d) The Large Intestine (Colon)
Sodium is actively transported from the mucosal to serosal side of the colon in anurans (USSING and ANDERSEN , 1955; COOPERSTEIN and HOGBEN, 1959). In the toad ,
Bufo marinus, this process can be increased by aldosterone and neurohypophysial
peptides (COFRE and CRABBE, 1965; 1967). FERREIRA and SMITH (1968) found that
when they placed these toads in dilute saline for two to three weeks, the sodium
transport across the colon (in vitro) reversed so that it was occuring from the serosa
to mucosa. This resulted in the sodium concentration at the mucosal surface being
higher than it was in the plasma. It would thus seem that in such saline adapted
toads, there is an extrarenal sodium excretion across the solon, but whether or not
this plays any role in the life of anurans that live in salt water is unknown. This
very interesting directional change in sodium transport across the colon is, as we
shall see later, reminiscent of the reversal in ion transfer that may occu r when euryhaline fishes are transferred from fresh water to sea-water.
3. Nitrogen Metabolism
The end-products of nitrogen metabolism require water for their excretion and
the amount that is required depends mainly on whether ammonia, urea or uric acid
is formed. The formation of uric acid is a reptilian and avian prerogative, while
the Amphibia, like the lungfishes, can form both ammonia and urea. I
The formation of ammonia requires th e presence of abundant water for its excret ion, as it is very to xic; this occurs in larval amphibians and in the neotenous
urodele, Necturus maculosus. In the latter about 90% of the total waste nitrogen
is excreted as ammonia, not through the gills or kidneys, but through the skin
(FANELLI and GOLDSTEIN, 1964). Even normal adult forms of amphibians can excrete large amounts of ammonia; the toad, Xenopus laevis, do es this when it is in
fresh water, but if kept out of water or in dilute saline solutions, it accumulates
urea instead (BALINSKY, CRAGG, and BALDWIN, 1961). This is similar to what happens in the African lungfish, Protopterus, when it aestivates buried in mud. In
Xenopus the increased formation and accumulation of urea is accompanied by a
two-fold increase in the activity of the ornithine-urea cycle enzymes present in the
liver (McBEAN and GOLDSTEIN, 1967). Rana pipiens can also be persuaded to accumulate extra urea when it is kept in dilute saline solutions, but this does not appear
to be influenced by the pituitary hormones as it is not significantly changed after
hypophysectomy (SCHEER and MARKEL, 1962).
As described earlier, some amphibians have a remarkable ability to accumulate
and withstand high concentrations of urea in their body fluids . This is seen in aestivating spadefoot toads and in the marine crab-eating frog. In the latter the ability
t J. P. Loveridge (Arnoldia, Publication of National Museums of Rhodesia, Vol. 5, 1-6
(1970) has recently shown that a Rhodesian frog, Chiromantis xerampelina, excretes substantial amounts of uric acid in its urine. This fascinating xerophilous frog survivesfor prolonged periods in dry exposed situations. It loses water by evaporation very slowly (comparable with that in a local lizard, Chamaeleo dilepis) but can absorb water very rapidly
through its ventral cutaneous surface.
184
Sodium is actively transported from the mucosal to serosal side of the colon in anurans (USSING and ANDERSEN , 1955; COOPERSTEIN and HOGBEN, 1959). In the toad ,
Bufo marinus, this process can be increased by aldosterone and neurohypophysial
peptides (COFRE and CRABBE, 1965; 1967). FERREIRA and SMITH (1968) found that
when they placed these toads in dilute saline for two to three weeks, the sodium
transport across the colon (in vitro) reversed so that it was occuring from the serosa
to mucosa. This resulted in the sodium concentration at the mucosal surface being
higher than it was in the plasma. It would thus seem that in such saline adapted
toads, there is an extrarenal sodium excretion across the solon, but whether or not
this plays any role in the life of anurans that live in salt water is unknown. This
very interesting directional change in sodium transport across the colon is, as we
shall see later, reminiscent of the reversal in ion transfer that may occu r when euryhaline fishes are transferred from fresh water to sea-water.
3. Nitrogen Metabolism
The end-products of nitrogen metabolism require water for their excretion and
the amount that is required depends mainly on whether ammonia, urea or uric acid
is formed. The formation of uric acid is a reptilian and avian prerogative, while
the Amphibia, like the lungfishes, can form both ammonia and urea. I
The formation of ammonia requires th e presence of abundant water for its excret ion, as it is very to xic; this occurs in larval amphibians and in the neotenous
urodele, Necturus maculosus. In the latter about 90% of the total waste nitrogen
is excreted as ammonia, not through the gills or kidneys, but through the skin
(FANELLI and GOLDSTEIN, 1964). Even normal adult forms of amphibians can excrete large amounts of ammonia; the toad, Xenopus laevis, do es this when it is in
fresh water, but if kept out of water or in dilute saline solutions, it accumulates
urea instead (BALINSKY, CRAGG, and BALDWIN, 1961). This is similar to what happens in the African lungfish, Protopterus, when it aestivates buried in mud. In
Xenopus the increased formation and accumulation of urea is accompanied by a
two-fold increase in the activity of the ornithine-urea cycle enzymes present in the
liver (McBEAN and GOLDSTEIN, 1967). Rana pipiens can also be persuaded to accumulate extra urea when it is kept in dilute saline solutions, but this does not appear
to be influenced by the pituitary hormones as it is not significantly changed after
hypophysectomy (SCHEER and MARKEL, 1962).
As described earlier, some amphibians have a remarkable ability to accumulate
and withstand high concentrations of urea in their body fluids . This is seen in aestivating spadefoot toads and in the marine crab-eating frog. In the latter the ability
t J. P. Loveridge (Arnoldia, Publication of National Museums of Rhodesia, Vol. 5, 1-6
(1970) has recently shown that a Rhodesian frog, Chiromantis xerampelina, excretes substantial amounts of uric acid in its urine. This fascinating xerophilous frog survivesfor prolonged periods in dry exposed situations. It loses water by evaporation very slowly (comparable with that in a local lizard, Chamaeleo dilepis) but can absorb water very rapidly
through its ventral cutaneous surface.
184