“Tight junctions” between cells in some epithelia actually provide the main route of passive ion permeation. The degree of junctional tightness may underlie important functional differences between different epithelia.

Route of passive ion permeation in epithelia.

GRANTHAM, JARED J., AND MAURICE B. BURG. Effect ofuasopressin and cyclic AMP on permeability of isolated collecting tubules. Am. J. Physiol. 21 I (I) : 255-259. rg66.-The effect of vasopressin and cyclic 3’,5’-AMP on the permeability to water and urea of the isolated perfused rabbit collecting tubule was measured. When added to the solution bathing the outside of the tubule, these agents regularly increased both net water absorption along an osmotic gradient and the diffusional permeability to water as measured with THO in the absence of an osmotic gradient. When added only to the solution bathing the luminal border of the cells, vasopressin was ineffective. Urea permeability was not significantly altered by either vasopressin or cyclic 3’) $-AMP despite simultaneous increases in water permeability.

Effect of vasopressin and cyclic AMP on permeability of isolated collecting tubules

1 Amiloride inhibits Na transport and short-circuit current (SCC) across the toad bladder. It is 1000 times more active at the mucosal than serosal surface. The lowest effective concentration was 10−7 M.

2. The inhibition was non-competitive with the sodium on the mucosal side of the bladder.

3. Vasopressin, cyclic adenosine monophosphate (AMP) and aldosterone increased Na transport and SCC across the bladder and these effects were inhibited by amiloride.

4. The antagonism of amiloride for vasopressin was non-competitive.

5. Amphotericin B also increases Na transport across the bladder but its action was not changed by amiloride.

6. Amiloride was without effects on SCC and diffusion potentials in bladders metabolically inhibited with CN− and iodoacetic acid (IAA).

7. Neither plasma albumin, Ca2+ nor adenosine triphosphate (ATP) altered the effects of amiloride.

8. The only structural analogue of amiloride found to reduce SCC similarly was guanidine which was 1000 times less active. Pyrazine and a substituted pyrazine analogue were without effect. Neither guanidine nor the substituted pyrazine compound were competitive with amiloride.

9. Amiloride had no effect on the osmotic permeability of the toad bladder either in the presence or absence of vasopressin.

10. Na transport across the toad colon was also reduced by 10−5 M amiloride at the mucosal surface.

11. The possible mechanism of action of amiloride is discussed.

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1968.sp008460

Amiloride: a potent inhibitor of sodium transport across the toad bladder

Summary

1. Urinary bladders are found in the amphibia, chelonian reptiles and mammals. In these orders liquid urine is stored in the bladder and eliminated at intervals from the body by micturation.



2. In the amphibia and chelonian reptiles, the urinary bladder is a functional extension of the renal tubules. The composition of the urine in the bladder is modified by the active movement of water and ions across the bladder wall, and these transporting processes are under hormonal control. The bladder acts as a water reservoir which can be drawn upon in times of water shortage.



3. The mammalian bladder separates two widely differing water phases, namely the urine which is frequently hypertonic to the blood and the tissue fluids which are isotonic. Its function is uniquely one of storage, and no adjustment to the composition of the urine is made by active transport of either water or ions across the bladder wall.



4. The epithelium lining the mammalian bladder is the site of the osmotic barrier between urine and tissue fluids. This functional barrier is dependent on the structure of the epithelium and is maintained despite large alterations in the surface area of the epithelium as the bladder rapidly contracts, or slowly dilates.



5. The epithelium is of mixed mesodermal and endodermal origin, is transitional in type and is usually 3 or 4 cell-layers thick. If this urothelium is damaged, it has a high capacity for regeneration and rapidly re-establishes an intact barrier over the luminal surface.



6. The superficial cell layer of this epithelium is composed of large, polyploid, highly differentiated squamous cells which have a long life span. These cells are limited on their free surface by an unusual, angular, semi-rigid luminal membrane. This membrane is assembled in the Golgi complex.



7. The luminal membrane is composed of thickened, discoidal plaques, separated by narrow bands of thinner membrane. When the bladder contracts, the membrane folds along the thinner ‘hinge’ regions, and the rigid discoidal plates invaginate to form fusiform, cytoplasmic vacuoles. The thickened plaques contain a hexagonal lattice of sub-units, spaced at 14 nm centre-to-centre. Each sub-unit in the lattice is itself composed of 12 smaller particles. These particles may be envisaged as small rods 3 nm in diameter and 12 nm long, and are inserted into matrix from which they project on the luminal face by about 3 nm. Each rod has a central hydrophobic portion separating distal hydrophilic ends.



8. The chemical composition of this luminal membrane is unusual. Cerebroside is a major component of the polar lipid fraction and there is an unusually high proline content in the protein fraction. When the mucoproteins are adequately dispersed, and the proteins separated by electrophoresis, a few major proteins are revealed in 33000–80000 dalton range of molecular weight.



9. If the normal structure of the luminal membrane is altered, either by physical damage or by failure of the cells to produce it, the barrier function of the epithelium is lost.



10. The structure and function of this membrane depend ultimately on its chemical composition. Cerebroside is known to decrease the permeability of lipid bi-layers to water, but for maximum impermeability a lipid bi-layer must be maintained in a condensed configuration. The stresses of bladder distension and contraction might be expected to disrupt the bi-layer, and it is suggested that the function of the rigid plaque regions is to reduce mechanical stresses in the membrane to a minimum. The plaque areas occupy between 73 and 90 % of the membrane surface, and only the remaining 10–27% of the membrane is thus subject to bending and distortion when the bladder contracts or expands. The structure of the plaque areas is probably determined by the nature of the complex proteins which form the sub-units. Proline is known to confer rigidity on polypeptide chains, and may play an important role in ordering the structure of the plaques.



11. The bladder epithelium, though normally differentiated as a transitional epithelium, has other biologicai potentialities. It can undergo squamous metaplasia to form a stratified cornified epithelium in response to mechanical irritation and/or vitamin A deficiency. If transplanted from its normal location, it can induce other supporting mesenchyme tissues to lay down bone. When regenerating in response to damage, the newly formed transitional cells can act as phagocytes and engulf and digest damaged or dying cells. In the normal animal the epithelium is largely protected from tumour formation by cell-mediated immunological surveillance. The defensive mechanisms are triggered by tissue-type specific antigens which develop in neoplastic bladder epithelial cells.

The mammalian urinary bladderan accommodating organ

Stoichiometry and localization of adenosine triphosphate-dependent sodium and potassium transport in the erythrocyte.

Studies were made of the active ion transport by the isolated urinary bladder of the European toad, Bufo bufo, and the large American toad, Bufo marinus. The urinary bladder of the toad is a thin membrane consisting of a single layer of mucosal cells supported on a small amount of connective tissue. The bladder exhibits a characteristic transmembrane potential with the serosal surface electrically positive to the mucosal surface. Active sodium transport was demonstrated by the isolated bladder under both aerobic and anaerobic conditions. Aerobically the mean net sodium flux across the bladder wall measured with radioactive isotopes, Na(24) and Na(22), just equalled the simultaneous short-circuit current in 42 periods each of 1 hour's duration. The electrical phenomenon exhibited by the isolated membrane was thus quantitatively accounted for solely by active transport of sodium. Anaerobically the mean net sodium flux was found to be slightly less than the short-circuit current in 21 periods of observation. The cause of this discrepancy is not known. The short-circuit current of the isolated toad bladder was regularly stimulated with pure oxytocin and vasopressin when applied to the serosal surface under aerobic and anaerobic conditions. Adrenaline failed to stimulate the short-circuit current of the toad bladder.

Lot B. Page

Papers

Active sodium transport by the isolated toad bladder.

Respiration and active sodium transport of isolated toad bladder.

Childhood Hypertension Etiology, Investigation and Treatment