Branchial and renal handling of urea in the gulf toadfish, Opsanus beta: The effect of exogenous urea loading

The objective of this study was to determine whether the pulsatile facilitated diffusion transport mechanism (tUT) found in the gills of the gulf toadfish (Opsanus beta) and the active secretion transporter thought to be present in its kidney could be saturated when faced with elevated plasma urea concentrations. Toadfish were infused with four consecutive exogenous urea loads at a rate of 0, 150, 300 and 600 μmol kg^-1 h^-1. Initial plasma and urine urea concentrations were 8.1±0.9 and 12.4±1.5 mmol l^-1, respectively, and steadily increased with increasing infused loads of urea to a maximum of 36.8±2.8 mmol l^-1 in the plasma and 39.8±6.5 mmol l^-1 in the urine. There was only a very weak relationship (r=0.17) between pulse size (measured as branchial excretion during pulsatile excretion of urea) and plasma urea concentration (slope=9.79 μmol-N kg^-1 per mmol-N l^-1; P<0.05) suggesting that the branchial excretion mechanism was already saturated at normal plasma urea concentrations. Urine flow rate (0.15±0.03 ml kg^-1 h^-1) and glomerular filtration rate (0.025±0.004 ml kg^-1 h^-1) remained constant throughout the experiment despite the increased volume load. Renal urea secretion rate maintained a strong linear relationship (r=0.84) to plasma urea levels (slope=0.391 μmol-N kg^-1 h^-1 per mmol-N l^-1; P<0.001) with no observable transport maximum, suggesting that the renal secretory transport mechanism was not saturated even at plasma urea levels well above normal, in contrast to the branchial excretion mechanism.