The regulatory role of glucocorticoid and mineralocorticoid receptors in pulsatile urea excretion of the gulf toadfish,Opsanus beta

Abstract

SUMMARYGulf toadfish, Opsanus beta, are one among a group of unusual teleosts that excrete urea as their predominant nitrogen end product in response to stressful conditions. Under conditions of crowding or confinement, fasted toadfish excrete the majority of their nitrogen waste in large pulses of urea (>90% of total nitrogen) lasting up to 3 h. An earlier study demonstrated that cortisol has an inhibitory influence on urea pulse size. The present study tested the hypothesis that cortisol mediates changes in urea pulse size in ureotelic toadfish through the glucocorticoid receptor (GR) and not the mineralocorticoid receptor (MR). In vivo pharmacological investigations were used to manipulate the corticosteroid system in crowded toadfish, including experimentally lowering plasma cortisol levels by the injection of metyrapone, blocking cortisol receptors through exposure to either RU-486 (GR antagonist) and spironolactone (MR antagonist), or through exogenous infusion of the tetrapod mineralocorticoid aldosterone (tetrapod MR agonist). The data demonstrate that lowering the activity of cortisol, either by inhibiting its synthesis or by blocking its receptor, resulted in a two-to threefold increase in pulse size with no accompanying change in pulse frequency. Treatment with spironolactone elicited a minor (~1.5-fold) reduction in pulse size, as did aldosterone treatment, suggesting that the antimineralocorticoid spironolactone has an agonistic effect in a piscine system. In summary, the evidence suggests that urea transport mechanisms in pulsing toadfish are upregulated in response to low cortisol, mediated primarily by GRs, and to a lesser extent MRs.

The Effect of Stress on Gill Basolateral Membrane Binding Kinetics of 5‐HT2 Receptor Ligands: Potential Implications for Urea Excretion Mechanisms

“…Similar to previous studies, crowding resulted in elevated cortisol levels in toadfish compared to toadfish kept in uncrowded conditions (Wood et al, ; McDonald et al, ). Metyrapone treatment was also successful in reducing circulating cortisol levels compared to saline‐injected controls as previously described by Rodela et al, . Also consistent with previous work, the 1 week of crowding experienced by crowded, saline‐treated controls and metyrapone‐treated fish of the present experiment resulted in an increase in plasma urea levels compared to presumably non‐ureotelic, uncrowded fish, likely as a result of an upregulation in urea production via the ornithine‐urea cycle (Walsh et al, ; Hopkins et al, ; Laberge et al, ).…”

“…However, the apparent differential effects of stress on [ 3 H]‐ketanserin binding mirrors the relationship that exists in toadfish between cortisol and urea excretion. Specifically, that chronic elevation of cortisol in response to a stressor initiates the upregulation in tUT mRNA expression (McDonald et al, ) as well as upregulates the basolateral membrane recruitment of tUT (Rodela et al, ). However, subsequent elevations in cortisol then inhibit membrane insertion and/or activation of tUT (Wood et al, ; McDonald et al, ; Rodela et al, ).…”

“…Specifically, that chronic elevation of cortisol in response to a stressor initiates the upregulation in tUT mRNA expression (McDonald et al, ) as well as upregulates the basolateral membrane recruitment of tUT (Rodela et al, ). However, subsequent elevations in cortisol then inhibit membrane insertion and/or activation of tUT (Wood et al, ; McDonald et al, ; Rodela et al, ). Therefore, we believe the findings of the present study may suggest that chronic stress associated with crowding results in an increase in 5‐HT 2A receptor number or basolateral membrane recruitment of the 5‐HT 2A receptor initially but then, similar to observations with tUT, sustained elevations in stress or circulating cortisol levels may inhibit the continued installment of the receptor to the basolateral membrane.…”

“…Glucocorticoids, specifically the stress hormone, cortisol, have been shown to play a key role in the regulation of this pulsatile process (Wood et al, ; McDonald et al, ; Rodela et al, ). While the chronic elevation of cortisol in response to a stressor initiates the production of urea (Walsh et al, ; Hopkins et al, ; Laberge et al, ) as well as an upregulation in tUT mRNA expression (McDonald et al, ) and initial membrane recruitment (Rodela et al, ), cortisol also inhibits pulsatile urea excretion as the activation or insertion of tUT into the gill membrane appears to only occur when cortisol levels are periodically reduced (Wood et al, ; McDonald et al, ; Rodela et al, ). The drop in cortisol alone does not trigger the urea pulse since a natural drop in cortisol can be observed without a corresponding pulse occurring (Wood et al, ).…”

Interactions between cortisol and Rhesus glycoprotein expression in ureogenic toadfish, Opsanus beta

“…The observed accumulation of plasma urea likely represented the net influence of enhanced urea production and decreased rates of urea excretion. Previous studies have reported that the amount of urea released during discrete urea pulse events is dependent on the concentration of plasma glucocorticoids; in particular, elevated cortisol inhibits tUT function McDonald et al, 2009;Rodela et al, 2009b).…”