and renal tubules are relevant to our ideas about the regulation of renal function.
A renal portal system, such as occurs in non-mammalian tetrapods, allows for the
separate functioning of glomeruli and tubules. Many fish have a renal portal blood
supply, but this may be absent in some, even within groups like the Teleostei (see
GERARD, 1954).
The rate of urine flow is very much greater in fish living in fresh water than
in those in the sea and th is reflects the excessive accumulation of water in the former
environment (Table 7.7). The high rate of urine flow in freshwater fishes is accompanied by an elevated GFR and a relatively restricted rate of water reabsorption
from the renal tubule. This usually amounts to less than 50% of that filtered across
the glomerulus, but may vary from about 25% in the eel living in fresh water, to
69% in the African lungfish. In fishe s, alterations in the urine volume are mainly
determined by changes in the GFR, but some changes in the rate of water
reabsorption across the tubules are also apparent (Table 7.7). This is in direct contrast to mammals, where tubular water reabsorption predominates, though in other
vertebrates, like the reptiles and amphibians, changes in the GFR are also prominent. The rainbow trout, when kept in fresh water, reabsorbs 38% of the water
filtered at the glomerulus, but in sea-water this rises to 93% (Table 7.7) indicating
that the glomerulus is not in exclusive control.
Euryhaline fishes in nature experience the contrast of the osmotic circumstances
of fresh water and sea-water. When such fish are transferred from fresh water to
sea-water the urine flow decreases . This is not an immediate effect but takes place
slowl y and in the flounder is not complete for two days (LAHLOU, 1967). This suggests that physiological changes are occurring and that the altered ur ine flow is not
simply the direct effect of a decrease in the amount of filterable water being delivered to the kidney. Killifish , Fundulus kan sae, when transferred from fresh water
to sea-water show, over a period of several hours, a gradual drop in the rate of urine
formation (FLEMINGand STANLEY, 1965). Eels, Anguilla anguilla , placed in sea-water also show a gradual decline in urine flow that does not stabilize for about 6
h (CHESTER ]qNES, CHAN, and RANKIN, 1969a). In the first 20 min there is an increased GFR rate in eels and this presumably reflects the initial increase in arte rial
blood pressure. Thereafter the GFR and the blood pressure gradually decrease until
a stable level is reached about 5 h later. Using Japanese eels, Anguilla japonica,
aIDE and UTIDA (1968) also observed such a decline in urine flow GFR, but after
10 days the GFR, but not the urine flow , of these fish returned to the levels seen
in fresh water. The reabsorption of water across the renal tubule increased so that
the urine flow remained low .
The physiological control of GFR in fishes is not clear, but several factors could
be influencing it. The rate of filtration across individual glomeruli could be influenced by changes in the osmotic pressure of the plasma proteins as would be
expected if water were added to, or subtracted from, the plasma and this has been
shown in eels (KEYS, 1933). As described above, the blood pressure of eels placed
in sea-water decreases and this could also reduce filtration. Apart from changes
in such balances of hydrostatic force across the glomerular membrane, alterations
in the number of functioning glomeruli would be reflected by changes in the total
GFR of the fish . MAETZ (1963) suggested that such 'glomerular recruitment' occurs
in the goldfish and LAHLOU (1966) has also demonstrated it in the flounder. HICK216
A renal portal system, such as occurs in non-mammalian tetrapods, allows for the
separate functioning of glomeruli and tubules. Many fish have a renal portal blood
supply, but this may be absent in some, even within groups like the Teleostei (see
GERARD, 1954).
The rate of urine flow is very much greater in fish living in fresh water than
in those in the sea and th is reflects the excessive accumulation of water in the former
environment (Table 7.7). The high rate of urine flow in freshwater fishes is accompanied by an elevated GFR and a relatively restricted rate of water reabsorption
from the renal tubule. This usually amounts to less than 50% of that filtered across
the glomerulus, but may vary from about 25% in the eel living in fresh water, to
69% in the African lungfish. In fishe s, alterations in the urine volume are mainly
determined by changes in the GFR, but some changes in the rate of water
reabsorption across the tubules are also apparent (Table 7.7). This is in direct contrast to mammals, where tubular water reabsorption predominates, though in other
vertebrates, like the reptiles and amphibians, changes in the GFR are also prominent. The rainbow trout, when kept in fresh water, reabsorbs 38% of the water
filtered at the glomerulus, but in sea-water this rises to 93% (Table 7.7) indicating
that the glomerulus is not in exclusive control.
Euryhaline fishes in nature experience the contrast of the osmotic circumstances
of fresh water and sea-water. When such fish are transferred from fresh water to
sea-water the urine flow decreases . This is not an immediate effect but takes place
slowl y and in the flounder is not complete for two days (LAHLOU, 1967). This suggests that physiological changes are occurring and that the altered ur ine flow is not
simply the direct effect of a decrease in the amount of filterable water being delivered to the kidney. Killifish , Fundulus kan sae, when transferred from fresh water
to sea-water show, over a period of several hours, a gradual drop in the rate of urine
formation (FLEMINGand STANLEY, 1965). Eels, Anguilla anguilla , placed in sea-water also show a gradual decline in urine flow that does not stabilize for about 6
h (CHESTER ]qNES, CHAN, and RANKIN, 1969a). In the first 20 min there is an increased GFR rate in eels and this presumably reflects the initial increase in arte rial
blood pressure. Thereafter the GFR and the blood pressure gradually decrease until
a stable level is reached about 5 h later. Using Japanese eels, Anguilla japonica,
aIDE and UTIDA (1968) also observed such a decline in urine flow GFR, but after
10 days the GFR, but not the urine flow , of these fish returned to the levels seen
in fresh water. The reabsorption of water across the renal tubule increased so that
the urine flow remained low .
The physiological control of GFR in fishes is not clear, but several factors could
be influencing it. The rate of filtration across individual glomeruli could be influenced by changes in the osmotic pressure of the plasma proteins as would be
expected if water were added to, or subtracted from, the plasma and this has been
shown in eels (KEYS, 1933). As described above, the blood pressure of eels placed
in sea-water decreases and this could also reduce filtration. Apart from changes
in such balances of hydrostatic force across the glomerular membrane, alterations
in the number of functioning glomeruli would be reflected by changes in the total
GFR of the fish . MAETZ (1963) suggested that such 'glomerular recruitment' occurs
in the goldfish and LAHLOU (1966) has also demonstrated it in the flounder. HICK216