3. FISH PROLACTIN AND GROWTH HORMONE
211
of the frog prolactin in any essential way. Parallel responses to homogenates of P. latipinnu pituitary were also obtained, which permitted the
bioassay of the activity in the fish gland ( Ensor and Ball, 1968a). The
question then arose whether the activity should be ascribed to a single
hormone in the fish gland, which could be designated fish prolactin, or
whether there were any reasons for thinking that mammalian prolactin in
the experiments was mimicking the physiological action of two or more
fish hormones in combination, or of some quite different fish hormone
such as ACTH.
By bioassay, it was shown that there is approximately double the
“prolactin” activity in pituitary glands taken from P . latipinnu adapted to
freshwater compared with glands taken from fish adapted to dilute seawater, this increased activity being already established 72 hr after the fish
enters freshwater, coincident with restoration to normal of the initially
declining plasma sodium ( Ensor and Ball, 1968a; Ball, 1969). In this
respect, the activity behaves more as though it were the property of a
single hormone rather than of a combination of hormones. The parallelism
between the responses to fish and frog pituitary homogenates supports
this idea, since there is no reason to attribute the activity in the amphibian
gland to anything other than a single hormone ( Ensor and Ball, 1968a).
Additional evidence comes from ectopic pituitary transplants in P. formom In these preparations some functions pass into total or partial
abeyance (gonadotropins, ACTH, growth hormone, and MSH), but the
sodium-conserving activity persists, together with TSH secretion (Ball
et ul., 1965; Ball, 1968). One would not expect these results if the sodiumconserving activity were owing to ACTH, gonadotropins, MSH, or growth
hormone, alone or in combination; and the involvement of endogenous
TSH is unlikely, since treatment with thiourea or thyroxine does not impair survival in freshwater ( Ball, 1968).
Considering now the work on pituitary histophysiology in P. latipinnu
in relation to freshwater survival, we h d a situation that parallels the
specificity screening of the mammalian pituitary hormones. The origin
of the fish prolactinlike activity has been experimentally ascribed to one
cell type in the rostra1 pars distalis, the 9 cell (see chapter by Ball and
Baker, Volume 11, Section 11, B, 1 ) . This is distinct from the cell type that
secretes growth hormone, just as the prolactin activity is distinct from
growth promotion (Ball, 1965a), and is distinct from the ACTH cells, the
TSH cells, and the gonadotrophs and pars intermedia cells (see chapter
by Ball and Baker, Volume 11). Furthermore, the 9 cells, but no others,
become activated during the first 72 hr after transfer to freshwater, in
correlation with the increase in prolactinlike activity in the gland and
with the restoration of the initially falling plasma sodium levels and the
211
of the frog prolactin in any essential way. Parallel responses to homogenates of P. latipinnu pituitary were also obtained, which permitted the
bioassay of the activity in the fish gland ( Ensor and Ball, 1968a). The
question then arose whether the activity should be ascribed to a single
hormone in the fish gland, which could be designated fish prolactin, or
whether there were any reasons for thinking that mammalian prolactin in
the experiments was mimicking the physiological action of two or more
fish hormones in combination, or of some quite different fish hormone
such as ACTH.
By bioassay, it was shown that there is approximately double the
“prolactin” activity in pituitary glands taken from P . latipinnu adapted to
freshwater compared with glands taken from fish adapted to dilute seawater, this increased activity being already established 72 hr after the fish
enters freshwater, coincident with restoration to normal of the initially
declining plasma sodium ( Ensor and Ball, 1968a; Ball, 1969). In this
respect, the activity behaves more as though it were the property of a
single hormone rather than of a combination of hormones. The parallelism
between the responses to fish and frog pituitary homogenates supports
this idea, since there is no reason to attribute the activity in the amphibian
gland to anything other than a single hormone ( Ensor and Ball, 1968a).
Additional evidence comes from ectopic pituitary transplants in P. formom In these preparations some functions pass into total or partial
abeyance (gonadotropins, ACTH, growth hormone, and MSH), but the
sodium-conserving activity persists, together with TSH secretion (Ball
et ul., 1965; Ball, 1968). One would not expect these results if the sodiumconserving activity were owing to ACTH, gonadotropins, MSH, or growth
hormone, alone or in combination; and the involvement of endogenous
TSH is unlikely, since treatment with thiourea or thyroxine does not impair survival in freshwater ( Ball, 1968).
Considering now the work on pituitary histophysiology in P. latipinnu
in relation to freshwater survival, we h d a situation that parallels the
specificity screening of the mammalian pituitary hormones. The origin
of the fish prolactinlike activity has been experimentally ascribed to one
cell type in the rostra1 pars distalis, the 9 cell (see chapter by Ball and
Baker, Volume 11, Section 11, B, 1 ) . This is distinct from the cell type that
secretes growth hormone, just as the prolactin activity is distinct from
growth promotion (Ball, 1965a), and is distinct from the ACTH cells, the
TSH cells, and the gonadotrophs and pars intermedia cells (see chapter
by Ball and Baker, Volume 11). Furthermore, the 9 cells, but no others,
become activated during the first 72 hr after transfer to freshwater, in
correlation with the increase in prolactinlike activity in the gland and
with the restoration of the initially falling plasma sodium levels and the