The final confirmatory evidence required to establish the endocrine role of the
neurohypophysis in osmoregulation is the identification of the putative hormone
in the circulation. It should be present in amounts consistent with the sensitivity
of its supposed receptors and should be released in response to ph ysiologicall y appropriate stimuli. Dehydration has been shown to reduce greatl y the amounts of
neurohypophysial pep tides in the pituitaries of Rana pipiens and Bufo bufo
(LEVINSKY and SAWYER, 1953; JORGENSEN et al., 1956) and this probably reflects
a release into the circulation. DICKER and ELLIOTT (1970a), on the other hand,
found that the neurohypophysis of crab-eating frogs, adapted to living in dilute
sea-water (2/3 concentration), contained twice as much peptide activity as when
they were living in fresh water. It is uncertain whether this reflects a decreased release or an increased rate of synthesis, possibly combined with higher levels in the
circulation. SHOEMAKER (1965) found that when he injected blood plasma from
dehydrated toads, Bufo marinus, into toads kept in water, they hydrated at an increased rate, just as though vasotocin were injected. I have collected blood from
several anurans, Bufo marinus, Rana catesbeiana and Xenopus laevis, when they
were dehydrated or had spent an hour or so swimming in hypertonic saline. The
plasma of all such anurans treated in this manner, contains activity identical in its
pharmacological behaviour, and interactions with various enzymes, to vasotocin
(BENTLEY, 1969 a). It is present (if it is vasotocin) at a concentration of 109
to 1O-10M
which would stimulate adequately the various receptor tissues. It is interesting that
Xenopus laevis, which lacks such receptor tissues, still releases this peptide in response to a deh ydrational stress. I could not find vasotocin-like activity in the blood
of Necturus maculosus , even when the animals were placed in saline solutions. Indeed the role of the neurohypophysis in the ph ysiology of aquatic amphibians
remains an enigma. It has been formerly suggested that it may playa role in electrolyte metabolism (MAETZ, 1963; HELLER and BENTLEY, 1963), but this has not
been substantiated. One cannot even venture, at present, to suggest a role for the
other neurohypophysial peptide, mesotocin, in any of these tetrapods.
Despite some initial disadvantages, like a highl y permeable skin and aquatic
larvae, the amphibians have adapted to a number of different habitats with contrasting osmotic conditions . Their manner of doing this necessarily differs from
that of other tetrapod vertebrates, but the result is often as effective . When living
in fresh water the Amphibia depend more on extrarenal mechanisms in osmoregulation. This feature is shared with the fishes, though the principal sites in
each, skin or gills, differ. When on land many amphibians have a remarkable ability
to survive after being excessively dehydrated but they are generally more dependent
on the proximity of fresh water than other tetrapods. Amphibians in contrast to
the fishes, have a distensable cloacal urinary bladder that can be used to store large
amounts of water. If dry conditions become extreme they may aestivate entombed
under the ground for man y months, and with the aid of this store of water, await
the return of more favourable conditions.
The amphibian endocrine system is clearly involved in many osmotic adjustments. It has the usual complement of tetrapod hormones most of which have,
apparently, been inherited from the fishes. Aldosterone, however, seems to make
its initial hormonal appearance in this group. Corticosterone is also present in the
fishes as well as in the phyletically subsequent reptiles and birds. The peptide hor196
neurohypophysis in osmoregulation is the identification of the putative hormone
in the circulation. It should be present in amounts consistent with the sensitivity
of its supposed receptors and should be released in response to ph ysiologicall y appropriate stimuli. Dehydration has been shown to reduce greatl y the amounts of
neurohypophysial pep tides in the pituitaries of Rana pipiens and Bufo bufo
(LEVINSKY and SAWYER, 1953; JORGENSEN et al., 1956) and this probably reflects
a release into the circulation. DICKER and ELLIOTT (1970a), on the other hand,
found that the neurohypophysis of crab-eating frogs, adapted to living in dilute
sea-water (2/3 concentration), contained twice as much peptide activity as when
they were living in fresh water. It is uncertain whether this reflects a decreased release or an increased rate of synthesis, possibly combined with higher levels in the
circulation. SHOEMAKER (1965) found that when he injected blood plasma from
dehydrated toads, Bufo marinus, into toads kept in water, they hydrated at an increased rate, just as though vasotocin were injected. I have collected blood from
several anurans, Bufo marinus, Rana catesbeiana and Xenopus laevis, when they
were dehydrated or had spent an hour or so swimming in hypertonic saline. The
plasma of all such anurans treated in this manner, contains activity identical in its
pharmacological behaviour, and interactions with various enzymes, to vasotocin
(BENTLEY, 1969 a). It is present (if it is vasotocin) at a concentration of 109
to 1O-10M
which would stimulate adequately the various receptor tissues. It is interesting that
Xenopus laevis, which lacks such receptor tissues, still releases this peptide in response to a deh ydrational stress. I could not find vasotocin-like activity in the blood
of Necturus maculosus , even when the animals were placed in saline solutions. Indeed the role of the neurohypophysis in the ph ysiology of aquatic amphibians
remains an enigma. It has been formerly suggested that it may playa role in electrolyte metabolism (MAETZ, 1963; HELLER and BENTLEY, 1963), but this has not
been substantiated. One cannot even venture, at present, to suggest a role for the
other neurohypophysial peptide, mesotocin, in any of these tetrapods.
Despite some initial disadvantages, like a highl y permeable skin and aquatic
larvae, the amphibians have adapted to a number of different habitats with contrasting osmotic conditions . Their manner of doing this necessarily differs from
that of other tetrapod vertebrates, but the result is often as effective . When living
in fresh water the Amphibia depend more on extrarenal mechanisms in osmoregulation. This feature is shared with the fishes, though the principal sites in
each, skin or gills, differ. When on land many amphibians have a remarkable ability
to survive after being excessively dehydrated but they are generally more dependent
on the proximity of fresh water than other tetrapods. Amphibians in contrast to
the fishes, have a distensable cloacal urinary bladder that can be used to store large
amounts of water. If dry conditions become extreme they may aestivate entombed
under the ground for man y months, and with the aid of this store of water, await
the return of more favourable conditions.
The amphibian endocrine system is clearly involved in many osmotic adjustments. It has the usual complement of tetrapod hormones most of which have,
apparently, been inherited from the fishes. Aldosterone, however, seems to make
its initial hormonal appearance in this group. Corticosterone is also present in the
fishes as well as in the phyletically subsequent reptiles and birds. The peptide hor196