the excess solute through extrarenal channels, presumably the orbital 'salt' gland
(BENTLEY et al., 1967a). Some reptiles lack extrarenal channels for salt excretion
and have a poor tolerance to the presence of such solutes in their body fluids; we
have found that young caiman die within 24 h after being placed in 3.3% sodium
chloride solutions (BENTLEY and SCHMIDT-NIELSEN, 1965). The reptile kidney, in
the absence of adequate osmotically-free water has a very poor ability to excrete
sodium.
c) Cloaca and Urinary Bladder
The urine of reptiles, like that of birds, passes into a cloaca where it may be accumulated prior to periodic expulsion to the outside. In certain species it may pass
from the cloaca to be stored in a urinary bladder. The composition of the urine,
including its electrolyte content, may be altered by both of these organs. Urine
is stored for about 4 h in the cloaca of the caiman, during which time the sodium
concentration decreases by 50% (BENTLEY and SCHMIDT-NIELSEN, 1965). This
probably affords these crocodilians a substantial reduction in renal sodium loss .
Such an economy has also been observed in the crocodile, Crocodilus acutus (B.
SCHMIDT-NIELSEN and SKADHAUGE, 1967). The cloaca of the caiman, in vitro, has
an electrical p.d.of about 10 mV (mucosa negative) across its wall which is consistent with the process of active sodium transport. The cloaca and large intestine
of the Ophidia and the Chelonia cannot be readily separated for in vitro observations, but active sodium transport also appears to occur in this region of the gut
of various snakes and in Testudo graeca (JUNQUEIRA et al., 1966; BAILLIEN and
SCHOEFFENIELS, 1961; BENTLEY, 1962 b). In Testudo this process is not affected
in vitro by the presence of vasotocin or aldosterone. As we shall see in the next
chapter sodium transport across the large intestine of toads can be facilitated by
such hormones, suggesting that the actions of these substances on the cloaca and
large intestine of reptiles may be worth further investigation.
The urinary bladder of the chelonians, Pseudemys scripta and Testudo graeca,
also has the ability to transport sodium actively from the mucosal to the serosal
surfaces (BRODSKY and SCHILB, 1960; BENTLEY, 1962 b) thus further aiding conservation of sodium from the urine. In Testudo graeca this process is unaffected,
in vitro, by the presence of vasotocin, an observation that contrasts with the effects
of this peptide hormone in the Amphibia. However, like in frogs and toads, aldosterone increases the rate of sodium transport across the bladder of the Greek tortoise,
while if these reptiles are pretreated with an antialdosterone drug (spirolactone),
sodium transport is reduced. These observations need to be extended, especially
to other species of reptiles, as they suggest that endogenous aldosterone may physiologically facilitate sodium transport across the urinary bladder of reptiles.
The urinary bladder and cloaca of turtles may also be involved in the accumulation of sodium from the external environment. Pseudemys scripta exchange
sodium with bathing fluids at the rate of 0.04 to 10 fl-moles/100g h (DUNSON,
1967). When the cloaca is blocked, this rate of exchange is reduced by 70%,
indicating that the processes of sodium transfer in the cloaca and urinary bladder
may be involved. Turtles are known to irrigate their cloacal regions with the fluid
151
(BENTLEY et al., 1967a). Some reptiles lack extrarenal channels for salt excretion
and have a poor tolerance to the presence of such solutes in their body fluids; we
have found that young caiman die within 24 h after being placed in 3.3% sodium
chloride solutions (BENTLEY and SCHMIDT-NIELSEN, 1965). The reptile kidney, in
the absence of adequate osmotically-free water has a very poor ability to excrete
sodium.
c) Cloaca and Urinary Bladder
The urine of reptiles, like that of birds, passes into a cloaca where it may be accumulated prior to periodic expulsion to the outside. In certain species it may pass
from the cloaca to be stored in a urinary bladder. The composition of the urine,
including its electrolyte content, may be altered by both of these organs. Urine
is stored for about 4 h in the cloaca of the caiman, during which time the sodium
concentration decreases by 50% (BENTLEY and SCHMIDT-NIELSEN, 1965). This
probably affords these crocodilians a substantial reduction in renal sodium loss .
Such an economy has also been observed in the crocodile, Crocodilus acutus (B.
SCHMIDT-NIELSEN and SKADHAUGE, 1967). The cloaca of the caiman, in vitro, has
an electrical p.d.of about 10 mV (mucosa negative) across its wall which is consistent with the process of active sodium transport. The cloaca and large intestine
of the Ophidia and the Chelonia cannot be readily separated for in vitro observations, but active sodium transport also appears to occur in this region of the gut
of various snakes and in Testudo graeca (JUNQUEIRA et al., 1966; BAILLIEN and
SCHOEFFENIELS, 1961; BENTLEY, 1962 b). In Testudo this process is not affected
in vitro by the presence of vasotocin or aldosterone. As we shall see in the next
chapter sodium transport across the large intestine of toads can be facilitated by
such hormones, suggesting that the actions of these substances on the cloaca and
large intestine of reptiles may be worth further investigation.
The urinary bladder of the chelonians, Pseudemys scripta and Testudo graeca,
also has the ability to transport sodium actively from the mucosal to the serosal
surfaces (BRODSKY and SCHILB, 1960; BENTLEY, 1962 b) thus further aiding conservation of sodium from the urine. In Testudo graeca this process is unaffected,
in vitro, by the presence of vasotocin, an observation that contrasts with the effects
of this peptide hormone in the Amphibia. However, like in frogs and toads, aldosterone increases the rate of sodium transport across the bladder of the Greek tortoise,
while if these reptiles are pretreated with an antialdosterone drug (spirolactone),
sodium transport is reduced. These observations need to be extended, especially
to other species of reptiles, as they suggest that endogenous aldosterone may physiologically facilitate sodium transport across the urinary bladder of reptiles.
The urinary bladder and cloaca of turtles may also be involved in the accumulation of sodium from the external environment. Pseudemys scripta exchange
sodium with bathing fluids at the rate of 0.04 to 10 fl-moles/100g h (DUNSON,
1967). When the cloaca is blocked, this rate of exchange is reduced by 70%,
indicating that the processes of sodium transfer in the cloaca and urinary bladder
may be involved. Turtles are known to irrigate their cloacal regions with the fluid
151