glycogenolysis, plays an established biochemical role . In other instances, when the
result is a change in membrane permeability, the nature of the final steps is unknown, though they are the subject of much speculation.
a) Mechanism of Action of Neurohypophysial Peptides and Aldosterone
The mechanism of action of two groups of hormones, the peptides from the neurohypophysis and steroids from the adrenal cortex, is particularly relevant to our
theme of osmoregulation. These hormones can alter the transfer of water, sodium
and potassium across epithelial membranes.
Our knowledge of the mechanism of action of the neurohypophysial and cor -
ticosteroid hormones is derived principally from in vitro studies of the urinary
bladder of anuran amphibians. This membrane survives and functions for many
hours in physiological-saline solutions (such as RINGER solution).
EWER (1952 a) showed that an African toad can reabsorb water from its urinary
bladder at an accelerated rate when dehydrated, or injected with neurohypophysial
pep tides. Apart from being physiologically useful to many of the Amphibia, this
organ furnishes an admirable, and unique, preparation for studying hormonal effects on membrane permeability. The anuran urinary bladder in vitro can actively
transport sodium from mucosal to serosal side, a function that is accelerated by
neurohypophysial hormones (LEAF, ANDERSON, and PAGE, 1959) and aldosterone
(CRABBE, 1961 a, b). It is also osmotically permeable to water and this is increased
by neurohypophysial pep tides (BENTLEY, 1958).
Structurally the anuran urinary bladder is made up of a layer of epithelial cells,
supported by a thin serosal membrane, containing connective tissue, blood vessels
and interdigitating smooth muscle fibres. The epithelial cells line the inside of the
bladder, their outer borders (mucosal side) being bathed in vivo by the urine. The
epithelial cells are the portion of the tissue that is responsible for the special properties of the membrane with respect to movements of water and sodium, and are
the effector sites for the hormones. Three different types of cells make up this layer.
The predominant cells present (about 85%) are termed'granulated epithelial cells'
and are characterized by apical concentrations of small dense granules and a paucity
of mitochondria. About 10% of the epithelial cells contain numerous mitochondria
and are called 'mitochondria-rich cells'. In addition to these two cell-types, there
are also a few mucous cells present. It is uncertain whether the 'mitochondria rich'
and 'granulated cells' have special functions, but they exhibit different osmotic behaviour. 'Mitochondria-rich cells' swell more than 'granulated cells' when exposed
to toxic concentrations of the polyene antibiotic amphotericin B, an effect related
to their sodium permeability (SALADINO, BENTLEY, and TRUMP, 1969). On the
other hand, 'granulated cells', but not 'mitochondria-rich cells', swell when they
are exposed to vasopressin, an effect resulting from an increase in their osmotic
permeability (DIBONA, CIVAN, and LEAF, 1969). It is likely that the 'mitochondriarich cells' and 'granulated cells' playa special role in the response of the tissue to
hormones, but at this time the evidence is incomplete.
Molecules may conceivably move across the anuran urinary bladder by intercellular and intracellular routes. The magnitude of intercellular transfer is un82
result is a change in membrane permeability, the nature of the final steps is unknown, though they are the subject of much speculation.
a) Mechanism of Action of Neurohypophysial Peptides and Aldosterone
The mechanism of action of two groups of hormones, the peptides from the neurohypophysis and steroids from the adrenal cortex, is particularly relevant to our
theme of osmoregulation. These hormones can alter the transfer of water, sodium
and potassium across epithelial membranes.
Our knowledge of the mechanism of action of the neurohypophysial and cor -
ticosteroid hormones is derived principally from in vitro studies of the urinary
bladder of anuran amphibians. This membrane survives and functions for many
hours in physiological-saline solutions (such as RINGER solution).
EWER (1952 a) showed that an African toad can reabsorb water from its urinary
bladder at an accelerated rate when dehydrated, or injected with neurohypophysial
pep tides. Apart from being physiologically useful to many of the Amphibia, this
organ furnishes an admirable, and unique, preparation for studying hormonal effects on membrane permeability. The anuran urinary bladder in vitro can actively
transport sodium from mucosal to serosal side, a function that is accelerated by
neurohypophysial hormones (LEAF, ANDERSON, and PAGE, 1959) and aldosterone
(CRABBE, 1961 a, b). It is also osmotically permeable to water and this is increased
by neurohypophysial pep tides (BENTLEY, 1958).
Structurally the anuran urinary bladder is made up of a layer of epithelial cells,
supported by a thin serosal membrane, containing connective tissue, blood vessels
and interdigitating smooth muscle fibres. The epithelial cells line the inside of the
bladder, their outer borders (mucosal side) being bathed in vivo by the urine. The
epithelial cells are the portion of the tissue that is responsible for the special properties of the membrane with respect to movements of water and sodium, and are
the effector sites for the hormones. Three different types of cells make up this layer.
The predominant cells present (about 85%) are termed'granulated epithelial cells'
and are characterized by apical concentrations of small dense granules and a paucity
of mitochondria. About 10% of the epithelial cells contain numerous mitochondria
and are called 'mitochondria-rich cells'. In addition to these two cell-types, there
are also a few mucous cells present. It is uncertain whether the 'mitochondria rich'
and 'granulated cells' have special functions, but they exhibit different osmotic behaviour. 'Mitochondria-rich cells' swell more than 'granulated cells' when exposed
to toxic concentrations of the polyene antibiotic amphotericin B, an effect related
to their sodium permeability (SALADINO, BENTLEY, and TRUMP, 1969). On the
other hand, 'granulated cells', but not 'mitochondria-rich cells', swell when they
are exposed to vasopressin, an effect resulting from an increase in their osmotic
permeability (DIBONA, CIVAN, and LEAF, 1969). It is likely that the 'mitochondriarich cells' and 'granulated cells' playa special role in the response of the tissue to
hormones, but at this time the evidence is incomplete.
Molecules may conceivably move across the anuran urinary bladder by intercellular and intracellular routes. The magnitude of intercellular transfer is un82