b) Kidney
The urine of amphibians kept in fresh water contains little salt. Thus the ureteral
urine from Rana esculenta living in fresh water contains only about 5 rn-equiv/l
sodium (JARD and MOREL, 1963) while in urodeles, such as Triturus, the sodium
levels in the bladder urine are similar to this. Analysis of the glomerular filtrate
obtained by micropuncture of the renal tubules indicates that sodium and chloride
are progressively reabsorbed as they pass down the nephron, this occurring mainly
in the proximal but also in the distal segment (WALKER et al., 1937).
The role of hormones in regulating these processes in amphibians is not clear.
While from the mammalian evidence it seems likely that corticosteroids influence
sodium reabsorption and potassium excretion this has not been directly shown in
frogs and toads. Injections of aldosterone into Bufo marinus have no consistent
effect on renal sodium losses (MIDDLER et al., 1969). MAYER (1963; 1969) also found
this to be so in Rana esculenta. Information about the role of corticosteroids in
controlling renal electrolyte excretion in the Amphibia is still sparse.
Vasotocin increases sodium reabsorption by the renal tubule of Rana esculenta
(JARD and MOREL, 1963; lARD, 1966), an observation similar to that described previously in the snake, Natrix sipedon. Vasotocin also decreases the urinary sodium
loss in Triturus (BENTLEY and HELLER, 1964). The injection of large amounts of
mammalian neurohypophysial peptides, as well as mesotocin, increases the renal
sodium excretion in Rana pipiens which have been 'loaded' with saline (BOYD and
WHYTE, 1939; SAWYER and SAWYER, 1952). These effects probably result from an
increased GFR, perhaps reflecting the vasoactive properties of these peptides when
present at high concentration. The natriuretic effects of such pep tides are only of
pharmacological significance but their role in promoting renal sodium conservation
.
.
IS uncertain,
c) The Urinary Bladder
The urinary bladder of the Amphibia, in addition to performing the function of
a water reservoir, can decrease the sodium content of the urine. Active sodium
transport has been shown (in vitro) to take place across the urinary bladders of
frogs, Rana esculenta, and toads, Bufo marinus (LEAF et al., 1958).
Sodium transport across the urinary bladders of Bufo marinus and Rana ridibunda is increased by aldosterone, either when the animals are injected 24 h prior
to measuring sodium transport in vitro, or after the tissue is directly exposed to
the steroid's action (CRABBE, 1961 b, a; 1963). Other steroids, including corticosterone, are not as active and at very high concentrations (100 times as great) may
inhibit its action. Changes in the sodium concentration of the media bathing the
toads influences the rate of sodium transport across the bladder in a manner that
parallels that of aldosterone in the blood; a low external sodium level increases aldosterone in the blood and stimulates sodium transport.
LEAF et ai. (1958) found that oxytocin increases sodium transport across the
toad urinary bladder and vasotocin has been found to be very active in this process
in frogs (JARD et al., 1960). In Bufo marinus vasotocin is about 100 times more
182
The urine of amphibians kept in fresh water contains little salt. Thus the ureteral
urine from Rana esculenta living in fresh water contains only about 5 rn-equiv/l
sodium (JARD and MOREL, 1963) while in urodeles, such as Triturus, the sodium
levels in the bladder urine are similar to this. Analysis of the glomerular filtrate
obtained by micropuncture of the renal tubules indicates that sodium and chloride
are progressively reabsorbed as they pass down the nephron, this occurring mainly
in the proximal but also in the distal segment (WALKER et al., 1937).
The role of hormones in regulating these processes in amphibians is not clear.
While from the mammalian evidence it seems likely that corticosteroids influence
sodium reabsorption and potassium excretion this has not been directly shown in
frogs and toads. Injections of aldosterone into Bufo marinus have no consistent
effect on renal sodium losses (MIDDLER et al., 1969). MAYER (1963; 1969) also found
this to be so in Rana esculenta. Information about the role of corticosteroids in
controlling renal electrolyte excretion in the Amphibia is still sparse.
Vasotocin increases sodium reabsorption by the renal tubule of Rana esculenta
(JARD and MOREL, 1963; lARD, 1966), an observation similar to that described previously in the snake, Natrix sipedon. Vasotocin also decreases the urinary sodium
loss in Triturus (BENTLEY and HELLER, 1964). The injection of large amounts of
mammalian neurohypophysial peptides, as well as mesotocin, increases the renal
sodium excretion in Rana pipiens which have been 'loaded' with saline (BOYD and
WHYTE, 1939; SAWYER and SAWYER, 1952). These effects probably result from an
increased GFR, perhaps reflecting the vasoactive properties of these peptides when
present at high concentration. The natriuretic effects of such pep tides are only of
pharmacological significance but their role in promoting renal sodium conservation
.
.
IS uncertain,
c) The Urinary Bladder
The urinary bladder of the Amphibia, in addition to performing the function of
a water reservoir, can decrease the sodium content of the urine. Active sodium
transport has been shown (in vitro) to take place across the urinary bladders of
frogs, Rana esculenta, and toads, Bufo marinus (LEAF et al., 1958).
Sodium transport across the urinary bladders of Bufo marinus and Rana ridibunda is increased by aldosterone, either when the animals are injected 24 h prior
to measuring sodium transport in vitro, or after the tissue is directly exposed to
the steroid's action (CRABBE, 1961 b, a; 1963). Other steroids, including corticosterone, are not as active and at very high concentrations (100 times as great) may
inhibit its action. Changes in the sodium concentration of the media bathing the
toads influences the rate of sodium transport across the bladder in a manner that
parallels that of aldosterone in the blood; a low external sodium level increases aldosterone in the blood and stimulates sodium transport.
LEAF et ai. (1958) found that oxytocin increases sodium transport across the
toad urinary bladder and vasotocin has been found to be very active in this process
in frogs (JARD et al., 1960). In Bufo marinus vasotocin is about 100 times more
182