Polyvalent cation receptor proteins (CaRs) are salinity sensors in fish

Area of Science:

• Comparative physiology
• Molecular biology
• Environmental adaptation

Background:

• Calcium-sensing receptors (CaRs) are known to detect divalent cations.
• Fish osmoregulation involves complex physiological adjustments to varying salinity levels.
• The role of CaRs in sensing environmental salinity in fish remained largely unexplored.

Purpose of the Study:

• To investigate whether calcium-sensing receptors (CaRs) function as salinity sensors in fish.
• To characterize the tissue expression patterns of CaRs in different fish species.
• To explore the functional implications of CaRs in fish osmoregulation.

Main Methods:

• Homology-based cloning of a dogfish shark kidney CaR cDNA.
• Expression studies in human embryonic kidney cells.
• Reverse transcriptase-PCR for tissue expression analysis in teleost fish.
• Functional assays involving CaR agonists in winter flounder urinary bladder.

Main Results:

• Shark kidney CaR senses combinations of Ca(2+), Mg(2+), and Na(+) ions at physiological concentrations.
• CaRs are expressed in multiple osmoregulatory organs in dogfish sharks, including kidney, rectal gland, gill, and brain.
• Similar CaR tissue expression patterns were observed in teleost fish (winter flounder and Atlantic salmon).
• CaR agonists inhibited volume transport in winter flounder urinary bladder, and plasma ion concentrations increased in Atlantic salmon upon seawater transfer.

Conclusions:

• Calcium-sensing receptors (CaRs) act as critical salinity sensors in both teleost and elasmobranch fish.
• CaR expression in osmoregulatory organs suggests a role in detecting and responding to changes in environmental salinity.
• Findings provide insights into CaR functions in fish osmoregulation and potentially in terrestrial animals.