Regulation of Bicarbonate Secretion in Marine Fish Intestine by the Calcium-Sensing Receptor

Regulation of Stanniocalcin Secretion by Calcium and PTHrP in Gilthead Seabream (Sparus aurata)

Calcium balance is of paramount importance for vertebrates. In fish, the endocrine modulators of calcium homeostasis include the stanniocalcin (STC), and some members of the parathyroid hormone (PTH) family, such as the PTH-related protein (PTHrP), acting as antagonists. STC is ubiquitously expressed in higher vertebrates. In turn, bony fish exhibit specific STC-producing glands named the corpuscles of Stannius (CS). Previous studies pointed to a calcium-sensing receptor (CaSR) involvement in the secretion of STC, but little is known of the involvement of other putative regulators. The CS provides a unique model to deepen the study of STC secretion. We developed an ex vivo assay to culture CS of fish and a competitive ELISA method to measure STC concentrations. As expected, STC released from the CS responds to CaSR stimulation by calcium, calcimimetics, and calcilytic drugs. Moreover, we uncover the presence (by PCR) of two PTHrP receptors in the CS, e.g., PTH1R and PTH3R. Thus, ex vivo incubations revealed a dose-response inhibition of STC secretion in response to PTHrP at basal Ca2+ concentrations. This inhibition is achieved through specific and reversible second messenger pathways (transmembrane adenylyl cyclases and phospholipase C), as the use of specific inhibitors highlights. Together, these results provide evidence for endocrine modulation between two antagonist hormones, STC and PTHrP.

Calcium-sensing receptor (CaSR) agonist R568 inhibits small intestinal motility of mice through neural and non-neural mechanisms

Gastrointestinal motility (GI) disorder causes symptoms such as dyspepsia, abdominal distention, and constipation and severely affects quality of life. The calcium (Ca²⁺)-sensing receptor (CaSR) expressed in the digestive tract can be activated by amino acids and participates in GI motility regulation. This study is designed to explore the effect and underlying mechanism of CaSR agonist R568 on the small intestine motility of mice in vivo and ex vivo. R568 was given to male C57BL/6 mice by gavage or incubated with isolated jejunum and ileum segments to observe its effects on GI motility and the involved neurons, neurotransmitters and hormones were detected by fluorescence immunohistochemistry and enzyme-linked immunosorbent assays. The in vivo results showed that the intestinal propulsive rate reduced in response to oral intake of R568. R568 treatment increased the numbers of nitric oxide synthase-positive neurons and nitric oxide release but decreased the choline acetyl transferase-positive neurons and acetylcholine release in the myenteric plexuses. R568 increased the secretion of cholecystokinin in the intestinal tissues and serum but had no effect on the secretion of glucagon like peptide-1. Ex vivo results showed that R568 inhibited the contractility of intestinal strips from the jejunum and ileum. Nitric oxide synthase (NOS) inhibitor L-nitroarginine methyl ester (L-NAME), M-receptor antagonist-atropine, and tetrodotoxin (TTX) failed to block the effect of R568. CaSR co-expressed with interstitial cells of Cajal (ICCs) in the myenteric plexus suggests the possibility that ICCs mediated the effect of R568. Our findings demonstrate that CaSR activation inhibited intestinal motility, and both the enteric nervous system and non-neural mechanism are involved in this process.

Salinity-dependent expression of calcium-sensing receptors in Atlantic salmon (Salmo salar) tissues

Multiple reports suggest that calcium-sensing receptors (CaSRs) are involved in calcium homeostasis, osmoregulation, and/or salinity sensing in fish (Loretz 2008, Herberger and Loretz 2013). We have isolated three unique full-length CaSR cDNAs from Atlantic salmon (Salmo salar) kidney that share many features with other reported CaSRs. Using anti-CaSR antibodies and PCR primers specific for individual salmon CaSR transcripts we show expression in osmoregulatory, neuroendocrine and sensory tissues. Furthermore, CaSRs are expressed in different patterns in salmon tissues where mRNA and protein expression are modified by freshwater or seawater acclimation. For example, in seawater, CaSR mRNA and protein expression is increased significantly in kidney as compared to freshwater. Electrophysiological recordings of olfactory responses produced upon exposure of salmon olfactory epithelium to CaSR agonists suggest a role for CaSRs in chemoreception in this species consistent with other freshwater, anadromous, and marine species where similar olfactory responses to divalent and polyvalent cations have been reported. These data provide further support for a role of CaSR proteins in osmoregulatory and sensory functions in Atlantic salmon, an anadromous species that experiences a broad range of environmental salinities in its life history.

Increased intestinal carbonate precipitate abundance in the sea bream (Sparus aurata L.) in response to ocean acidification

Marine fish contribute to the carbon cycle by producing mineralized intestinal precipitates generated as by-products of their osmoregulation. Here we aimed at characterizing the control of epithelial bicarbonate secretion and intestinal precipitate presence in the gilthead sea bream in response to predicted near future increases of environmental CO₂. Our results demonstrate that hypercapnia (950 and 1800 μatm CO₂) elicits higher intestine epithelial HCO₃^- secretion ex vivo and a subsequent parallel increase of intestinal precipitate presence in vivo when compared to present values (440 μatm CO₂). Intestinal gene expression analysis in response to environmental hypercapnia revealed the up-regulation of transporters involved in the intestinal bicarbonate secretion cascade such as the basolateral sodium bicarbonate co-transporter slc4a4, and the apical anion transporters slc26a3 and slc26a6 of sea bream. In addition, other genes involved in intestinal ion uptake linked to water absorption such as the apical nkcc2 and aquaporin 1b expression, indicating that hypercapnia influences different levels of intestinal physiology. Taken together the current results are consistent with an intestinal physiological response leading to higher bicarbonate secretion in the intestine of the sea bream paralleled by increased luminal carbonate precipitate abundance and the main related transporters in response to ocean acidification.

Molecular and functional regionalization of bicarbonate secretion cascade in the intestine of the European sea bass (Dicentrarchus labrax)

In marine fish the intestinal HCO₃^- secretion is the key mechanism to enable luminal aggregate formation and water absorption. Using the sea bass (Dicentrarchus labrax), the present study aimed at establishing the functional and molecular organization of different sections of the intestine concerning bicarbonate secretion and Cl^- movements. The proximal intestinal regions presented similar HCO₃^- secretion rates, while differences were detected in the molecular expression of the transporters involved and on regional HCO₃^- concentrations. The anterior region presented significantly higher Na⁺/K⁺-ATPase activity, Cl^- transepithelial transport and basolateral slc4a4, apical slc26a6 and slc26a3 expression levels. In the mid intestine, the total HCO₃^- content was significantly increased in the fluid as in the carbonate aggregates. In the rectum no HCO₃^- secretion was observed and was characterized by the diminished HCO₃^- total content, residual molecular expression of slc4a4, slc26a6 and slc26a3, higher H⁺-ATPase activity and expression, suggesting the existence of a different bicarbonate handling mechanism. The possible regulation of HCO₃^- secretion by extracellular HCO₃^- and increased intracellular cAMP levels were also investigated. cAMP did not affect HCO₃^- secretion, although Cl^- secretion was enhanced by cftr. HCO₃^- secretion rise due to the HCO₃^- basolateral increment showed that at resting levels slc4a4 was not a limiting step for secretion. The transcellular/intracellular dependence of apical HCO₃^- secretion differed between the proximal regions. In conclusion, intestinal HCO₃^- secretion has a functional region-dependent organization that was not reflected by the anterior-posterior regionalization on HCO₃^- secretion and expression profiles of chloride/water absorption related genes.

Zinc uptake in fish intestinal epithelial model RTgutGC: Impact of media ion composition and methionine chelation

Apical uptake of zinc as ionic Zn(II) or as Zn-methionine (Zn-Met) was studied in RTgutGC cell line in vitro under media compositions mirroring the gut luminal ionic concentration of freshwater (FW) and seawater (SW) acclimated salmonids. Viability of the RTgutGC cells exposed to experimental media preparations showed a time-dependent decrease in SW treated cells, with the effect being significant at 48 h (P < 0.01), but not at 12 h or 24 h. Half effective concentration of Zn exposure over 12 h (EC₅₀, in μM) was not differentially affected by media composition (FW, 59.7 ± 12.1 or SW, 83.2 ± 7.2; mean ± SE, P = 0.43). Zinc (⁶⁵Zn) influx in RTgutGC was not different between FW or SW treated cells, but increased significantly in the presence of methionine (2 mM, L-Met or DL-Met). An interaction effect was observed between Zn concentration and media ionic composition on the impact of Met on apical Zn uptake (L-met, P < 0.001; DL-met, P = 0.02). In the presence of Met, apical Zn uptake in SW medium was significantly lower compared to FW, but only at higher Zn concentrations (12 and 25 μM, P < 0.01). Further, Met facilitated Zn uptake was reduced in cells treated with an amino acid transport system blocker with the effect being more significant and stereospecific in SW ionic conditions. The findings of this study showed that (i) Zn speciation in the presence of Met improved apical Zn uptake in RTgutGC cells and Zn-Met species were possibly taken up through Met uptake system. (ii) The effect was differentially affected by the ionic composition of the medium. Implications and limitations of the observations towards practical Zn nutrition of salmonids are discussed.