Endocrine and environmental influences upon plasma cortisol concentrations and plasma renin activity of the eel, Anguilla anguilla L.

The plasma concentrations of cortisol, sodium, potassium and calcium and plasma osmolarity were determined in freshwater silver eels, after intravascular injections of eel renin preparations, mammalian ACTH, mammalian angiotensin II and eel muscle extracts. Arterial blood specimens were taken before and after injection of test substances. Partially purified eel and rat renal renins gave prolonged pressor responses in intact and hypophysectomized eels and in the nephrectomized rat anaesthetized with sodium pentobarbitone. Angiotensin, but not ACTH, produced obvious pressor responses in intact and hypophysectomized eels and in eels without their corpuscles of Stannius. Hypophysectomized eels 4-8 days after operation had reduced plasma cortisol concentrations. No change in cortisol occurred in eels after removal of the corpuscles of Stannius. Eel renin preparations and ACTH gave increased concentrations of plasma cortisol 30 min after injection into hypophysectomized and intact eels. In general, the length of the renin-generated pressor response and the increased cortisol concentration were concomitant occurrences. Angiotensin injected into eels with corpuscles of Stannius removed and into hypophysectomized eels also increased cortisol levels. Control muscle extracts produced no significant changes. There were no acute changes in plasma electrolyte concentrations after the injections. Plasma renin activity measured indirectly by bioassay of angiotensin generated in vitro was more than twice as great in eels adapted to seawater than in eels in fresh water. Plasma renin activity gradually fell when eels were transferred from seawater to fresh water, and increased when the reverse transfer was carried out.