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Am J Physiol Regul Integr Comp Physiol 297: R313-R322, 2009. First published May 20, 2009; doi:10.1152/ajpregu.90894.2008
0363-6119/09 $8.00
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ARTICLES

Cortisol-sensitive urea transport across the gill basolateral membrane of the gulf toadfish (Opsanus beta)

Tamara M. Rodela,1 Kathleen M. Gilmour,1 Patrick J. Walsh,1 and M. Danielle McDonald2

1Department of Biology, University of Ottawa, Ottawa, Ontario, Canada; and 2Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida

Submitted 3 November 2008 ; accepted in final form 20 May 2009

Gulf toadfish (Opsanus beta) use a unique pulsatile urea excretion mechanism that allows urea to be voided in large pulses via the periodic insertion or activation of a branchial urea transporter. The precise cellular and subcellular location of the facilitated diffusion mechanism(s) remains unclear. An in vitro basolateral membrane vesicle (BLMV) preparation was used to test the hypothesis that urea movement across the gill basolateral membrane occurs through a cortisol-sensitive carrier-mediated mechanism. Toadfish BLMVs demonstrated two components of urea uptake: a linear element at high external urea concentrations, and a phloretin-sensitive saturable constituent (Km = 0.24 mmol/l; Vmax = 6.95 µmol·mg protein–1·h–1) at low urea concentrations (<1 mmol/l). BLMV urea transport in toadfish was unaffected by in vitro treatment with ouabain, N-ethylmaleimide, or the absence of sodium, conditions that are known to inhibit sodium-coupled and proton-coupled urea transport in vertebrates. Transport kinetics were temperature sensitive with a Q10 > 2, further suggestive of carrier-mediated processes. Our data provide evidence that a basolateral urea facilitated transporter accelerates the movement of urea between the plasma and gills to enable the pulsatile excretion of urea. Furthermore, in vivo infusion of cortisol caused a significant 4.3-fold reduction in BLMV urea transport capacity in lab-crowded fish, suggesting that cortisol inhibits the recruitment of urea transporters to the basolateral membrane, which may ultimately affect the size of the urea pulse event in gulf toadfish.

basolateral plasma membrane; urea permeability; facilitated diffusion; phloretin; membrane vesicles


Address for reprint requests and other correspondence: Tamara M. Rodela, Dept. of Biology, Univ. of Ottawa, Ottawa, Ontario, Canada K1N 6N5 (e-mail: trode075{at}uottawa.ca)






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