doses of vasotocin (but not isotocin) when injected, increase the urine flow of th e
goldfish. Vasotocin (but not mesotocin) also has such a diuretic effect in the African
lungfish (SAWYER, 1966 a). This action has also been demonstrated in the eel but
is absent in the flounder and lamprey (Table 7.8). The diuretic effect of
neurohypophysial peptides is the direct result of an increased GFR, the rate of
tubular water reabsorption being unchanged. The reason for the elevated GFR is
uncertain, but, at least in eels, it cannot be always accounted for by changes in the
blood pressure (CHESTER JONES, CHAN, and RANKIN, 1969b). However, a local
vasodilatatory action in the kidney could mediate its effect especially if it resulted
in a ' recruitment' of further glomeruli into activity. In the African lungfish the diuresis is initially associated with an increased arterial pressure (SAWYE R, 1966b) but
this does not persist for the duration of the effect (SAWYER, 1970). There are many
difficulties in considering the possible ph ysiological significance of the diuretic action of neurohypophysial peptides in fishes . The effect is not seen in all species,
while the sparse (and flimsy!) available evidence suggests that if the hormone is
released at all it may be in response to dehydration. This is not a physiologically
appropriate stimulus for a diuretic response. At present, judgement about such a
possible role should be reserved. From the phyletic standpoint it is, however, very
interesting that while the neurohypophysial peptides are present in both fishes and
tetrapods they only exert an antidiuretic action in the terrestrial group.
The control of urine volume in fishes may not depend on neurohypophysial
peptides, but could be dependent on an, as yet, unidentified hormone. It conceivably could also be directly controlled by regional changes in the blood supply to
the kidney that could result from haemodynamic effects dictated by changes in
circulating catecholamines and the volume and salt content of the fishes' body
fluids.
3. Salt Exchanges
Fishes may gain salts from, or lose them into, the fluids that bathe them . The
principal exchanges involve sodium and chloride and, to a much lesser extent,
potassium.
On simple physico-chemical grounds a net loss of sodium and chloride would
be expected to occur across the integument of fishes into fresh water, while a gain
of these ions may be expected to take place from sea-water. In addition, accumulation of salts occurs as a result of drinking and feeding. Earlier experiments, especially those of KROGH (1939), indicated that such movements of sod ium chloride
do indeed take place , but the technical methods then available did not allow a
prompt assessment of the relative magnitude of the various exchanges to be made.
The sub sequent ready availability of radio-isotopes considerably facilitated such
measurements. In 1950 MULLINS used 24Na to measure the total rate of salt exchange
in sticklebacks, Gasterosteus aculeatus. The isotopes were placed in the external
bathing fluids (alternatively they can be injected into the fish), and the amounts
accumulated by the fish were measured at intervals. If the fish are in salt balance
the total influx and outflux of the ion will be similar and can be taken to indicate
the total rate of its turnover by the fish. This has been called the rate constant (K)
218
goldfish. Vasotocin (but not mesotocin) also has such a diuretic effect in the African
lungfish (SAWYER, 1966 a). This action has also been demonstrated in the eel but
is absent in the flounder and lamprey (Table 7.8). The diuretic effect of
neurohypophysial peptides is the direct result of an increased GFR, the rate of
tubular water reabsorption being unchanged. The reason for the elevated GFR is
uncertain, but, at least in eels, it cannot be always accounted for by changes in the
blood pressure (CHESTER JONES, CHAN, and RANKIN, 1969b). However, a local
vasodilatatory action in the kidney could mediate its effect especially if it resulted
in a ' recruitment' of further glomeruli into activity. In the African lungfish the diuresis is initially associated with an increased arterial pressure (SAWYE R, 1966b) but
this does not persist for the duration of the effect (SAWYER, 1970). There are many
difficulties in considering the possible ph ysiological significance of the diuretic action of neurohypophysial peptides in fishes . The effect is not seen in all species,
while the sparse (and flimsy!) available evidence suggests that if the hormone is
released at all it may be in response to dehydration. This is not a physiologically
appropriate stimulus for a diuretic response. At present, judgement about such a
possible role should be reserved. From the phyletic standpoint it is, however, very
interesting that while the neurohypophysial peptides are present in both fishes and
tetrapods they only exert an antidiuretic action in the terrestrial group.
The control of urine volume in fishes may not depend on neurohypophysial
peptides, but could be dependent on an, as yet, unidentified hormone. It conceivably could also be directly controlled by regional changes in the blood supply to
the kidney that could result from haemodynamic effects dictated by changes in
circulating catecholamines and the volume and salt content of the fishes' body
fluids.
3. Salt Exchanges
Fishes may gain salts from, or lose them into, the fluids that bathe them . The
principal exchanges involve sodium and chloride and, to a much lesser extent,
potassium.
On simple physico-chemical grounds a net loss of sodium and chloride would
be expected to occur across the integument of fishes into fresh water, while a gain
of these ions may be expected to take place from sea-water. In addition, accumulation of salts occurs as a result of drinking and feeding. Earlier experiments, especially those of KROGH (1939), indicated that such movements of sod ium chloride
do indeed take place , but the technical methods then available did not allow a
prompt assessment of the relative magnitude of the various exchanges to be made.
The sub sequent ready availability of radio-isotopes considerably facilitated such
measurements. In 1950 MULLINS used 24Na to measure the total rate of salt exchange
in sticklebacks, Gasterosteus aculeatus. The isotopes were placed in the external
bathing fluids (alternatively they can be injected into the fish), and the amounts
accumulated by the fish were measured at intervals. If the fish are in salt balance
the total influx and outflux of the ion will be similar and can be taken to indicate
the total rate of its turnover by the fish. This has been called the rate constant (K)
218