A Ca(2+)-sensing receptor modulates shark rectal gland function

Area of Science:

• Physiology
• Comparative Endocrinology
• Ion Transport

Background:

• Elasmobranchs like Squalus acanthias maintain plasma osmolality via rectal gland secretion.
• A correlation between extracellular and intracellular calcium ([Ca2+]i) suggests a role for calcium-sensing mechanisms.

Purpose of the Study:

• Investigate the presence and function of a calcium-sensing receptor (CaSR) in the rectal gland artery and tubules of Squalus acanthias.
• Determine the signaling pathways involved in CaSR-mediated regulation of rectal gland function.

Main Methods:

• Measured cytosolic Ca2+ ([Ca2+]i) in rectal gland tissues in response to varying extracellular Ca2+ and agonists.
• Utilized specific inhibitors (thapsigargin, ryanodine, TMB-8, 2-APB, nifedipine) to probe signaling pathways including IP3 generation and Ca2+ store depletion.
• Assessed the effect of ionic strength on CaSR activity.

Main Results:

• Extracellular Ca2+ linearly increased [Ca2+]i in rectal gland tissues.
• CaSR agonists (spermine, Gd3+, Ni2+) elevated [Ca2+]i, with responses partially inhibited (~50%) by agents blocking IP3 generation and Ca2+ stores.
• Nifedipine blockade of L-channels had no significant effect on [Ca2+]i response.

Conclusions:

• A functional calcium-sensing receptor (CaSR) is present in the rectal gland artery and tubules of Squalus acanthias.
• CaSR activation leads to Ca2+-mediated vasoconstriction and reduced salt secretion, likely during non-feeding periods.
• Increased ionic strength, associated with feeding, inhibits CaSR, promoting rectal gland salt secretion.