acts for 3 to 6 h, when injected into amphibians, so that the extended period of
action is rather surprising. Furthermore, I have been unable to detect any action
of this peptide on sodium transfer in vitro across the skin of adult Ambystoma tigrinum (BENTLEY, 1969 b). It seems likely that some other factor is involved in vivo
and an endogenous release of aldosterone would be more consistent with the time
course of the change in sodium metabolism. In jected aldosterone can increase the
influx of sodium into larval Ambystoma (ALVARADO and KIRSCHNER, 1964).
The osmoregulation of larval amphibians seems to be well adapted to their
aquatic life but they nevertheless still exhibit some similarities, such as a 'water
balance effect', to their terrestrial adult forms . This response is poorly developed
in the larvae, but in bullfrog tadpoles and in the axolotl (H. HELLER unpublished
observations quoted by BENTLEY and HELLER, 1964) they can be increased by accelerating metamorphosis by administration of thyroid hormone. These animals
thus offer a unique opportunity for studying the changes in the osmoregulatory
processes that are associated with changing from an aquatic to a terrestrial mode
of life.
7. Function of the Amphibian Adrenocortical Tissue
in Relation to Osmoregulation
Amphibian adrenocortical tissue secretes principally corticosterone and aldosterone, and in some instances cortisol, (Table 6.4). Aldosterone, and corticosterone
(less strongly) have been shown, both in vivo and in vitro, to increase the rate of
sodium transport across the skin and urinary bladder of frogs and toads . In addition, the corticosteroid levels in the circulation change in response to alterations
in the sodium content of the bathing media. This suggests that these steroids may
be involved in assisting the regulation of amphibian salt metabolism .
The rates of secretion of corticosteroids are partly under the control of the adenohypophysis. CARSTENSEN et al. (1959; 1961) found that when adrenocortical tissue from bullfrogs was incubated with either mammalian cort icotrophin or an extract of the frogs anterior pituitary, the rate of corticosteroid production increased;
aldo sterone being produced at 4 times the rate of corticosterone. The circulating
levels of these corticosteroids have since been measured in bullfrogs, corticosterone
being found to be predominant (JOHNSTON et al., 1967). CRABBE(1961 a) has also
shown this to be so in Bufo marinus. Injection of mammalian corticotrophin into
the bullfrogs produced a ten-fold increase in the aldosterone level in the blood,
while corticosterone increased three-fold. However, in hypophysectomized bull -
frog s the amount of aldosterone in the blood was similar to that in normal animals
but the corticosterone level was reduced 5-fold. For the higher tetrapods there is
evidence indicating that aldosterone secretion ma y be predominantly influenced
by the release of renin from the kidneys. This enz yme initiates the formation of
angiotensin in the blood and so stimulates steroid secretion from the gland. JOHNSTON et at. (1967) extracted a renin-like mat erial from frog kidneys, wh ich, when
injected into hypophysectomized bullfrogs, increased the secretion of aldosterone,
but left that of corticosterone unchanged. The renin activity is greater if the frogs
are kept in distilled water instead of 0.02 % sodium chloride solution (CAPELLI et
189
action is rather surprising. Furthermore, I have been unable to detect any action
of this peptide on sodium transfer in vitro across the skin of adult Ambystoma tigrinum (BENTLEY, 1969 b). It seems likely that some other factor is involved in vivo
and an endogenous release of aldosterone would be more consistent with the time
course of the change in sodium metabolism. In jected aldosterone can increase the
influx of sodium into larval Ambystoma (ALVARADO and KIRSCHNER, 1964).
The osmoregulation of larval amphibians seems to be well adapted to their
aquatic life but they nevertheless still exhibit some similarities, such as a 'water
balance effect', to their terrestrial adult forms . This response is poorly developed
in the larvae, but in bullfrog tadpoles and in the axolotl (H. HELLER unpublished
observations quoted by BENTLEY and HELLER, 1964) they can be increased by accelerating metamorphosis by administration of thyroid hormone. These animals
thus offer a unique opportunity for studying the changes in the osmoregulatory
processes that are associated with changing from an aquatic to a terrestrial mode
of life.
7. Function of the Amphibian Adrenocortical Tissue
in Relation to Osmoregulation
Amphibian adrenocortical tissue secretes principally corticosterone and aldosterone, and in some instances cortisol, (Table 6.4). Aldosterone, and corticosterone
(less strongly) have been shown, both in vivo and in vitro, to increase the rate of
sodium transport across the skin and urinary bladder of frogs and toads . In addition, the corticosteroid levels in the circulation change in response to alterations
in the sodium content of the bathing media. This suggests that these steroids may
be involved in assisting the regulation of amphibian salt metabolism .
The rates of secretion of corticosteroids are partly under the control of the adenohypophysis. CARSTENSEN et al. (1959; 1961) found that when adrenocortical tissue from bullfrogs was incubated with either mammalian cort icotrophin or an extract of the frogs anterior pituitary, the rate of corticosteroid production increased;
aldo sterone being produced at 4 times the rate of corticosterone. The circulating
levels of these corticosteroids have since been measured in bullfrogs, corticosterone
being found to be predominant (JOHNSTON et al., 1967). CRABBE(1961 a) has also
shown this to be so in Bufo marinus. Injection of mammalian corticotrophin into
the bullfrogs produced a ten-fold increase in the aldosterone level in the blood,
while corticosterone increased three-fold. However, in hypophysectomized bull -
frog s the amount of aldosterone in the blood was similar to that in normal animals
but the corticosterone level was reduced 5-fold. For the higher tetrapods there is
evidence indicating that aldosterone secretion ma y be predominantly influenced
by the release of renin from the kidneys. This enz yme initiates the formation of
angiotensin in the blood and so stimulates steroid secretion from the gland. JOHNSTON et at. (1967) extracted a renin-like mat erial from frog kidneys, wh ich, when
injected into hypophysectomized bullfrogs, increased the secretion of aldosterone,
but left that of corticosterone unchanged. The renin activity is greater if the frogs
are kept in distilled water instead of 0.02 % sodium chloride solution (CAPELLI et
189