certain, but for reasons of metabolic involvement associated with active sodium
transport and hormonal stimulation, a substantial movement would appear to take
place in this way. This is thought to involve at least two major steps, entry into
the epithelial cell across its mucosal (outer) border, and exit through the opposite
or lateral borders. LEAF in 1959 showed that the lactate that accumulates in the
bladder under anaerobic conditions, passes more readily into the serosal than into
the mucosal bathing fluid . This suggests differences in the permeability of the two
surfaces of the epithelial cells, and also that the mucosal barrier may have a limiting
action on molecular transfers. Net water movement onl y occurs down its concentration gradient, so there is no need to postulate a special energetic mechanism
for its transfer. Sodium, however, moves against a concentration gradient, so that
the presence of at least one sodium ' pump' is postulated, and this is usually considered to be present at the serosal boundary of the cell. Sodium entry from the
external solution (urine in vivo) is widely thought to take place by diffusion ;
though as this process exhibits saturation kinetics with respect to the external
sodium levels, some interaction of sodium with the membrane presumably occurs.
Sodium can be transported from external solutions as dilute as 104
M , but chemical
and electrical measurements of the intracellular media do not indicate that a sufficient gradient for sodium diffusion under such conditions is present. There may,
however, be a compartmentalization of cell sodium, as suggested in the frog skin
by CEREIJIDO and ROTUNNO (1967) and this may result in favourable local gradients
for diffus ion of sod ium into cells. Alternatively there could be a sodium 'pump'
at the mucosal barrier.
.
Such is the current view of the stage on which water, sodium, and the
neurohypophysial and adrenocortical hormones play. The anuran urinary bladder
and the tetrapod kidney share many physiological properties, including responses
to the same hormones, so that information about the amphibian membrane throws
light on the corresponding renal mechanisms.
a) Neurohypophysial Hormones. Eight of these hormones have been described in
different vertebrates, 8-arginine vasotocin (vasotocin, AVT) has the widest distribution while the vasopressins (ADH) are the most celebrated, probably because
of their pr esence in man and other mammals. These hormones are octapeptides
(see Page 55) containing a disulphide bridge, the breaking of which abolishes the
specific biological activity. They can increase the permeability of the tetrapod kidney tubule, and amphibian urinary bladder and skin, to water. In some instances
they also incre ase the rates of sodium and urea movement across the amphibian
membranes, while effects on the permeability of the gills of fish have also been
described. These pept ides have an add itional ability to constrict, or dilate, peripheral blood vessels, and to contract the smooth muscle of the uterus and gut.
The biological potency of the various natural hormones and their synthetic analogues varies considerably. Thus, when tested for its ability to increase the osmotic
permeability of the toad bladder, vasotocin is 200 times as active as the other amphibian neurohypophysial peptides, oxytocin and mesotocin, The antidiuretic effect of vasopressin in rats is 100 times greater than oxytocin. Vasopressin also acts
on the toad bladder, but is 100 times less active than vasotocin. These variations
in potency are thus associated with contrasting species, and with differences of one
83
transport and hormonal stimulation, a substantial movement would appear to take
place in this way. This is thought to involve at least two major steps, entry into
the epithelial cell across its mucosal (outer) border, and exit through the opposite
or lateral borders. LEAF in 1959 showed that the lactate that accumulates in the
bladder under anaerobic conditions, passes more readily into the serosal than into
the mucosal bathing fluid . This suggests differences in the permeability of the two
surfaces of the epithelial cells, and also that the mucosal barrier may have a limiting
action on molecular transfers. Net water movement onl y occurs down its concentration gradient, so there is no need to postulate a special energetic mechanism
for its transfer. Sodium, however, moves against a concentration gradient, so that
the presence of at least one sodium ' pump' is postulated, and this is usually considered to be present at the serosal boundary of the cell. Sodium entry from the
external solution (urine in vivo) is widely thought to take place by diffusion ;
though as this process exhibits saturation kinetics with respect to the external
sodium levels, some interaction of sodium with the membrane presumably occurs.
Sodium can be transported from external solutions as dilute as 104
M , but chemical
and electrical measurements of the intracellular media do not indicate that a sufficient gradient for sodium diffusion under such conditions is present. There may,
however, be a compartmentalization of cell sodium, as suggested in the frog skin
by CEREIJIDO and ROTUNNO (1967) and this may result in favourable local gradients
for diffus ion of sod ium into cells. Alternatively there could be a sodium 'pump'
at the mucosal barrier.
.
Such is the current view of the stage on which water, sodium, and the
neurohypophysial and adrenocortical hormones play. The anuran urinary bladder
and the tetrapod kidney share many physiological properties, including responses
to the same hormones, so that information about the amphibian membrane throws
light on the corresponding renal mechanisms.
a) Neurohypophysial Hormones. Eight of these hormones have been described in
different vertebrates, 8-arginine vasotocin (vasotocin, AVT) has the widest distribution while the vasopressins (ADH) are the most celebrated, probably because
of their pr esence in man and other mammals. These hormones are octapeptides
(see Page 55) containing a disulphide bridge, the breaking of which abolishes the
specific biological activity. They can increase the permeability of the tetrapod kidney tubule, and amphibian urinary bladder and skin, to water. In some instances
they also incre ase the rates of sodium and urea movement across the amphibian
membranes, while effects on the permeability of the gills of fish have also been
described. These pept ides have an add itional ability to constrict, or dilate, peripheral blood vessels, and to contract the smooth muscle of the uterus and gut.
The biological potency of the various natural hormones and their synthetic analogues varies considerably. Thus, when tested for its ability to increase the osmotic
permeability of the toad bladder, vasotocin is 200 times as active as the other amphibian neurohypophysial peptides, oxytocin and mesotocin, The antidiuretic effect of vasopressin in rats is 100 times greater than oxytocin. Vasopressin also acts
on the toad bladder, but is 100 times less active than vasotocin. These variations
in potency are thus associated with contrasting species, and with differences of one
83