Effect of aldosterone on active and passive conductance andE Na in the toad bladder

Abstract

Summary. It has been demonstrated previously that aldosterone increases the electrical conductance of the toad bladder in association with the stimulation of active sodium transport. In the present study the concurrent measurement of electrical quantities and ion tracer flux distinguishes effects on active and passive pathways. Lack of an effect on passive Na + or CI-tracer flux in hemibladders preselected to eliminate large artefactual leaks indicates that aldosterone has no influence on physiological passive conductance. Thus, the enhancement of electrical conductance is entirely attributable to the active pathway. The magnitude of the increase in the active conductance was estimated. The data permitted also the comparison of effects on the flux ratio of Na + at short circuit (fo) and the electrical potential difference adequate to abolish active sodium transport (ENa). Even in membranes with minimal leakage the flux ratio does not reliably reflect ENa. Aldosterone increased mean fo from 11 to 22, but did not affect ENa.Active sodium transport across the toad bladder is believed to comprise two processes: (1) "passive" movement across the apical (mucosal) surface into the cell [13], followed by (2) "active" extrusion at the basal-lateral (serosal) surface [14]. Accordingly, in attempting to explain how aldosterone facilitates sodium transport, two main hypotheses have been advanced. Crabb6 [5] and Sharp and Leaf [21,22] have suggested that aldosterone increases mucosal permeability, permitting more rapid passive inflow and elevation of the intracellular sodium concentration, with resultant enhancement of active transport across the serosal surface. Edelman and his co-workers [12,20], on the other hand, have urged the importance of energetic factors. In this view aldosterone might increase sodium transport either by increasing the supply of metabolic energy to the sodium "pump," or by facilitating the linkage of metabolism to transport. Recently, it has been suggested that both permeability and energetic factors may be involved [18].

Na+ transport by rabbit urinary bladder, a tight epithelium

“…Where tested, the effects of choline-for-Na § replacement, amiloride, aldosterone, ouabain, pH and Ca 2 + on Isr and R are parallel in the three tissues. In toad bladder (Saito & Essig, 1973) as in rabbit bladder, dMs of 22Na is linearly related to G, while JsM is independent of G or Isc.…”

Relationship of transient electrical properties to active sodium transport by toad urinary bladder

“…This approach has indeed proved fruitful. For example, measurement of changes induced in DC resistance has helped characterize sites and modes of action of the hormones vasopressin (Civan, Kedem & Leaf, 1966) and aldosterone (Civan & Hoffman, 1971 ;Saito & Essig, 1973). On the other hand, a number of investigators have remarked on…”

A thermodynamic analysis of the correlation between active Na+ transport and the rate of oxygen consumption in epithelia

“…This is not the case with the other drugs. It should be remarked that some of the values cited in Table 4 for changes in L+ are taken from the toad bladder [6,18], and it is not improbable that different epithelia react differently to some of the drugs, as was indeed found when using ouabain [10].…”