Abstract
In marine fish, high epithelial bicarbonate secretion by the intestine generates luminal carbonate precipitates of divalent cations that play a key role in water and ion homeostasis. In vitro studies highlight the involvement of the calciotropic hormones PTHrP (parathyroid hormone-related protein) and stanniocalcin (STC) in the regulation of epithelial bicarbonate transport. The present study tested the hypothesis that calciotropic hormones have a regulatory role in carbonate precipitate formation in vivo. Sea bream (Sparus aurata) juveniles received single intraperitoneal injections of piscine PTHrP(1-34), the PTH/PTHrP receptor antagonist PTHrP(7-34) or purified sea bream STC, or were passively immunized with polyclonal rabbit antisera raised against sea bream STC (STC-Ab). Endocrine effects on the expression of the basolateral sodium bicarbonate co-transporter (Slc4a4.A), the apical anion exchangers Slc26a6.A and Slc26a3.B, and the V-type proton pump β-subunit (Atp6v1b) in the anterior intestine were evaluated. In keeping with their calciotropic nature, the hypocalcaemic factors PTHrP(7-34) and STC up-regulated gene expression of all transporters. In contrast, the hypercalcaemic factor PTHrP(1-34) and STC antibodies down-regulated transporters involved in the bicarbonate secretion cascade. Changes in intestine luminal precipitate contents provoked by calcaemic endocrine factors validated these results: 24 h post-injection either PTHrP(1-34) or immunization with STC-Ab reduced the carbonate precipitate content in the sea bream intestine. In contrast, the PTH/PTHrP receptor antagonist PTHrP(7-34) increased not only the precipitated fraction but also the concentration of HCO3(-) equivalents in the intestinal fluid. These results confirm the hypothesis that calciotropic hormones have a regulatory role in carbonate precipitate formation in vivo in the intestine of marine fish. Furthermore, they illustrate for the first time in fish the counteracting effect of PTHrP and STC, and reveal an unexpected contribution of calcaemic factors to acid-base balance.
Highlights
Marine teleosts sustain an ionic equilibrium with seawater to keep their plasma osmolality within narrow limits
In addition to regulation of the amount of water ingested by endocrine and environmental factors (Fuentes et al, 1996; Fuentes and Eddy, 1997a; Fuentes and Eddy, 1997b; Guerreiro et al, 2004; Guerreiro et al, 2001), the processing of imbibed fluid has a major impact in fish ion regulation
STC antiserum validation Western blot analysis of sea bream serum with an antibody generated against purified sea bream STC identified a single immunoreactive band (Fig. 1)
Summary
Marine teleosts sustain an ionic equilibrium with seawater to keep their plasma osmolality within narrow limits. Water replacement by drinking becomes essential to sustain ion regulation. In addition to regulation of the amount of water ingested by endocrine and environmental factors (Fuentes et al, 1996; Fuentes and Eddy, 1997a; Fuentes and Eddy, 1997b; Guerreiro et al, 2004; Guerreiro et al, 2001), the processing of imbibed fluid has a major impact in fish ion regulation. Ingested water is first processed in the oesophagus, where it undergoes selective absorption of NaCl (Hirano and Mayer-Gostan, 1976; Parmelee and Renfro, 1983), which lowers fluid osmolality and is believed to enhance water absorption in the intestine
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