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THIRST AND VOLUME, ELECTROLYTE HOMEOSTASIS

Cortisol alters carbonic anhydrase-mediated renal sulfate secretion

¹Department of Physiology and Neurobiology, University of Connecticut, Storrs, Connecticut 06269; and ²Mount Desert Island Biological Laboratory, Salisbury Cove, Maine 04672

Submitted 17 June 2003 ; accepted in final form 5 August 2003

Active transepithelial sulfate secretion rate by winter flounder renal proximal tubule epithelium in primary culture (fPTC) is dependent on intracellular carbonic anhydrase (CA) and enhanced by cortisol. To further evaluate this relationship, a partial cDNA clone (327 bp) of carbonic anhydrase II (CAII) with high sequence similarity to CAII from numerous species including fish, chicken, and human was obtained from fPTCs. The majority of CA activity and CAII protein was present in the cytosol of fPTCs; however, significant amounts of both (in addition to SDS-resistant CA activity, i.e., CAIV-like isoform) were present in concentrated plasma membranes. CAII from concentrated membranes migrated differently than purified CAII on nondenaturing PAGE gels, suggesting that CAII associates with another membrane component. Treatment of fPTCs with the cell-soluble CA inhibitor methazolamide (100 µM) caused a 58% reduction in active transepithelial SO₄^2- secretion. fPTCs that were previously cultured under high-cortisol concentrations, when subjected to 5 days of low physiological levels of cortisol, had decreased CA activity (28%), CAII protein abundance (65%), and net active SO₄^2- secretion (28%), with no effect on epithelial differentiation. Methazolamide and low-cortisol treatment in combination inhibited net active SO₄^2- secretion 56%, which was not different than the effect of methazolamide treatment alone. These data indicate that cortisol directly increases renal CA activity, CAII protein abundance, and CA-dependent SO₄^2- secretion in the marine teleost renal proximal tubule.

renal proximal tubule; marine teleost; carbonic anhydrate II; primary culture

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