Basolateral potassium (IKCa) channel inhibition prevents increased colonic permeability induced by chemical hypoxia.

Major liver resection is associated with impaired intestinal perfusion and intestinal ischemia, resulting in decreased mucosal integrity, increased bacterial translocation, and an increased risk of postoperative sepsis. However, the mechanism by which ischemia impairs intestinal mucosal integrity is unclear. We therefore evaluated the role of Ca(2+)-sensitive, intermediate-conductance (IK(Ca)) basolateral potassium channels in enhanced intestinal permeability secondary to chemical hypoxia. The effects of chemical hypoxia induced by 100 µM dinitrophenol (DNP) and 5 mM deoxyglucose (DG) on basolateral IK(Ca) channel activity and whole cell conductance in intact human colonic crypts, and paracellular permeability (G(S)) in isolated colonic sheets, were determined by patch-clamp recording and transepithelial electrical measurements, respectively. DNP and DG rapidly stimulated IK(Ca) channels in cell-attached basolateral membrane patches and elicited a twofold increase (P = 0.004) in whole cell conductance in amphotericin B-permeabilized membrane patches, changes that were inhibited by the specific IK(Ca) channel blockers TRAM-34 (100 nM) and clotrimazole (CLT; 10 µM). In colonic sheets apically permeabilized with nystatin, DNP elicited a twofold increase (P = 0.005) in G(S), which was largely inhibited by the serosal addition of 50 µM CLT. We conclude that, in intestinal epithelia, chemical hypoxia increases G(S) through a mechanism involving basolateral IK(Ca) channel activation. Basolateral IK(Ca) channel inhibition may prevent or limit increased intestinal permeability during liver surgery.

Regulation of colonic apical potassium (BK) channels by cAMP and somatostatin.

High-conductance apical K+ (BK) channels are present in surface colonocytes of mammalian (including human) colon. Their location makes them well fitted to contribute to the excessive intestinal K(+) losses often associated with infective diarrhea. Since many channel proteins are regulated by phosphorylation, we evaluated the roles of protein kinase A (PKA) and phosphatases in the modulation of apical BK channel activity in surface colonocytes from rat distal colon using patch-clamp techniques, having first increased channel abundance by chronic dietary K+ enrichment. We found that PKA activation using 50 micromol/l forskolin and 5 mmol/l 3-isobutyl-1-methylxanthine stimulated BK channels in cell-attached patches and the catalytic subunit of PKA (200 U/ml) had a similar effect in excised inside-out patches. The antidiarrheal peptide somatostatin (SOM; 2 micromol/l) had a G protein-dependent inhibitory effect on BK channels in cell-attached patches, which was unaffected by pretreatment with 10 micromol/l okadaic acid (an inhibitor of protein phosphatase type 1 and type 2A) but completely prevented by pretreatment with 100 micromol/l Na+ orthovanadate and 10 micromol/l BpV (inhibitors of phosphoprotein tyrosine phosphatase). SOM also inhibited apical BK channels in surface colonocytes in human distal colon. We conclude that cAMP-dependent PKA activates apical BK channels and may enhance colonic K+ losses in some cases of secretory diarrhea. SOM inhibits apical BK channels through a phosphoprotein tyrosine phosphatase-dependent mechanism, which could form the basis of new antidiarrheal strategies.

Sodium transport in a mouse model of colonic carcinogenesis.

Following 4 weeks of s.c. injections of 1,2-dimethylhydrazine, a carcinogen that produces colon cancer in CF1 mice, an increase in the unidirectional mucosal to serosal flux and net absorption of sodium was observed in the distal colon. This increase in sodium transport was amiloride sensitive. 1,2-Dimethylhydrazine treatment had no effect on sodium transport in the distal colon of DBA/2 mice, a strain which does not develop colonic malignant transformation. Although stimulation of sodium transport has been observed in cultured cell systems exposed to growth factors, similar changes in sodium transport have not previously been demonstrated in an intact epithelium at an early stage of carcinogenesis. The present study in mouse distal colon demonstrates that sodium transport is altered in 1,2-dimethylhydrazine-induced malignant transformation of the large bowel.

Diarrhea in ulcerative colitis. The role of altered colonic sodium transport.

In normal human colon, water and sodium (Na+) absorption are directly related. Defective Na+ absorption may therefore be an important factor in the pathogenesis of diarrhea in ulcerative colitis (UC). Electrophysiological studies have revealed profound decreases in channel-mediated apical Na+ entry and Na(+)-K(+)-ATPase-mediated basolateral Na+ extrusion in surface epithelial cells in inflamed human distal colon. Recent molecular biological studies indicate that mucosal inflammation in UC leads to significant decreases in Na+ channel beta- and gamma-subunit expression in the apical membrane of surface colonocytes, with a marked reduction in the levels of beta- and gamma-subunit-specific mRNAs. In addition, basolateral expression of the Na(+)-K(+)-ATPase alpha 1-isoform is reduced along the surface cell-crypt cell axis in UC, although there is no change in the level of the corresponding mRNA. Diarrhea in ulcerative colitis is therefore related, at least in part, to a major defect in electrogenic Na+ absorption, which reflects changes in the levels of expression of critical subunits of both the apical Na+ channel and basolateral Na(+)-K(+)-ATPase.

Rapid stimulation of human renal ENaC by cAMP in Xenopus laevis oocytes.

Among the compensatory mechanisms restoring circulating blood volume after severe haemorrhage, increased vasopressin secretion enhances water permeability of distal nephron segments and stimulates Na(+) reabsorption in cortical collecting tubules via epithelial sodium channels (ENaC). The ability of vasopressin to upregulate ENaC via a cAMP-dependent mechanism in the medium to long term is well established. This study addressed the acute regulatory effect of cAMP on human ENaC (hENaC) and thus the potential role of vasopressin in the initial compensatory responses to haemorrhagic shock. The effects of raising intracellular cAMP (using 5 mmol/L isobutylmethylxanthine (IBMX) and 50 µmol/L forskolin) on wild-type and Liddle-mutated hENaC activity expressed in Xenopus oocytes and hENaC localisation in oocyte membranes were evaluated by dual-electrode voltage clamping and immunohistochemistry, respectively. After 30 min, IBMX + forskolin had stimulated amiloride-sensitive Na(+) current by 52% and increased the membrane density of Na(+) channels in oocytes expressing wild-type hENaC. These responses were prevented by 5 µmol/L brefeldin A, which blocks antegrade vesicular transport. By contrast, IBMX + forskolin had no effects in oocytes expressing Liddle-mutated hENaC. cAMP stimulated rapid, exocytotic recruitment of wild-type hENaC into Xenopus oocyte membranes, but had no effect on constitutively over-expressed Liddle-mutated hENaC. Extrapolating these findings to the early cAMP-mediated effect of vasopressin on cortical collecting tubule cells, they suggest that vasopressin rapidly mobilises ENaC to the apical membrane of cortical collecting tubule cells, but does not enhance ENaC activity once inserted into the membrane. We speculate that this stimulatory effect on Na(+) reabsorption (and hence water absorption) may contribute to the early restoration of extracellular fluid volume following severe haemorrhage.

Apical potassium (BK) channels and enhanced potassium secretion in human colon.

The human colon has the capacity to secrete potassium (K(+)) ions and enhanced K(+) secretion is a feature of a variety of diarrhoeal diseases. Recent work points to K(+) secretion in human colon being mediated by high conductance (BK) K(+) channels located in the apical membrane of colonic epithelial cells. The aim of this review is to highlight the importance of these channels in maintaining K(+) homoeostasis in health and disease.

Altered cryptal expression of luminal potassium (BK) channels in ulcerative colitis.

Decreased sodium (Na(+)), chloride (Cl(-)), and water absorption, and increased potassium (K(+)) secretion, contribute to the pathogenesis of diarrhoea in ulcerative colitis. The cellular abnormalities underlying decreased Na(+) and Cl(-) absorption are becoming clearer, but the mechanism of increased K(+) secretion is unknown. Human colon is normally a K(+) secretory epithelium, making it likely that K(+) channels are expressed in the luminal (apical) membrane. Based on the assumption that these K(+) channels resembled the high conductance luminal K(+) (BK) channels previously identified in rat colon, we used molecular and patch clamp recording techniques to evaluate BK channel expression in normal and inflamed human colon, and the distribution and characteristics of these channels in normal colon. In normal colon, BK channel alpha-subunit protein was immunolocalized to surface cells and upper crypt cells. By contrast, in ulcerative colitis, although BK channel alpha-subunit protein expression was unchanged in surface cells, it extended along the entire crypt irrespective of whether the disease was active or quiescent. BK channel alpha-subunit protein and mRNA expression (evaluated by western blotting and real-time PCR, respectively) were similar in the normal ascending and sigmoid colon. Of the four possible beta-subunits (beta(1-4)), the beta(1)- and beta(3)-subunits were dominant. Voltage-dependent, barium-inhibitable, luminal K(+) channels with a unitary conductance of 214 pS were identified at low abundance in the luminal membrane of surface cells around the openings of sigmoid colonic crypts. We conclude that increased faecal K(+) losses in ulcerative colitis, and possibly other diseases associated with altered colonic K(+) transport, may reflect wider expression of luminal BK channels along the crypt axis.

Enhanced large intestinal potassium permeability in end-stage renal disease.

The capacity of the colon for potassium (K+) secretion increases in end-stage renal disease (ESRD), to the extent that it makes a substantial contribution to K+ homeostasis. This colonic K+ adaptive response may reflect enhanced active K+ secretion, and be associated with an increase in apical membrane K+ permeability. In this study, this hypothesis was tested in patients with normal renal function or ESRD, by evaluating the effect of barium ions (a K+ channel inhibitor) on rectal K+ secretion using a rectal dialysis technique, and the expression of high conductance (BK) K+ channel protein in colonic mucosa by immunohistochemistry. Under basal conditions, rectal K+ secretion was almost threefold greater (p < 0.02) in ESRD patients (n = 8) than in patients with normal renal function (n = 10). Intraluminal barium (5 mmol/l) decreased K+ secretion in the ESRD patients by 45% (p < 0.05), but had no effect on K+ transport in patients with normal renal function. Immunostaining using a specific antibody to the BK channel alpha-subunit revealed greater (p < 0.001) levels of BK channel protein expression in surface colonocytes and crypt cells in ESRD patients (n = 9) than in patients with normal renal function (n = 9), in whom low levels of expression were mainly restricted to surface colonocytes. In conclusion, these results suggest that enhanced colonic K+ secretion in ESRD involves an increase in the apical K+ permeability of the large intestinal epithelium, which most likely reflects increased expression of apical BK channels.

Segmental heterogeneity of basal and aldosterone-induced electrogenic Na transport in human colon.

Recent in vitro studies in human colon have demonstrated marked segmental differences in electrogenic Na transport. In the present study, the Na channel blocker amiloride was used further to characterise basal and aldosterone-induced electrogenic Na transport in isolated human distal and proximal colon. Bathed in NaCl Ringer solution, distal and proximal colon exhibited similar basal electrical properties, but the amiloride-sensitive short-circuit current (Isc) was 200% greater in the distal than in the proximal segment. Bathed in choline-Cl Ringer solution, total Isc decreased by 97% in distal colon and by 88% in proximal colon, indicating that Na dependent transport process(es) account almost entirely for the Isc in both segments. Substituting Na2SO4 for NaCl Ringer solution (i) increased amiloride-sensitive Isc by 56% (p less than 0.01) in distal colon but had no effect on amiloride-sensitive Isc in proximal colon, and (ii) decreased amiloride-insensitive Isc in distal and proximal colon by 52% (p less than 0.05) and 81% (p less than 0.001) respectively. After the addition of nystatin to the apical membrane, the relationship between total Isc and mucosal Na concentration indicated that the activity of the basolateral membrane Na pump was similar in both colonic segments. In a further series of experiments, exposure of distal colon to 1 mumol/l aldosterone for 5 h increased total Isc by 52% (p less than 0.05), which reflected stimulation of its amiloride-sensitive component; in contrast, aldosterone had no effect on proximal colon.(ABSTRACT TRUNCATED AT 250 WORDS)

Segmental variability of glucocorticoid induced electrolyte transport in rat colon.

Recent studies suggest that the ability of glucocorticoids to reduce diarrhoea in active colitis may reflect their direct effects on distal colonic electrogenic Na+ transport and water absorption, as well as their anti-inflammatory action. To determine whether glucocorticoids induce similar changes in proximal colon, specific Na+ and K+ channel blockers (amiloride and tetraethylammonium chloride (TEA) respectively) were used to evaluate the cation transport properties of rat proximal and distal colon in vitro after three days treatment with the glucocorticoid agonist dexamethasone (600 micrograms/100 g/day). In the proximal colon, dexamethasone increased short circuit current (Isc) 2.3 fold (p less than 0.025) and total conductance (Gt) by 87% (p less than 0.015), but had negligible effects on the maximum activity of the basolateral membrane Na(+)-K+ pump and the baseline Na+ and K+ conductive properties of the apical membrane. Additional studies with diphenylamine-2-carboxylic acid (a Cl- channel blocker) suggested that the dexamethasone induced increases in Isc and Gt in proximal colon reflected stimulation of an electrogenic Cl- secretory process. In contrast, in the distal colon dexamethasone increased Isc 10 fold (p less than 0.025), Gt by 100% (p less than 0.015), and the maximum activity of the basolateral Na(+)-K+ pump by 200% (p less than 0.05), and induced substantial Na+ and K+ conductances in the apical membrane. These results indicate that dexamethasone stimulates electrogenic Na+ transport and water absorption to a significant degree only in the distal segment of rat colon. Thus in patients with active colitis, that part of the antidiarrhoeal action of glucocorticoids that reflects stimulation of electrogenic Na+ transport (and hence water absorption) may be restricted to the descending colon and rectum.

Acute effects of dexamethasone on cation transport in colonic epithelium.

Single pharmacological doses of glucocorticoid hormones stimulate net Na+ and water absorption, K+ secretion and electrical potential difference in rat distal colon and human rectum after five hours. To determine the cellular basis of these effects, the Na+ and K+ transport properties of epithelial cell membranes in rat distal colon were studied in vitro five hours after in vivo treatment with dexamethasone 600 micrograms/100 g body weight. Compared with control tissues, dexamethasone increased transepithelial voltage 3.5-fold (p less than 0.001) and short circuit current 4.5-fold (p less than 0.001), and decreased total resistance by 20% (p less than 0.005). Measurements of cell membrane voltages obtained with intracellular microelectrodes indicated that the dexamethasone-induced rise in transepithelial voltage reflected a significant decrease (p less than 0.05) in apical membrane voltage, consistent with the induction of apical Na+ channels and the stimulation of electrogenic Na+ absorption. Apical addition of 10(-4) mol/l amiloride (a Na+ channel blocker) and then 30 mmol/l tetraethylammonium chloride (TEA; a K+ channel blocker) to control tissues had little or no effect on transepithelial electrical parameters, indicating the absence of significant apical Na+ and K+ conductances. In contrast, in dexamethasone treated tissues, amiloride and TEA produced electrical changes that were consistent with the inhibition of glucocorticoid-induced apical Na+ and K+ conductances. Kinetic studies of the basolateral membrane Na+-K+ pump revealed that five hours after administration, dexamethasone had no effect on the maximum capacity of the pump for Na+ transport, but significantly increased the affinity of the pump for Na+, and the number of Na+ ions binding to each pump site. Thus, the acute stimulatory effects of dexamethasone on distal colonic Na+ absorption and K+ secretion reflect increased apical membrane conductance to Na+ and K+, and an increase in the 'efficiency' of the basolateral membrane Na+-K+ pump.

Segmental variability of membrane conductances in rat and human colonic epithelia. Implications for Na, K and Cl transport.

The membrane conductances in proximal and distal segments of rat and human colon were studied with microelectrodes, nystatin, ion channel blockers and Cl replacement. The results reveal that (1) in rat colon, total conductance (Gt) is greater in the proximal segment than in the distal segment, reflecting greater values of apical (Ga) and paracellular shunt (Gs) conductances in the proximal segment; in contrast, in human colon, Gt and its individual membrane components are similar in the proximal and distal segments, and lower than the corresponding values in rat colon; (2) amiloride sensitive apical Na conductances are absent in rat proximal colon, rat distal colon, and human proximal colon, but in human distal colon amiloride produces changes consistent with blockade of an apical Na conductance and inhibition of electrogenic Na transport; (3) a TEA-sensitive apical K conductance may be present in rat proximal colon (a K secretory epithelium), but not in rat distal colon (a K absorptive epithelium) or in either segment of human colon; and (4) in rat colon, replacement of mucosal and serosal Cl produces changes consistent with a substantial paracellular shunt permeability to Cl which is more marked in the proximal segment, whereas in human colon Cl replacement results in changes which suggest a relatively small paracellular shunt permeability to Cl which is similar in both segments. These data indicate marked segmental differences in Na, K and Cl transport in rat and human colon, and emphasise the hazards of applying models of colonic electrolyte transport in one species to another.

Complications of laxative abuse.

Laxative abuse is an uncommon but clinically important cause of chronic diarrhea, a condition often associated with other gastrointestinal symptoms, as well as with disturbances in electrolyte and acid-base balance. A high index of suspicion, a detailed history, and the detection of laxative in stool and/or urine will establish the diagnosis once routine laboratory, endoscopic, and radiologic investigations have excluded common causes of chronic diarrhea. Confirmation of the diagnosis may rule out the need for more extensive, invasive, and costly investigations. However, management is frequently difficult owing to the laxative abuser's complex underlying psychopathology.

Comparison of aldosterone- and RU-28362-induced apical Na+ and K+ conductances in rat distal colon.

In mammalian distal colon, aldosterone induces electrogenic Na+ absorption and electrogenic K+ secretion, whereas the sole transport effect of specific glucocorticoid agonists is thought to be stimulation of electroneutral NaCl absorption. In this study, intracellular microelectrodes and Na(+)- and K(+)-channel blockers were used to compare the effects of aldosterone and RU-28362 (a specific glucocorticoid agonist) on apical Na+ and K+ conductances in surface cells and upper crypt cells in the most distal colonic segment from adrenalectomized rats. In control animals, surface cells and crypt cells were devoid of apical Na+ and K+ conductances. In aldosterone-treated animals (70 micrograms.100 g body wt-1.day-1 for 7 days), Na+ conductances were induced in 88% of surface cells but only 40% of crypt cells, and the distribution of K+ conductances was similar (82% of surface cells and 50% of crypt cells). The same dose of RU-28362 also induced Na+ conductances in 82% of surface cells and 50% of crypt cells, which tended to be smaller than those induced by aldosterone. RU-28362, in contrast to aldosterone, had no effect on apical K+ conductance in surface cells or crypt cells. Concurrent treatment with the mineralocorticoid antagonist RU-28318 (3.5 mg.100 g body wt-1.day-1 for 7 days) inhibited Na(+)-channel expression in aldosterone-treated animals but had no effect in RU-28362-treated animals. We conclude that in the most distal segment of rat colon, aldosterone acts via mineralocorticoid receptors to induce apical Na+ and K+ conductances, which are only fully expressed in the surface cell population.(ABSTRACT TRUNCATED AT 250 WORDS)

Potassium secretion in rat distal colon during dietary potassium loading: role of pH regulated apical potassium channels.

BACKGROUND: Chronic dietary K+ loading increases the abundance of large conductance (210 pS) apical K+ channels in surface cells of rat distal colon, resulting in enhanced K+ secretion in this epithelium. However, the factors involved in the regulation of these K+ channels are at present unclear. AIMS: To evaluate the effect of dietary K+ loading on intracellular pH and its relation to large conductance apical K+ channel activity in surface cells of rat distal colon. METHODS/RESULTS: As assessed by fluorescent imaging, intracellular pH was higher in K+ loaded animals (7.48 (0.09)) than in controls (7.07 (0.04); p<0.01) when surface cells were bathed in NaCl solution, and a similar difference in intracellular pH was observed when cells were bathed in Na2SO4 solution (7.67 (0.09) and 6.92 (0.05) respectively; p<0.001). Ethylisopropylamiloride (EIPA; an inhibitor of Na+-H+ exchange; 1 microM) decreased intracellular pH when surface cells from K+ loaded animals were bathed in either solution, although the decrease was greater when the solution contained NaCl (DeltapH 0.50 (0.03)) rather than Na2SO4 (DeltapH 0. 18 (0.02); p<0.05). In contrast, EIPA had no effect in cells from control animals. As assessed by patch clamp recording techniques, the activity of large conductance K+ channels in excised inside-out membrane patches from distal colonic surface cells of K+ loaded animals increased twofold when the bath pH was raised from 7.40 to 7. 60. As assessed by cell attached patches in distal colonic surface cells from K+ loaded animals, the addition of 1 M EIPA decreased K+ channel activity by 50%, consistent with reversal of Na+-H+ exchange mediated intracellular alkalinisation. CONCLUSION: Intracellular alkalinisation stimulates pH sensitive large conductance apical K+ channels in rat distal colonic surface cells as part of the K+ secretory response to chronic dietary K+ loading. Intracellular alkalinisation seems to reflect an increase in EIPA sensitive Na+-H+ exchange, which may be a manifestation of the secondary hyperaldosteronism associated with this model of colonic K+ adaptation.

Electrophysiology of rat distal colon after partial nephrectomy. Implications for K transport.

Previous in vivo studies in rat and man indicate that chronic renal insufficiency leads to an increase in the capacity of the large intestine for K secretion. The present studies were performed in isolated rat distal colon with conventional and K-sensitive microelectrodes to determine the cellular basis for enhanced colonic K secretion after 70% nephrectomy. The data revealed that in animals fed a regular diet, nephrectomy had no effect on the Na or K conductance of the apical membrane, or the kinetics of the basolateral membrane Na-K pump, but intracellular K activity decreased from 70 +/- 4 mmol/l to 58 +/- 4 mmol/l (P less than 0.005). In control (non-nephrectomised) animals, feeding a diet modestly (4-fold) enriched with K resulted in small but significant increases in the Na and K conductance of the apical membrane, no change in the kinetics of the basolateral membrane Na-K pump, and a rise in intracellular K activity from 70 +/- 4 mmol/l to 94 +/- 7 mmol/l (P less than 0.005). In contrast, in animals fed the K enriched diet, nephrectomy resulted in (i) large, amiloride-sensitive increases in transepithelial voltage and total tissue conductance (consistent with an appreciable degree of secondary hyperaldosteronism), (ii) marked increases in the Na and K conductance of the apical membrane, (iii) significant hyperpolarization of the basolateral membrane, (iv) a 100% increase (P less than 0.02) in the maximum activity of the basolateral membrane Na-K pump, and (v) a rise in intracellular K activity from 94 +/- 7 mmol/l to 129 +/- 7 mmol/l (P less than 0.0025). These data suggest that the combination of modest dietary K enrichment and 70% nephrectomy stimulated an active K secretory process which reflected an increase in the K excretory load applied to the colonic mucosa, and the effects of aldosterone. In this model of renal insufficiency, enhanced K secretion by the transcellular and paracellular (potential-dependent) pathways results in a marked rise in the K excretory capacity of the colon.

A cAMP-activated chloride channel in the plasma membrane of cultured human gastric cells (HGT-1).

Hydrochloric acid (HCl) secretion by gastric parietal cells involves an apical Cl- conductance, the properties of which have not been defined. In the present study, forskolin and histamine [agonists that increase intracellular cyclic adenosine monophosphate (cAMP)], and dibutyryl cAMP, activated channels in previously quiescent cell-attached membrane patches on cultured human gastric cells (HGT-1). In the cell-attached configuration (Cl- 149 mmol/l in bath and pipette), channels exhibited outward rectification, voltage dependence, inward current (-0.7 pA) at zero holding potential and a reversal potential of +24 mV, consistent with the presence of a Cl- conductive pathway. In excised inside-out patches, channels (i) exhibited degrees of outward rectification and voltage dependence that were comparable to those seen in cell-attached patches, (ii) demonstrated a -21 mV shift of their reversal potential when bath Cl- was decreased from 149 mmol/l to 53 mmol/l (calculated Cl-:cation permeability ratio 17:1), and (iii) were highly sensitive to the Cl- channel blocker diphenylamine-2-carboxylic acid (DPC, 10(-3) mol/l). This cAMP-activated Cl- channel bears many similarities to other Cl- channels within intestinal epithelia, and may represent the apical Cl- channel operating in HCl-secreting gastric parietal cells.

Properties of a potassium channel in cultured human gastric cells (HGT-1) possessing specific omeprazole binding sites.

The HGT-1 human gastric cell line is similar to acid secreting parietal cells in that it possesses H2 receptors, histamine sensitive adenyl cyclase, and Cl- channels, which are activated by histamine by a cyclic adenosine monophosphate (cAMP) dependent mechanism. To discover if HGT-1 cells have additional properties found in parietal cells, [3H]omeprazole and patch clamp recording techniques were used to evaluate specific omeprazole binding sites and K+ channels in the plasma membrane. HGT-1 cells exhibited [3H]omeprazole binding in the non-stimulated state, which increased 100% in the presence of 1 mM histamine. High conductance (about 155 pS) K+ channels were active spontaneously in 17% of cell attached or excised inside out patches in non-stimulated subconfluent HGT-1 cells. In inside out patches, channel activity increased fivefold during depolarisation, ion substitution experiments confirmed that the channels were highly selective for K+, and channel activity was almost abolished by removal of Ca2+ or addition of 5 mM Ba2+. In quiescent cell attached patches, 0.1 mM dibutyryl cAMP failed to activate K+ channels. In contrast, 6.7 microM A23187 (a Ca2+ ionophore) increased intracellular Ca2+ concentration from mean (SEM) 14 (3) nM to 248 (30) nM and activated K+ channels in 21% of patches. It is concluded that the plasma membrane of HGT-1 cells possesses (a) specific 3H-omeprazole binding sites, which may reflect the omeprazole sensitive H+,K(+)-ATPase present in gastric parietal cells; and (b) Ca(2+)-activated K+ channels, which may be located in the basolateral membrane of human gastric parietal cells and play a part in acid secretion triggered by Ca(2+)-mediated secretory agonists.

Inter-relationships between the absorptions of hydrocortisone, sodium, water and actively transported organic solutes in the human jejunum.

The effect of intraluminal hydrocortisone (100 mg/l) on sodium and water transport in the small intestine was investigated by jejunal perfusion (flow rate 15 ml/min) of healthy subjects with normal saline and saline containing 56 mmol/l galactose or alanine. Minimal absorption of sodium and water occurred with normal saline and did not change significantly in the presence of hydrocortisone. Galactose and alanine enhanced sodium and water absorption and further significant increases occurred in the presence of hydrocortisone. Glucocorticoid induced increases in absorption were detected within 20-30 min, while plasma cortisol concentrations were in the normal range. 43% of the perfused dose of hydrocortisone was absorbed with normal saline (p less than 0.01). There was a significant positive correlation (p less than 0.0025) between hydrocortisone and water absorption. Thus, in the presence of actively absorbed organic solutes, hydrocortisone rapidly increased sodium absorption and the concurrent increase in water absorption appears to have facilitated passive absorption of hydrocortisone.

The effect of hydrocortisone on the transport of water, sodium, and glucose in the jejunum. Perfusion studies in normal subjects and patients with coeliac disease.

During perfusion of an isotonic solution containing 56 mmol/l glucose and 122 mmol/l sodium chloride, net movements of water and sodium and glucose absorption were measured in the absence and presence of intraluminal hydrocortisone along 20-cm segments of proximal jejunum in five normal subjects and five patients with active coeliac disease. In normal subjects, the mean net absorption of water and sodium was significantly increased in the presence of hydrocortisone decreased. In the coeliac patients, the mean net secretion of water and sodium was significantly decreased. In neither group was the effect of hydrocortisone associated with a significant increase in glucose absorption. Similar changes in net movements of water and sodium occurred in both groups, even though absorption of hydrocortisone in coeliac patients was 44% of the control value. These results suggest that hydrocortisone may have exerted a local effect on the mucosal transport of water and sodium.