Transport physiology of the urinary bladder in teleosts: a suitable model for renal urea handling?

The transport physiology of the urinary bladder of both the freshwater rainbow trout (Oncorhychus mykiss) and the marine gulf toadfish (Opsanus beta) was characterized with respect to urea, and the suitability of the urinary bladder as a model for renal urea handling was investigated. Through the use of the in vitro urinary bladder sac preparation urea handling was characterized under control conditions and in the presence of pharmacological agents traditionally used to characterize urea transport such as urea analogues (thiourea, acetamide), urea transport blockers (phloretin, amiloride), and hormonal stimulation (arginine vasotocin; AVT). Na(+)-dependence and temperature sensitivity were also investigated. Under control conditions, the in vitro trout bladder behaved as in vivo, demonstrating significant net reabsorption of Na(+), Cl(-), water, glucose, and urea. Bladder urea reabsorption was not affected by pharmacological agents and, in contrast to renal urea reabsorption, was not correlated to Na(+). However, the trout bladder showed a threefold greater urea permeability compared to artificial lipid bilayers, a prolonged phase transition with a lowered E(a) between 5 degrees C and 14 degrees C, and differential handling of urea and analogues, all suggesting the presence of a urea transport mechanism. The in vitro toadfish bladder did not behave as in vivo, showing significant net reabsorption of Na(+) but not of Cl(-), urea, or water. As in the trout bladder, pharmacological agents were ineffective. The toadfish bladder showed no differential transport of urea and analogues, consistent with a low permeability storage organ and intermittent urination. Our results, therefore, suggest the possibility of a urea transport mechanism in the urinary bladder of the rainbow trout but not the gulf toadfish. While the bladders may not be suitable models for renal urea handling, the habit of intermittent urination by ureotelic tetrapods and toadfish seems to have selected for a low permeability storage function in the urinary bladder.