or no evidence to indicate that it increases sodium conservation by the kidney. Physiologically this may be unnecessary owing to the presence of extrarenal sites where
regulation may also be effected . Possibly the acquisition of a renal site of action represents a phyletic advance that evolved subsequently in the other tetrapods.
8. Physiology of the Amphibian Neurohypophysis
in Relation to Osmoregulation
Vasotocin and mesotocin have been identified in a variety of anuran and urodele
amphibians, and oxytocin has also been found in Rana pipiens (Table 6.3). Vasotocin is the predominant peptide in the neurohypophysis, usually being present at
more than twice the concentration of mesotocin. The amounts of these materials
found in the neurohypophysis are similar to those in other tetrapods, 1(JS to
10- 9 M/ kg body weight (Table 6.8). There are, however, some interesting exceptions , as in the aquatic urodele, Necturus maculosus, and the bullfrog tadpole in
which the neurohypophysis contains only about 20 % of the usual quantity of
vasotocin. These amounts are similar to those usually observed in the fishes and it
is tempting to suggest that they may reflect their aquatic habitats. Other aquatic
amphibians, like Xenopus laevis and Siren lacertina , however, have neurohypophysial levels of vasotocin that are comparable with those of terrestrial species . As
the tadpole is a larval form and Necturus is neotenous, .the differences between
these and other amphibians probably reflect the stage of their development.
Injections of vasotocin cause water retention in most amphibians and , as predicted by the results of STEGGERDA (1937), aquatic species respond poorly as compared with terrestrial ones (Table 6.9). We (HELLER and BENTLEY, 1965) also noted
that urodeles generally do not respond as well as anurans even when they come
from similar habitats. Vasotocin may affect the water (and sodium) metabolism
of amphibians by an action at several distinct sites:
(i) The kidney, by reducing the flow of urine and sodium loss.
(ii) The urinary bladder, by promoting reabsorption of water and sodium.
(iii) The colon, where vasopressin has been shown to have a small effect in promoting reabsorption of water and sodium from the contents of the gut.
(iv) The skin, where it promotes uptake of water and sodium from the external
solutions.
Not all of these effector sites are present in every amphibian and differences
in the distribution of these account partly for the variations that are observed in
the magnitude of the effects of injected vasotocin. Thus in the urodeles, neither
the skin, nor the bladder (with one exception) undergo an increase in osmotic permeability in the presence of vasotocin. Indeed urodeles usually exhibit a renal response (antidiuresis) to such pep tide s; the only exception is the fire-salamander
which, instead, increases the osmotic permeability of its urinary bladder. Most
anurans, on the other hand, reduce their urine volume and also increase the osmotic
permeability of their skin and bladders in response to vasotocin. The aquatic toad,
Xenopus laevis, is the only known exception; water transfer in thi s animal is unaffected by vasotocin, though sodium transport across the skin is increased.
192
regulation may also be effected . Possibly the acquisition of a renal site of action represents a phyletic advance that evolved subsequently in the other tetrapods.
8. Physiology of the Amphibian Neurohypophysis
in Relation to Osmoregulation
Vasotocin and mesotocin have been identified in a variety of anuran and urodele
amphibians, and oxytocin has also been found in Rana pipiens (Table 6.3). Vasotocin is the predominant peptide in the neurohypophysis, usually being present at
more than twice the concentration of mesotocin. The amounts of these materials
found in the neurohypophysis are similar to those in other tetrapods, 1(JS to
10- 9 M/ kg body weight (Table 6.8). There are, however, some interesting exceptions , as in the aquatic urodele, Necturus maculosus, and the bullfrog tadpole in
which the neurohypophysis contains only about 20 % of the usual quantity of
vasotocin. These amounts are similar to those usually observed in the fishes and it
is tempting to suggest that they may reflect their aquatic habitats. Other aquatic
amphibians, like Xenopus laevis and Siren lacertina , however, have neurohypophysial levels of vasotocin that are comparable with those of terrestrial species . As
the tadpole is a larval form and Necturus is neotenous, .the differences between
these and other amphibians probably reflect the stage of their development.
Injections of vasotocin cause water retention in most amphibians and , as predicted by the results of STEGGERDA (1937), aquatic species respond poorly as compared with terrestrial ones (Table 6.9). We (HELLER and BENTLEY, 1965) also noted
that urodeles generally do not respond as well as anurans even when they come
from similar habitats. Vasotocin may affect the water (and sodium) metabolism
of amphibians by an action at several distinct sites:
(i) The kidney, by reducing the flow of urine and sodium loss.
(ii) The urinary bladder, by promoting reabsorption of water and sodium.
(iii) The colon, where vasopressin has been shown to have a small effect in promoting reabsorption of water and sodium from the contents of the gut.
(iv) The skin, where it promotes uptake of water and sodium from the external
solutions.
Not all of these effector sites are present in every amphibian and differences
in the distribution of these account partly for the variations that are observed in
the magnitude of the effects of injected vasotocin. Thus in the urodeles, neither
the skin, nor the bladder (with one exception) undergo an increase in osmotic permeability in the presence of vasotocin. Indeed urodeles usually exhibit a renal response (antidiuresis) to such pep tide s; the only exception is the fire-salamander
which, instead, increases the osmotic permeability of its urinary bladder. Most
anurans, on the other hand, reduce their urine volume and also increase the osmotic
permeability of their skin and bladders in response to vasotocin. The aquatic toad,
Xenopus laevis, is the only known exception; water transfer in thi s animal is unaffected by vasotocin, though sodium transport across the skin is increased.
192