, Verbost, P M
• Format: Article
• Language: English
• Subjects: Freshwater; Fundulus heteroclitus
• ISSN: 0174-1578

We examined transepithelial transport of Ca super(2+) across the isolated opercular epithelium of the euryhaline killifish adapted to fresh water. The opercular epithelium, mounted in vitro with saline on the serosal side and fresh water (0.1 nmol/l Ca super(2+)) bathing the mucosal side, actively transported Ca super(2+) in the uptake direction; net flux averaged 20-30 nmol/cm super(2)/h. The rate of Ca super(2+) uptake varied linearly with the density of mitochondria-rich cells in the preparations. Ca super(2+) uptake was saturable, apparent K sub( one half ) of 0.348 mmol/l, indicative of a multistep transcellular pathway. Ca super(2+) uptake was inhibited partially by apically added 0.1 mmol/l La super(3+) and 1.0 mmol/l Mg super(2+). Addition of dibutyryl-cyclic adenosine monophosphate (0.5 mmol/l) + 0.1 mmol/l 3-isobutyl-1-methylxanthine inhibited Ca super(2+) uptake by 54%, but epinephrine, clonidine and isoproterenol were without effect. Agents that increase intracellular Ca super(2+), thapsigargin (1.0 mu mol/l, serosal side), ionomycin (1.0 mu mol/l, serosal side) and the calmodulin blocker trifluoperazine (50 mu mol/l, mucosal side) all partially inhibited Ca super(2+) uptake. In contrast, apically added ionomycin increased mucosal to serosal unidirectional Ca super(2+) flux, indicating Ca super(2+) entry across the apical membrane is rate limiting in the transport. Verapamil (10-100 mu mol/l, mucosal side), a Ca super(2+) channel blocker, had no effect. Results are consistent with a model of Ca super(2+) uptake by mitochondria rich cells that involves passive Ca super(2+) entry across the apical membrane via verapamil-insensitive Ca super(2+) channels, intracellular complexing of Ca super(2+) by calmodulin and basolateral exit via an active transport process. Increases in intracellular Ca super(2+) invoke a downregulation of transcellular Ca super(2+) transport, implicating Ca super(2+) as a homeostatic mediator of its own transport.