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Uptakes of 22Na, 36Cl, and 86Rb into isolated bovine tracheal epithelial cells were measured in the presence and absence of 10 microM bumetanide. Preincubation with ouabain (0.5 mM) did not alter initial rates of Na and Cl uptakes but prolonged from 0.5 or less to 2 min the period in which Na uptake is linear with time. Initial rates of bumetanide-inhibitable Na and Cl uptakes (influxes), measured for 2 min in identical solutions, were similar in magnitude (bumetanide-sensitive Cl influx/bumetanide-sensitive Na influx = 1.2). Omission of K did not affect bumetanide-sensitive Na or Cl influx. Cl influx was not affected by 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid. Amiloride (0.1 mM) partially inhibited the bumetanide-insensitive Na influx but had no effect on the bumetanide-sensitive Na influx. Rb influx was not affected by bumetanide but was markedly reduced by ouabain and slightly reduced by Ba, the combination being additive. Half-maximally inhibitory concentration values for inhibition of Cl influx by 4 sulfamoylbenzoic acid derivatives were as follows (in mumol/l): 0.125 benzmetanide; 0.64 bumetanide; 16 piretanide; and 26 furosemide. Affinities of Na and Cl for the bumetanide-inhibitable cotransport process were determined by measuring bumetanide-sensitive Cl influx at varying [Na] and bumetanide-sensitive Na influx at varying [Cl]. Both plots were hyperbolic, and the K0.5 values for Na and Cl were 4.1 and 53.9 mM, respectively. Bumetanide-inhibitable Cl influx was not altered by secretory stimuli (epinephrine, A23187) but was more than doubled by osmotic shrinkage (200 mM mannitol or sucrose).(ABSTRACT TRUNCATED AT 250 WORDS)

Phorbol esters, specific activators of protein kinase C, inhibit amiloride-sensitive Na uptake from the mucosal medium in intact intestinal mucosa as well as in isolated chicken villus enterocytes. In isolated cells, maximal inhibition is observed at 60 s, and influx returns to control values within 15 min. This effect can be measured either as initial 22Na influx rates or by following changes in intracellular pH using the pH-sensitive fluorescent dye 5,6-carboxyfluorescein. The effects of amiloride and phorbol esters were not additive, suggesting inhibition of a common transport system, i.e., Na-H exchange. In brush-border membrane vesicles (BBMV) made from villus enterocytes, amiloride-sensitive Na-H exchange activity was significantly inhibited in phorbol ester-treated cells. The degree of inhibition of 22Na uptake by BBMV had the same time course and dose-effect relationship as phorbol ester-inhibited cellular Na uptake. Similarly, the time course of protein kinase C translocation from cytosol to particulate or brush-border membrane fractions correlated with Na uptake measurements made in whole cells and BBMV. These results suggest that protein kinase C activation in chicken villus enterocytes inhibits brush-border membrane Na-H exchange activity.

Na+/H+ exchangers (NHEs) are integral membrane proteins that exchange Na+ for H+ across membranes. Four isoforms have been cloned (NHE-1-4). NHE-3 localizes to the apical domain, and its expression is increased in dexamethasone-treated rats by Northern analysis. This stimulatory effect on expression is region and tissue specific, being present in ileum and proximal colon, but not in jejunum, distal colon, or kidney. The increase in transcript expression in ileum correlates with an increase in protein expression by immunoblotting. Changes in apical Na+/H+ exchange activity, as measured by 22Na uptake into brush-border membrane vesicles, correlate with expression differences, with significant increases observed in ileum and proximal colon. In situ hybridization showed NHE-3 mRNA only in villus and absorptive cells of control rats, the pattern not being altered by dexamethasone treatment. This suggests that dexamethasone does not increase expression by inducing crypt cells to express NHE-3 prematurely. We conclude that glucocorticoids selectively increase intestinal NHE-3 activity in a region-specific manner and that this effect also appears to be tissue specific.

Nitrovasodilators, such as sodium nitroprusside (SNP), release nitric oxide (NO) and stimulate intestinal electrolyte transport. However, the second messengers involved in this process are unknown. NO stimulates soluble guanylate cyclase activity in other tissues, but stimulation of this enzyme has not previously been described for intestine. We report a 20-fold increase in guanosine 3',5'-cyclic monophosphate (cGMP) production by radioimmunoassay in colonic mucosal strips stimulated with SNP. SNP also caused a significant increase in prostaglandin (PG) E2 release but did not stimulate release of the prostanoids thromboxane B2 or 6-keto-PGF1alpha. Stimulation of isolated colonic crypts and the remaining subepithelial mucosa demonstrated that the latter was the major source of the increases in cGMP and PGE2. Immunostaining of colonic mucosa revealed minimal basal cGMP immunoreactivity but large increases in abundance, localizing to the subepithelium, after SNP treatment. Under basal conditions, there was diffuse immunostaining for constitutive NO synthase in both the epithelial and subepithelial compartments, which was corroborated with NADPH diaphorase staining. In conclusion, SNP was an NO donor stimulates production of cGMP and PGE2 from the subepithelium. NO may be an important mediator of colonic secretion and other processes predominantly via its direct effects on cells of the lamina propria.

The granulocyte-derived oxidant, monochloramine (NH2Cl), is known to stimulate chloride ion secretion in rat distal colonic mucosa mounted in Ussing chambers, through mechanisms that are sensitive and insensitive to tetrodotoxin (TTX). The possible role of intracellular thiols, in the mechanism of action of NH2Cl as a secretagogue, was evaluated with the thiol-oxidizing agent diamide and by measuring tissue sulfhydryl levels in response to NH2Cl. Serosal exposure to the antioxidant glutathione (0.25 mM), 5 min before NH2Cl (50 microM) addition, decreased the maximal effect of 50 microM NH2Cl on short-circuit current (Isc). The NH2Cl-stimulated increase in Isc was not affected by mucosal amiloride (5 microM). Pretreatment with 0.1 mM diamide shortened the lag period before the increase in Isc in response to NH2Cl, but it did not affect the maximal increase in Isc. Although TTX (0.5 microM) increased the lag time for achievement of the maximal Isc response to NH2Cl, the neurotoxin did not inhibit the effect of diamide, suggesting that diamide acts primarily on the nonneural component of NH2Cl-stimulated secretion. Incubation of colonic mucosa with NH2Cl, with or without diamide, decreased cellular acid-soluble sulfhydryl concentrations. Taken together, the results support a role for epithelial cell thiols in NH2Cl-stimulated electrolyte secretion by the rat colon.

The potential importance of inducible heat shock proteins (HSPs) in conferring protection to intestinal epithelial cells was investigated using IEC-18 cells. To establish optimal HSP induction, [35S]methionine and [35S]cysteine incorporation was assessed. Two 35S-labeled proteins of 70 and 90 kDa were preferentially synthesized. Western and Northern blot analyses confirmed induction of HSP70. This induction provided significant protection to the cells treated with the oxidant monochloramine or lethal heat (49 degrees C). To better establish the protective role of HSP70, cells were stably transfected with human HSP70 under control of the lac operator. When these cells were subjected to injury, they were protected by isopropylthiogalactoside-stimulated HSP70 induction under nonstress conditions. Additionally, the rate of protein synthesis (assessed by [3H]leucine incorporation) was protected. These results demonstrate that HSPs are preferentially and rapidly induced in IEC-18 cells and that induction of inducible HSP70 is important in promoting protection against cellular injury.