small increases in the rates of water absorption by dehydrated animals, the maximum rate of water uptake being observed in Ambystoma opacum, and this was
only 10 ,ullcm
2h.
Urodeles also only accumulate water slowly after being injected
with neurohypophysial peptides (see HELLER and BENTLEY 1965).
Whether or not the rate of water absorption through the skin following dehydration can be related to the habitat of the species is not clear . Such a correlation
certainly does not exist among all species. N evertheless, considerable diversity does
exist, and this, reflects differences in the permeability of the skin that range from
about 4 ,ullcm
2h
in dehydrated Xenopus to as much as 420 ,u1/cm
2h
in the ventral
pelvic skin in Bufo punctatus (MCCLANAHAN and BALDWIN, 1969). In addition,
the permeability of the skin may be augmented during dehydration, in such a manner as to suggest the presence of a regulatory mechanism. In view of these observations I feel that it is reasonable to suppose that such differences arose and persisted in response to ecological stresses on the Amphibia during their evolutionary
history, even though they may not today be precisely related to the life of all contemporary species.
The rate at which amphibians may gain water is directly related to the rate that
water passes across their skin . This may depend on several factors. As shown by
SCHMID'S (1965) studies, the permeability to the skin of anurans may differ even
in the absence of dehydration, and this will, of course, contribute to the differences
obs erved in the rates of rehydration. The permeability of the skin to water may,
as we have seen, be augmented in dehydrated animals and this could result from
several effects . Due to the increased concentration of the body fluid s, the osmotic
gradient across the skin increases during dehydration, and water would be expected
to move along this gradient more rapidly. Such effects must be relatively small,
but could contribute to minor increases in water absorption as SPIGHT observed
among urodeles . The increased osmotic pressure of the body fluids probably also
exerts a dire ct action on the skin and increases its permeability to water, such as has
been observed in the urinary bladders of toads and frogs (BENTLEY, 1964; PARISI,
RIPOCHE, and BOURGUET, 1969). In 1936 NOVELLI found that neurohypophysial
extracts could increase the permeability of the skin of anurans to water; this action
has since been shown to be widespread within this group (Table 6.5). Mammalian
neurohypophysial hormones, like vasopressin (AD H) can exert such an action on
anuran skin but the amphibian peptide, vasotocin, is far more active. BOURGUET
and MAETZ (1961) found that it increased the permeability of the skin of Rana esculenta even when present at the concentration of only 1O10M.
Vasotocin is released
into the blood of dehydrated frogs and toads in which it is present at a concentration
of 109
to 1010
M, which should be adequate to mediate increases in the permeability of the skin of dehydrated anurans. Other hormones possibly could also
be involved ; thus adrenaline has been shown to increase in vivo the osmotic permeability of the skin of toads, Bufo melanostictus, (ELLIOTT, 1968) and frogs in
vitro (JARD et al., 1968). If this catecholamine were released into the circulation
during dehydration it could facilitate the action of vasotocin.
Vasotocin, and related peptides from the neurohypophysis, increase the permeability of the skin of anurans to water, the only known exceptions being, notably, Xenopus laevis and the crab-eating frog, Rana cancrivora (Table 6.5). The
skins of a variety of urodeles have been examined for this response but none of
172
only 10 ,ullcm
2h.
Urodeles also only accumulate water slowly after being injected
with neurohypophysial peptides (see HELLER and BENTLEY 1965).
Whether or not the rate of water absorption through the skin following dehydration can be related to the habitat of the species is not clear . Such a correlation
certainly does not exist among all species. N evertheless, considerable diversity does
exist, and this, reflects differences in the permeability of the skin that range from
about 4 ,ullcm
2h
in dehydrated Xenopus to as much as 420 ,u1/cm
2h
in the ventral
pelvic skin in Bufo punctatus (MCCLANAHAN and BALDWIN, 1969). In addition,
the permeability of the skin may be augmented during dehydration, in such a manner as to suggest the presence of a regulatory mechanism. In view of these observations I feel that it is reasonable to suppose that such differences arose and persisted in response to ecological stresses on the Amphibia during their evolutionary
history, even though they may not today be precisely related to the life of all contemporary species.
The rate at which amphibians may gain water is directly related to the rate that
water passes across their skin . This may depend on several factors. As shown by
SCHMID'S (1965) studies, the permeability to the skin of anurans may differ even
in the absence of dehydration, and this will, of course, contribute to the differences
obs erved in the rates of rehydration. The permeability of the skin to water may,
as we have seen, be augmented in dehydrated animals and this could result from
several effects . Due to the increased concentration of the body fluid s, the osmotic
gradient across the skin increases during dehydration, and water would be expected
to move along this gradient more rapidly. Such effects must be relatively small,
but could contribute to minor increases in water absorption as SPIGHT observed
among urodeles . The increased osmotic pressure of the body fluids probably also
exerts a dire ct action on the skin and increases its permeability to water, such as has
been observed in the urinary bladders of toads and frogs (BENTLEY, 1964; PARISI,
RIPOCHE, and BOURGUET, 1969). In 1936 NOVELLI found that neurohypophysial
extracts could increase the permeability of the skin of anurans to water; this action
has since been shown to be widespread within this group (Table 6.5). Mammalian
neurohypophysial hormones, like vasopressin (AD H) can exert such an action on
anuran skin but the amphibian peptide, vasotocin, is far more active. BOURGUET
and MAETZ (1961) found that it increased the permeability of the skin of Rana esculenta even when present at the concentration of only 1O10M.
Vasotocin is released
into the blood of dehydrated frogs and toads in which it is present at a concentration
of 109
to 1010
M, which should be adequate to mediate increases in the permeability of the skin of dehydrated anurans. Other hormones possibly could also
be involved ; thus adrenaline has been shown to increase in vivo the osmotic permeability of the skin of toads, Bufo melanostictus, (ELLIOTT, 1968) and frogs in
vitro (JARD et al., 1968). If this catecholamine were released into the circulation
during dehydration it could facilitate the action of vasotocin.
Vasotocin, and related peptides from the neurohypophysis, increase the permeability of the skin of anurans to water, the only known exceptions being, notably, Xenopus laevis and the crab-eating frog, Rana cancrivora (Table 6.5). The
skins of a variety of urodeles have been examined for this response but none of
172