Effects of vasopressin and aldosterone on amiloride binding in toad bladder epithelial cells

The amiloride-sensitive epithelial Na+ channel: binding sites and channel densities

The amiloride-sensitive Na+ channel found in many transporting epithelia plays a key role in regulating salt and water homeostasis. Both biochemical and biophysical approaches have been used to identify, characterize, and quantitate this important channel. Among biophysical methods, there is agreement as to the single-channel conductance and gating kinetics of the highly selective Na+ channel found in native epithelia. Amiloride and its analogs inhibit transport through the channel by binding to high-affinity ligand-binding sites. This characteristic of high-affinity binding has been used biochemically to quantitate channel densities and to isolate presumptive channel proteins. Although the goals of biophysical and biochemical experiments are the same in elucidating mechanisms underlying regulation of Na+ transport, our review highlights a major quantitative discrepancy between methods in estimation of channel densities involved in transport. Because the density of binding sites measured biochemically is three to four orders of magnitude in excess of channel densities measured biophysically, it is unlikely that high-affinity ligand binding can be used physiologically to quantitate channel densities and characterize the channel proteins.

Single-channel recordings from the apical membrane of the toad urinary bladder epithelial cell

The patch-clamp technique for the recording of single-channel currents was used to investigate the activity of ion channels in the intact epithelium of the toad urinary bladder. High resistance seals were obtained from the apical membrane of tightly stretched tissue. Single-channel recordings revealed the activity of a variety of ion channels that could be classified in 4 groups according to their mean ion conductances, ranging from 5 to 59 pS. In particular, we observed highly selective, amiloride-sensitive Na channels with a mean conductance of 4.8 pS, channels with a similar conductance that were not Na-selective and channels with mean conductance values of 17-58 pS that were mostly seen after stimulation of the tissue with vasopressin or cAMP. When inside-out patches from the apical membrane were exposed to 110 mM fluoride, large conductances (86-490 pS) appeared.

Aldosterone regulation of Na+ transport and Na+-K+-ATPase in A6 cells: role of growth conditions

The effects of aldosterone on transepithelial sodium transport (measured by the short-circuit current (SCC) and on Na+-K+-adenosine triphosphatase (ATPase) biogenesis have been studied in A6 kidney cells grown on collagen-coated filters in two different media. In medium A, base-line SCCA was close to zero but transmural electrical resistance (RA) was high. Aldosterone (100 nM, t24h) drastically increased SCCA and RA, but only after a 4-h latent period. In medium B, base-line SCCB and RB were significantly higher than in medium A. Aldosterone significantly enhanced SCCB and to a lesser extent RB after a much shorter latent period (approximately 45 min) than in medium A. In medium A, aldosterone elicited a fourfold increase in the relative rate of synthesis of alpha- and beta-subunits of Na+-K+-ATPase. A twofold increase was already observed within the observed latent period. This time course suggests that de novo synthesis of sodium pumps might be one of the critical factors underlying the increase in sodium transport in this growth medium. In medium B, aldosterone elicited a two- to fourfold increase in the relative rate of synthesis of the alpha- and beta-subunits of Na+-K+-ATPase that paralleled SCCB. Thus de novo synthesis of Na+-K+-ATPase is clearly not a prerequisite for the early mineralocorticoid response (t90 min - t180 min), but still could be part of the late mineralocorticoid response (t3 h - t24 h). In both media, the immunochemical cellular pool of Na+-K+-ATPase was apparently not modulated by aldosterone for up to 48 h of incubation.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of standard diuretics and RPH 2823 on transepithelial Na+ transport in isolated frog skin

Short-circuit current (SCC) techniques were used to monitor the effects of various diuretic agents on Na+ transport in isolated frog skin, a model for the late distal tubule and the collecting duct of the mammalian kidney. Acetazolamide, hydrochlorothiazide, torasemide, and ethacrynic acid did not affect sodium transport (as indicated by the SCC) or transepithelial electrical resistance when added either to the apical (outer) or to the inner (basolateral, corial) bathing solution of the tissue. However, Na+ transport was sensitive to amiloride, the triamterene derivate dimethylamino-hydroxypropoxytriamterene (RPH 2823), and to furosemide. Whereas apical amiloride, and RPH 2823 induced a dose-dependent decrease in SCC and increase in transepithelial electrical resistance, apical furosemide resulted in a dose-dependent increase in SCC and a decrease in electrical resistance. None of the three diuretic agents caused a significant change in SCC when applied to the inner bathing Ringer's solution. The small furosemide-induced decrease in resistance compared with the huge increase in SCC suggests that furosemide affects Cl- permeability as well as Na+ permeability. Evidence for this notion was achieved by the following findings: The decrease in resistance after furosemide was more pronounced in tissues bathed in Cl(-)-free solutions compared with Cl(-)-containing solutions. n contrast, SCC stimulation by apical furosemide is Cl(-)-ion independent, but strongly Na+-ion dependent. SCC stimulation by furosemide is amiloride-sensitive. With respect to the onset, locus, and reversibility of action, it seems reasonable to assume that amiloride, RPH 2823, and furosemide all influence transepithelial Na+ transport by interacting with the Na+ channel or a regulator site of it within the apical membrane. The stoichiometry of the amiloride (RPH 2823)-receptor site interaction revealed Hill-coefficient(s) of less than 1, indicating a negative cooperativity among the receptor sites. The interaction between Na+ ions and amiloride or RPH 2823 displayed mixed competitive-noncompetitive inhibition. Taken together, these results support the hypothesis that amiloride and Na+ as well as RPH 2823 and Na+ may act at different loci on the apical entry mechanism in Rana esculenta skin.

Thyroid hormone antagonizes an aldosterone-induced protein: a candidate mediator for the late mineralocorticoid response

In the urinary bladder of the toad Bufo marinus, the basal rate of synthesis of a number of proteins was modulated in a bidirectional way (i.e., induced or repressed) by aldosterone and by triiodothyronine (T3). Each hormone was therefore characterized by a distinct domain of response. When both hormones were added simultaneously, the two domains consistently overlapped at least for one protein, termed AIP-1, or aldosterone-induced protein 1 (Mr approximately 65 kilodaltons, pi = 6.7, as analyzed by two-dimension gel electrophoresis). The physiological role of AIP-1 is unknown, but could be related to the late mineralocorticoid response. In five experiments, T3 (60 nM, 18-hr incubation) consistently repressed AIP-1, while aldosterone-dependent sodium transport (late response) was significantly inhibited, as previously described. The repression of AIP-1 was also observed as early as 6 hr after aldosterone addition. In addition, sodium butyrate (3 mM), which was previously shown to also selectively inhibit the late mineralocorticoid response, was also able to repress AIP-1. Our results suggest that AIP-1 is one of the proteins involved in the mediation of the late mineralocorticoid response.

Binding of 3H-phenamil, an irreversible amiloride analog, to toad urinary bladder: effects of aldosterone and vasopressin

Phenamil, an analog of amiloride, has previously been shown to bind specifically to sodium channels in toad bladder (J.L. Garvin et al., J. Membrane Biol. 87:45-54, 1985). In this paper, 3H-phenamil was used to measure sodium channel density in both isolated epithelial cells and intact bladders. From the specific binding to intact bladders, a channel density of 455 +/- 102 channels/micron2 was calculated. No correlation between specific binding and the magnitude of irreversible inhibition of short-circuit current was found. Pretreatment of intact bladders with 1 mg/ml trypsin reduced specific binding to isolated cells by 82 +/- 5%. In isolated cells, neither aldosterone nor vasopressin had any significant effect on specific phenamil binding. It is inferred that phenamil binds to both open and closed channels which may be either in the mucosal membrane or in the submembrane space. Finally, and rather surprisingly, we found that 3H-phenamil binds irreversibly to the basolateral membrane at concentrations as low as 4 X 10(-7) M. Therefore, care must be used in interpreting binding studies with amiloride or its analog at such concentrations.

Kinetics of the effect of amiloride on the permeability of the apical membrane of rabbit descending colon to sodium

The effects of the addition of graded concentrations of amiloride, (A)m, to the mucosal bathing solution on the permeability of the apical membrane of rabbit descending colon to Na (PmNa) were determined when the Na activity in the mucosal bathing solution, (Na)m, was 18, 32 or 100 mM. PmNa was obtained from current-voltage relations determined on tissues bathed with a high-K serosal solution before and after the addition of a maximally inhibitory concentration of amiloride to the mucosal solution as described by Turnheim et al. (Turnheim, K., Thompson, S.M., Schultz, S.G. 1983. J. Membrane Biol. 76:299-309). The results indicate that: (1) As demonstrated previously (Turnheim et al., 1983), PmNa decreases with increasing (Na)m. (2) PmNa also decreases hyperbolically with increasing (A)m. Kinetic analyses of the effect of amiloride on PmNa are consistent with the conclusions that: (i) the stoichiometry between the interaction of amiloride with apical membrane receptors that results in a decrease in PmNa is one-for-one; (ii) there is no evidence for cooperativity between amiloride and these binding sites; (iii) the value of (A)m needed to halve PmNa at a fixed (Na)m is 0.6-1.0 microM; and, (iv) this value is independent of (Na)m over the fivefold range studied. These findings are consistent with the notion that the sites with which amiloride interacts to bring about closure of the channels through which Na crosses the apical membrane are kinetically distinct from the sites with which (Na)m interacts to bring about closure (i.e., "self-inhibition"). In short, the effects of (Na)m and (A)m on PmNa in this tissue appear to be independent and additive.

Intracellular electrolyte concentrations in the frog skin epithelium: effect of vasopressin and dependence on the Na concentration in the bathing media

The intracellular electrolyte concentrations of the frog skin epithelium have been determined in thin freeze-dried cryosections using the technique of electron microprobe analysis. Stimulation of the transepithelial Na transport by arginine vasopressin (AVP) resulted in a marked increase in the Na concentration and a reciprocal drop in the K concentration in all epithelial cell layers. The effects of AVP were cancelled by addition of amiloride. It is concluded from these results that the primary mechanism by which AVP stimulates transepithelial Na transport is an increase in the Na permeability of the apical membrane. However, also some evidence has been obtained for an additional stimulatory effect of AVP on the Na pump. In mitochondria-rich cells and in gland cells no significant concentration changes were detected, supporting the view that these cells do not share in transepithelial Na transport. Furthermore, the dependence of the intracellular electrolyte concentrations upon the Na concentration in the outer and inner bathing solution was evaluated. Both in control and AVP-stimulated skins the intracellular Na concentration showed saturation already at low external Na concentrations, indicating that the self-inhibition of transepithelial Na transport is due to a reduction of the permeability of the apical membrane. After lowering the Na concentration in the internal bath frequently a Na increase in the outermost and a drop in the deeper epithelial layers was observed. It is concluded that partial uncoupling of the transport syncytium occurs, which may explain the inhibition of the transepithelial Na transport and blunting of the AVP response under this condition.

Voltage-dependent block by amiloride and other monovalent cations of apical Na channels in the toad urinary bladder

Inhibition of the Na conductance of the apical membrane of the toad urinary bladder by amiloride, alkali cations and protons was voltage dependent. Bladders were bathed with a high K-sucrose serosal medium to reduce series basal-lateral resistance and potential difference. Transepithelial current-voltage relationships were measured over a voltage range of +/- 200 mV with a voltage ramp of frequency 0.5 to 1 Hz. Na channel I-V relationships were obtained by subtraction of currents measured in the presence of maximal doses of amiloride (10 to 20 microM). With submaximal doses of amiloride (0.05 to 0.5 microM), the degree of inhibition of the Na channel current (INa) increased as the mucosal potential was made more positive. The data can be reasonably well explained by assuming that amiloride blocks Na transport by binding to a site which senses approximately 12% of the transmembrane voltage difference. INa was reduced in a qualitatively similar voltage-dependent manner by mucosal K, Rb, Cs and Tl (approximately 100 mM) and by mucosal H (approximately 1 mM). Block by these cations cannot be explained in terms of interactions with a single membrane-voltage-sensing site; a model in which there are two or more blocking sites in series provides a better description of the data. On the other hand, amiloride block was reduced competitively by mucosal Na and K, suggesting that occupation of the channel by one cation excludes occupancy by the others. ADH and ouabain also reduce the apparent affinity of amiloride for its blocking site. Thus, intracellular Na may also compete with amiloride for occupancy of the channel.

Control of sodium permeability of the outer barrier in toad skin

The 24Na efflux (JNaeff) (i.e., the rate of appearance of 24Na in the outer compartment) in the isolated short-circuited toad skin bathed by NaCl-Ringer's solution on both sides is composed of para- and transcellular components of almost equal magnitudes. This relies on the assumption that amiloride acts on the transcellular component only and could block it completely. Ouabain induces a large transient increase of the transcellular component. This increase, which starts within a few minutes after the addition of ouabain, is due to electrical depolarization of the outer barrier, rather than a consequence of blocking Na recirculation across the inner barrier. The subsequent decline of JNaeff, which takes place after the ouabain-induced JNaeff peak, is due to a progressive block of outer barrier Na channels with time, which can eventually be complete, depending on the duration of action of ouabain. As the external Na concentration was always kept high and constant in these experiments, the results indicate that a rise in cell Na concentration, and not in the outer bathing solution, is the signal that triggers the reduction of outer barrier Na permeability (PNao). Ouabain has no effect upon JNaeff with Na-free solution bathing the outer and NaCl-Ringer's solution the inner skin surface, showing the importance of Na penetration across the outer barrier, and not across the inner barrier due to its low Na permeability, in the process of closing the Na channels of this structure. Step changes from Na 115 mM to Na-free external solution, or vice-versa, may affect both the outer barrier electrical potential difference (PDo) and cell Na concentration (Na)c. Therefore, the behavior of JNaeff depends on which variable (if PDo or (Na)c regulated outer barrier Na permeability) is most affected by step changes in outer bathing solution Na concentration. Amiloride in the control condition blocks the transcellular component of JNaeff. However, in the condition of approximate short-circuiting of the outer barrier and high cellular Na concentration induced by long term effects of ouabain, when the Na channels of the outer barrier are already blocked by elevated cell Na concentration, amiloride may induce the opposite effect, increasing Na permeability of the outer barrier. With outer barrier Na channels completely blocked by high cell Na concentration, PCMB in the outer bathing medium induces a large increase of JNaeff, rendering these channels again amiloride sensitive. The results are consistent with the notion that Na efflux from cell compartment to the outer bathing solution goes through the amiloride-sensitive Na channels of the apical border of the superficial cell layer of toad skin, with an apparent Na permeability modulated by cell ionic environment, most probably the cell Na concentration.

Cystic fibrosis: a disorder of calcium-stimulated secretion and transepithelial sodium transport?

Both increased epithelial reabsorption of sodium and raised intracellular calcium have been implicated in the pathogenesis of cystic fibrosis. An intracellular calcium-stimulated increase in sodium reabsorption through an amiloride-sensitive pathway and the consequent obligatory reabsorption of water could explain the thick tenacious sections that characterise the disease. In the pancreatic ducts and airways increased intracellular calcium could exacerbate the problem of hyperviscous blockage by inducing acinar hypersecretion. Hypersecretion by the salivary and sweat glands would lead to excessive release of a factor which blocks sodium reabsorption by the cells in the ducts of these glands; this would lead to raised ion concentrations in sweat and saliva.

The role of sodium-channel density in the natriferic response of the toad urinary bladder to an antidiuretic hormone

Urinary bladders of Bufo marinus were depolarized, by raising the serosal K concentration, to facilitate voltage-clamping of the apical membrane. Passive Na transport across the apical membrane was then studied with near-instantaneous current-voltage curves obtained before and after eliciting a natriferic response with oxytocin. Fitting with the constant-field equation showed that the natriferic effect is accounted for by an increase in the apical Na permeability. It is accompanied by a small increase in cellular Na activity. Furthermore, fluctuation analysis of the amiloride-induced shot-noise component of the short-circuit current indicated that the permeability increase is not due to increased Na translocation through those Na channels which were already conducting prior to hormonal stimulation. Rather, the natriferic effects is found to be based on an increase in the population of transporting channels. It appears that, in response to the hormone, Na channels are rapidly "recruited" from a pool of electrically silent channels.

Identification of potential components of the transport mechanism for Na+ in the hen colon and coprodaeum

The binding of 3H-benzamil to homogenates of epithelium removed from the colon and coprodaeum of hens was studied. A low capacity high affinity binding component was detected. The binding constants for benzamil and amiloride to this component were similar to those obtained when these ligands were used to inhibit transport in intact epithelia. The mean potency ratio of benzamil to amiloride was 12.8 measured by binding compared with 11.6 from functional inhibition. Binding activity was present in tissues taken from animals fed on low sodium diets and those containing a normal sodium content. In the presence of sodium the affinity of benzamil was slightly reduced, but only in tissues taken form animals on a low salt diet. Only a small fraction of the total binding activity was present in the apical surface of low salt tissues indicating that in homogenates a small percentage of the total activity is associated with functional sodium entry sites. It is suggested that the major part of the binding activity detected in homogenates represents components of the sodium ion translocation mechanism which are en route for the apical membrane.

Kinetics of amiloride action in the hen coprodaeum in vitro

The kinetics of amiloride action on the isolated epithelium of the hen coprodaeum are reported. Tissues were taken from birds fed on low salt diets for 9-10 days, conditions which induce a high resting short circuit current due to sodium and sensitive to amiloride. The relation between the inhibition of amiloride sensitive short circuit current and blocker concentration obeyed simple mass laws with an apparent stoichiometry of 1:1 between amiloride and the sodium entry sites. The concentration of amiloride producing its half maximal effect (Ki) was 1.77 +/- 0.20 microM at a sodium concentration of 130 mM. There was a shallow dependence of Ki on sodium concentration, the value of Ki falling to 0.78 +/- 0.1 microM at 1.3 mM Na. The relation of Ki to Na concentration was linear indicating competitive antagonism. The sodium concentration which half saturates the amiloride site (KNa) was 80 mM. This value is very different from the concentration of sodium which half saturates SCC (Kscc = 5-7 mM) suggesting there are at least two sites at which sodium can modify the transporting characteristics. These data are compared to those for other epithelia where Kscc and KNa are rather similar. The benzyl derivative of amiloride (benzamil) was found to be 11.6 times more potent than amiloride on this tissue. The potency ration is similar to that for other sodium transporting epithelia suggesting that the structure of the ion translocation mechanism is partly conserved between species although the Ki for amiloride may vary by an order of magnitude.

Aldosterone control of the density of sodium channels in the toad urinary bladder

Near-instantaneous current-voltage relationships and shot-noise analysis of amiloride-induced current fluctuations were used to estimate apical membrane permeability to Na (PNa), intraepithelial Na activity (Nac), single-channel Na currents (i) and the number of open (conducting) apical Na channels (N0), in the urinary bladder of the toad (Bufo marinus). To facilitate voltage-clamping of the apical membrane, the serosal plasma membranes were depolarized by substitution of a high KCl (85 mM) sucrose (50 mM) medium for the conventional Na-Ringer's solution on the serosal side. Aldosterone (5 X 10(-7) M, serosal side only) elicited proportionate increases in the Na-specific current (INa and in PNa, with no significant change in the dependence of PNa on mucosal Na (Nao). PNa and the control of PNa by aldosterone were substrate-dependent: In substrate-depleted bladders, pretreatment with aldosterone markedly augmented the response to pyruvate (7.5 X 10(-3) M) which evoked coordinate and equivalent increases in INa and PNa. The aldosterone-dependent increase in PNa was a result of an equivalent increase in the area density of conducting apical Na channels. The computed single-channel current did not change. We propose that, following aldosterone-induced protein synthesis, there is a reversible metabolically-dependent recruitment of preexisting Na channels from a reservoir of electrically undetectable channels. The results do not exclude the possibility of a complementary induction of Na-channel synthesis.

Amiloride sensitivity of the transepithelial electrical potential and of sodium and potassium transport in rat distal colon in vivo

1. The effect of amiloride within the gut lumen on the transepithelial electrical potential difference (p.d.) and Na and K transport by the distal colon of adrenalectomized (dexamethasone-maintained), normal, aldosterone-infused and Na-depleted groups of rats was examined. 2. Amiloride had no effect in adrenalectomized rats; in normal rats, only the p.d. was significantly reduced. 3. In the group given a short (2 hr) aldosterone infusion, amiloride reduced the elevated p.d. and K secretion rate to normal levels. There was no change in apparent K permeability of the epithelium. 4. In the Na-depleted group, p.d. and Na absorption were virtually abolished by amiloride but although K secretion was reduced it still remained much above normal levels. Adrenalectomy prevented the effects of Na depletion. 5. P.d. change occurred rapidly when amiloride was added to the perfusate. Increasing the Na concentration in the perfusate reduced the sensitivity to amiloride. Apparent 'Km' values estimated from p.d. changes (luminal Na, 50 mM) were similar for aldosterone-infused (7.6 X 10(-6) M) and Na-depleted (5.4 X 10(-6) M) rats. 6. Aldosterone appears to be essential for the induction of amiloride-sensitive Na paths in the mucosal plasma membrane of rat colonic epithelial cells. Prolonged aldosterone stimulation, as in the Na-depleted rats, increases the amiloride-sensitive Na paths while largely suppressing the amiloride-insensitive Na paths; in addition, the K/Na clearance rate ratio of the epithelium is increased. AMiloride interacts only with one set of Na paths and does not interact directly with K paths.

Mechanism of action of aldosterone on active sodium transport across toad skin

Epithelium of the abdominal skin of the toad, Bufo marinus, has been studied by microelectrode impalement. Using an electrical equivalent circuit model, effective EMF's and specific conductances of the apical and basolateral membrane could be calculated. The skin was divided into 2 fragments for incubation in the presence, or not, of aldosterone (greater than or equal to 0.1 microM). After incubation overnight, sodium transport by the hormone-treated piece was increased 2.7-fold on average, compared to the untreated control. Concomitantly, conductance of the apical border increased more than 3-fold. Furthermore, mean conductance and electromotive force at the basolateral border increased by 80% and by 10%, respectively. Whether the latter changes merely represent delayed adaptation to increased apical conductance, cannot be settled from the data available.

Decreased sensitivity to amiloride of amphibian epithelia treated with aldosterone. Further evidence for an apical hormonal effect

The reversible inhibition of transepithelial sodium transport achieved with amiloride (and triamterene) was evaluated in amphibian preparations stimulated with aldosterone so as to provide further information regarding a possible influence of this hormone on the apical border of target cells. When aldosterone secretion was enhanced by withdrawal of sodium from toad (Bufo marinus) habitat, sensitivity of abdominal skin to amiloride decreased; the same occurred in skin and bladder preparations incubated with aldosterone for several hours. Amiloride proved a less efficient blocker of sodium transport by toad skin exposed to vasopressin and to ouabain; both substances are capable or raising cell sodium content. It is therefore proposed that the decrease in sensitivity to amiloride of amphibian epithelial treated with aldosterone results from an increase in target cell sodium, itself due to a hormone-induced increas in sodium conductance at the apical cell border. Glucose, which enhanced markedly the rate of sodium transport in preparations treated with aldosterone for several hours, failed to decrease any further the response to amiloride; this is taken as an argument for an additional (? secondary) influence of aldosterone on the cell's metabolic machinery connected with the operation of the sodium 'pump'.

Effect of amiloride on the apical cell membrane cation channels of a sodium-absorbing, potassium-secreting renal epithelium

The effect of the K-sparing diuretic amiloride was assessed electrophysiologically in the isolated cortical collecting tubule of the rabbit, a segment which absorbs Na and secretes K. Low concentrations of amiloride in the perfusate caused a rapid, reversible, decrease in the magnitude of the lumen negative transepithelial potential difference, Vte, transepithelial conductance Gte, and equivalent short-circuit current, Isc, with an apparent K1/2 of approximately 7 X 10(-8) M. The effects of a maximum inhibitory concentration of amiloride (10(-5) M) were identical to those observed upon Na removal from lumen and bath (Na removal from the bath alone has no effect). Removal of Na in the presence of 10(-5) M amiloride had no affect on Vte, Gte, or Isc, and is consistent with the view that amiloride blocks the Na conductive pathways of the apical cell membrane. Further, in the absence of Na, the subsequent addition of amiloride had no influence. In tubules where active Na absorption was either spontaneously low, or abolished by removal of Na from lumen and bath, the elevation of K from 5 to 155 meq/liter in the perfusate caused a marked change of the Vte in the negative direction and an increase in the Gte. These effects could be attributed to a high K permeability of the apical cell membrane and not of the tight junctions. Amiloride (10(-5) M) had no effect on these responses to K. It is concluded that amiloride selectively blocks the apical cell membrane Na channels but has no effect on the K conductive pathway(s). This selective nature of amiloride may indicate that Na and K are transported across the apical cell membrane via separate conductive pathways.

Synthesis, properties and biological activity of tritiated N-benzylamidino-3,5-diamino-6-chloro-pyrazine carboxamide -- a new ligand for epithelial sodium channels

A method is described for the synthesis and purification of tritiated N-benzylamidino-3,5-diamino-6-chloro-pyrazine carboxamide (benzamil). The tritium was inserted at the meta position of the benzyl ring, from which it apparently does not exchange with solvent hydrogen. When stored in ethanol at -4 degrees C the radioligand remains stable for at least 15 months. The pharmacology of benzamil is very similar to that of amiloride in terms of its effects on sodium transporting epithelia except that it has a higher affinity. The affinity of benzamil for sodium channels in amphibian epithelia in the absence of sodium is approximately 10(9) M-1. The new ligand can be used to label sodium channels in epithelia, and may be useful in channel isolation procedures.

Effect of aldosterone on incorporation of amino acids into renal medullary proteins

Studies on the effects of pretreatment with aldosterone on the incorporation of 3H leucine or 3H methionine into proteins in renal slices were carried out in Joklik-modified minimal essential medium. Administration of aldosterone (2microgram/100 g body wt) to adrenalectomized rats increased 3H leucine incorporation into trichloroacetic acid insoluble fractions of crude homogenates of cortical slices by 15.5 +/- 0.4% and of medullary slices by 53.5 +/- 1.3%. No increase in isotope incorporation was observed in slices of renal papilla or spleen prepared from the same rats. Aldosterone had no effect on the 3H-leucine content of the trichloroacetic acid-soluble fractions of all three renal zones and the spleen. The dose of aldosterone that elicited a half-maximal increase in 3H-methionine incorporation into proteins of renal medullary slices (0.45 microgram of aldosterone/100 g body wt) was indistinguishable from that needed to elicit a halt-maximal increase in the urinary K+/Na+ ratio (0.35 microgram of aldosterone/100 g body wt). Dexamethasone, a potent glucocorticoid, at a dose of 0.8 microgram/100 g body wt did not augment 3H-leucine incorporation into renal medullary proteins but was effective at 8 microgram/100 g body wt. Spirolactone (SC-26304), a potent anti-mineralocorticoid, abolished the effect of aldosterone on amino acid incorporation into medullary proteins when administered at a 100-fold higher dosage [i.e., 80 microgram (per 100 g body wt)]. These results imply that the action of aldosterone on amino acid incorporation is mediated by the mineralocorticoid rather than the glucocorticoid pathway, presumably the mineralocorticoid receptors. Moreover, pretreatment of the rats with actinomycin D (70--80 microgram/100 g body wt) erased the effect of aldosterone (0.8 microgram/100 g body wt) on amino acid incorporation into medullary proteins. In paired experiments with 3H and 35S methionine, aldosterone (0.8 microgram/100 g body wt) increased methionine incorporation into trichloroacetic acid precipitable proteins of subcellular fractions of the renal medulla. The effect of aldosterone on incorporation of methionine into medullary cytosol proteins was analyzed further by polyacrylamide gel electrophoresis at pH 8.3 in tris-glycine buffer. The gel profiles indicate that aldosterone significantly increased methionine incorporation into at least one protein (independent of the isotope) with a molecular weight of approximately 31,000. This increase was inhibited by either pretreatment of the rat with actinomycin D (70--80 microgram/100 g body wt or SC-26304 (80 microgram/100 g body wt). Dexamethasone (0.8 microgram/100 g body wt) did not increase incorporation of methinine into the medullary cytosol proteins resolved by polyacrylamide gel electrophoresis.

Effects of some pyrazinecarboxamides on sodium transport in frog skin

1 The inhibitory effect of amiloride (N-amidino-3,5-diamino-6-chloropyrazinecarboxamide) on sodium transport in isolated skin of frog has been compared with 17 of its analogues. The dissociation constant of amiloride for passive sodium channels was 181.9 +/- 8.9 nM, and the maximal percentage inhibition of sodium transport was 101.3 +/- 0.4% (means of 123 measurements) when measured at a sodium concentration of 111 mM. 2 The N-benzylamidino and N-o-chlorobenzylamidino compounds had affinities approximately 20 times larger than those for amiloride, and produced maximal inhibition of transport. 3 Substitution of chlorine in the 6-position by other halogens showed that the bromo-compound was equally active to amiloride, whereas the iodo derivative had an affinity equal to 15% of that for amiloride. 4 Substitution in the 5-amino group in 10 compounds reduced the affinities to less than 1% of that of amiloride, without affecting their ability to produce complete inhibition of transport. 5 N-Amidino-3,5-diaminopyrazinecarboxamide was unique in that it produced an unusual concentration-response relationship.

Effect of aldosterone on ion transport by rabbit colon in vitro

Segments of descending colon obtained from rabbits, that had been maintained on drinking water containing 25 mM NaCl and an artificial diet which contains 1% Na and is nominally K-free, respond to aldosterone in vitro (after a 30 to 60-min lag period) with a marked increase in the short-circuit current (Isc), an equivalent increase in the rate of active Na absorption (JNa net) and a decline in tissue resistance (Rt). Aldosterone also brings about a marked increase in the unidirection influx of Na into the cells across the mucosal membrane ("zero-time" rate of uptake) which does not differ significantly from the inrease m Isc. Treatment of control tissues with amphotericin B brings about sustained increases in Isc and JNa net to levels observed in aldosterone-treated tissues. However, addition of amphotericin B to the mucosal solution of aldosterone-treated tissues does not result in a sustained increase in Isc or JNa net and these values do not differ markedly from those observed in control tissues treated with amphotericin B. These findings, together with other evidence that Na entry in the presence of amphotericin B is sufficiently rapid to saturate the active Na extrusion mechanism at the baso-lateral membrane, are consistent with the notion that the aldosterone-induced protein increases the permeability of the mucosal membrane to Na but does not increase the "saturation level" of the active Na "pump" within the time-frame of these studies (3 hr). Finally, aldosterone has no effect on the bidirectional or net transepithelial movements of K under short-circuit conditions, suggesting that the enhanced secretion of K observed in vivo is the result of increased diffusion of K from plasma to lumen via paracellular pathways in response to an increased transepithelial electrical potential difference (lumen negative).

Induction of transporting sites in a sodium transporting epithelium

1. Frogs (Rana temporaria) were bathed for 1 week in solutions containing 1-1 mM sodium chloride and either one or both of amiloride (10(-4)M) and spironolactone (10(-5r both of amiloride (10(-4) M) and spironolactone (10(-5) M). This procedure was designed to deplete the sodium transporting compartment of the skin epithelium of sodium, while at the same time antagonizing the effects of endogenous aldosterone. 2. After 1 week the skins were used in vitro to measure the level of sodium transport (short-circuit current) and the density of sodium entry sites in the mucosal surface of the epithelium ([14C]amiloride binding). 3. Sodium deprivation for 1 week caused approximately a doubling of both sodium transport and the density of sodium entry sites in the mucosal surface of the epithelium compared to control skins. 4. When the results for sodium deprived and control skins were pooled there was a highly significant correlation between the density of sodium entry sites and sodium transport. 5. Mechanisms by which sodium deprivation leads to an increase in the density of sodium entry sites are discussed.

Characteristics of the entry process for sodium in transporting epithelia as revealed with amiloride

1. The permeation of sodium ions trhough the mucosal surface of frog skin epithelium at different transepithelial potentials has been investigated using the blocking drug amiloride. 2. An increase in serosal negativity in voltage-clamped skins was associated with an increase in the absolute amount of inhibition caused by a fixed concentration of amiloride. Hyperpolarizing or depolarizing skins with respect to the short-circuited condition did not affect the apparent affinity of amiloride for the entry sites. 3. When skins were voltage clamped at -50 mV (serosa negative) the specific binding of amiloride to sodium entry sites was increased by 77% compared to the short-circuited condition. Skins clamped at +50 mV had only 72% of the specific binding found in short-circuited skins. Experiments with a second blocking drug, triamterene, indicated that the extra binding sites appearing at -50mV were similar to those found under short-circuit conditions. The appearance and disappearance of binding sites may reflect changes in cell volume. 4. The findings suggest that the increased sodium current which flows when skins are clamped at -50 mV results from an increase in the number of entry sites, and perhaps also to a voltage sensitive increase in flux through each entry site.

Estimation of the lifespan of amiloride binding sites in the membranes of toad bladder epithelial cells

1. Sodium entry sites in the membranes of isolated epithelial cells prepared from bladders of toads (Bufo marinus) have been labelled with amiloride. The number of binding sites remained constant in suspensions for up to 100 hr. 2. In the presence of a protein synthesis inhibitor (cycloheximide, 0-5 mug/ml.) there was a decline in the density of binding sites was approximately exponential. Regression analysis gave a half-life of approximately 60 hr. 3. Aldosterone (5 X 10(-8) M) caused a significant (P less than 0-001) increase (50%) in the density of amiloride binding sites. Cells which had been treated with aldosterone had populations of binding sites which declined, in the presence of cycloheximide, at rates indistinguishable from those of untreated cells.