Irreversible inhibition of sodium entry sites in frog skin by a photosensitive amiloride analog

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.

Identification of the renal Na+/H+ exchanger with N,N'-dicyclohexylcarbodiimide (DCCD) and amiloride analogues

Dicyclohexylcarbodiimide (DCCD) and the 5-ethyl-isopropyl-6-bromo-derivative of amiloride (Br-EIPA) have been used as affinity and photoaffinity labels of the Na+/H+ exchanger in rat renal brush-border membranes. Intravesicular acidification by the Na+/H+ exchanger was irreversibly inhibited after incubation of vesicles for 30 min with DCCD. The substrate of the antiporter, Na+, and the competitive inhibitor, amiloride, protected from irreversible inhibition. The Na+-dependent transport systems for sulfate, dicarboxylates, and neutral, acidic, and basic amino acids were inhibited by DCCD, but not protected by amiloride. An irreversible inhibition of Na+/H+ exchange was also observed when brush-border membrane vesicles were irradiated in the presence of Br-EIPA. Na+ and Li+ protected. [14C]-DCCD was mostly incorporated into three brush-border membrane polypeptides with apparent molecular weights of 88,000, 65,000 and 51,000. Na+ did not protect but rather enhanced labeling. In contrast, amiloride effectively decreased the labeling of the 65,000 molecular weight polypeptide. In basolateral membrane vesicles one band was highly labeled by [14C]DCCD that was identified as the alpha-subunit of the Na+,K+-ATPase. [14C]-Br-EIPA was mainly incorporated into a brush-border membrane polypeptide with apparent molecular weight of 65,000. Na+ decreased the labeling of this protein. Similar to the Na+/H+ exchanger this Na+-protectable band was absent in basolateral membrane vesicles. We conclude that a membrane protein with an apparent molecular weight of 65,000 is involved in rat renal Na+/H+ exchange.

Amiloride-sensitive Na+-H+ antiporter in Escherichia coli

In everted vesicles of Escherichia coli, delta pH caused by H+ efflux through the Na+/H+ antiporter was measured by using a fluorescent dye. Amiloride inhibited the activity of the Na+/H+ antiporter. Kinetic studies showed that amiloride competed with Na+. The inhibition constant of 40 microM was obtained.

Structure-activity relationship of amiloride analogs as blockers of epithelial Na channels: I. Pyrazine-ring modifications

The overall on- and off-rate constants for blocking epithelial Na channels by amiloride analogs were estimated by noise analysis of frog skin epithelium. The substituents at position-5 and -6 of the pyrazine ring of amiloride were varied in order to obtain the structure/rate constant relationship. (1) The off-rate constant increases with halo-substitutions at position-6 in the order Cl less than Br less than l less than F less than H. Substitution of Cl by H lowers the standard free energy of activation of the off-step by 2.3 kcal mol-1. The on-rate constant is not affected. Apparently the substituent at ring position-6 controls the duration of attachment in the blocking position. pKa considerations show that the duration is longer when the 6-substituent is more negatively polarized. We suggest that this substituent binds to the receptor by virtue of its electronegativity. (2) In contrast, replacement of the adjacent 5-amino group (electron donor) by H or Cl affects both the on-rate and the off-rate. The dual effect may be explained by a decrease of the electronic charge at more remote parts of the molecule (on-rate decrease), as well as at the 6-position (off-rate increase). Apparently the 5-amino group stabilizes the blocking position by increasing the electron density on the 6-ligand.

Phenamil: an irreversible inhibitor of sodium channels in the toad urinary bladder

Several new amiloride analogues and two reported photoaffinity analogues were tested for irreversible inhibition of short-circuit current, Isc, in toad bladder. Bromoamiloride, a photoaffinity analogue, induced 40% irreversible inhibition at 500 microM after irradiation with ultraviolet light greater than or equal to 320 nm. Iodoamiloride caused no irreversible inhibition. Of the new analogues tested, only 3,5-diamino-6-chloro-N-[(phenylamino) amino-methylene] pyrazinecarboxamide, phenamil, irreversibly inhibited Isc at concentrations of 0.05 to 5 microM when added to the mucosal solution. Irreversible inhibition of Isc by phenamil may be attributed to specific blockage of the mucosal sodium channels, which depended on: 1) time of exposure; 2) mucosal pH; 3) mucosal sodium concentration. For example, 5 microM phenamil irreversibly inhibited Isc by 38% in 103 mM Na at pH 8.6 and nearly 75% in 30 mM Na at pH 6.4 after a 40-min exposure. Irreversible inhibition occurred in two phases with time constants of less than or equal to 10 min and approximately 140 min. Due to its irreversible nature, phenamil may be used to measure channel density.

Synthesis and characterization of methylbromoamiloride, a potential biochemical probe of epithelial Na+ channels

We report the synthesis of a radioactive, methylated analog of bromoamiloride which inhibits the amiloride-sensitive, epithelial Na+ channel reversibly and with high affinity. This synthesis was achieved by methylation of a nitrogen in the acylguanidinium moiety with tritiated methyliodide of high specific activity. This methylated bromoamiloride molecule (CH3BrA) was purified by both thin layer and high performance liquid chromatography. Proton nuclear magnetic resonance and mass spectroscopy techniques were used to determine the structure of this analog. This compound inhibited both short-circuit current of in vitro frog skin and 22Na+ influx into apical plasma membrane vesicles made from cultured toad kidney cells (line A6) with the same or lower apparent inhibitory dissociation constant as bromoamiloride. Irradiation with ultraviolet light rendered this inhibition irreversible in both A6 vesicles and frog skin. Preparation of radioactive CH3BrA yielded specific activities in excess of 1 Ci/mmol. We suggest that this compound will be useful in the isolation and purification of this ubiquitous Na+ channel.

The intracellular localization of amiloride in frog skin

The diuretic compound amiloride is often used as a specific inhibitor of the passive Na+ entry step in the transepithelial transport of Na+ across frog skin. We have utilized the fluorescence properties of amiloride to study the distribution of this transport inhibitor in the ventral skin of Rana pipiens. After a 30 s exposure of 1-100 microM amiloride to the external surface of frog skin, amiloride fluorescence was evident in the cytoplasm of all cell layers of the epidermis and alveolar gland epithelium. Changes in the conditions of incubation which alter the pharmacological activity of amiloride did not affect the intracellular distribution of amiloride or the washout profile of [14C]amiloride. The presence of amiloride fluorescence in the cytoplasm prevented our examination of changes in the amiloride fluorescence at the cell surface with various conditions of incubation. Four derivatives of amiloride that differed in their ability to inhibit short-circuit current were also localized intracellularly but varied in their relative distribution among the cell layers of the epidermis. Our results indicate that when incubated at concentrations from 1 to 100 microM, a large fraction of the amiloride taken up by frog skin is not directly involved with the inhibition of passive Na+ transport at the apical surface of the stratum granulosum. The mechanism of intracellular uptake of amiloride is not clear. However, the cytoplasmic localization of amiloride could explain the action of the drug on intracellular enzymes and may account for the large proportion of non-displaceable [14C]amiloride that has been observed in frog skin.

Irreversible inhibition of epithelial sodium channels by ultraviolet irradiation

1 The effects of u.v. irradiation at 254 nm and 350 nm on sodium transport across frog skin epithelium have been investigated. 2 Irradiation at 254 nm but not at 350 nm produces a dose-dependent, functionally selective blockade of sodium transport. The effect is apparently due to the irreversible closure of apical sodium channels. 3 The amiloride-sensitive conductance was directly related to sodium transport as measured by short circuit current (SCC) both in normal and irradiated tissues, although both conductance and current were reduced in irradiated tissues. 4 The sensitivity of epithelia to irradiation at 254 nm was defined from the rate constants for the decline in SCC during three 2 min periods of irradiation at 1850 microW cm-2. The rate constant for the initial 2 min irradiation was 0.093 +/- 0.008 min-1. 5 Lowering the sodium concentration to 5.5 mM from 110 mM increased the rate constant to 0.141 +/- 0.014 min-1, consistent with the view that more functional sodium channels exist at lowered sodium concentration. 6 Lowering the temperature to 7 degrees C from 23 degrees C reduced the rate constant to 0.032 +/- 0.007 min-1 suggesting that blockade of channels is not due to a direct interaction with photons. 7 Using a variety of experimental protocols we were unable to demonstrate that bromamiloride or iodoamiloride can act as photoligands for sodium channels in the epithelium of Rana temporaria. This is in contrast to earlier reports with other epithelia.

Amiloride: a molecular probe of sodium transport in tissues and cells

The potassium-sparing diuretic amiloride has proven to be a useful pharmacological tool for elucidating the molecular basis and physiological regulation of facilitated sodium entry in tissues and cells. There are two general classes of Na+ transport mechanisms which are sensitive to this drug: 1) a conductive Na+ entry pathway found in electrically high resistance epithelia and 2) a Na+-H+ electroneutral exchange system found in certain leaky epithelia such as the renal proximal tubule. This latter system is also found in many different cellular preparations and seems to function in cell proliferation and differentiation, volume regulation, and intracellular pH regulation. In these cells, this exchange pathway becomes operational usually after some external stimuli. Much higher concentrations of amiloride are required to inhibit the exchange pathway than those required to inhibit the Na+ entry pathway. This drug is the most potent and specific inhibitor of Na+ entry found to date and thus affords the opportunity to be used as a label for the isolation of these transport moieties.

Irreversible inhibition of sodium transport by the toad urinary bladder following photolysis of amiloride analogs

Active sodium transport was completely and irreversibly inhibited in toad urinary bladders photolyzed in the presence of iodoamiloride.

Reduction of sodium dependent stump currents disturbs urodele limb regeneration

We have asked the question whether the natural electric currents which leave urodele limb stumps are in any way needed for their regeneration. As an initial test, we have greatly reduced such currents in the tiger salamander, Ambystoma tigrinum, by applying 0.5 mM amiloride to the stump skin or by immersion of the animals in sodium depleted media. We have also reduced such currents in the red spotted newt, Notophthalmus viridescens, by such immersion. Limb regeneration in half of the amiloride-treated animals was either entirely blocked or grossly deficient, while the others regenerated normally. Limb regeneration in sodium depleted media was consistently inhibited for some weeks but then recovered. These results are consistent with the hypothesis that stump currents are in some way needed for normal regeneration.