Reversible stimulation of sodium transport in the toad bladder by stretch

Structural and functional responses of the bullfrog urinary bladder to distension caused by hydrostatic pressure gradients

The responses to mucosal pressure elevation (physiological pressure: PP) were compared to responses to serosal pressure elevation (non-physiological pressure: NPP) in bullfrog urinary bladders (Rana catesbeiana). The bladders were mounted on vertical chambers as flat sheets. Distension was applied with 98.07 Pa. pressure gradients. PP resulted in increases in transepithelial electrical potential difference (TEP) and short-circuit current (SCC). Electrical resistance (R), urea permeability (P(urea)) and net water flux (J( v)) were not effected. NPP resulted in decreases in TEP (38%), SCC (13%), and R (36%). While P(urea) (97%) and J(v) (96%) increased. PP caused little or no change in the electron microscopic structure of frog bladder while NPP caused irreversible dilation of the lateral intercellular spaces. There were no observable changes in tight junctions under PP or NPP. The subepithelial elements of the bladder became detached from the epithelial layer during NPP suggesting a role for them during PP.

Effect of brefeldin A on ADH-induced transport responses of toad bladder

We have used brefeldin A (BFA) to examine the role of membrane traffic in the short-circuit current (ISC) and water permeability responses of the toad urinary bladder. BFA treatment of 1 or 5 micrograms/ml had a complex effect on the response of the ISC to antidiuretic hormone (ADH) or forskolin stimulation. Although the responses to initial challenges by ADH were not impaired by BFA, subsequent ISC responses were progressively reduced. Similarly, while the response to an initial challenge by forskolin was modestly reduced by BFA, subsequent responses were markedly reduced. Inhibition of protein synthesis with cycloheximide (CHM) affected ISC responses similarly. Neither BFA nor CHM had an effect on water permeability responses. These observations show that although the membrane traffic responsible for the water permeability response is insensitive to inhibition by BFA or CHM, the stimulation of Na+ transport becomes increasingly sensitive to these inhibitors with successive challenges by ADH or forskolin. Although initial increases in Na+ transport utilize preexisting components, subsequent responses appear to require an intact system for membrane biogenesis.

Effects of thyromimetic drugs on aldosterone-dependent sodium transport in the toad bladder

Aldosterone increases transepithelial Na+ transport in the urinary bladder of Bufo marinus. The response is characterized by 3 distinct phases: 1) a lag period of about 60 min, ii) an initial phase (early response) of about 2 hr during which Na+ transport increases rapidly and transepithelial electrical resistance falls, and iii) a late phase (late response) of about 4 to 6 hr during which Na+ transport still increases significantly but with very little change in resistance. Triiodothyronine (T3, 6 nM) added either 2 or 18 hr before aldosterone selectively antagonizes the late response. T3 per se (up to 6 nM) has no effect on base-line Na+ transport. The antagonist activity of T3 is only apparent after a latent period of about 6 to 8 hr. It is not rapidly reversible after a 4-hr washout of the hormone. The effects appear to be selective for thyromimetic drugs since reverse T3 (rT3) is inactive and isopropyldiiodothyronine (isoT2) is more active than T3. The relative activity of these analogs corresponds to their relative affinity for T3 nuclear binding sites which we have previously described. Our data suggest that T3 might control the expression of aldosterone by regulating gene expression, e.g. by the induction of specific proteins, which in turn will inhibit the late mineralocorticoid response, without interaction with the early response.

Energetics of sodium transport in the urinary bladder of the toad. Effect of aldosterone and sodium cyanide

Experiments were designed to determine whether the stimulatory effect of aldosterone on sodium transport involves an increase in tissue ATP. Urinary bladders that were removed from toads presoaked in 0.6% saline for 48-72 h, mounted as sacs, and maintained in open circuit except for brief observation of short circuit current every 30 min responded to 100 nM aldosterone added to the serosal bath with an increase in short circuit current to 170% of control hemibladders, which plateaus at 2-3 h. Tissue (ATP)/(ADP) X (Pi) measured in perchloric acid extracts increased to a maximum of 208% of controls (P less than 0.001) and ATP increased to 116% of controls (P less than 0.01) at 180 min. The short circuit current response to aldosterone paralleled the increase in ATP and (ATP)/(ADP) X (Pi) measured at 75, 120, 180, and 240 min. In bladders clamped at -150 mV, the short circuit current response to aldosterone was greater: 280% of controls (P less than 0.001) and tissue (ATP)/(ADP) X (Pi) increased to 191% of controls (P less than 0.001). In continuously short circuited bladders and bladders clamped at +75 mV, the short circuit current response to aldosterone and the change in ATP, ADP, or Pi were markedly diminished. 100 microM amiloride added to mucosal bath decreased the short circuit current to zero and inhibited the short circuit current response to aldosterone, whereas tissue ATP increased to 141% (P less than 0.05). 100, 250, and 500 microM NaCN dropped the short circuit current to 59, 35, and 24% of control values, respectively. Concurrently, tissue ATP measured at 60 min after the addition of NaCN dropped to 79, 66, and 56% of control values, respectively, and tissue ATP/ADP dropped to 68, 50, and 40%, respectively. The data revealed significant correlation between the change in the rate of sodium transport produced by aldosterone or NaCN as measured by the short circuit current and the concentration of ATP (r = 0.96, P less than 0.001), as well as ATP/ADP (r = 0.95, P less than 0.001). In conclusion, these results support the view that the stimulatory effects of aldosterone on sodium transport involve an increase in ATP or (ATP)/(ADP) X (Pi).

Effect of distension on ADH-induced osmotic water flow in toad urinary bladder

We recently described a method by which the resistance to water flow of the luminal membrane of ADH-stimulated toad bladder can be quantitatively distinguished from that of barriers lying in series with it. This method requires estimated of both total bladder water permeability (assessed by transbladder osmotic water flow at constant gradient) and luminal membrane water permeability (assessed by quantitation of the frequency of ADH-induced luminal membrane particle aggregates). In the present study we examined the effect of bladder distension on transepithelial osmotic water flow before and during maximal ADH stimulation. Base-line water flow was unaffected by bladder distension, but hormonally stimulated flow increased systematically as bladders became more distended. Distension had no effect on the frequency of ADH-induced intramembranous particle aggregates. By comparing the relationships between aggregate frequency and hormonally induced water permeability in distended and undistended bladders, we found that distension appeared to enhance ADH-stimulated water flow by decreasing the resistance of the series permeability barrier while the apparent water permeability associated with each single luminal membrane aggregate was unaffected. In that bladder distension caused tissue thinning, the series resistance limiting ADH-stimulated water flow appears to be accounted for by deformable barriers within the bladder tissue itself, probably unstirred layers of water.

The short-circuited everted sac of rat colon mucosa

A short-circuited preparation of everted rat colon sacs is described. The serosal current electrode is a AgAgCl wire. A cylindrical agar bridge or AgAgCl electrode may be employed on the mucosal side. Effects of Ag+ ions liberated from the electrodes on ion transport could not be demonstrated. Fluid and sodium are absorbed ad bicarbonate secreted. Potassium and chloride movements are not significantly different form zero. The preparation remains stable for at least 2 h. Sodium absorption is diminished by 50% and bicarbonate secretion abolished in the absence of glucose. In principle, similar ion transport properties were found as in Ussing-chamber preparations. The advantage of the everted sac is the capability of measuring net transport of fluid and electrolytes simultaneously and directly because of the large surface/inner volume ratio of the sac.

Transepithelial current-voltage relationships of toad urinary bladder and colon. Estimates of ENaA and shunt resistance

Studies were done to investigate the transepithelial current-voltage (IT-VT) relationships of urinary bladder and colon of the toad Bufo marinus. Like several other Na transporting epithelia, the IT-VT plots characteristically showed a break at voltage E1, averaging near 124 mV for urinary bladder and 110 mV for colon. With bladders treated with antidiuretic hormone, estimates of ENa and shunt resistance, Rs, were obtained according to a method outlined by Yonath and Civan, 1971 (J Membr. Biol. 5:336-385). Our results not only confirmed their observations, but were consistent with the notion that the values of E1 (IT-VT plots) were the same as those of ENa. In addition, the values of Rs were found to be the same as those estimated from the quotient E1/I1 obtained from the voltage and current coordinates at the break of the IT-VT plot of bladders studied in both stretched and unstretched states. Amiloride at concentrations up to 10(-5) M caused a small decrease of both E1 and E1/I1 of urinary bladder. Similarly, amiloride caused small but significant changes of ENa and RNa of the colon. For both epithelia, the values of E1 and E1/I1 of the IT-VT plots were the same as those of ENa and Rs estimated by an independent method. In general, these findings are similar to those of several other epithelia where the ENa and Rs can be estimated directly from their IT-VT relationships.

Isolation and separation of toad bladder epithelial cells

The epithelium of the urinary bladder of Bufo marinus is composed of 5 cell types, i.e., granular (Gr), mitochondria-rich (MR) and goblet (G) cells which face the urinary lumen, microfilament-rich (MFR) and undifferentiated cells (Un) located basally. The epithelium was dissociated by collagenase and EGTA treatment. Fractionation of dispersed cells by isopycnic centrifugation on dense serum albumin solutions yielded 4 fractions: (i) a very light fraction (p approximately equal to 1.025) enriched in MR and MFR cells; (ii) a light fraction (p approximately equal to 1.045) enriched in vacuolated Gr cells; (iii) a heavy fraction (p approximately equal to 1.065) composed essentially of aggregated Gr cells, and (iv) a pellet (p approximately equal to 1.085) enriched in G and undifferentiated cells. Recoveries were based on cell counts and DNA measurements. DNA content per cell was 13.2 pg +/- 0.9 (n = 37). From 1 g fresh tissue, 62 +/- 5 x 10(6) (n = 10) cells were recovered before isopycnic centrifugation of which about 70% excluded Trypan blue. After centrifugation, 90 to 95% of the cells excluded the vital dye and approximately 3(9) x 10(6) cells were recovered from the gradient. Cell metabolism in each fraction was estimated by oxygen consumption measurements in absence or presence of ouabain, acetazolamide, and dinitrophenol. The consumption measurements in absence or presence of ouabain, acetazolamide, and dinitrophenol. The consumption was threefold higher in the very light and light fractions when compared to the heavy and pellet fractions. Ouabain sensitive oxygen consumption (QO2) represented 12 to 35% of the total O2 consumption depending on the cell fraction, and acetazolamide sensitive QO2 varied from -0.8% in the heavy fractions to 20% in the lighter fractions. DNP increased QO2 in all fractions by 20 to 50%. Finally, the cells were able to reaggregate and form junctional complexes upon addition of calcium to the medium.

Microfilament-rich cells in the toad bladder epithelium

Basal cells of the bladder epithelium of Bufo marinus have been found heterogenous and consist of microfilament-rich cells (MFR-cell) and undifferentiated cells (Un-cell). The MFR-cell, which represents approximately 20% of the epithelial cell population, lies between the epithelial layer lining the urinary space and the basement membrane; it extends under several epithelial cells by processes of varying widths and lengths which contact, via desmosomes, other MFR-cells, as well as cells in the superficial layer, i.e., granular and mitochondria-rich cells. The cytoplasm of MFR-cell is filled with intermediate filaments arranged in bundles which run parallel to the plane of the epithelium and no dense granules, typical of granular cells, have been detected. Strong immunofluorescence for actin is associated with cells which occupy the same basal position as MFR-cells. Undifferentiated cells have no contact via desmosomes with adjacent cells and their cytoplasm is filled with free ribosomes; they lack bundles of intermediate filaments and possess no specialized organelles. After a 4-hr pulse of 3H-thymidine, 1.5% of epithelial cells incorporate thymidine into nuclear DNA, out of which 3/4 are basally and 1/4 are apically located. Identification of cell types by electron microscopy reveals that approximately 10% of undifferentiated basal cells are labeled, whereas less than 0.1% of granular cells and no MFR-cells incorporate 3H-thymidine into DNA. When dissociated from the epithelium and separated by isopycnic centrifugation, MFR-cells possess a mean buoyant density of approximately 1.025, cosediment with mitochondria-rich cells and exhibit a strong immunofluorescence for actin. The function of MFR-cells remains unknown; however, they may play a role in cell coupling and responses to hormonal and physical factors.

Radio-iodination of plasma membranes of toad bladder epithelium

The present report describes high yield enzymatic radio-iodination of the apical and basal-lateral plasma membranes of toad bladder epithelium, by a procedure that does not breach the functional integrity of the epithelium, as assessed by the basal and vasopressin-sensitive short-circuit current (SCC). Restriction of the label to the membrane surface, was ascertained by light and electron-microscopic autoradiographs. On the apical surface, the grains were over the glycocalyx and the plasma membrane. Analysis of the labeled glycocalyx by agarose gel filtration, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), as well as enzymatic and pH-dependent hydrolysis indicated that the glycocalyx is a trichloro-acetic acid-soluble macromolecular complex of high molecular weight composed of a peptide moiety attached to large prosthetic groups (presumably carbohydrates) by O-glycosidic bonds. Analysis of the labeled apical plasma membrane components by agarose gel filtration and SDS-PAGE disclosed the presence of six major species of apparent molecular weights: 23,000, 28,000, 37,000, 44,000, 68,000, and 95,000. More than half of the membrane-associated radio-iodine was in two bands of molecular weights 37,000 and 44,000. Concentrations of vasopressin and cyclic AMP sufficient to increase the SCC significantly did not modify the extent of membrane labeling or the distribution of the label among the apical membrane components (presumably proteins) as assessed by SDS-PAGE. Iodination in the presence of amiloride inhibited incorporation but did not change the pattern of the distribution of the label among the components resolved by SDS-PAGE. Iodination of basal-lateral plasma membranes, at a yield comparable to that obtained with apical labeling, was attained after about 30 min of exposure of the intact bladder to the labeling solutions. Approximately 25% of the basal-lateral labeling was lost when the epithelial cells were harvested after collagenase treatment, implying that some iodination of the basement membrane had taken place. Less than 10% of iodination of the apical or basal-lateral surfaces was accounted for by lipid-labeling. Analysis of the labeled apical and basal-lateral species by enzymatic digestion and thin layer chromatography disclosed that virtually all the radioactivity was present as mono-iodotyrosine (MIT).

Cellular and shunt conductances of toad bladder epithelium

Toad urinary bladders were mounted in Ussing-type chambers and volt-age-clamped. At nonzero voltages only, small fluctuations in current, delta I, and therefore in tissue conductance, delta Gt, were detected. These fluctuations were caused by the smooth muscle of the underlying tissue which could be monitored continuously and simultaneously with the current, I. Inhibition of the smooth muscle contraction with verapamil (2 X 10(-5) M) abolished the fluctuations in I and Gt. Amiloride (10(-4) M) had no significant effect on the magnitude of delta Gt, oxytocin increased Gt without affecting delta Gt, and mucosal hypertonicity produced by mannitol increased delta Gt. These results are consistent with the hypothesis that two parallel pathways exist for passive current flow across the toad urinary bladder: one, the cellular pathway, was not affected by smooth muscle activity; the other, the paracellular pathway, was the route whose conductance was altered by the action of the smooth muscle. Thus the relationship between the cellular and shunt conductances of the epithelium of the toad urinary bladder, under a variety of conditions, can be investigated by utilizing the effects of the movement of the smooth muscle.

Segmental heterogeneity of epithelial transport in rat large intestine

Functionally isolated segments of rat colon and rectum were perfused in situ in a closed loop system. Rectum was defined as the lower 25--35% of the length of large intestine (cecum excluded). Perfusion conditions were optimized at 0.5 ml.min-1 and 3 cm H2O luminal pressure. Variation of perfusion rate between 0.2 and 2 ml.min-1 did not influence net volume transport (JNV). Luminal distension following elevation of hydrostatic pressure to 18 cm H2O reversibly increased Jnv. Under control conditions Jnv and Na+-transport rates (JnNa) of colon were 2--3 times higher than those of rectum. In colon transepithelial electrical potential difference (psims) was time independent --12 mV (lumen negative) whereas rectal psims increased with time from --6 mV, reaching a plateau of --67 mV within 6 h. Amiloride 10(-4) mol.l-1 had no effect on psims, Jnv, and JnNa in colon but did slightly depress K+-secretion in colon descendens. In contrast, psims in rectum was dose-dependently depressed, being reversed to +7 mV at 10(-4) mol.l-1. Jnv and JnNa were decreased by half. Acetazolamide in addition to amiloride lowered the positive post-amiloride rectal psims by half. Adrenalectomy had no effect on colonic psims, but abolished psims of the rectum. A single dose of 40 microgram.kg-1 b.w. aldosterone during the experiment restored the typical time course of rectal psims, but did not affect psims in colon. It is concluded that aldosterone induces an amiloride-sensitive Na+-pathway only in rectum, but not in colon, and that colon and rectum differ basically in their transport properties, quantitatively as well as qualitatively, as do the kidney distal convoluted tubule and the cortical collecting duct.

Binding of aldosterone to cytoplasmic and nuclear receptors of the urinary bladder epithelium of Bufo marinus

Binding of aldosterone to cytoplasmic and nuclear sites in urinary bladder epithelia of Bufo marinus (Dominican variant) is saturable and dependent upon steroid concentration. Scatchard analysis of specific cytoplasmic binding yielded a maximal binding capacity (N) of 14.5 X 10(-14) mol/mg protein and an apparent equilibrium dissociation constant (Kd) of 1.4 X 10(-8) M. Since Scatchard analysis of specific nuclear binding was complex, this binding was resolved by a computer-generated cirve-fitting technique which analyzed total aldosterone bound. Nuclear binding was resolved into three sites: a nonsaturable site that was linearly dependent upon aldosterone concentration, and two saturable sites (types I and II). Type I sites had relatively low capacity for aldosterone (N = 31 +/- 1 X 10(-14) mol/mg DNA) and high affinity (Kd = 2.5 +/- 0.5 X 10(-9 M); tffininty (Kd = 8.6 +/- 1.7 X 10(-7) M). Competition for [3H]aldosterone binding by dexamethasone, corticosterone, cortisol, progesterone, testosterone, and 17 beta-estradiol demonstrated that type I nuclear sites have higher affinity for aldosterone than for other steroids. The findings are consistent with the inference that the type I site is the mineralocorticoid receptor.

Aldosterone receptor occupancy and sodium transport in the urinary bladder of Bufo marinus

The concentration dependence of binding of [3H]aldosterone to cytoplasmic and nuclear receptors was evaluated in urinary bladder epithelial cells of Colombian toads. One class of specific sites (sensitive to displacement by excess aldosterone) was detected in the cytosol. However, two classes of specific nuclear [3H]aldosterone binding sites were evident. In the nucleus, high-affinity (Kd = 2.7 X 10(-9) M), low-capacity (N = 15 X 10(-14) mol/mg DNA) sites (type I) were completely saturated at approximately 4 X 10(-8) M aldosterone, a concentration which gave a maximal increase in short-circuit current (SCC). Occupancy of low-affinity (Kd = 4.6 X 10(-7) M), high-capacity sites (N = 150 X 10(-14) mol/mg DNA) (type II) occurred at higher steroid concentrations and did not correlate with further increase in SCC. Binding parameters of Colombian and Dominican variants of Bufo marinus were compared. In both variants, the SCC increase elicited by aldosterone correlated with accumulation of type I complexes in the nucleus, but the relationship was markedly nonlinear. Various alternatives were considered as the basis for a curvilinear dependence of the increment in Na+ transport on abundance of nuclear type I complexes.

Evidence for a transcellular component to the transepithelial sodium efflux in toad skin

The transepithelial efflux of sodium, from the inner to the outer surface was measured across the isolated toad skin, mostly after abolition of the electrochemical gradient. The effects on this efflux of several agents and manipulations were studied in order to make a distinction between the paracellular component and a hypothetical transcellular one. Amiloride decreased the transepithelial efflux, while ouabain and cyanide increased it. From the known mode of action of those agents, it was inferred that part of the efflux occurred across the cell. Removal of sodium from the external solution interfered apparently with both components of the transepithelial efflux, while the action of external hypertonicity seemed to be restricted to the paracellular shunt pathway. Access of sodium from the internal solution to the active transport pool is thus suggested, with consequent increase in metabolic cost of transport. Yet, compared with the net influx, the amounts involved are very small; consequently, they escape detection by oxygen consumption measurements.

Effects of 3'-deoxycytidine on rRNA synthesis in toad bladder: analysis of response to aldosterone

Previous studies showed that aldosterone augments transepithelial active Na+ transport and the incorporation of [3H]uridine into polyadenylated RNA (poly(A)(+)-RNA) (putatively mRNA) early in the latent period. Soon thereafter, incorporation of [methyl-14C] groups, as well as [3H]uridine into rRNA is also increased. To evaluate the role of rRNA in mineralocorticoid action, the inhibitor 3'-deoxycytidine was used in studies on the urinary bladder of the toad Bufo marinus. 3'-deoxycytidine suppressed the incorporation of [methyl-14C] and [3H]uridine into nuclear precursors of rRNA and subunits of cytoplasmic rRNA. In contrast, 3'-deoxycytidine inhibited incorporation of ]3H]uridine into cytoplasmic poly(A)(+)-RNA minimally. In control experiments, 3'-deoxycytidine had no significant effect on Na+ transport, measured as the short-circuit current (scc), when given alone. 3'-Deoxycytidine also had no significant effect on the aldosterone-dependent increase in scc. In the presence of 3'-deoxycytidine, aldosterone enhanced both the scc and the incorporation of [3H]uridine into poly(A)(+)-RNA significantly. We conclude that during the first 3 h, the mineralocorticoid action of aldosterone is not sensitive to inhibition of rRNA synthesis. Previous studies, however, implicate mRNA synthesis in this early response.

Spironolactone antagonism of aldosterone action on Na+ transport and RNA metabolism in toad bladder epithelium

In earlier studies, aldosterone increased the incorporation of precursors into a class of cytoplasmic RNA with the characteristics of messenger RNA (mRNA), in toad bladder epithelium. In the present studies, this effect was analyzed further with a competitive antagonist, spironolactone (SC-9420). Paired hemibladders were labeled with 3H-uridine (30 min pulse - 140 min chase), with or without aldosterone (3.5 x 10(-8) M, 7 X 10(-8) M) in the presence or absence of SC-9420 (7 X 10(-6) M, 2.5 X 10(-5) M) at molar ratios of 200:1 to 280:1. Cytoplasmic RNA, either the total phenol-SDS extract or polyadenylated-RNA (poly(A)(+)-RNA) obtained by oligo-deoxythymidylate-cellulose (oligo(dT)-cellulose) chromatography was analyzed in linear 5 -- 20% sucrose gradients. Eight sets of experiments were completed in which the short-circuit current (scc) was monitored for 180 min and the incorporation of 3H-uridine (30 min pulse -- 150 min chase) was simultaneously determined on pools of epithelia from 5 to 10 hemibladders. The fractional change in scc correlated linearly with the fractional change in 3H-uridine of 12S cytoplasmic RNA (r=0.95, p less than 0.001). The poly(A)(+)-RNA fraction had no detectable rRNA or tRNA and gave a heterogeneous pattern, typical of mRNA, in the sucrose gradients. In the presence of exogenous aldosterone, SC-9420 inhibited the incorporation of 3H-uridine into poly(A)(+)-RNA (particularly 12S). These results support the inference that induction of mRNA mediates the action of aldosterone on Na+ transport.

Relationship of aggregated intramembranous particles to water permeability in vasopressin-treated toad urinary bladder

It has been previously demonstrated with freeze-fracture electron microscopy that vasopressin induces specific structural alterations of the luminal membrane of granular cells from toad urinary bladder in a dose-dependent fashion. These alterations consist of aggregated intramembranous particles and are observed both in the presence and absence of an osmotic gradient. We examined the effect of methohexital, a selective inhibitor of vasopressin-stimulated water flow, and the effect of phloretin, a selective inhibitor of urea permeability, on the structure of the granular cell luminal membrane. Methohexital treatment of the vasopressin-stimulated toad bladder reduced both the osmotic water flow and vasopressin-induced alterations of membrane structure to the same extent. Phloretin reduced urea permeability but not water flow or particle aggregation. Since neither agent affects vasopressin-stimulated sodium movement, these findings indicate that the phenomenon of particle aggregation is specifically related to vasopressin-induced water permeability and not to changes in urea or sodium permeability.

Actions of aldosterone on polyadenylated ribonucleic acid and Na+ transport in the toad bladder

Polyadenylated ribonucleic acid [poly(A)(+)-RNA] has been isolated from the cytoplasm of the epithelium of the urinary bladder of the toad (Bufo marinus) by oligo-(deoxythymidylate)cellulose chromatography. Aldosterone increased the incorporation of [3H]uridine, and of [3H]uridine and [3H]adenosine (given in combination) into 7S, 12S, and 18S poly(A)(+)-RNA during the first 30 min of the action of the hormone, as defined by either a pulse or pulse-chase sequence. The quantity of cytoplasmic poly(A)(+)-RNA that hybridized to [3H]poly(uridine) was also increased by aldosterone. These results are consistent with the inference of induction of messenger RNA synthesis. This effect was most marked during the first 30 min of the action of the hormone.

Actions of aldosterone on rRNA and Na+ transport in the toad bladder

Aldosterone increased methylation by [methyl-3H]methionine of nuclear ribosomal ribonucleic acid (rRNA) sedimenting at 18S, 28S, and 40S within 90 min and of 28S cytoplasmic RNA within 240 min of continuous exposure to the precursor and the hormone, in the toad bladder. In addition, incorporation of [14C]uridine into cytoplasmic 4S transfer RNA, and 18S and 28S rRNA was enhanced after 240 min of continuous exposure to the precursor and the hormone. Aldosterone had minimal effects on the 3H or 14C-labeled, acid-soluble pools. These results suggest that aldosterone augments the synthesis of rRNA at the transcriptional level. Ribosomes isolated 240 min after treatment of the toad bladder with aldosterone showed increased incorporation of [3H]-phenylalanine into peptides with both endogenous messenger RNA (mRNA) and exogenous mRNA (i.e., poly(uridylic acid) in an in vitro assay. Inhibition of reinitiation with NaF or poly (AUG) reduced the rate of amino acid polymerization by 45% but the aldosterone to control ratio remained significantly high. These results imply an increase in active ribosomes and perhaps in endogenous mRNA content. Our findings, however, do not distinguish between a steroid-dependent increase in the total number of active ribosomes and an increase in translational activity per ribosome.

A new model for regulation of sodium transport in high resistance epithelia

A new three barrier, four compartment model for sodium transport in high resistance urinary epithelia is presented. This model provides a unified and simplified mechanistic explanation for sodium transport and its quantitative regulation. Sodium enters the epithelial cell by passive diffusion. Active extrusion occurs across the lateral cell membrane into the lateral intercellular space (LICS). Sodium movement from the LICS into the serosal compartment is not free and unobstructed as in the models for low resistance epithelia, but rather occurs through a regulatory channel of the LICS passing through desmosomes and the basilar slit. The exact configuration of this regulatory channel controls the rate of sodium movement from the LICS into the serosal compartment. Thus, the configuration of the regulatory channel controls the afterload on the sodium pump and thus ultimately controls the rate of transepithelial sodium transport. Antidiuretic hormone could act by increasing the effective width of this regulatory channel by contraction of intracellular microtubules or microfilaments. Present theories for regulation of transepithelial sodium transport in high resistance epithelia invoke a regulatory barrier at the apical cell membrane or at the active sodium pump located in the basolateral cell membrane. The hypothetical model presented here invokes a new alternative: regulation of the active pump rate by the sodium concentration in the LICS serving as an afterload on the pump; sodium escape from the LICS into the serosal compartment thus becomes the regulatory step for transepithelial transport.

Electrical properties of amphibian urinary bladder epithelia. I. Inverse relationship between potential difference and resistance in tightly mounted preparations

In an attempt to find a high-resistance epithelium suitable for microelectrode work, we have studied the electrical properties of Necturus and Amphiuma urinary bladders in comparison to toad bladder. Improved mounting techniques were developed, which yield better reproducible degrees of distension and prevent electrical leaks around the edge of the preparation in the Ussing chamber. Transepithelial potential difference and resistance was measured with NaCl Ringer's on either surface of the epithelium, as well as under conditions of ion substitutions and in the presence of amiloride. Compared to data from conventionally mounted toad bladders reported in the literature, our experiments yielded higher potential differences and resistances in all three species. In Necturus values of up to 175 mV and 75 komega cm2 were recorded. Furthermore an inverse relationship was observed between potential difference and resistance, which was not noticed previously with the conventional mounting technique. The relationship is discussed quantitatively in terms of the two-membrane model of active Na+ transport, for which it provides further supportive evidence.

Kinetics of RNA labeling in toad bladder epithelium: effects of aldosterone and related steroids

Sucrose gradient analysis revealed an aldosterone-dependent increase in the incorporation of [(3)H]uridine into a nonmethylated, rapidly labeled RNA that sedimented at 9-12 S. This effect was antagonized by an anti-mineralocorticoid, spironolactone (SC 9420), and was not elicited either by a glucocorticoid, cortisol, or by the inactive isomer, 17-alpha-isoaldosterone. These findings are in accord with the inference that mRNA is induced by aldosterone during the latent period and that the induced mRNA mediates the action on sodium transport.

Effect of phloretin on water and solute movement in the toad bladder

It is generally believed that urea crosses the cell membrane through aqueous channels, and that its movement across the membrane is accelerated in the direction of net water flow (solvent drag effect). The present report presents evidence for a vasopressin-sensitive pathway for the movement of urea, other amides, and certain non-amides, which is independent of water flow. Phloretin, when present at 10(-4) M concentration in the medium bathing the luminal surface of the toad bladder, strongly inhibits the movement of urea, acetamide, and propionamide across the toad bladder, both in the absence and presence of vasopressin. The vasopressin-stimulated movement of formaldehyde and thiourea is also reduced. Osmotic water flow, on the other hand, is not affected; nor is the movement of ethanol and ethylene glycol, or the net transport of sodium. On the basis of these studies we would conclude that the movement of many, if not all, solutes across the cell membrane is independent of water flow, and that a vasopressin-sensitive carrier may be involved in the transport of certain solutes across the cell membrane.

Aldosterone action and sodium- and potassium-activated adenosine triphosphatase in toad bladder

Urinary hemibladders obtained from toads soaked in water or saline were treated with aldosterone, 10(-6) M, either 1(1/2) or 16 h after mounting. After 2(1/2) h exposure to the hormone, short-circuit current was increased by 110-192% and open-circuit potential by 20-44% as compared with untreated paired hemibladders. Mucosal cells were then assayed for sodium-potassium-stimulated adenosine triphosphatase (ATPase). No increase occurred in activity per milligram protein or in the portion of total activity dependent on sodium. Activity at low sodium concentrations was also measured and analyzed by means of the Hill equation in terms of K, the apparent dissociation constant of the enzyme-sodium complex, and n, a number that expresses the degree of interaction between binding sites. Neither K nor n was significantly altered by aldosterone. A few experiments were also carried out at low ATP concentrations (0.3 mM); again no change in sodium-dependent activity was noted. The results indicate that aldosterone does not stimulate sodium transport by increasing the quantity of sodium-potassium adenosine triphosphatase in mucosal cells or the dependence of this activity on sodium or ATP concentrations.

Components of sodium and chloride flux across toad bladder

The effect of transepithelial potential difference (psi) on Na and Cl flux across toad bladder was assessed by measuring isotopic flux between identical media at various values of psi. The contribution of edge damage to ionic permeability was eliminated, resulting in relatively high spontaneous psi (-97 +/-4 mv) and low electrical conductance g. Bidirectional Na fluxes were measured simultaneously. Unidirectional Cl fluxes were measured in paired hemibladders at psi = 0 mv or -97 mv. Net Na flux J(Na), at psi = 0 mv, was slightly less than short-circuit current (SCC). At psi = -97 mv, J(Na) averaged 17% of SCC, and was sometimes zero. DeltaJ(Na)/Deltapsi (= g(+)) averaged 60% of g between -97 mv and +75 mv; at -150 mv, g(+) fell, indicating rectification. Analysis of unidirectional Na fluxes indicates low passive conductance (1.5 mumho/mg wet weight), a bidirectional, electrically neutral flux of approximately 0.13 mua/mg, and relatively large conductance of the active transport path at psi >/= -97 mv. The absence of appreciable transstimulation of serosal (S)-to-mucosal (M) Na flux (in response to increasing mucosal Na concentration) indicates that the electrically neutral flux is not exchange diffusion in the usual sense. Analysis of Cl fluxes indicates similar values for passive conductance and neutral flux, suggesting linked neutral flux of Na and Cl. Either the electromotive force of the Na pump E, its conductance g(a), or both are strong functions of psi. The product of these two quantities, Eg(a), is a measure of the "transport capacity" at any given value of psi, independent of the direct effect of psi on J(Na) through the pump path. Eg(a) varies with psi. Hence estimation of the net Na flux or current at any one value of psi, including psi = 0, fails to reveal the maximal transport capacity of the pump, its resting electromotive force (when J(Na) = 0 through the pump), or the dependence of transport capacity on potential.