KREBS citric acid cycle. Fatty acids and amino acids may also enter the citric acid
cycle. As the supply of ATP is limiting to sodium transport, the reactions and substrates from which it is formed may influence the rate of sodium movement, and
it is at such sites that hormones which alter sodium transfer could be expected to
act.
Estimates of the number of ions transferred in relation to each molecule of ATP
hydrolysed depend principally on measurements of oxygen consumption in relation to the rate of sodium transport. Such estimates range from the transport of
4 to 28 sodium ions for each molecule of oxygen used, though 14 to 18 ions per
molecule of 02is more usual. It will be recalled that six molecules of ATP are produced for each molecule of oxygen utilized so that we can expect 2 or 3 sodium
ions to be transported for each molecule of ATP broken down.
The nature of the linkage of the phosphate bond energy provided by ATP, with
the 'ion jump' from one electro-chemical phase to another is not clear. SKOU in 1957
noted the association and similar properties of the enzyme Na-K ATPase with the
sodium 'pump' in peripheral nerves. This enzyme requires Mg2+ as well Na+ and
K+ for its activation and accelerates the breakdown of ATP to ADP with the release
of the energy and inorganic phosphate. The enzyme has been found in a wide variety of tissues noted for their abilit y to transport sodium actively and including,
apart from nerve, muscle, red blood cells and tissues especially noted for transepithelial ion transport, such as the kidney, 'salt' glands, gills and amphibian skin
and urinary bladder (see CSAKY, 1965). Histochemical and cell fractionation procedures indicate that the N a-K ATPase is localized in or near the cell membrane
across which 'ion jumps' may be expected to occur. In addition, the sodium ' pump'
and ATPase have a number of similar, rather specialized characteristics; both may
be inhibited in a similar way by the drug ouabain (strophanthin G) and can be
activated by sodium and potassium. It thus seems that ATPase and the Na ' pump'
are closely associat ed and it is possible that the enzyme itself acts as the final 'carrier'
for the ions, but the relationship and method of bringing about the final 'ion jump'
is unknown.
Hormones have not been shown to influence Na-K ATPase directly but may
do so indirectly by changing the local concentrations of the ion ic substrates and
possibly ATP. Levels of Na-K activated ATPase are somewhat labile depending
on environmental conditions. Increasing temperature can facilitate the amount of
this enzyme in the goldfish gut (SMITH, COLOMBO, and MUNN, 1968). Adaptation
to salt water increases N a-K ATPase in the gills of the eel (UTIDA et al., 1966) and
the killifish , Fundulus heteroclitus, (EpSTEIN, KATZ, and PICKFORD, 1967) and the
intestine of the eel, Anguilla japonica (OIDE, 1967). Ducks adapted to drinking
hypertonic saline solutions show an increased level of Na-K ATPase in their nasal
'salt' glands (FLETCHER, STAINER, and HOLMES, 1967). It is possible that hormones
may be involved in such changes .
Conclusion. For the purpose of our theme it is primarily important to know if solutes and water can move across a membrane in the animal, in which direction this
may take place, and whether such movement(s) require some metabolic intervention by the cell. The latter makes a 'control system' feasible, while precise information about such a metabolic link may suggest the nature of the ' signal' that
activates such a system.
14
cycle. As the supply of ATP is limiting to sodium transport, the reactions and substrates from which it is formed may influence the rate of sodium movement, and
it is at such sites that hormones which alter sodium transfer could be expected to
act.
Estimates of the number of ions transferred in relation to each molecule of ATP
hydrolysed depend principally on measurements of oxygen consumption in relation to the rate of sodium transport. Such estimates range from the transport of
4 to 28 sodium ions for each molecule of oxygen used, though 14 to 18 ions per
molecule of 02is more usual. It will be recalled that six molecules of ATP are produced for each molecule of oxygen utilized so that we can expect 2 or 3 sodium
ions to be transported for each molecule of ATP broken down.
The nature of the linkage of the phosphate bond energy provided by ATP, with
the 'ion jump' from one electro-chemical phase to another is not clear. SKOU in 1957
noted the association and similar properties of the enzyme Na-K ATPase with the
sodium 'pump' in peripheral nerves. This enzyme requires Mg2+ as well Na+ and
K+ for its activation and accelerates the breakdown of ATP to ADP with the release
of the energy and inorganic phosphate. The enzyme has been found in a wide variety of tissues noted for their abilit y to transport sodium actively and including,
apart from nerve, muscle, red blood cells and tissues especially noted for transepithelial ion transport, such as the kidney, 'salt' glands, gills and amphibian skin
and urinary bladder (see CSAKY, 1965). Histochemical and cell fractionation procedures indicate that the N a-K ATPase is localized in or near the cell membrane
across which 'ion jumps' may be expected to occur. In addition, the sodium ' pump'
and ATPase have a number of similar, rather specialized characteristics; both may
be inhibited in a similar way by the drug ouabain (strophanthin G) and can be
activated by sodium and potassium. It thus seems that ATPase and the Na ' pump'
are closely associat ed and it is possible that the enzyme itself acts as the final 'carrier'
for the ions, but the relationship and method of bringing about the final 'ion jump'
is unknown.
Hormones have not been shown to influence Na-K ATPase directly but may
do so indirectly by changing the local concentrations of the ion ic substrates and
possibly ATP. Levels of Na-K activated ATPase are somewhat labile depending
on environmental conditions. Increasing temperature can facilitate the amount of
this enzyme in the goldfish gut (SMITH, COLOMBO, and MUNN, 1968). Adaptation
to salt water increases N a-K ATPase in the gills of the eel (UTIDA et al., 1966) and
the killifish , Fundulus heteroclitus, (EpSTEIN, KATZ, and PICKFORD, 1967) and the
intestine of the eel, Anguilla japonica (OIDE, 1967). Ducks adapted to drinking
hypertonic saline solutions show an increased level of Na-K ATPase in their nasal
'salt' glands (FLETCHER, STAINER, and HOLMES, 1967). It is possible that hormones
may be involved in such changes .
Conclusion. For the purpose of our theme it is primarily important to know if solutes and water can move across a membrane in the animal, in which direction this
may take place, and whether such movement(s) require some metabolic intervention by the cell. The latter makes a 'control system' feasible, while precise information about such a metabolic link may suggest the nature of the ' signal' that
activates such a system.
14