malian or amphibian neurohypophyses may produce an antidiuresis, but amphibian vasotocin is most active . In Rana catesbeiana and R . esculenta this peptide reduces the urine flow when injected to give concentrations estimated to be 1010 to
1O-IlM(URANGAandSAwYER ,1960 ;]ARD,MAETz, and MOREL, 1960). These are similar to the concentrations of vasotocin in the blood of dehydrated frogs and toads
(BENTLEY, 1969 a). Vasotocin has a dual action on the kidney; it depresses the GFR
and increases the renal tubular reabsorption of water. The decreased GFR seems
usually to result from a reduction in the number of active glomerule, as has also
been observed among the reptiles. The early studies of WALKER et al. (1937) on
the composition of the fluid in the renal tubules of frogs suggests that the tubular
site of action of vasotocin is a distal one, like in the mammals. The other amphibian
neurohypophysial peptides, oxytocin and mesotocin, when present at high concentrations, exert various effects on the kidney. In the bullfrog, oxytocin may decrease or even, on occasions, increas e the flow of urine (URANGA and SAWYER,
1960). Mesotocin also has a diuretic effect when injected into Rana esculenta (lARD,
1966). Neither mesotocin nor oxytocin have a prominent effect on tubular water
reabsorption , (acting mainly on the glomeruli)- and indeed, as shown by MOREL
and ]ARD (1963), may even antagonize this action of vasotocin. It seems likely that
vasotocin performs the role of an antidiuretic hormone in many amphibians, especially the more terrestrial specie s. Nevertheless, it is also likely that other factors
may act to mediate changes in urine flow .
c) The Urinary Bladder
As a tetrapod innovation, a urinary bladder derived embryologically from the
cloaca, makes its phyletic debut in the Amphibia. It is a distensable sac into which
the urine passes , and in some species, especially those that live in arid conditions,
like the Australian leptodactylid frogs , it can hold fluid equivalent to 50% of the
body weight (Table 6.7). TOWNSON in 1799 described how frogs 'have power of
absorbing the fluids necessary for their support .. . through the external skin . . .
a large part of them appearing to be retained in the so-called urinary bladder, though
gradually thrown off again by the skin.'. The more recent experimental observations of EWER (1952a) and SAWYER and SCHISGALL (1956) have shown that toads
and frogs reabsorb water from their urinary bladders when they are deh ydrated
(see also RUIBAL, 1962; SHOEMAKER, 1964). The urinary bladder of man y terrestrial
amphibians holds large quantities of water compared with more aquatic species
(Table 6.7) and thus affords them a useful store of water, that can be used to replace
body water at times when other supplies are limited.
Before the discovery of vasotocin, mammalian neurohypophysial pep tides were
shown, when injected, to increase water reabsorption from the bladders of toads
and frogs (EWER, 1952a; SAWYER and SCHISGALL, 1956). Vasotocin has since been
shown to be particularly active in promoting water reabsorption from this organ,
being effective, in vitro, at concentrations of 1011
to 1012M
(lARD et al., 1960; SAWYER, 1960). Mesotocin and oxytocin also increase the permeability of the anuran
bladder to water but, in Bufo marinus, they are about 200 times less potent than
vasotocin (Fig. 6.2).
176
1O-IlM(URANGAandSAwYER ,1960 ;]ARD,MAETz, and MOREL, 1960). These are similar to the concentrations of vasotocin in the blood of dehydrated frogs and toads
(BENTLEY, 1969 a). Vasotocin has a dual action on the kidney; it depresses the GFR
and increases the renal tubular reabsorption of water. The decreased GFR seems
usually to result from a reduction in the number of active glomerule, as has also
been observed among the reptiles. The early studies of WALKER et al. (1937) on
the composition of the fluid in the renal tubules of frogs suggests that the tubular
site of action of vasotocin is a distal one, like in the mammals. The other amphibian
neurohypophysial peptides, oxytocin and mesotocin, when present at high concentrations, exert various effects on the kidney. In the bullfrog, oxytocin may decrease or even, on occasions, increas e the flow of urine (URANGA and SAWYER,
1960). Mesotocin also has a diuretic effect when injected into Rana esculenta (lARD,
1966). Neither mesotocin nor oxytocin have a prominent effect on tubular water
reabsorption , (acting mainly on the glomeruli)- and indeed, as shown by MOREL
and ]ARD (1963), may even antagonize this action of vasotocin. It seems likely that
vasotocin performs the role of an antidiuretic hormone in many amphibians, especially the more terrestrial specie s. Nevertheless, it is also likely that other factors
may act to mediate changes in urine flow .
c) The Urinary Bladder
As a tetrapod innovation, a urinary bladder derived embryologically from the
cloaca, makes its phyletic debut in the Amphibia. It is a distensable sac into which
the urine passes , and in some species, especially those that live in arid conditions,
like the Australian leptodactylid frogs , it can hold fluid equivalent to 50% of the
body weight (Table 6.7). TOWNSON in 1799 described how frogs 'have power of
absorbing the fluids necessary for their support .. . through the external skin . . .
a large part of them appearing to be retained in the so-called urinary bladder, though
gradually thrown off again by the skin.'. The more recent experimental observations of EWER (1952a) and SAWYER and SCHISGALL (1956) have shown that toads
and frogs reabsorb water from their urinary bladders when they are deh ydrated
(see also RUIBAL, 1962; SHOEMAKER, 1964). The urinary bladder of man y terrestrial
amphibians holds large quantities of water compared with more aquatic species
(Table 6.7) and thus affords them a useful store of water, that can be used to replace
body water at times when other supplies are limited.
Before the discovery of vasotocin, mammalian neurohypophysial pep tides were
shown, when injected, to increase water reabsorption from the bladders of toads
and frogs (EWER, 1952a; SAWYER and SCHISGALL, 1956). Vasotocin has since been
shown to be particularly active in promoting water reabsorption from this organ,
being effective, in vitro, at concentrations of 1011
to 1012M
(lARD et al., 1960; SAWYER, 1960). Mesotocin and oxytocin also increase the permeability of the anuran
bladder to water but, in Bufo marinus, they are about 200 times less potent than
vasotocin (Fig. 6.2).
176