Kinetics of amiloride action in the hen coprodaeum in vitro

Expression of amiloride-sensitive Na+ channels of hen lower intestine in Xenopus oocytes: electrophysiological studies on the dependence of varying NaCl intake

Epithelial Na+ channels were incorporated into the plasma membrane of Xenopus laevis oocytes after micro-injection of RNA from hen lower intestinal epithelium (colon and coprodeum). The animals were fed either a normal poultry food which contained NaCl (HS), or a similar food devoid of NaCl (LS). Oocytes were monitored for the expression of amiloride-sensitive sodium channels by measuring membrane potentials and currents. Oocytes injected with poly(A)+RNA prepared from HS animals or non-injected control oocytes showed no detectable sodium currents, whereas oocytes injected with LS-poly(A)+RNA had large amiloride-blockable sodium currents. These currents were almost completely saturated by sodium concentrations of 20 mM with a Km of about 2.6 mM sodium. Amiloride (10 microM) inhibits the expressed sodium channels entirely and examination of dose response relationships yielded a half-maximal inhibition concentration (Ki) of 120 nM amiloride. I-V difference curves in the presence or absence of sodium or amiloride (10 microM) indicate a potential dependence of the sodium transport which can be described by the Goldman equation. When Na+ is replaced by K+, no amiloride response was detected indicating a high selectivity for Na+ over K+. These results provide strong evidence that intestinal Na+ channels are regulated by dietary salt intake on the RNA level.

Na-transport during long-term incubation of the hen lower intestine: No aldosterone effect

1. In order to test the aldosterone effect in vitro, Na-transport of the coprodeal epithelium from hens on low-NaCl diet was measured in the Ussing chamber for up to 8 hr. Short-circuit current (SCC, near equal to the amiloride inhibitable Na-transport) was recorded. 2. Incubation media were either Krebs-phosphate or bicarbonate buffer with and without addition of beta-hydroxybutyrate, glutamine and mannose as "metabolic fuels". The media were replaced every hour. The Krebs-phosphate buffer was further tested with and without indomethacin and media replacement. Na-transport was best maintained in this buffer with replacement: SCC at 4 hr: 156 +/- 21 microA/cm2, 8 hr: 73 +/- 14 microA/cm2. 3. The aldosterone experiments were carried out on tissues from hens resalinated for 24 hr. No effects were demonstrated at concentrations up to 10(-5) M. The SCC showed an unexpected raise within 2-4 hr to a very high level (4 hr: 221 +/- 61 microA/cm2) both in the control and in all aldosterone-treated tissues. This SCC decreased slowly to 210 +/- 29 microA/cm2 at 8 hr. It was abolished by amiloride. 4. No increase in SCC was observed in tissues from hens after 48 and 72 hr of resalination either after aldosterone or on chronic high-NaCl diet.

Altered sensitivity to amiloride in cystic fibrosis. Observations using cultured sweat glands

1. Using cultured epithelia from sweat glands, derived from both cystic fibrosis and normal subjects, the relationship between amiloride concentration and the inhibition of electrogenic sodium transport was measured, under short circuit conditions. 2. The Kd for amiloride in cultures from normal subjects was 0.64 microM (n = 6) while in cultures derived from CF patients the value was 1.07 microM (n = 4). The values were significantly different (P less than 0.02, ANOVA). 3. In cultures from normal sweat glands, bathed in solutions free of permeable anions (chloride/bicarbonate), the Kd for amiloride rose to a value greater than found in CF tissues (2.3 microM). In CF epithelia subject to the same conditions the abnormally high value increased further, so that in solutions without permeable anions normal and CF cultures behaved similarly. 4. In cultures derived from normal and CF tissues lowering the sodium concentration to 10 mM also lowered the Kd for amiloride, however the shift was greater for CF cultures. 5. Several possible explanations for the results are discussed. The most probable is that the relatively more positive apical membrane potential in CF epithelia opposes the interaction of amiloride with the sodium channel, implying that complex formation is potential sensitive.

Electrophysiological analysis of sodium-transport in the colon of the frog (Rana esculenta). Modulation of apical membrane properties by antidiuretic hormone

Sodium transport and apical bioelectrical membrane properties were investigated in frog colonic epithelium in the absence and presence of the antidiuretic hormone arginine-vasotocin (AVT). Apical Na-permeability and intracellular Na-activity were evaluated by analysis of current-voltage relationships in the serosally K-depolarized tissue. Tissue- and apical membrane capacitance were measured by voltages step analysis. The frog colon was found to be a tight epithelium with a transepithelial resistance of 2.63 +/- 0.25 k omega.muF (n = 17). 85-90% of short circuit current (11.2 +/- 1.1 microA.microF.l-1; n = 17) was related to electrogenic Na-transport from mucosa to serosa. Graded doses of amiloride (less than 50 mumol.l-1) induced Michaelis-Menten-type inhibition kinetics. Serosal addition of 10(-6) mol.l-1 AVT induced a significant increase in sodium current (25%), apical sodium permeability (19%) and tissue capacitance (4.3%) whereas intracellular Na-activity remained unchanged. There was a good correlation between increased Na-current and apical Na-permeability. No correlation was found between Na-current and membrane capacitance. Our results demonstrate that in contrast to other species the amphibian colon shows a natriferic reaction to AVT. We suggest that the regulation of Na-transport in frog colon is similar to that in the toad urinary bladder. It is caused by an activation of preexisting apical Na-channels and not by fusion of subapical cytoplasmic vesicles with the apical membrane.

Modulation of Na and Cl transport by mineralocorticoids

1. The epithelia of the hen lower intestine show a Na-channel, Na-cotransport, chloride cells, and chloride absorption and secretion. 2. The short circuit current is affected by low salt levels, amiloride, glucose, lysine, leucine, galactose, ouabain, bumetanide, aldosterone, dexamethasone and spironolactone. 3. The properties of the different sodium and chloride channels are described.

Effects of adrenal steroids on Na transport in the lower intestine (coprodeum) of the hen

The influence of adrenal steroids on sodium transport in hen coprodeum was investigated by electrophysiological methods. Laying hens were maintained on low-NaCl diet (LS), or on high-NaCl diet (HS). HS hens were pretreated with aldosterone (128 micrograms/kg) or dexamethasone (1 mg/kg) before experiment. A group of LS hens received spironolactone (70 or 160 mg/kg, for three days). The effects of these dietary and hormonal manipulations on the amiloride-sensitive part of the short-circuit current were examined. This part is in excellent agreement with the net Na flux, and therefore a direct electrical measurement for Na transport. After depolarizing the basolateral membrane potential with a high K concentration, the apical Na permeability and the intracellular Na activity were investigated by current-voltage relations for the different experimental conditions. Plasma aldosterone concentrations (PA) were low in HS hens, dexamethasone-treated HS hens and spironolactone-treated LS hens (less than 70 pM). In contrast LS hens and aldosterone-treated HS hens had a PA concentration of 596 +/- 70 and 583 +/- 172 pM, respectively. LS diet (chronic stimulation) had the largest stimulatory effect on Na transport and apical Na permeability. Hormone-treated animals had three- to fourfold lower values. Spironolactone supply in LS hens decreased Na transport and apical Na permeability about 50%. The results provide evidence that both mineralo- and gluco-corticoids stimulate Na transport in this tissue by increasing the apical Na permeability. Quantitative differences between acute and chronic stimulation reveal a secondary slower adaptation in apical membrane properties.

Embryonic chick allantois: functional isolation and development of sodium transport

By removing the shell membranes from the chorioallantoic membrane, the chorion is damaged, as visualized by electron microscopy, and rendered permeable, as evidenced by penetration of horseradish peroxidase and increased inhibition of the allantoic Na+-K+ pump by ouabain applied on the chorionic side. The short-circuit current (SCC) of this functionally isolated allantoic epithelium is augmented by nystatin, a channel-forming ionophore, when applied to the mucosal surface. Electrical parameters were determined for three age groups between 12 and 19 days of incubation. The SCC approximately doubled from the youngest (12-13 days) to the oldest (18-19 days) groups, whereas the transepithelial resistance (Re) of 700-900 omega X cm2 remained the same. Amiloride, an inhibitor of apical Na+ uptake, inhibited 98-100% of the SCC at 10(-4) M in both 15-16 and 18-19 day epithelia. In the 12- to 13-day preparation 20-25% of the SCC was insensitive to 10(-3) M amiloride. The Ki's for amiloride were similar in all preparations, at about 5 X 10(-7) M. Determination of the Hill coefficients for inhibition revealed a lower value (0.75 +/- 0.03) for the 12-13 day preparation compared with the two older preparations with coefficients not significantly different from unity. Replacing Na+ in the bathing solutions abolished the SCC of 18-19 day epithelia, whereas about 15% of the SCC remained at 12-13 days. Thus, during development, the SCC of the allantoic epithelium increases in magnitude and becomes increasingly (to 100%) amiloride-sensitive and Na+-dependent.

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.

Competitive blocking of epithelial sodium channels by organic cations: the relationship between macroscopic and microscopic inhibition constants

Fluctuation analysis of Na current passing the apical membrane in the skin of Rana ridibunda was used to study the kinetics of Na-channel blocking by several organic cations present in the outer solution together with 60 mM Na. The ratios of the apparent off-rate and on-rate constants (the microscopic inhibition constants) thus obtained for triamterene, triaminopyrimidine (TAP), 5,6-diCl-amiloride, 5H-amiloride and amiloride itself are found to be in the mean about sevenfold smaller than the corresponding inhibition constants obtained from macroscopic dose-response curves. The apparent discrepancy is explicable by competition of the organic blocker with the channel block by Na ions (the self-inhibition effect). The type of interaction between extrinsic blockage and self-inhibition may be purely competitive or mixed. However, in case of mixed inhibition the competitive component must dominate the noncompetitive component by at least seven to one.

Conversion of sodium channels to a form sensitive to cyclic AMP by component(s) from red cells

Sodium transport has been measured in the isolated epithelium from colons of male Sprague-Dawley rats. Sodium transport in colons was induced by pretreating the animals with dexamethasone (6 mg kg-1) which caused the appearance of an amiloride-sensitive short circuit current within a few hours. Forskolin, a diterpene, which activates adenylate cyclase, was found to increase the cyclic adenosine monophosphate (cyclic AMP) content of rat colons and also to increase short circuit current at the same time. However, measurements of chloride and sodium fluxes across the epithelium indicated that forskolin activates chloride secretion but has no effect on sodium transport. In confirmation of (3) it was found that the amiloride-sensitive short circuit current was unchanged after the short circuit current had been increased by forskolin under a variety of conditions. The behaviour of the mammalian colon as indicated in (3) and (4) is unlike that of amphibian sodium transporting epithelia. It is shown that in toad urinary bladder forskolin increases amiloride-sensitive short circuit current. Procedures were investigated which might make sodium transport in the mammalian colon sensitive to cyclic AMP. Exposing the apical surface to sonicated suspensions of nucleated red cells (frog, toad and duck), followed by washing, gave preparations with amiloride-sensitive short circuit currents which were increased by forskolin or dibutyryl cyclic AMP. It would appear that the sodium channel in the mammalian colon, unlike that of amphibian tissues, has lost the ability to have its properties modified by cyclic AMP. Incubation of colons with sonicated suspensions of nucleated red cells apparently modifies the tissues such that sodium transport across the tissue becomes sensitive to the nucleotide.

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.