Increase In Sodium Transport Research Articles

Action of amphotericin B on the toad bladder: evidence for sodium transport along two pathways.

1. The relationship of short-circuit current (SCC, sodium transport) and potential difference (p.d.) across the toad bladder to the concentration of sodium on the mucosal side was measured. Maximal values were attained when the Na concentration was 10 mM for SCC and 30 mM for p.d.2. Amphotericin B increased SCC across bladders bathed on their mucosal surface with isotonic (115 mM) NaCl solution and this effect was not inhibited by cyanide and iodoacetate. The osmotic permeability of the bladder was also increased by amphotericin B.3. Vasopressin and aldosterone increased sodium transport from solutions of low Na concentration, but amphotericin B was ineffective until a level of about 40-60 mM was present.4. The evidence suggests two pathways for sodium transport across the bladder distinguished by differences in their electrochemical gradients with the fluid at the mucosal side.

On the Stimulation of Sodium Transport by Aldosterone

Aldosterone, the major sodium retaining hormone in man, will stimulate active transport of sodium across the urinary bladder of the toad, Bufo marinus in vitro, at physiological concentrations of the hormone. The in vitro action of aldosterone is mimicked by steroid hormones with known mineralocorticoid properties and it is competitively inhibited by other analogues, e.g. spironolactone and cortisone. Aldosterone is bound to physiological receptor sites within the transporting epithelial cells, chiefly within the nuclei, and is displaced from these binding sites specifically by structural analogues including other mineralocorticoids. Effects of aldosterone are dependent upon availability of metabolizable substrates to support the active transport of sodium. Although the stimulation of sodium transport by aldosterone can be specifically inhibited by actinomycin D, an inhibitor of RNA synthesis, and by puromycin, an inhibitor of protein synthesis, direct evidence of stimulation of new RNA and protein synthesis during the latent period with physiological concentrations of aldosterone is still lacking. It is possible, however, that the amounts of RNA and protein that are involved are too small to be detected by available techniques. Evidence is summarized which leads us to conclude that the increased sodium transport induced by aldosterone is the consequence of a reduced resistance of the apical plasma membrane of the transporting epithelia to the entry of sodium into the transport pathway.

Erythrocyte sodium transport in hereditary elliptocytosis.

The active and passive efflux of sodium from erythrocyte membranes was studied in 13 subjects with hereditary elliptocytosis and in 15 normal adult controls. The results indicate that the elliptocytic red ceil membrane is not abnormally permeable to sodium. The active sodium efflux is, however, significantly greater in elliptocytic erythrocyte membranes and suggests that a primary membrane abnormality is responsible for the increased sodium transport.

ACTION OF ALDOSTERONE ON THE BLADDER AND SKIN OF THE TOAD.

ALDOSTERONE stimulates the active transport of sodium across the isolated urinary bladder and ventral skin of the toad (Bufo marinus), after treatment of the animal with the hormone and incubation of these membranes in its presence1–3. With the toad bladder an interval of 1 h approximately elapses at room temperature between addition of aldosterone to the incubation fluid and the first noticeable increase in sodium transport, regardless of the concentration of the steroid. Yet, the amounts of aldosterone fixed on the bladder tissue exceeded medium concentrations after 15 min incubation2. It was, therefore, proposed that aldosterone exerts its influence on the process under study via the stimulation of synthesis of a substance involved in the active transport of sodium1. In view of the growing body of information pointing at a close relationship between protein synthesis and hormonal steroid action4,5 it was thought that aldosterone might exert its action on this metabolic pathway. Investigations were, therefore, made with antibiotics characterized by their inhibiting properties on definite steps of protein synthesis. ‘Puromycin’ is at present considered as blocking ribosomal elaboration of proteins6, while actinomycin D presumably acts in the cell nucleus on the DNA-dependent synthesis of RNA7,8.

Effects of neurohypophysial hormones on oxidative metabolism of the toad bladder in vitro.

Oxytocin and vasopressin stimulate the metabolism of C14-labeled substrates to C14O2 by the toad urinary bladder. The hormone effect is selective: the metabolism of glucose, pyruvate, lactate and palmitate is stimulated, while that of acetate and succinate is not. The metabolic effect of these hormones is not eliminated by alterations of the ionic medium which reduce active sodium transport to very low levels. It is suggested that the neurohypophysial hormones have a direct effect on toad bladder metabolism in addition to the induction of increased permeability and increased sodium transport.

Action des deux peptides neurohypophysaires sur le transport actif de sodium et le flux net d'eau à travers la peau de diverses espèces de batraciens anoures

The effect of oxytocin (extracted or synthesized) and of antidiuretic hormone (ADH) on the active transport of sodium (recorded by short-circuit current) and on the net flux of water has been studied in vitro on skins of three species of Anura: Rana esculenta, Rana temporaria and Bufo bufo. In Rana esculenta, at concentrations of 0.15 milliunits/ml or higher, oxytocin greatly increases sodium transport and net flux of water, while ADH has no effect. In Rana temporaria, the influence of oxytocin on the transport of sodium is less marked, but it is still far more clearly discernible than that of ADH. In Bufo bufo, the two hormones are equally effective, both increasing the net flux of sodium and of water. Apart from these differences as regards species, this investigation has demonstrated that the action of the hormones on the active uptake of sodium through the skin and on its filtration permeability go hand in hand, and are closely correlated. At least in Rana esculenta, the clear response to even the lowest doses of oxytocin argues in favour of the role attributed to neurohypophyseal peptides in the control of sodium metabolism. It seems, however, that the hormones physiologically secreted by the lower Vertebrates differ from those extracted from the posterior lobe of the hypophysis of Mammals.