there is also a small decrease in outflux (renal effect?). Experiments on the isolated
skin confirm that the site is not a cutaneous one. Perfusion of the tadpoles branchial
chambers suggests that the gills are the site of this active ion accumulation. Drinking
in these tadpoles was considerable and was equivalent to about 33 % of the body
weight each day .
The neurohypophysis of bullfrog tadpoles contains an activity that increases
the permeability of the toad bladder to water and this is presumably due to the
vasotocin which is known to be present in adult anurans (BENTLEY and GREENWALD, 1970). The activity present, is only 25% as much (activity/kg body weight)
as that in bullfrogs and, interestingly, is similar in quantity to that present in
the neurohypophysis of a neotenous form , the mudpuppy (FOLLETT and HELLER,
1964 a). When vasotocin is injected into these tadpoles it causes a water retention.
Th is is rather less than in adults, especially in the younger tadpoles, but increases
as metamorphosis approaches (ALVARADO and JOHNSON, 1966; BENTLEY and
GREENWALD, 1970). We have found a small increase in the osmotic permeability
of the skin (in vitro) when it is exposed to vasotocin, but the water retention in
vivo is almost certainly mainly due to a reduced urine flow. Vasotocin also causes
a net accumulation of sodium by bullfrog tadpoles, and this similarly results from
a decreased renal excretion. The cutaneous effect of vasotocin on sodium movement
in adults, is not apparent at this stage (ALVARADO and JOHNSON, 1966). The administration of thyroxine accelerates metamorphosis in amphibian larvae and when
this hormone is given to bullfrog tadpoles their skin develops an electrical p. d.
between its two sides, presumably reflecting the onset of active sodium transport
(TAYLOR and BARKER, 1965). It would not be surprising to find that corticosteroids
influence the electrolyte metabolism of the tadpole but I have no information about
this.
The tadpoles of the marine frog, Rana cancrzvora, live in the sea and so have
rather special osmoregulatory problems. In contrast to the adults that live in this
environment, they maintain their body fluids at a level that is hypoosmotic to the
sea-water (GORDON and TUCKER, 1965). How they manage to do this is uncertain,
but it was suggested that extrarenal salt excretion, possibly through the gills, may
occur. Needless to say these tadpoles are particularly interesting as their pattern
of osmoregulation shows a remarkable similarity to that of many marine fishes.
Larvae of the salamander, Ambystoma tigrinum, exchange water and sodium
with the fresh water in which they live (ALVARADO and KIRSCHNER, 1963). Water
uptake in these larvae is about 26% of their body weight in a day which is slightly
less than that observed in the adults. This water is excreted as a dilute urine that
also contains about 75% of the larva's total sodium efflux, compared with 55%
in adults. Sodium is also exchanged across the skin with an active uptake of sodium
and chloride.
ALVARADO and JOHNSON (1965) have compared the effects of vasotocin in larval
and adult salamanders. They found that while this peptide caused water retention
in both larvae and adults, the effect was less in the former . In both instances the
effect was due solely to a reduction in urine flow resulting (in the larvae) from a
decrease in the GFR. Vasotocin injections also resulted in the accumulation of
sodium, a prolonged effect that persisted for 43 h in the adults. Such an effect has
also been observed in the axolotl (JORGENSEN et al., 1946). Vasotocin usually only
188
skin confirm that the site is not a cutaneous one. Perfusion of the tadpoles branchial
chambers suggests that the gills are the site of this active ion accumulation. Drinking
in these tadpoles was considerable and was equivalent to about 33 % of the body
weight each day .
The neurohypophysis of bullfrog tadpoles contains an activity that increases
the permeability of the toad bladder to water and this is presumably due to the
vasotocin which is known to be present in adult anurans (BENTLEY and GREENWALD, 1970). The activity present, is only 25% as much (activity/kg body weight)
as that in bullfrogs and, interestingly, is similar in quantity to that present in
the neurohypophysis of a neotenous form , the mudpuppy (FOLLETT and HELLER,
1964 a). When vasotocin is injected into these tadpoles it causes a water retention.
Th is is rather less than in adults, especially in the younger tadpoles, but increases
as metamorphosis approaches (ALVARADO and JOHNSON, 1966; BENTLEY and
GREENWALD, 1970). We have found a small increase in the osmotic permeability
of the skin (in vitro) when it is exposed to vasotocin, but the water retention in
vivo is almost certainly mainly due to a reduced urine flow. Vasotocin also causes
a net accumulation of sodium by bullfrog tadpoles, and this similarly results from
a decreased renal excretion. The cutaneous effect of vasotocin on sodium movement
in adults, is not apparent at this stage (ALVARADO and JOHNSON, 1966). The administration of thyroxine accelerates metamorphosis in amphibian larvae and when
this hormone is given to bullfrog tadpoles their skin develops an electrical p. d.
between its two sides, presumably reflecting the onset of active sodium transport
(TAYLOR and BARKER, 1965). It would not be surprising to find that corticosteroids
influence the electrolyte metabolism of the tadpole but I have no information about
this.
The tadpoles of the marine frog, Rana cancrzvora, live in the sea and so have
rather special osmoregulatory problems. In contrast to the adults that live in this
environment, they maintain their body fluids at a level that is hypoosmotic to the
sea-water (GORDON and TUCKER, 1965). How they manage to do this is uncertain,
but it was suggested that extrarenal salt excretion, possibly through the gills, may
occur. Needless to say these tadpoles are particularly interesting as their pattern
of osmoregulation shows a remarkable similarity to that of many marine fishes.
Larvae of the salamander, Ambystoma tigrinum, exchange water and sodium
with the fresh water in which they live (ALVARADO and KIRSCHNER, 1963). Water
uptake in these larvae is about 26% of their body weight in a day which is slightly
less than that observed in the adults. This water is excreted as a dilute urine that
also contains about 75% of the larva's total sodium efflux, compared with 55%
in adults. Sodium is also exchanged across the skin with an active uptake of sodium
and chloride.
ALVARADO and JOHNSON (1965) have compared the effects of vasotocin in larval
and adult salamanders. They found that while this peptide caused water retention
in both larvae and adults, the effect was less in the former . In both instances the
effect was due solely to a reduction in urine flow resulting (in the larvae) from a
decrease in the GFR. Vasotocin injections also resulted in the accumulation of
sodium, a prolonged effect that persisted for 43 h in the adults. Such an effect has
also been observed in the axolotl (JORGENSEN et al., 1946). Vasotocin usually only
188