facie evidence thus suggests that neurohypophysial peptides have a physiological
role in controlling urine volume and concentration in rodents, just as they are
thought to do in other orders of placental mammals.
The ability to limit urinary water loss may vary among species in relation to
the dryness of the normal habitat. Extrarenal water loss, however, predominates,
and in white rats kept in the laboratory without water it is 5 times greater than
the urinary loss, while in the kangaroo rat it is 2.5 times as great (see DICKER and
NUNN, 1957). Under such conditions these rodents form hyperosmotic urine,
which in the white rat may be 9 times more concentrated than the plasma, and in
the kangaroo rat 14 times that level (SCHMIDT-NIELSEN, 1964a). In the absence of
neurohypophysial antidiuretic hormones, urine that is isoosmotic to plasma is
formed, so that the total water loss under such conditions would be expected to
increase considerably. Urinary loss would then be more than twice as great as the
extrarenal loss. The role of such hormones in forming a concentrated urine should
be kept in perspective as the primary mechanism for such water conservation exists
in the kidney itself, while it is the hormones' role to activate this .
The diversity in the habitats occupied by different rodents, and the variation
in their osmoregulatory problems, suggests that there may be associated differences
in neurohypophysial function. The known differences in ability to concentrate the
urine are related to the physiology of the kidney, not to quantitative differences
in circulating vasopressin levels, though the requirements for such peptides may
conceivably differ in contrasting osmotic circumstances. As we have seen, either
of two vasopressins may exist in mammals and it is pertinent to see if such a difference affects the water metabolism. Differences in the antidiuretic potency of
neurohypophysial peptides, if extreme, could result in a condition of diabetes insipidus, or if more moderate, necessitate differing circulating levels to produce
comparable effects. The antidiuretic potencies of lysine - and arginine - vasopressin
differ in various species . In the dog, which normally possesses the peptide containing arginine, the lysine analogue is only one-sixth as potent as the natural hormone,
while in man it is about one-half as active. The pig possesses lysine-vasopressin,
but its kidney, nevertheless, is only two-thirds as sensitive to this as it is to the exogenous arginine analogue (MUNSICK, SAWYER, and VAN DYKE, 1958). The antidiuretic potencies of arginine - and lysine - vasopressin have been examined in laboratory rats and the latter here also has only about two-thirds the activity of the
other peptide (SAWYER, 1958). Such differences in the relative potencies of these
natural pep tides are not great enough to affect the water metabolism of the species
possessing them. It is interesting that in some of the Suiformes, either or both such
peptide hormones may be present (FERGUSON and HELLER, 1965) in animals living
together in the same area. DOUGLAS FERGUSON (1969) has measured the distribution of these pep tides in a group of 18 warthogs, Phacochoerus aethiopicus,
from Rhodesia. Eight of these beasts contained arginine-vasopressin only, two had
lysine-vasopressin only, while eight were heterozygous and contained both peptides. This distribution appears to be random, and suggests that the possession of
a particular vasopressin confers no selective advantage to anyone warthog over
another with a different peptide. It is possible that in other populations of mammals, living in different circumstances, this could be of advantage, but it appears
to be unlikely.
106
role in controlling urine volume and concentration in rodents, just as they are
thought to do in other orders of placental mammals.
The ability to limit urinary water loss may vary among species in relation to
the dryness of the normal habitat. Extrarenal water loss, however, predominates,
and in white rats kept in the laboratory without water it is 5 times greater than
the urinary loss, while in the kangaroo rat it is 2.5 times as great (see DICKER and
NUNN, 1957). Under such conditions these rodents form hyperosmotic urine,
which in the white rat may be 9 times more concentrated than the plasma, and in
the kangaroo rat 14 times that level (SCHMIDT-NIELSEN, 1964a). In the absence of
neurohypophysial antidiuretic hormones, urine that is isoosmotic to plasma is
formed, so that the total water loss under such conditions would be expected to
increase considerably. Urinary loss would then be more than twice as great as the
extrarenal loss. The role of such hormones in forming a concentrated urine should
be kept in perspective as the primary mechanism for such water conservation exists
in the kidney itself, while it is the hormones' role to activate this .
The diversity in the habitats occupied by different rodents, and the variation
in their osmoregulatory problems, suggests that there may be associated differences
in neurohypophysial function. The known differences in ability to concentrate the
urine are related to the physiology of the kidney, not to quantitative differences
in circulating vasopressin levels, though the requirements for such peptides may
conceivably differ in contrasting osmotic circumstances. As we have seen, either
of two vasopressins may exist in mammals and it is pertinent to see if such a difference affects the water metabolism. Differences in the antidiuretic potency of
neurohypophysial peptides, if extreme, could result in a condition of diabetes insipidus, or if more moderate, necessitate differing circulating levels to produce
comparable effects. The antidiuretic potencies of lysine - and arginine - vasopressin
differ in various species . In the dog, which normally possesses the peptide containing arginine, the lysine analogue is only one-sixth as potent as the natural hormone,
while in man it is about one-half as active. The pig possesses lysine-vasopressin,
but its kidney, nevertheless, is only two-thirds as sensitive to this as it is to the exogenous arginine analogue (MUNSICK, SAWYER, and VAN DYKE, 1958). The antidiuretic potencies of arginine - and lysine - vasopressin have been examined in laboratory rats and the latter here also has only about two-thirds the activity of the
other peptide (SAWYER, 1958). Such differences in the relative potencies of these
natural pep tides are not great enough to affect the water metabolism of the species
possessing them. It is interesting that in some of the Suiformes, either or both such
peptide hormones may be present (FERGUSON and HELLER, 1965) in animals living
together in the same area. DOUGLAS FERGUSON (1969) has measured the distribution of these pep tides in a group of 18 warthogs, Phacochoerus aethiopicus,
from Rhodesia. Eight of these beasts contained arginine-vasopressin only, two had
lysine-vasopressin only, while eight were heterozygous and contained both peptides. This distribution appears to be random, and suggests that the possession of
a particular vasopressin confers no selective advantage to anyone warthog over
another with a different peptide. It is possible that in other populations of mammals, living in different circumstances, this could be of advantage, but it appears
to be unlikely.
106