a role in maintaining the integrity of the processes of sodium transport. Prolactin
may also influence the secretion from the avian salt gland since injections of ovine
prolactin have been found to facilitate the flow of this secretion in ducks (PEAKER,
PHILLIPS, and WRIGHT, 1970). It has been suggested that the release of this hormone may be associated with the migration of these birds from fresh water to marine
estuaries where it may facilitate the salt gland activity when the birds commence
to drink sea-water.
In summary, the process of salt secretion by the avian nasal glands would appear
to be initiated by stimulation of osmoreceptors associated with the nervous system,
and nerve impulses pass from these to the parasympathetic nerves supplying the
gland. Secretion follows, and this leads to a secondary vasodilatation in the gland,
which results in the admission of additional corticosterone to the secretory tissue.
This steroid may be necessary for the optimal function of the sodium transport
mechanisms involved, especially if they are to work at a high rate .
4. Adrenocortical Hormones and Salt Metabolism
A variety of corticosteroids have been isolated from the blood of birds, and from
the media incubating slices of their adrenocortical tissues (in vitro). The levels of
some of these steroids are very low so that there is doubt as to their exact identity.
The principal steroids that have been isolated, and which probably playa physiological role, are corticosterone and aldosterone (Table 4.3). Aldosterone can be
produced during corticosterone synthesis, so that there are even some doubts as
to its hormonal role, especially as it has only been identified in vivo in chicken blood
and at low concentrations. Corticosterone is the principle corticosteroid secreted
tn VIVO.
Corticosterone is produced at increased rates in the presence of mammalian
corticotrophin, both in vitro and in vivo. The avian hormone does not appear to
have been tested for such an action. Nevertheless, the bird adrenal has a substantial
degree of autonomy from the adenohypophysis. Indeed, at one time it appeared
that it may be almost completely independent of the pituitary, as injections of corticotrophin, or hypophysectomy, often failed to produce changes in the weight
of the adrenals, or the levels of cholesterol and ascorbic acid present (see for instance
ZARROW and BALDINI, 1952; NEWCOMER, 1959). This contrasts with observations
in mammals. These criteria of avian adrenocortical function are, however, somewhat misleading; the weight of the duck adrenal is unchanged after adenohypophysectomy but histological examination shows an atrophy of the inner parts of the
gland. (WRIGHT, PHILLIPS, and HUANG, 1966). When chickens are hypophysectomized, the circulating levels of corticosterone decrease by 40%, while injection
of corticotrophin (mammalian) elevates the levels of the steroid in the plasma
(NAGRA et a!', 1963). Corticotrophin also has this effect in pheasants, and indirect
evidence indicates that it has the same role in ducks (HOLMES, PHILLIPS, and BUTLER, 1961), in which it also increases the weight of the adrenal. When adrenocortical
tissue slices from chickens, ducks, pigeons and western gulls are exposed to corticotrophin in vitro, there is an increased rate of corticosterone production (DERoos, 1961) but aldosterone formation is little affected. In duck adrenal tissues,
125
may also influence the secretion from the avian salt gland since injections of ovine
prolactin have been found to facilitate the flow of this secretion in ducks (PEAKER,
PHILLIPS, and WRIGHT, 1970). It has been suggested that the release of this hormone may be associated with the migration of these birds from fresh water to marine
estuaries where it may facilitate the salt gland activity when the birds commence
to drink sea-water.
In summary, the process of salt secretion by the avian nasal glands would appear
to be initiated by stimulation of osmoreceptors associated with the nervous system,
and nerve impulses pass from these to the parasympathetic nerves supplying the
gland. Secretion follows, and this leads to a secondary vasodilatation in the gland,
which results in the admission of additional corticosterone to the secretory tissue.
This steroid may be necessary for the optimal function of the sodium transport
mechanisms involved, especially if they are to work at a high rate .
4. Adrenocortical Hormones and Salt Metabolism
A variety of corticosteroids have been isolated from the blood of birds, and from
the media incubating slices of their adrenocortical tissues (in vitro). The levels of
some of these steroids are very low so that there is doubt as to their exact identity.
The principal steroids that have been isolated, and which probably playa physiological role, are corticosterone and aldosterone (Table 4.3). Aldosterone can be
produced during corticosterone synthesis, so that there are even some doubts as
to its hormonal role, especially as it has only been identified in vivo in chicken blood
and at low concentrations. Corticosterone is the principle corticosteroid secreted
tn VIVO.
Corticosterone is produced at increased rates in the presence of mammalian
corticotrophin, both in vitro and in vivo. The avian hormone does not appear to
have been tested for such an action. Nevertheless, the bird adrenal has a substantial
degree of autonomy from the adenohypophysis. Indeed, at one time it appeared
that it may be almost completely independent of the pituitary, as injections of corticotrophin, or hypophysectomy, often failed to produce changes in the weight
of the adrenals, or the levels of cholesterol and ascorbic acid present (see for instance
ZARROW and BALDINI, 1952; NEWCOMER, 1959). This contrasts with observations
in mammals. These criteria of avian adrenocortical function are, however, somewhat misleading; the weight of the duck adrenal is unchanged after adenohypophysectomy but histological examination shows an atrophy of the inner parts of the
gland. (WRIGHT, PHILLIPS, and HUANG, 1966). When chickens are hypophysectomized, the circulating levels of corticosterone decrease by 40%, while injection
of corticotrophin (mammalian) elevates the levels of the steroid in the plasma
(NAGRA et a!', 1963). Corticotrophin also has this effect in pheasants, and indirect
evidence indicates that it has the same role in ducks (HOLMES, PHILLIPS, and BUTLER, 1961), in which it also increases the weight of the adrenal. When adrenocortical
tissue slices from chickens, ducks, pigeons and western gulls are exposed to corticotrophin in vitro, there is an increased rate of corticosterone production (DERoos, 1961) but aldosterone formation is little affected. In duck adrenal tissues,
125