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First published online May 13, 2004
Journal of Experimental Biology 207, 2003-2010 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00957

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Dogmas and controversies in the handling of nitrogenous wastes: 5-HT₂-like receptors are involved in triggering pulsatile urea excretion in the gulf toadfish, Opsanus beta

M. Danielle McDonald^* and Patrick J. Walsh

Division of Marine Biology and Fisheries, NIEHS Marine and Freshwater Biomedical Science Center, Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, 33149-1098, USA

View larger version (15K): [in a new window]   Fig. 1. (A) Injection of 8-OH-DPAT (N=6) had no effect on the appearance of urea in the water. (B) Fish injected with 8-OH-DPAT did show a significant increase in plasma cortisol levels; however, this increase was also measured in the saline-injected controls (N=16; ANOVA, P<0.001). All values are means ± 1 S.E.M.

View larger version (13K): [in a new window]   Fig. 2. Injection of -methyl-5-HT caused a significant increase in (A) the appearance of urea in the water (N=27) with no corresponding increase in (B) the appearance of NH3 (N=6) or (C) the appearance of [3H]PEG 4000 (N=8). All values are means ± 1 S.E.M.; *P<0.05, significantly different from value at t=0.

View larger version (21K): [in a new window]   Fig. 3. Water samples taken every 5 min from a separate group of -methyl-5-HT-injected fish, demonstrating that the agonist injection results in a significant and pulsatile increase in the excretion of urea within 5 min. Inset shows coarser time scale for comparison with Fig. 2. All values are means ± 1 S.E.M. (N=6); *P<0.05, significantly different from value at t=0.

View larger version (20K): [in a new window]   Fig. 4. (A) Pre-injection with ketanserin caused a significant inhibition of the -methyl-5-HT-mediated effect. (B) Ketanserin mediated a dose-dependent decrease in pulse size. (C) The percentage of fish that pulsed following agonist injection. All values are means ± 1 S.E.M.; *P<0.05, significantly different from value at t=0; P<0.05 compared with -methyl-5HT injection alone.