of sodium chloride by keeping them for several weeks in distilled water; at the end
of this time they were able to accumulate sodium chloride actively from solutions
as dilute as 105
to 104M.
The isolated gills of the eel can also actively absorb
sodium from dilute solutions (0.04 mM) while the intact fish can accumulate
sodium from fresh water containing only 0.05 mM sodium chloride (BELLAMY,
1961; GARCIA ROMEU and MOTAIS, 1966). Chloride is apparently not taken up
actively by the gills of the eel (KROGH, 1939; GARCIA ROMEU and MOTAIS, 1966).
When eels are kept in distilled water for three weeks, and then replaced in tap water,
the rate of branchial accumulation of sodium increases three-fold (HENDERSON and
CHESTER JONES, 1967). Such experiments are performed on fasting fish and it
should be remembered that feeding results in an accumulation of sodium through
the gut and that this normally contributes to the animals' needs. However, many
fish do normally undergo periods of prolonged fasting, and in these circumstances
the ability to accumulate sodium chloride actively may well be vit al to their survival.
The mechanism of sodium and chloride accumulation by the gills of fish in fresh
water involves exchanges with ions in the body. In the goldfish sodium enters the
gills in exchange for NH 4 + while external chloride exchanges with bicarbonate
(MAETZ and GARCIA ROMEU, 1964; GARCIA ROMEO and MAETZ, 1964). A similar
Na + - NH 4 + exchange occurs across the gills of eels (GARCIA ROMEU and MOTAIS,
1966). It is not known if N a- K activated ATPase is concerned in the mechanism
of active accumulation of sodium in fresh water as it is with extrusion of this ion
in the sea. This enzyme has been identified in the gills of killifish and European
eels adapted to fresh water (EpSTEIN et al., 1967; MOTAIS , 1970) though the level
of activity is less than half as great as in sea-water. However, the rate of accumulation of sodium in fresh water is far less than its rate of secretion in sea-water,
so the requirements in the former media are probably less. MOTAIS (1970) found
that while the levels of Na-K ATPase activity in the gills of eels in sea-water could
be reduced by treatment with actinomycin D, the freshwater fish was unaffected
by this antibiotic. He has suggested that there are two forms of N a-K ATPase
present. One, which is situated at the external border of the epithelial cells in the
gills, mediates sodium extrusion in sea-water, while the other, at the opposite side
of these cells, is involved with sodium absorption. As yet there is no direct evidence
to indicate that Na-K ATPase is concerned with active branchial sodium accumulation in fresh water but it seems likely that it may perform this role, just as it does
in so many other tissues .
Sodium-chloride depletion, as we have seen, stimulates the rate of sodium uptake by the gills of goldfish and eels. Injection of solutions that dilute the plasma
electrolytes (but not its osmotic concentration) increases absorption of sodium and
chloride by the gills of goldfish (BOURGUET, LAHLOU, and MAETZ. 1964). Whether
such effects are mediated by hormones is unknown but the injection of several ' putative' ones has a similar effect (Table 7.8). Isotocin, and to a lesser extent vasotocin, when so administered, stimulate the rate of sodium uptake across the gills of
the goldfish. It is po ssible that such effects are mediated by changes in the branchial
circulation (MAETZ and RANKIN, 1969). We (BENTLEY and FOLLETT, 1962; 1963)
could find no evidence that vasotocin altered sodium exchange across the gills of
lampreys. The effects of such peptides in chondrichthyeans do not seem to have
223
of this time they were able to accumulate sodium chloride actively from solutions
as dilute as 105
to 104M.
The isolated gills of the eel can also actively absorb
sodium from dilute solutions (0.04 mM) while the intact fish can accumulate
sodium from fresh water containing only 0.05 mM sodium chloride (BELLAMY,
1961; GARCIA ROMEU and MOTAIS, 1966). Chloride is apparently not taken up
actively by the gills of the eel (KROGH, 1939; GARCIA ROMEU and MOTAIS, 1966).
When eels are kept in distilled water for three weeks, and then replaced in tap water,
the rate of branchial accumulation of sodium increases three-fold (HENDERSON and
CHESTER JONES, 1967). Such experiments are performed on fasting fish and it
should be remembered that feeding results in an accumulation of sodium through
the gut and that this normally contributes to the animals' needs. However, many
fish do normally undergo periods of prolonged fasting, and in these circumstances
the ability to accumulate sodium chloride actively may well be vit al to their survival.
The mechanism of sodium and chloride accumulation by the gills of fish in fresh
water involves exchanges with ions in the body. In the goldfish sodium enters the
gills in exchange for NH 4 + while external chloride exchanges with bicarbonate
(MAETZ and GARCIA ROMEU, 1964; GARCIA ROMEO and MAETZ, 1964). A similar
Na + - NH 4 + exchange occurs across the gills of eels (GARCIA ROMEU and MOTAIS,
1966). It is not known if N a- K activated ATPase is concerned in the mechanism
of active accumulation of sodium in fresh water as it is with extrusion of this ion
in the sea. This enzyme has been identified in the gills of killifish and European
eels adapted to fresh water (EpSTEIN et al., 1967; MOTAIS , 1970) though the level
of activity is less than half as great as in sea-water. However, the rate of accumulation of sodium in fresh water is far less than its rate of secretion in sea-water,
so the requirements in the former media are probably less. MOTAIS (1970) found
that while the levels of Na-K ATPase activity in the gills of eels in sea-water could
be reduced by treatment with actinomycin D, the freshwater fish was unaffected
by this antibiotic. He has suggested that there are two forms of N a-K ATPase
present. One, which is situated at the external border of the epithelial cells in the
gills, mediates sodium extrusion in sea-water, while the other, at the opposite side
of these cells, is involved with sodium absorption. As yet there is no direct evidence
to indicate that Na-K ATPase is concerned with active branchial sodium accumulation in fresh water but it seems likely that it may perform this role, just as it does
in so many other tissues .
Sodium-chloride depletion, as we have seen, stimulates the rate of sodium uptake by the gills of goldfish and eels. Injection of solutions that dilute the plasma
electrolytes (but not its osmotic concentration) increases absorption of sodium and
chloride by the gills of goldfish (BOURGUET, LAHLOU, and MAETZ. 1964). Whether
such effects are mediated by hormones is unknown but the injection of several ' putative' ones has a similar effect (Table 7.8). Isotocin, and to a lesser extent vasotocin, when so administered, stimulate the rate of sodium uptake across the gills of
the goldfish. It is po ssible that such effects are mediated by changes in the branchial
circulation (MAETZ and RANKIN, 1969). We (BENTLEY and FOLLETT, 1962; 1963)
could find no evidence that vasotocin altered sodium exchange across the gills of
lampreys. The effects of such peptides in chondrichthyeans do not seem to have
223