From Freshwater Research Articles

Control of renal and adrenocortical function by the renin-angiotensin system in two euryhaline teleost fishes

Plasma ions and cortisol levels were measured sequentially during the adaptation of European eels ( Anguilla anguilla) from fresh water (FW) to sea water (SW). The importance of the renin-angiotensin system in the regulation of this adaptation was assessed using captopril (SQ14225, an inhibitor of angiotensin I-converting enzyme). The effects of captopril on renal function in FW- and SW-adapted trout were also examined. During the first 5 hr in sea water, plasma levels of cortisol in eels increased threefold, plasma sodium rose steadily from 137 to 156 mmol/l and plasma potassium fell from 2.1 to 1.6 mmol/l. In contrast, captopril-treated eels when adapted to sea water had plasma cortisol levels twice those of controls. Captopril treatment did not affect the electrolyte responses to seawater adaptation. Captopril injected into eels which were fully adapted to and wholly maintained in sea water had no effect on plasma levels of cortisol, sodium, and potassium. Plasma cortisol was 30% lower in freshwater eels 2 hr after an injection of captopril but plasma sodium and potassium levels were unchanged. In both FW- and SW-adapted trout, captopril infusions doubled the glomerular filtration and urine production rates and the tubular transport maxima for glucose without changes in plasma composition.

Cortisol and its effects on plasma thyroid hormone and electrolyte concentrations in fresh water and during seawater acclimation in yearling coho salmon, Oncorhynchus kisutch

Plasma cortisol, thyroxine (T 4), and triiodothyronine (T 3) concentrations increased during seawater (SW) acclimation in yearling coho salmon, Oncorhynchus kisutch. Maximal concentrations of cortisol (220 ng/ml) occurred within 1.5 hr after the ambient water was changed from fresh water (FW) to SW; after 21 days in SW, cortisol levels were still slightly elevated (23 ng/ml) compared to those in FW fish (8 ng/ml). Plasma T 4 concentrations peaked (14 ng/ml) at 12 and 72 hr after exposure to SW, and they were higher than those in FW fish (4 ng/ml) at all sample times. Maximal concentrations of T 3 (8 ng/ml) occurred within 12 hr after exposure to SW, followed by a return to FW control levels (4 ng/ml) within 24 hr. Chronic treatment with cortisol significantly lowered plasma T 3 concentrations in FW and during SW exposure, but it had no significant effect on T 4 concentrations. Cortisol treatment lowered gill NaK-ATPase activity in FW fish, but it did not affect plasma osmolarity, Na, K, Ca, or Mg in fish in FW or during SW acclimation.

The significance of cortisol for osmoregulation in carp ( Cyprinus carpio) and tilapia ( Sarotherodon mossambicus)

Changes in plasma cortisol and glucose concentration were studied in carp during acclimation from fresh water (FW) to 1.5% salt water and vice versa. There was an increase in cortisol and glucose concentration during acclimation from FW to salt water which lasted for several days. Reacclimation to FW did not cause clear changes in cortisol and glucose levels. One single injection of cortisol (0.2 mg/100 g or 1 mg/100 g) and additional transfer to salt water (1.5% for carp and 2.7% for tilapia) altered the changes caused by acclimation alone of cortisol, glucose, Na + concentration, and the osmolality in plasma. Gill NaK-ATPase activity was also influenced. The effects of cortisol on electrolyte concentrations during acclimation and on Na +K +-ATPase activity differed in both types of fish. Cortisol clearly lowered the increase in plasma Na + concentration of the stenohaline carp and increased the ATPase activity. The changes in plasma Na + concentration of the euryhaline tilapia was not clearly altered and the enzyme activity was inhibited. The significance of these cortisol effects is discussed.

The ultrastructure of the cell types and of the neurosecretory innervation in the pituitary of Mugil cephalus L. from fresh water, the sea, and a hypersaline lagoon : I. The rostral pars distalis

The ultrastructure of the cells of the rostral pars distalis and their neurosecretory innervation were studied in the pituitaries of Mugil cephalus from fresh water (FW), the sea (SW), and a hypersaline lagoon (HL). The secretory granules of the eta cells have a median diameter of 2600 Å in FW, 2300 Å in SW, and 1800 Å in HL. Large-sized nucleus and nucleolus, a well developed endoplasmic reticulum, and abundance in secretory granules in FW eta cells were interpreted as indicating high synthetic activity, while scarcity of granules, decrease of cellular size as well as reduction of the volume of the RPD in HL specimens were interpreted as indicating high release and inadequate synthesis. An intercellular channel system formed by specialized channel cells in the eta cell region is described. In the epsilon cell region, a channel system is formed by invaginating membranes of the epsilon cells. Neurosecretory granules in the rostral neurohypophysis are of two types: (1) presumably peptidergic, which accompany the capillaries and probably most of them empty into the capillaries before the latter penetrate the epithelial region; and (2) presumably aminergic granules which are close to the interface which separates the neurohypophysis from the rostral pars distalis. Neurosecretory elements in the adenohypophyseal region between the eta cells, are of the aminergic type. The possible roles of the different neurosecretory granules are discussed.

Measurement of plasma cortisol levels in the eel Anguilla anguilla in relation to osmotic adjustments.

SUMMARY The competitive protein-binding radioassay (CPB method) of Murphy (1967) has been adapted to determine total cortisol levels in the plasma of the eel, Anguilla anguilla L. Validation of the method for this species depended in part on the development of a chromatographic—fluorimetric technique for eel cortisol, following classical procedures and using radioactive tracers; by this means, the specificity of the CPB method for cortisol in eel plasma was established. Accuracy, precision and sensitivity of the CPB method were also investigated and were shown to be satisfactory. Plasma total cortisol levels were determined in eels during osmotic adjustments after transfers from fresh water (FW) to sea-water (SW) and vice versa, and from FW to distilled water. Plasma osmotic pressure and/or sodium levels were monitored simultaneously, to follow the progress of osmotic regulation. In only one of the transfer situations did the plasma cortisol level change significantly, showing a marked transitory increase during the first few days after transfer from FW to SW, corresponding to the development and correction of an 'osmotic crisis'. Plasma cortisol levels were the same in eels adapted for long periods to FW and to SW. Plasma cortisol fell to extremely low levels after hypophysectomy. These results are discussed in the light of the literature on hormonal control of osmoregulatory mechanisms in the eel, with particular emphasis on the role of adrenocorticosteroids in ionic regulation of animals in SW.