Cortisol, a Sodium Excreting Factor in the Eel (Anguilla anguilla L.) adapted to Sea Water

Abstract

ALTHOUGH aldosterone has been identified in those representatives of the Tetrapoda so far examined, it does not appear to be a natural hormone in teleost fish. In the eel^1,2 and in some other teleosts³ but not all⁴, cortisol is the major corticosteroid secreted by the adrenal cortex. The level of cortisol in peripheral blood can be significantly altered by such experimental manipulations as hypo-physectomy or shock^5,6. The role of the adrenal cortex in fish, however, especially its role in osmoregulatory mechanism, is still not clear^7–9, and only recently has a surgical procedure for adrenalectomy of the eel been described¹⁰. Furthermore, isotope techniques have now been devised which help in a better understanding of ion exchange mechanisms in the gill of fresh water or sea water fishes and in euryhaline forms during transfer from one medium to the other^11,12. The use of the “eel-tube” preparation¹³ allowed the study of adreno-cortical function in fresh water eels and those depleted of sodium by adaptation to distilled water^14–16. Adrenalectomy either by surgical or pharmacological means greatly reduced the net sodium adsorption through the gills. Aldosterone or low doses of cortisol within the physiological range restored or enhanced net sodium gain, while high doses of cortisol gave net sodium loss. It seems possible, therefore, that cortisol may act on the gills of fresh water eels to aid in the absorption of sodium. In sea water fish, while there is renal reabsorption of sodium^9,17, the gills are the site of extrarenal sodium excretion where a factor promoting sodium absorption would be irrelevant to the physiological needs in a medium hypertonic to the body fluids. The present report considers adrenocortical function in eels adapted to sea water and on transfer from one medium to the other.