a high content of sodium chloride (BURNS, 1956). The adrenal cortex of laboratory
rats secretes aldosterone and corticosterone, but little, if any, cortisol. BUSH (1953)
found that the ratio in the secretion of cortisol /corticosterone (compounds FIB)
was less than 0.05. The banner-tailed kangaroo rat, on the other hand, also forms
cortisol and has an FIB ratio of about 1.4. BUSH, after comparing FIB ratios in
a number of species of placenrals which normally have a differing diet and way
of life, concluded that such differences are probably genetically determined variations of the biosynthetic pathways in the adrenal cortex, and cannot be related
to differences in the role of the corticosteroids.
The rates of corticosteroid secretion in desert rodents have not been directly
determined. Indirect evidence, however, suggests that the kangaroo rat may have
low circulating levels of cortisol and corticosterone but a high concentration of
aldosterone (COLE et al., 1963). These conclusions are tentative and largely based
on the renal deficiencies in excreting water and sodium, a facility in secreting
potassium and a ready response to anti-aldosterone drugs. Aldosterone could contribute to the kangaroo rat's ability to concentrate urine by facilitating gradients
of sodium concentration associated with the counter current-multiplier system of
the renal tubule. CRABBE (1962) has shown that aldosterone increases the ability
of man to concentrate urine and has suggested such a mechanism . Adrenalectomized rats have a decreased ability to concentrate urine (SIGLER, FORREST, and ELKINGTON, 1965) an observation that would be consistent with the above hypothesis.
Corticotrophin initiates release of corticosterone and aldosterone in rats, but
the role of the renin-angiotensin system has been in doubt as injection of angiotensin has little effect on release of these hormones. KINSON and SINGER (1968)
have confirmed these observations but find that they are confined to sodium-replete rats, for if sodium-deficient animals are used, angiotensin stimulates release of
aldosterone (but not corticosterone). The role of corticotrophin and renin in controlling corticosteroid secretion in the Rodentia thus appears to conform to the
usual placental pattern. There may, nevertheless, be some differences in die physiological and biochemical details, that result in variations in sensitivity to these trophic hormones.
While there is probably more information about osmoregulation in diverse species of the Rodentia than in any other mammalian order, this is unfortunately still
very incomplete. Research has been mainly directed towards laboratory species
like the rat and a small number of more exotic animals that live principally in desert
areas. The comparative information about osmoregulation in the Rodentia is thus
somewhat lopsided, as, with the exception of Aplodontia, we have no such knowledge about species living at the aquatic extremes of the range. Such water dwellers
are numerous among the rodents. These species are not only inherently interesting,
but may provide information that can be used to assess better the significance of
physiological and endocrine variations thought to be associated with the osmoregulatory problems of the animals. Thus measurements of evaporative water
loss, urine concentrations, storage and secretion of neurohypophysial pep tides and
adrenocorticosteroids in aquatic rodents would yield interesting data for comparison with similar measurements in desert rodents.
The sea provides an ecological niche for many mammals and although fragmentary information about the water and salt metabolism of such species is avail109
rats secretes aldosterone and corticosterone, but little, if any, cortisol. BUSH (1953)
found that the ratio in the secretion of cortisol /corticosterone (compounds FIB)
was less than 0.05. The banner-tailed kangaroo rat, on the other hand, also forms
cortisol and has an FIB ratio of about 1.4. BUSH, after comparing FIB ratios in
a number of species of placenrals which normally have a differing diet and way
of life, concluded that such differences are probably genetically determined variations of the biosynthetic pathways in the adrenal cortex, and cannot be related
to differences in the role of the corticosteroids.
The rates of corticosteroid secretion in desert rodents have not been directly
determined. Indirect evidence, however, suggests that the kangaroo rat may have
low circulating levels of cortisol and corticosterone but a high concentration of
aldosterone (COLE et al., 1963). These conclusions are tentative and largely based
on the renal deficiencies in excreting water and sodium, a facility in secreting
potassium and a ready response to anti-aldosterone drugs. Aldosterone could contribute to the kangaroo rat's ability to concentrate urine by facilitating gradients
of sodium concentration associated with the counter current-multiplier system of
the renal tubule. CRABBE (1962) has shown that aldosterone increases the ability
of man to concentrate urine and has suggested such a mechanism . Adrenalectomized rats have a decreased ability to concentrate urine (SIGLER, FORREST, and ELKINGTON, 1965) an observation that would be consistent with the above hypothesis.
Corticotrophin initiates release of corticosterone and aldosterone in rats, but
the role of the renin-angiotensin system has been in doubt as injection of angiotensin has little effect on release of these hormones. KINSON and SINGER (1968)
have confirmed these observations but find that they are confined to sodium-replete rats, for if sodium-deficient animals are used, angiotensin stimulates release of
aldosterone (but not corticosterone). The role of corticotrophin and renin in controlling corticosteroid secretion in the Rodentia thus appears to conform to the
usual placental pattern. There may, nevertheless, be some differences in die physiological and biochemical details, that result in variations in sensitivity to these trophic hormones.
While there is probably more information about osmoregulation in diverse species of the Rodentia than in any other mammalian order, this is unfortunately still
very incomplete. Research has been mainly directed towards laboratory species
like the rat and a small number of more exotic animals that live principally in desert
areas. The comparative information about osmoregulation in the Rodentia is thus
somewhat lopsided, as, with the exception of Aplodontia, we have no such knowledge about species living at the aquatic extremes of the range. Such water dwellers
are numerous among the rodents. These species are not only inherently interesting,
but may provide information that can be used to assess better the significance of
physiological and endocrine variations thought to be associated with the osmoregulatory problems of the animals. Thus measurements of evaporative water
loss, urine concentrations, storage and secretion of neurohypophysial pep tides and
adrenocorticosteroids in aquatic rodents would yield interesting data for comparison with similar measurements in desert rodents.
The sea provides an ecological niche for many mammals and although fragmentary information about the water and salt metabolism of such species is avail109