5-Hydroxytryptamine initiates pulsatile urea excretion from perfused gills of the gulf toadfish (Opsanus beta)

Abstract

When stressed, toadfish become ureotelic and excrete almost all of their nitrogenous waste in 1–3 daily pulses of urea-N across the gills. Intravascular injections of 5-hydroxytyptamine (5-HT; serotonin) and analogues also elicit marked excretory pulses of urea-N from toadfish in vivo, suggesting that 5-HT release is the proximate trigger for spontaneous pulses. However it is unclear whether 5-HT is acting on the gills directly or elsewhere to cause the effect indirectly. A perfused whole gill preparation which maintained normal pressure relationships and stable vascular resistance was employed to address this question. Bolus injections into the ventral aortic perfusate of either 5-HT (1, 10μmolkg−1) or the specific 5-HT2 receptor agonist α-methyl 5-HT (1, 10μmolkg−1) elicited rapid urea-N pulses from perfused toadfish gills. The effective doses, the post-injection delays (5.5±1.3min, range=2–22), the percent occurrences (57–85%), and the magnitude of the induced urea-N pulses (615.4±131.3μmol-Nkg−1, range 66.0–2634.0), were all similar to those previously reported when these agents were injected in vivo. Bolus injections of 5-HT and α-methyl 5-HT also elicited a biphasic response in ventral aortic pressure, reflecting an initial rapid short-lived vasodilation and a subsequent longer-lasting vasoconstriction. These events were similar to those which have been recorded to occur at a greater frequency during spontaneous urea-N pulsing in vivo. Neither the urea-N pulsing nor the cardiovascular responses to 5-HT were inhibited by the 5-HT2A receptor subtype blocker, ketanserin (pre-injection with 10μmolkg−1 plus 33μmolL−1 in the perfusate). Overall, these results provide strong support for the idea that the proximate stimulus for natural urea pulsing in vivo is 5-HT mobilization, acting directly in the gills.