ing adaption of fish to fresh water or sea-water is unknown. Unequivocal evidence
of their physiological action is indeed lacking. Measurements of their circulating
levels during such transitional adjustments would help substantiate such a role. ENSOR and BALL (1968a) have promised to measure circulating prolactin levels in fish,
and it seems more reasonable to think that this should also be po ssible for corticosteroids . This evidence must be awaited . In the meantime we can speculate as to
their roles in the physiological adjustments that are known to occur when fish are
transferred from sea-water to fresh water. The prompt decrease in branchial permeability to sodium, which is observed on such occasions, probably does not depend on hormones. Endocrines may, however, playa role in the subsequent 'delayed' secondary decrease in branchial permeability (MOTAIS et al., 1966). The
long-term adaptation of the Na-K activated ATPase levels and the sodiumexchange diffusion 'carrier' (or its inhibitor) could also be influenced by the
endocrines. Hypophysectomy has, for instance, been shown to interfere with
alterations in the branchial Na-K ATPase levels in killifish (EpSTEIN et al., 1967) ;
a deficiency that is restored by the injection of cortisol (PICKFORD et al., 1970).
The action of prolactin could be more banal and involve the integrity and secretion
of the branchial mucous glands that have been observed to degenerate after hypophysectomy (BURDEN, 1956; PICKFORD, PANG, and SAWYER, 1966). Other hormones could also be released during transitions between fresh water and sea-water.
It is notable that injections of vasotocin, corticotrophin and cortisol can facilitate
the exchange of sodium that occurs across the gills of the flounder when it is transferred from fresh water to sea-water (MOTAIS and MAETZ, 1967; MAYER and
MAETZ, 1967). Cortisol probably acts to stimulate ion extrusion from the 'chloride
secreting cells' while vasotocin may increase the blood supply to these cells (MAETZ
and RANKIN, 1969). A final judgement of the role of these ho rmones must however,
await measurement of their circulating levels in the various osmotic circumstances.
Preliminary results in Japanese eels show that plasma cortisol levels double when
they are transferred from fresh water to sea-water (HIRANO,1969). This effect ma y
be mediated through the pituitary as it was not seen when hypophysectomized
eels were treated in this manner. After 2 months in sea-water the circulating corticoid levels declined and did not change significantly when the eels were replaced
in fresh water.
While the kidneys appear to play little role in the adjustments of electrolyte
metabolism that accompany movements between fresh andsalt water, they are vitally concerned in regulating body water. In fresh water urine flow is high , in order
to excrete excess water, while in the sea it is low , reflecting its relative sparsity.
Although, as discussed earlier, vasotocin can in some circumstances increase the
flow of urine in freshwater fish this property is shared by other substances such
as adrenaline and angiotensin which also increase the blood pressure (CHESTER
JONES et al., 1969 b). As described below, the control of urine volume in fish ma y
well be mediated haemodynamically, and while vasoactive substances in the circulation could be involved in this, neural mechanisms would be equally feasible .
There has recently been a revival of interest in the role of vascular changes and
vasoactive substances in the osmoregulation of fish. KEYS and BATEMAN (1932)
found that adrenaline produced a vasodilatatory action in the perfused gills of the
eel, while vasopressin (from mammals) had little effect . When the gills were bathed
242
of their physiological action is indeed lacking. Measurements of their circulating
levels during such transitional adjustments would help substantiate such a role. ENSOR and BALL (1968a) have promised to measure circulating prolactin levels in fish,
and it seems more reasonable to think that this should also be po ssible for corticosteroids . This evidence must be awaited . In the meantime we can speculate as to
their roles in the physiological adjustments that are known to occur when fish are
transferred from sea-water to fresh water. The prompt decrease in branchial permeability to sodium, which is observed on such occasions, probably does not depend on hormones. Endocrines may, however, playa role in the subsequent 'delayed' secondary decrease in branchial permeability (MOTAIS et al., 1966). The
long-term adaptation of the Na-K activated ATPase levels and the sodiumexchange diffusion 'carrier' (or its inhibitor) could also be influenced by the
endocrines. Hypophysectomy has, for instance, been shown to interfere with
alterations in the branchial Na-K ATPase levels in killifish (EpSTEIN et al., 1967) ;
a deficiency that is restored by the injection of cortisol (PICKFORD et al., 1970).
The action of prolactin could be more banal and involve the integrity and secretion
of the branchial mucous glands that have been observed to degenerate after hypophysectomy (BURDEN, 1956; PICKFORD, PANG, and SAWYER, 1966). Other hormones could also be released during transitions between fresh water and sea-water.
It is notable that injections of vasotocin, corticotrophin and cortisol can facilitate
the exchange of sodium that occurs across the gills of the flounder when it is transferred from fresh water to sea-water (MOTAIS and MAETZ, 1967; MAYER and
MAETZ, 1967). Cortisol probably acts to stimulate ion extrusion from the 'chloride
secreting cells' while vasotocin may increase the blood supply to these cells (MAETZ
and RANKIN, 1969). A final judgement of the role of these ho rmones must however,
await measurement of their circulating levels in the various osmotic circumstances.
Preliminary results in Japanese eels show that plasma cortisol levels double when
they are transferred from fresh water to sea-water (HIRANO,1969). This effect ma y
be mediated through the pituitary as it was not seen when hypophysectomized
eels were treated in this manner. After 2 months in sea-water the circulating corticoid levels declined and did not change significantly when the eels were replaced
in fresh water.
While the kidneys appear to play little role in the adjustments of electrolyte
metabolism that accompany movements between fresh andsalt water, they are vitally concerned in regulating body water. In fresh water urine flow is high , in order
to excrete excess water, while in the sea it is low , reflecting its relative sparsity.
Although, as discussed earlier, vasotocin can in some circumstances increase the
flow of urine in freshwater fish this property is shared by other substances such
as adrenaline and angiotensin which also increase the blood pressure (CHESTER
JONES et al., 1969 b). As described below, the control of urine volume in fish ma y
well be mediated haemodynamically, and while vasoactive substances in the circulation could be involved in this, neural mechanisms would be equally feasible .
There has recently been a revival of interest in the role of vascular changes and
vasoactive substances in the osmoregulation of fish. KEYS and BATEMAN (1932)
found that adrenaline produced a vasodilatatory action in the perfused gills of the
eel, while vasopressin (from mammals) had little effect . When the gills were bathed
242