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

Binding sites for amiloride in intact epithelia

The following is the abstract of the article discussed in the subsequent letter:

Blazer-Yost, Bonnie L., and Sandy I. Helman.The amiloride-sensitive epithelial Na+ channel: binding sites and channel densities. Am. J. Physiol. 272 ( Cell Physiol. 41): C761–C769, 1997.—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.

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.

Immunocytochemical localization of amiloride-sensitive sodium channels in the lower intestine of the hen

We have used polyclonal antibodies generated against purified bovine renal amiloride-sensitive Na+ channels to localize amiloride-sensitive Na+ channels within the lower intestine (colon and coprodeum) of the hen. These antibodies cross-reacted with two polypeptides exhibiting M(r)'s of 235 and 150 kDa on immunoblots of detergent-solubilized apical membrane fractions from both the colon and coprodeum. The apparent molecular masses of theses polypeptides are in agreement with the M(r)'s of 2 of the subunits of the renal high amiloride-affinity Na+ channel, namely the alpha and the beta (= amiloride binding) subunits. The cellular distribution of Na+ channels was determined by immunoperoxidase and indirect immunofluorescence cytochemical techniques. The apical (luminal) membrane and cytoplasm of villar principal cells in both colon and coprodeum exhibited immunoreactivity, whereas goblet cells were negative. Both principal and goblet cells of the crypts were also negative. We conclude that the amiloride-sensitive Na+ channels are localized to the principal cells of the intestinal villi and that these cells are responsible for intestinal Na+ absorption.

Structural and enzymatic studies on the plasma membrane domains and sodium pump enzymes of absorptive epithelial cells in the avian lower intestine

The coprodaeum of the domestic hen maintained on a low-NaCl diet adapts by enhanced sodium transport. This study examines the adaptive response at the single cell and whole organ levels. Surface areas of apical (microvillous) and basolateral plasma membranes of columnar absorptive epithelial cells were estimated by use of ultrastructural stereology. The activities of succinic dehydrogenase (a mitochondrial enzyme) and ouabain-sensitive, potassium-dependent paranitrophenyl phosphatase (a sodium pump enzyme) were determined in tissue homogenates. Sodium, potassium-ATPase (pump enzyme) activity in cell membranes was localized by ultrastructural cytochemistry. Apical and basolateral membranes responded differently. In high-NaCl hens, the membrane signature of the average cell was 32 microns 2 (apical), 932 microns 2 (lateral) and 17 microns 2 (basal). Cells from low-NaCl hens had more apical membrane (49 microns 2 per cell) but essentially the same area of basolateral membrane. However, total surfaces per organ were greater for all membranes. Sodium pump enzymes were localized in basolateral membranes. Enzyme activities per unit mitochondrial volume and per unit basolateral membrane surface were higher in low-NaCl birds. These findings are discussed in the context of known mechanisms of transcellular sodium transport via apical ion channels and basolateral pumps.

A low-salt diet facilitates Cl secretion in hen lower intestine

The regulation of sodium and chloride transport in hen coprodeum by mineralocorticoids was investigated with isolated epithelia under short-circuit conditions. Unidirectional fluxes of Na and Cl were measured by isotopes and modulated by amiloride, theophylline and bumetanide. Hens were maintained either on low-NaCl diet (LS) or on high-NaCl diet (HS). Plasma aldosterone (PA) levels of these groups were measured with radioimmunoassay. A group of HS hens received injections of aldosterone on a 6-hr schedule before experiments. Another group of LS hens was resalinated, and experiments carried out on a 24-hr interval. Salt deprivation stimulated PA levels ninefold, compared to HS hens. Na absorption was stimulated according to previous reports. Electrogenic Cl secretion was elicited by theophylline and partially inhibited by bumetanide. Modulation of PA levels by diet, resalination or aldosterone injection changed the magnitude of electrogenic Cl secretion in parallel between 0.5 mueq/cm2.hr (HS) and 4 mueq/cm2.hr (LS), with pronounced alteration in tissue resistance. The results demonstrate a new action of aldosterone which besides stimulating Na absorption also directly or indirectly elicits Cl secretion. Evidence is presented for a hormonal adaptation of chloride transport in this epithelium. There was a morphological change of the apical plasma membrane and further experiments will have to clarify the exact cellular nature of this process.

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.

Relationships among sodium current, permeability, and Na activities in control and glucocorticoid-stimulated rabbit descending colon

Effects of a potent synthetic glucocorticoid, methylprednisolone (MP), on transepithelial Na transport were examined in rabbit descending colon. Current-voltage (I-V) relations of the amiloride-sensitive apical Na entry pathway were measured in colonic tissues of control and MP-treated (40 mg im for 2 days) animals. Tissues were bathed mucosally by solutions of various Na activities, (Na)m, ranging from 6.2 to 75.6 mM, and serosally by a high K solution. These I-V relations conformed to the "constant field" flux equation permitting determination of the permeability of the apical membrane to Na, PmNa, and the intracellular Na activity, (Na)c. The following empirical relations were observed for both control and MP-treated tissues: Na transport increases hyperbolically with increasing (Na)m obeying simple Michaelis-Mentin kinetics; PmNa decreased hyperbolically with increasing (Na)m, but was unrelated to individual variations in (Na)c; (Na)c increased hyperbolically with (Na)m; both spontaneous and steroid-stimulated variations in Na entry rate could be attributed entirely to parallel variations in PmNa at each mucosal Na activity. Comparison of these empirical, kinetic relations between control and MP-treated tissues revealed: maximal Na current and PmNa were greater in MP tissues, but the (Na)m's at which current and PmNa were half-maximal were markedly reduced; (Na)c was significantly increased in MP tissues at each (Na)m while the (Na)m at half-maximal (Na)c was unchanged. These results provide direct evidence that glucocorticoids cause marked stimulation of Na absorption across rabbit colon primarily by increasing the Na permeability of the apical membrane. While the mechanism for the increased permeability remains to be determined, the altered relation between PmNa and (Na)m suggests possible differences in the conformation or environment of the Na channel in MP-treated tissues.