measurements in vitro of any substantial active sodium transport across the skin
of the aquatic urodeles, Necturus maculosus, Siren lacertina or Amphiuma means
(BENTLEY, 1969 b) but this could be due to damage that occurs during the removal
of the skin from these animals. Such transport could also be obscured if sodium
and an accompanying anion were transported in electrically equivalent quantities
so that no electrical p. d. would be observed.
a) Aldosterone. The rate of sodium transport across anuran skin in vitro increases
when the frogs or toads have been previously maintained for several weeks in distilled water, while it decreases when they are kept in 0.7% sodium chloride solution
(MAETZ, 1959; see also QABBE, 1966; HORNBY and THOMAS, 1969). MAETZ and
his collaborators also found that the prior injection of aldosterone into frogs, Rana
esculenta kept in saline greatly increased the rate of sodium transport that was subsequently observed across the skin (in vitro), but it had a much smaller effect in frogs
that had been kept in fresh water. This suggests that aldosterone is involved in the
adaptational changes that are observed in vivo. Aldosterone has since been shown
to increase sodium transport in vitro across the ventral skin of Rana esculenta and
Bufo marinus (CRABBE, 1964). Previous to this TABENHAUS, FRITZ, and MORTON
(1956), forecast such an action in what appears to be a relatively forgotten series
of observations. They found that desoxycorticosterone and cortisol promoted fluid
transfer across the isolated skin of Rana pipiens. Sodium was not actually determined but from measurements of the osmotic pressure of the transported fluid it
clearly was transferred. The levels of aldosterone in the blood of Bufo marinus kept
in distilled water are far higher than in those kept in dilute saline (CRABBE, 1961 a).
Aldosterone thus would seem to assist the regulation of sodium transport across
the skin of anurans. There is, however, little information about its role in urodeles.
As many species of this group live an almost exclusively aquatic existence, such a
regulatory mechanism could be useful, though it is conceivably not vital. If, for instance, aquatic amphibians normally took up sodium at a constant (and maximal)
rate through their skin, any excess salt, if modest, could be excreted through the
kidneys just like excess water.
Frogs that enter, or live in, saline solutions would seem to have some special
problems with regard to the permeability of their skin to salt. ADOLPH (1933) described how leopard frogs, Rana pipiens, when placed in hypoosmotic saline solutions gained weight due to the accumulation of salt. When anuran skin is exposed,
in vitro, to external solutions of increasing sodium chloride concentration, the rate
of sodium transport increases, due, presumably, to a depression of its chemical
gradient across the membrane. It seems likely that the salt accumulation observed
by ADOLPH results from such increased permeability to salt , in conjunction with
an inability of the kidneys to excrete it. I have observed such salt accumulation
in the anurans, Rana catesbeiana, Bufo marinus and Xenopus laevis, kept in 0.5%
sodium chloride solutions (but not in the urode1es Necturus, Amphiuma or Siren).
For these anurans a reduction in the permeability of the skin to sodium may be
useful in such circumstances. When the green toad, Bufo viridis, adapts to living
in 2% sodium chloride solutions, the electrical properties of its skin indicate a reduction of about 95 % in the sodium transport (GORDON, 1962). Information about
the skin of the crab-eating frog in sea-water is sparse, but the results of GORDON
180
of the aquatic urodeles, Necturus maculosus, Siren lacertina or Amphiuma means
(BENTLEY, 1969 b) but this could be due to damage that occurs during the removal
of the skin from these animals. Such transport could also be obscured if sodium
and an accompanying anion were transported in electrically equivalent quantities
so that no electrical p. d. would be observed.
a) Aldosterone. The rate of sodium transport across anuran skin in vitro increases
when the frogs or toads have been previously maintained for several weeks in distilled water, while it decreases when they are kept in 0.7% sodium chloride solution
(MAETZ, 1959; see also QABBE, 1966; HORNBY and THOMAS, 1969). MAETZ and
his collaborators also found that the prior injection of aldosterone into frogs, Rana
esculenta kept in saline greatly increased the rate of sodium transport that was subsequently observed across the skin (in vitro), but it had a much smaller effect in frogs
that had been kept in fresh water. This suggests that aldosterone is involved in the
adaptational changes that are observed in vivo. Aldosterone has since been shown
to increase sodium transport in vitro across the ventral skin of Rana esculenta and
Bufo marinus (CRABBE, 1964). Previous to this TABENHAUS, FRITZ, and MORTON
(1956), forecast such an action in what appears to be a relatively forgotten series
of observations. They found that desoxycorticosterone and cortisol promoted fluid
transfer across the isolated skin of Rana pipiens. Sodium was not actually determined but from measurements of the osmotic pressure of the transported fluid it
clearly was transferred. The levels of aldosterone in the blood of Bufo marinus kept
in distilled water are far higher than in those kept in dilute saline (CRABBE, 1961 a).
Aldosterone thus would seem to assist the regulation of sodium transport across
the skin of anurans. There is, however, little information about its role in urodeles.
As many species of this group live an almost exclusively aquatic existence, such a
regulatory mechanism could be useful, though it is conceivably not vital. If, for instance, aquatic amphibians normally took up sodium at a constant (and maximal)
rate through their skin, any excess salt, if modest, could be excreted through the
kidneys just like excess water.
Frogs that enter, or live in, saline solutions would seem to have some special
problems with regard to the permeability of their skin to salt. ADOLPH (1933) described how leopard frogs, Rana pipiens, when placed in hypoosmotic saline solutions gained weight due to the accumulation of salt. When anuran skin is exposed,
in vitro, to external solutions of increasing sodium chloride concentration, the rate
of sodium transport increases, due, presumably, to a depression of its chemical
gradient across the membrane. It seems likely that the salt accumulation observed
by ADOLPH results from such increased permeability to salt , in conjunction with
an inability of the kidneys to excrete it. I have observed such salt accumulation
in the anurans, Rana catesbeiana, Bufo marinus and Xenopus laevis, kept in 0.5%
sodium chloride solutions (but not in the urode1es Necturus, Amphiuma or Siren).
For these anurans a reduction in the permeability of the skin to sodium may be
useful in such circumstances. When the green toad, Bufo viridis, adapts to living
in 2% sodium chloride solutions, the electrical properties of its skin indicate a reduction of about 95 % in the sodium transport (GORDON, 1962). Information about
the skin of the crab-eating frog in sea-water is sparse, but the results of GORDON
180