3. FISH PROLACTIN AND GROWTH HORMONE
215
effective ACTH therapy (Ball and Ensor, 1969). It is noteworthy that the
prolactin cells in the pituitary of this species are not totally regressed in
this dilute seawater as they are in full-strength seawater, and the data
suggest that in both dilute seawater and freshwater the physiological action of fish prolactin lies in reducing the passive permeability of the
integument ( presumably especially the gills ) to sodium. It would seem
that the importance of this “impermeabilization” action of fish prolactin
in relation to other components of the osmoregulatory machinery varies
from one species to another, and that this may be the determinant of, for
example, the drastically rapid loss of sodium in hypophysectomized
P . latipinnu in freshwater compared to the slow sodium loss suffered by
the eel in like circumstances, with Xiphophorus and the goldfish coming
somewhere between these two extremes.
The pituitary gland of the marine form of the stickleback, Gasterosteus
aculeatus ( trachurus), seems unable during the winter to secrete paralactin in amounts adequate for survival in freshwater, although competent to
do so in spring and summer, a seasonal difference probably triggered by
photoperiodic changes (Lam and Hoar, 1967). Thus, intact winter fish
may be regarded as being “physiologically hypophysectomized,” at least
where paralactin is concerned. In nature, the fish lives in seawater or
brackish water during the winter, and migrates to freshwater to breed
in spring or early summer. Winter fish transferred from seawater to freshwater suffer a high mortality which can be reduced by prolactin treatment
(Lam and Leatherland, 1969), and they also display a greater fall in
plasma osmolality and a smaller fall in urine osmolarity than spring fish
transferred to freshwater in the same way (Lam and Hoar, 1967). The
fall in plasma osmolality in winter fish after transfer to freshwater was
paralleled by a rapid drop in plasma sodium and chloride, which could
be corrected by a single injection of prolactin given 2 4 hr before the
transfer (Lam, 1968). It will be seen that although not surgically hypophysectomized, the winter stickleback behaves remarkably like
hypophysectomized P. latipinnu (Ball and Ensor, 1965, 1967). In the
stickleback, the loss of electrolytes after transfer to freshwater, and the
prevention of this loss by prolactin, is paralleled by the behavior of the
gill mucous cells, which were increased in density by prolactin treatment
( Leatherland and Lam, 1968). Lam ( 1968) quotes preliminary studies
showing that prolactin reduces extrarenal outflux of sodium in sticklebacks transferred to freshwater, as in F . heteroclitus and P. latipinnu; and
renal effects of prolactin also seem operative in this species (Lam and
Hoar, 1967).
A striking illustration of the fact that even within the same genus
different species may vary greatly in their physiological mechanisms is
215
effective ACTH therapy (Ball and Ensor, 1969). It is noteworthy that the
prolactin cells in the pituitary of this species are not totally regressed in
this dilute seawater as they are in full-strength seawater, and the data
suggest that in both dilute seawater and freshwater the physiological action of fish prolactin lies in reducing the passive permeability of the
integument ( presumably especially the gills ) to sodium. It would seem
that the importance of this “impermeabilization” action of fish prolactin
in relation to other components of the osmoregulatory machinery varies
from one species to another, and that this may be the determinant of, for
example, the drastically rapid loss of sodium in hypophysectomized
P . latipinnu in freshwater compared to the slow sodium loss suffered by
the eel in like circumstances, with Xiphophorus and the goldfish coming
somewhere between these two extremes.
The pituitary gland of the marine form of the stickleback, Gasterosteus
aculeatus ( trachurus), seems unable during the winter to secrete paralactin in amounts adequate for survival in freshwater, although competent to
do so in spring and summer, a seasonal difference probably triggered by
photoperiodic changes (Lam and Hoar, 1967). Thus, intact winter fish
may be regarded as being “physiologically hypophysectomized,” at least
where paralactin is concerned. In nature, the fish lives in seawater or
brackish water during the winter, and migrates to freshwater to breed
in spring or early summer. Winter fish transferred from seawater to freshwater suffer a high mortality which can be reduced by prolactin treatment
(Lam and Leatherland, 1969), and they also display a greater fall in
plasma osmolality and a smaller fall in urine osmolarity than spring fish
transferred to freshwater in the same way (Lam and Hoar, 1967). The
fall in plasma osmolality in winter fish after transfer to freshwater was
paralleled by a rapid drop in plasma sodium and chloride, which could
be corrected by a single injection of prolactin given 2 4 hr before the
transfer (Lam, 1968). It will be seen that although not surgically hypophysectomized, the winter stickleback behaves remarkably like
hypophysectomized P. latipinnu (Ball and Ensor, 1965, 1967). In the
stickleback, the loss of electrolytes after transfer to freshwater, and the
prevention of this loss by prolactin, is paralleled by the behavior of the
gill mucous cells, which were increased in density by prolactin treatment
( Leatherland and Lam, 1968). Lam ( 1968) quotes preliminary studies
showing that prolactin reduces extrarenal outflux of sodium in sticklebacks transferred to freshwater, as in F . heteroclitus and P. latipinnu; and
renal effects of prolactin also seem operative in this species (Lam and
Hoar, 1967).
A striking illustration of the fact that even within the same genus
different species may vary greatly in their physiological mechanisms is