The significance of changes in thermodynamic affinity induced by aldosterone in sodium-transporting epithelia

Aldosterone-induced glycoproteins: further characterization

Aldosterone induces the synthesis of a group of glycoproteins (GP65,70) in toad urinary bladders which are potential effectors of the natriferic action of this hormone. The GP65,70 complex is composed of two molecular weight classes of proteins (Mr 65 and 70 kDa), each class being composed of several discrete proteins of varying isoelectric points (5.8-6.2). These proteins can be partially enriched (approximately 20-fold) using wheat germ agglutinin-sepharose affinity chromatography, are neuraminidase-resistant, and can be N-deglycosylated by endoglycosidase-H and N-glycanase. Treatment with N-glycanase leads to the appearance of a microheterogeneous group of proteins, all having the same Mr (approximately 40 kDa). From these studies it can be concluded that these particular aldosterone-induced proteins: (1) are heavily glycosylated, (2) contain multiple high mannose and hybrid oligosaccharides side chains, and (3) contain similar (if not identical) peptide backbones. Post-translational N-glycosylation accounts, at least in part, for their electrophoretic polymorphism (variation in Mr) but not for their electrophoretic microheterogeneity (variation in pI). The latter may reflect other types of post-translational modification (e.g. O-glycosylation, phosphorylation) or may be due to subtle differences in amino acid composition. The partial purification and biochemical characterization of GP65,70 should ultimately lead to a better understanding of the function of these putative "effectors" of aldosterone-stimulated Na+ transport.

Oxygen consumption and active sodium and chloride transport in bovine tracheal epithelium

The O2 consumption (Jr) and the short-circuit current (Ji) were measured simultaneously in bovine tracheal epithelium in vitro. In this tissue, Ji is the sum of two active transport processes, Cl- secretion and Na+ absorption. Jr was determined from the decrease of PO2 in the incubation solution, at 37 +/- 0.05 degrees C and at a PO2 around 600 torr. Microbial contamination and leaks of dissolved O2 from the solution never exceeded 4% of the rate of PO2 decrease due to the O2 consumption of the tissue. Ji and Jr were stable over 5 h of incubation under standard conditions. Ji was 106 +/- 4 nequiv min-1 cm-2 and Jr was 39.8 +/- 1.1 nmol O2 min-1 cm-2 (mean +/- S.E., n = 46). Ji was varied with several agents known to affect ion transport across the tracheal epithelium. Na+ absorption was inhibited partly with amiloride or completely following Na+ substitution with choline. Cl- secretion was selectively suppressed by furosemide. Ji was also reduced to a very low level, using ouabain or K+ suppression to inhibit the Na+-K+-ATPase. All these manoeuvres resulted in significant reductions of both Ji and Jr. Basal Jr was not affected when Ji was modified. A plot of the relative change in suprabasal Jr versus the relative change of Ji gave a straight line (r = 0.98, n = 60). A plot using absolute values yielded a stoichiometric ratio of 13.9 ions per O2 molecule, for Na+ as well as for Cl-. The stoichiometric ratio was also calculated for each experiment. Its mean value was 14.9 ions per O2 molecule. The population of the ratios was widely dispersed, but this was explained as a predictable statistical phenomenon.

Opposite effects of indacrinone (MK-196) on sodium and chloride conductance of amphibian skin

Indacrinone, a drug chemically related to ethacrynic acid, usually stimulated reversibly short circuit current and sodium influx when applied to the epithelial surface of amphibian skin. Concomitantly, transepithelial conductance, gt, decreased, provided chloride was the main anion in the incubation fluid. Electrophysiological analysis including microelectrode impalement indicated that the drug increased the sodium-conductance at the apical border of the impaled (most likely granular) cells. The decrease in gt thus points at shunt conductance being reduced with indacrinone, sometimes drastically. Decrease of transepithelial chloride flux with the drug as well as lack of effect of the drug on gt in the absence of chloride on the epithelial side demonstrate the influence of indacrinone on a chloride specific pathway. Whether this is along a paracellular route or through a cellular compartment not coupled to granular cells (mitochondria-rich cells?) cannot be decided on the basis of the present data.

Effects of hormonal and electrical stimulation of sodium transport on metabolism of toad urinary bladder

The carbon dioxide produced by toad urinary bladders bathed on their mucosal surfaces by sodium Ringer solution and on their serosal surfaces by modified Leibovitz tissue culture medium was analysed by multiple regression on both sodium transport and time. The fractions contributed by metabolism related to transport and by basal metabolism were assessed, and the extent to which these might vary with time was determined. This analytical method, which improves the accuracy with which suprabasal metabolism is estimated, was used to examine the effects on metabolism of vasopressin, aldosterone, and mucosa-positive voltage-clamping. Vasopressin (0.05 u./ml), which on average increased sodium transport 2.9 times and concurrently increased the rate of carbon dioxide production in these transporting tissues, also altered the carbon dioxide production of non-transporting, amiloride-treated control hemibladders. For each hemibladder the ratio of sodium transported to suprabasal carbon dioxide produced after vasopressin was compared with that observed before vasopressin. Differences between the ratios were much reduced when the carbon dioxide productions of the paired transporting hemibladders were corrected for the effects of vasopressin on basal carbon dioxide production. With such analysis, it was confirmed that vasopressin did not alter the stoichiometry of sodium transport. A 30 mV, mucosa-positive voltage clamp, applied near the peak of the response to vasopressin, further increased both sodium transport and carbon dioxide production. No alterations of the ratio of sodium to suprabasal carbon dioxide were seen under these conditions where the maximal rate of active sodium transport in this tissue must have been approached. Active sodium transport was more than doubled some 4 h after adding aldosterone (10(-7) M). However, the related increase in suprabasal carbon dioxide production was greater than threefold. Therefore, whereas the stimulation resulting from vasopressin and voltage clamping had no effect on the ratio of sodium transported to suprabasal carbon dioxide produced, this ratio was reduced significantly by aldosterone. When the sodium transport of aldosterone-treated bladders was increased further by voltage clamping, the ratio of sodium transported to suprabasal carbon dioxide production remained at the reduced value. Sodium transport was increased by approximately 35% more when aldosterone-treated hemibladders were voltage clamped after vasopressin, the control paired hemibladders being exposed to vasopressin and voltage clamping alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for concerted effects of aldosterone on a target sodium-transporting epithelium

The sodium-transporting activity of toad skin is stimulated in vitro with aldosterone in the absence of energy-providing substrate; it can be stimulated further upon addition of glucose after prolonged (overnight) incubation. The magnifying effect exerted by glucose in these conditions could be blocked by inhibitors of ribonucleic acid and protein biosynthesis. In addition, exposure to cycloheximide prevented the increase in thermodynamic affinity resulting from aldosterone treatment. A synthetic 19-nor steroid, (RU 24411), dimethyl-2,2-hydroxy-21-nor-19-pregnene-4-dione-3,20, also stimulated sodium transport by toad skin incubated in the absence of glucose, but there was no magnifying effect of this substrate. Furthermore, there was no change in thermodynamic affinity with RU 24411. Therefore, the magnifying effect seen with glucose and the increase in thermodynamic affinity are not necessarily integral parts of the response of sodium-transporting epithelial to "mineralocorticoids."