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The Journal of Experimental Biology 205, 1889-1897 (2002)
© 2002 The Company of Biologists Limited

A Ca²⁺-sensing receptor modulates shark rectal gland function

Susan K. Fellner^1,2,* and Laurel Parker²

¹ Department of Cell and Molecular Physiology, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
² Mount Desert Island Biological Laboratory, Salisbury Cove, ME 04672, USA

^* Author for correspondence (e-mail: sfellner{at}med.unc.edu )

Accepted 17 April 2002

The elasmobranch Squalus acanthias controls plasma osmolality and extracellular fluid volume by secreting a hypertonic fluid from its rectal gland. Because we found a correlation between extracellular Ca²⁺ concentration and changes in cytosolic Ca²⁺ ([Ca²⁺]_i), we sought the possible presence of a calcium-sensing receptor in rectal gland artery and tubules. Cytosolic Ca²⁺ of both tissues responded to the addition of external Ca²⁺ (0.8-5.3 mmol l^-1) in a linear fashion. Spermine, Gd³⁺ and Ni²⁺, known agonists of the calcium-sensing receptor, increased [Ca²⁺]_i. To assess the participation of inositol triphosphate (IP₃) generation, sarcoplasmic/endoplasmic reticulum (SR/ER) Ca²⁺ depletion, and activation of store-operated Ca²⁺ entry, we utilized thapsigargin and ryanodine to deplete Ca²⁺ SR/ER stores and the inhibitory reagents TMB-8 and 2-APB to block IP₃ receptors. In each case, these agents inhibited the [Ca²⁺]_i response to agonist stimulation by approximately 50 %. Blockade of L-channels with nifedipine had no significant effect. Increases in ionic strength are known to inhibit the calcium-sensing receptor. We postulate that the CaSR stimulates Ca²⁺-mediated constriction of the rectal gland artery and diminishes cyclic AMP-mediated salt secretion in rectal gland tubules during non-feeding conditions. When the shark ingests sea water and fish, an increase in blood and interstitial fluid ionic strength inhibits the activity of the calcium-sensing receptor, relaxing the rectal gland artery and permitting salt secretion by the rectal gland tubules.

Key words: dogfish, Squalus acanthias, norepinephrine, procaine, cyclic AMP, ryanodine, rectal gland

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