moment they afford the strongest available evidence for a urophysial role in piscine
osmoregulation. Electrical recordings from caudal neurosecretory cells have shown
that they can respond to osmotic changes in the fish. Injection of distilled water
into the marine teleost, Paralichthys dentatus, increased the rate of discharge of
these cells (BENNETf and Fox, 1962), while in the freshwater species, Tilapia mossambica, Y AGI and BERN (1963) found that there was an increased rate of discharge
when the gills were exposed to saline . These effects were further examined in Tilapia and it was found that some of the neuro-secretory fibres were stimulated by
high sodium concentrations while others responded to sodium-free solutions
(YAGI and BERN, 1965). The specificity of such stimuli to the particular neural activity is in question.
The biological activity of urophysial lobe extracts has been examined pharmacologically in order to characterize it, and in an attempt to identify it with other
known biological agents. Possible homologies with the neurohypophysial peptides
have been explored without success (SAWYER and BERN, 1963), though the extracts
do exhibit an ability to increase the permeability of the frog urinary bladder to
water and this effect has also been shown in toads (LACANILAO, 1969). Extracts
of this tissue also have the ability to produce water retention in toads and elevate
blood pressure of eels j the latter effect was not abolished by incubation with
sodium thioglycollate which distinguishes it from neurohypophysial peptides
(BERN et al., 1967). The active material thus mimics neurohypophysial peptides
in some of its actions. The effects on anuran membranes are particularly interesting,
as apart from neurohypophysial-type peptides, only the nucleotide cyclic AMP, in
high concentrations, is known to change their osmotic permeability. The chemical
nature of the materials in the urophysis will thus be of considerable interest.
10. Mechanisms of Hormone Action
(with special reference to osmoregulation)
The responses of 'target' or 'effector' organs to hormones can be classified broadly
into four groups:
1. Trophic (or tropic) actions on other endocrine glands, such as the effect of
gonadotrophins on the gonads and corticotrophin on the adrenal cortex.
2. Metabolic effects, such as the formation of proteins and glucose from amino
acids, interconversions between fatty acids, glucose and glycogen, and the increased
catabolism of substrates which provide energy. This category can also include the
interconversions and transfer of mineral reserves such as calcium and phosphate
from 'fixed stores', as in the bones, to a more mobile form in the plasma. Hormones
that act in this way include insulin, growth hormone, the various gonadal and adrenocortical steroids, as well as the thyroid and parathyroid secretions.
3. Mechanical responses, like the contraction and relaxation of the smooth
(nonstriated, involuntary) muscle that surrounds the blood vessels, oviducts, uterus
and the gut. Adrenaline, oxytocin and vasopressin may initiate such effects.
4. Permeability changes in membranes, which result in changes in movements
of water and ions across the tubules of secretory glands, like the kidneys, the wall
80
osmoregulation. Electrical recordings from caudal neurosecretory cells have shown
that they can respond to osmotic changes in the fish. Injection of distilled water
into the marine teleost, Paralichthys dentatus, increased the rate of discharge of
these cells (BENNETf and Fox, 1962), while in the freshwater species, Tilapia mossambica, Y AGI and BERN (1963) found that there was an increased rate of discharge
when the gills were exposed to saline . These effects were further examined in Tilapia and it was found that some of the neuro-secretory fibres were stimulated by
high sodium concentrations while others responded to sodium-free solutions
(YAGI and BERN, 1965). The specificity of such stimuli to the particular neural activity is in question.
The biological activity of urophysial lobe extracts has been examined pharmacologically in order to characterize it, and in an attempt to identify it with other
known biological agents. Possible homologies with the neurohypophysial peptides
have been explored without success (SAWYER and BERN, 1963), though the extracts
do exhibit an ability to increase the permeability of the frog urinary bladder to
water and this effect has also been shown in toads (LACANILAO, 1969). Extracts
of this tissue also have the ability to produce water retention in toads and elevate
blood pressure of eels j the latter effect was not abolished by incubation with
sodium thioglycollate which distinguishes it from neurohypophysial peptides
(BERN et al., 1967). The active material thus mimics neurohypophysial peptides
in some of its actions. The effects on anuran membranes are particularly interesting,
as apart from neurohypophysial-type peptides, only the nucleotide cyclic AMP, in
high concentrations, is known to change their osmotic permeability. The chemical
nature of the materials in the urophysis will thus be of considerable interest.
10. Mechanisms of Hormone Action
(with special reference to osmoregulation)
The responses of 'target' or 'effector' organs to hormones can be classified broadly
into four groups:
1. Trophic (or tropic) actions on other endocrine glands, such as the effect of
gonadotrophins on the gonads and corticotrophin on the adrenal cortex.
2. Metabolic effects, such as the formation of proteins and glucose from amino
acids, interconversions between fatty acids, glucose and glycogen, and the increased
catabolism of substrates which provide energy. This category can also include the
interconversions and transfer of mineral reserves such as calcium and phosphate
from 'fixed stores', as in the bones, to a more mobile form in the plasma. Hormones
that act in this way include insulin, growth hormone, the various gonadal and adrenocortical steroids, as well as the thyroid and parathyroid secretions.
3. Mechanical responses, like the contraction and relaxation of the smooth
(nonstriated, involuntary) muscle that surrounds the blood vessels, oviducts, uterus
and the gut. Adrenaline, oxytocin and vasopressin may initiate such effects.
4. Permeability changes in membranes, which result in changes in movements
of water and ions across the tubules of secretory glands, like the kidneys, the wall
80