An update on acid secretion

Gastric acid secretion is a complex process that requires hormonal, neuronal, or calcium-sensing receptor activation for insertion of pumps into the apical surface of the parietal cell. Activation of any or all these pathways causes the parietal cell to secrete concentrated acid with a pH at or close to 1. This acidic fluid combines with enzymes that are secreted from neighbouring chief cells and passes out of the gland up through a mucous gel layer covering the surface of the stomach producing a final intragastric pH of less than 4 during the active phase of acid secretion. Defects in either the mucosal barrier or in the regulatory mechanisms that modulate the secretory pathways will result in erosion of the barrier and ulcerations of the stomach or esophagus. The entire process of acid secretion relies on activation of the catalytic cycle of the gastric H+,K+-ATPase, resulting in the secretion of acid into the parietal cell canaliculus, with K+ being the important and rate-limiting ion in this activation process. In addition to K+ as a rate limiter for acid production, Cl- secretion via an apical channel must also occur. In this review we present a discussion of the mechanics of acid secretion and a discussion of recently identified transporter proteins and receptors. Included is a discussion of some of the recent candidates for the apical K' recycling channel, as well as two recently identified apical proteins (NHE-3, PAT-1), and the newly characterized calcium-sensing receptor (CaSR). We hope that this review will give additional insight into the complex process of acid secretion.

The calcium-sensing receptor acts as a modulator of gastric acid secretion in freshly isolated human gastric glands

Gastric acid secretion is activated by two distinct pathways: a neuronal pathway via the vagus nerve and release of acetylcholine and an endocrine pathway involving gastrin and histamine. Recently, we demonstrated that activation of H(+)-K(+)-ATPase activity in parietal cells in freshly isolated rat gastric glands is modulated by the calcium-sensing receptor (CaSR). Here, we investigated if the CaSR is functionally expressed in freshly isolated gastric glands from human patients undergoing surgery and if the CaSR is influencing histamine-induced activation of H(+)-K(+)-ATPase activity. In tissue samples obtained from patients, immunohistochemistry demonstrated the expression in parietal cells of both subunits of gastric H(+)-K(+)-ATPase and the CaSR. Functional experiments using the pH-sensitive dye 2',7'-bis-(2-carboxyethyl)-5-(and 6)-carboxyfluorescein and measurement of intracellular pH changes allowed us to estimate the activity of H(+)-K(+)-ATPase in single freshly isolated human gastric glands. Under control conditions, H(+)-K(+)-ATPase activity was stimulated by histamine (100 microM) and inhibited by omeprazole (100 microM). Reduction of the extracellular divalent cation concentration (0 Mg(2+), 100 microM Ca(2+)) inactivated the CaSR and reduced histamine-induced activation of H(+)-K(+)-ATPase activity. In contrast, activation of the CaSR with the trivalent cation Gd(3+) caused activation of omeprazole-sensitive H(+)-K(+)-ATPase activity even in the absence of histamine and under conditions of low extracellular divalent cations. This stimulation was not due to release of histamine from neighbouring enterochromaffin-like cells as the stimulation persisted in the presence of the H(2) receptor antagonist cimetidine (100 microM). Furthermore, intracellular calcium measurements with fura-2 and fluo-4 showed that activation of the CaSR by Gd(3+) led to a sustained increase in intracellular Ca(2+) even under conditions of low extracellular divalent cations. These experiments demonstrate the presence of a functional CaSR in the human stomach and show that this receptor may modulate the activity of acid-secreting H(+)-K(+)-ATPase in parietal cells. Furthermore, our results show the viability of freshly isolated human gastric glands and may allow the use of this preparation for experiments investigating the physiological regulation and properties of human gastric glands in vitro.

An amino acid transporter involved in gastric acid secretion

Gastric acid secretion is regulated by a variety of stimuli, in particular histamine and acetyl choline. In addition, dietary factors such as the acute intake of a protein-rich diet and the subsequent increase in serum amino acids can stimulate gastric acid secretion only through partially characterized pathways. Recently, we described in mouse stomach parietal cells the expression of the system L heteromeric amino acid transporter comprised of the LAT2-4F2hc dimer. Here we address the potential role of the system L amino acid transporter in gastric acid secretion by parietal cells in freshly isolated rat gastric glands. RT-PCR, western blotting and immunohistochemistry confirmed the expression of 4F2-LAT2 amino acid transporters in rat parietal cells. In addition, mRNA was detected for the B(0)AT1, ASCT2, and ATB(0+) amino acid transporters. Intracellular pH measurements in parietal cells showed histamine-induced and omeprazole-sensitive H+-extrusion which was enhanced by about 50% in the presence of glutamine or cysteine (1 mM), two substrates of system L amino acid transporters. BCH, a non-metabolizable substrate and a competitive inhibitor of system L amino acid transport, abolished the stimulation of acid secretion by glutamine or cysteine suggesting that this stimulation required the uptake of amino acids by system L. In the absence of histamine glutamine also stimulated H+-extrusion, whereas glutamate did not. Also, phenylalanine was effective in stimulating H+/K+-ATPase activity. Glutamine did not increase intracellular Ca2+ levels indicating that it did not act via the recently described amino acid modulated Ca2+-sensing receptor. These data suggest a novel role for heterodimeric amino acid transporters and may elucidate a pathway by which protein-rich diets stimulate gastric acid secretion.

L-type amino acids stimulate gastric acid secretion by activation of the calcium-sensing receptor in parietal cells

Parietal cells are the primary acid secretory cells of the stomach. We have previously shown that activation of the calcium-sensing receptor (CaSR) by divalent (Ca(2+)) or trivalent (Gd(3+)) ions stimulates acid production in the absence of secretagogues by increasing H(+),K(+)-ATPase activity. When overexpressed in HEK-293 cells, the CaSR can be allosterically activated by L-amino acids in the presence of physiological concentrations of extracellular Ca(2+) (Ca(o)(2+); 1.5-2.5 mM). To determine whether the endogenously expressed parietal cell CaSR is allosterically activated by L-amino acids, we examined the effect of the amino acids L-phenylalanine (L-Phe), L-tryptophan, and L-leucine on acid secretion. In ex vivo whole stomach preparations, exposure to L-Phe resulted in gastric luminal pH significantly lower than controls. Studies using D-Phe (inactive isomer) failed to elicit a response on gastric pH. H(+)-K(+)-ATPase activity was monitored by measuring the intracellular pH (pH(i)) of individual parietal cells in isolated rat gastric glands and calculating the rate of H(+) extrusion. We demonstrated that increasing Ca(o)(2+) in the absence of secretagogues caused a dose-dependent increase in H(+) extrusion. These effects were amplified by the addition of amino acids at various Ca(o)(2+) concentrations. Blocking the histamine-2 receptor with cimetidine or inhibiting system L-amino acid transport with 2-amino-2-norbornane-carboxylic acid did not affect the rate of H(+) extrusion in the presence of L-Phe. These data support the conclusion that amino acids, in conjunction with a physiological Ca(o)(2+) concentration, can induce acid secretion independent of hormonal stimulation via allosteric activation of the stomach CaSR.

The B1-subunit of the H(+) ATPase is required for maximal urinary acidification

The multisubunit vacuolar-type H(+)ATPases mediate acidification of various intracellular organelles and in some tissues mediate H(+) secretion across the plasma membrane. Mutations in the B1-subunit of the apical H(+)ATPase that secretes protons in the distal nephron cause distal renal tubular acidosis in humans, a condition characterized by metabolic acidosis with an inappropriately alkaline urine. To examine the detailed cellular and organismal physiology resulting from this mutation, we have generated mice deficient in the B1-subunit (Atp6v1b1(-/-) mice). Urine pH is more alkaline and metabolic acidosis is more severe in Atp6v1b1(-/-) mice after oral acid challenge, demonstrating a failure of normal urinary acidification. In Atp6v1b1(-/-) mice, the normal urinary acidification induced by a lumen-negative potential in response to furosemide infusion is abolished. After an acute intracellular acidification, Na(+)-independent pH recovery rates of individual Atp6v1b1(-/-) intercalated cells of the cortical collecting duct are markedly reduced and show no further decrease after treatment with the selective H(+)ATPase inhibitor concanamycin. Apical expression of the alternative B-subunit isoform, B2, is increased in Atp6v1b1(-/-) medulla and colocalizes with the H(+)ATPase E-subunit; however, the greater severity of metabolic acidosis in Atp6v1b1(-/-) mice after oral acid challenge indicates that the B2-subunit cannot fully functionally compensate for the loss of B1. Our results indicate that the B1 isoform is the major B-subunit isoform that incorporates into functional, plasma membrane H(+)ATPases in intercalated cells of the cortical collecting duct and is required for maximal urinary acidification.

Insulin receptor (IR) and glucose transporter 2 (GLUT2) proteins form a complex on the rat hepatocyte membrane

The hepatic glucose transporter, GLUT2, facilitates bidirectional glucose transport across the hepatocyte plasma membrane under insulin regulation. We studied the interactions of IR and GLUT2 proteins to determine whether they are physically coupled in a receptor-transporter complex. By comparing endosome and plasma membrane IR and GLUT2 ratios before and after feeding, it was determined that IR and GLUT2 are internalized in a fixed ratio. When solubilized hepatocytes were immunoprecipitated with antibodies against either IR or GLUT2, both proteins co-precipitated. The association of IR and GLUT2 was further assessed by confocal microscopy. Sections of fed liver were incubated with fluorescein-tagged anti-GLUT2 or Texas Red-tagged anti-IR. Colocalization was observed both at the plasma membrane and in the cytosol. Fluorescence-resonance energy transfer studies further confirmed this association. We conclude that IR and GLUT2 form a receptor-transporter complex in hepatocytes, which forms a mechanism of insulin-mediated hepatic glucose regulation.

Bicarbonate secretion: a neglected aspect of colonic ion transport

Understanding of the mechanism of colonic electrolyte transport has markedly increased over the past three decades. This article provides a brief summary of the critical features of Na, Cl, and K transport in the large intestine and how these processes may be altered in diarrhea. Less understood is the mechanism of colonic HCO3 secretion. Recent progress in the regulation of HCO3 secretion in the distal colon is summarized with emphasis on the interrelationship between Cl-dependent, short-chain fatty acid (SCFA)-dependent, and cAMP-induced HCO3 secretion. cAMP down-regulates Cl-dependent HCO3 secretion, while SCFA stimulates HCO3 secretion but also inhibits both Cl-dependent and cAMP-induced HCO3 secretion. As SCFAs are the primary anions in stool, it is likely that SCFA-dependent HCO3 secretion is the primary mechanism of HCO3 secretion in the mammalian colon. Future studies will undoubtedly provide increased understanding of the mechanism of HCO3 secretion in health and disease.

SECRETION AND ABSORPTION BY COLONIC CRYPTS

The intestines play an important role in the absorption and secretion of nutrients. The colon is the final area for recapturing electrolytes and water prior to excretion, and in order to maintain this electrolyte homeostasis, a complex interaction between secretory and absorptive processes is necessary. Until recently it was thought that secretion and absorption were two distinct processes associated with either crypts or surface cells, respectively. Recently it was demonstrated that both the surface and crypt cells can perform secretory and absorptive functions and that, in fact, these functions can be going on simultaneously. This issue is important in the complexities associated with secretory diarrhea and also in attempting to develop treatment strategies for intestinal disorders. Here, we update the model of colonic secretion and absorption, discuss new issues of transporter activation, and identify some important new receptor pathways that are important modulators of the secretory and absorptive functions of the colon.

Fluorescent imaging of Cl- in Amphiuma red blood cells: how the nuclear exclusion of Cl- affects the plasma membrane potential

In this work, we test the idea that most, if not all, cellular Cl- of Amphiuma red blood cells is contained in the cytoplasm. If true, this could resolve the difference between the measured plasma membrane potential (Em) and that expected from the Donnan equilibrium distribution of Cl-. We studied the changes in the fluorescence intensity of the Cl- -sensitive dye, MQAE, entrapped in red cells that occurred when intracellular Cl- was exchanged with NO3-. We could thus monitor the distribution of Cl- between the nuclear and cytoplasmic compartments. We found that essentially all of the cell's Cl- resides in the cytoplasm. Knowing the volume of the cell occupied by the nucleus, we could accordingly correct the measured values of cell Cl-. This resulted in establishing a concordance between the measured values of Em and those calculated from the corrected values of the Cl- ratio, thus explaining the discrepancy. The exclusion of Cl- from the nucleus may result from its unusually high content of "excess" DNA that imposes an imbalance of net negative charge.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

Nongenomic stimulation of vacuolar H+-ATPases in intercalated renal tubule cells by aldosterone

Renal collecting ducts play a critical role in acid-base homeostasis by establishing steep transepithelial pH gradients necessary for the almost complete reabsorption of bicarbonate and the effective secretion of ammonium into the urine. The mechanisms of urine acidification in collecting ducts involve active, electrogenic hydrogen (H+) secretion and, less importantly, potassium (K+)-H+ exchange. Deranged renal acidification and the inability to lower urine pH are hallmarks of distal tubular acidosis and often result from inborn errors of metabolism involving vacuolar H+-ATPase subunits in the collecting ducts. Three factors regulate H+-ATPase activity in intercalated cells of collecting ducts: the acid-base status, angiotensin II, and aldosterone. Most effects of aldosterone involve activation of the mineralocorticoid receptor and genomic changes in transcription and protein synthesis. Here we demonstrate a nongenomic pathway of vacuolar H+-ATPase activation in intercalated cells of isolated mouse outer medullary collecting ducts (OMCD). In vitro exposure of isolated outer medullary collecting ducts to aldosterone (10 nM) for times as short as 15 min increases vacuolar H+-ATPase activity approximately 2- to 3-fold. Neither inhibition of mineralocorticoid receptors nor of transcription and protein synthesis prevented aldosterone-induced stimulation of H+-ATPase. Incubation with colchicine, however, abolished the stimulatory effect of aldosterone, suggesting a role of the microtubular network for H+-ATPase stimulation. Immunohistochemistry in kidneys from aldosterone-injected mice showed increased apical H+-ATPase staining in OMCD-intercalated cells. The stimulatory effect of aldosterone was associated with a transient rise in intracellular Ca2+ and required intact PKC. Thus, rapid nongenomic modulation of vacuolar H+-ATPase activity in OMCD-intercalated cells by aldosterone may play an additional role in hormonal control of systemic acid-base homeostasis.

Molecular cloning and characterization of Atp6v1b1, the murine vacuolar H+ -ATPase B1-subunit

The multisubunit vacuolar-type proton-translocating ATPases (H(+)-ATPases) mediate the acidification of various intracellular organelles. In a subset of tissues, they also mediate H(+) secretion at the plasma membrane. Two isoforms of the H(+)-ATPase B-subunit exist in humans; we have shown that mutations in ATP6V1B1, encoding the B1-isoform, cause the clinical condition distal renal tubular acidosis. Here we report the cloning and characterization of murine Atp6v1b1, which encodes a 513-amino acid (aa) protein with 93% identity to human ATP6V1B1. Genomic organization is conserved between the murine and human H(+)-ATPase B1-subunits, and Atp6v1b1 maps to a region of mouse chromosome 6 syntenic to human 2p13, the location of ATP6V1B1. Northern blotting detects a 2.2-kb Atp6v1b1 transcript in the kidney and testis, but not other major organs. In mouse kidney, the B1-subunit localizes to intercalated cells of the cortical and medullary collecting duct. B1 protein levels were not increased in either mouse renal cortex or medulla after either 2 or 7 days of oral acid loading. These results demonstrate that Atp6v1b1 encodes the murine ortholog of human ATP6V1B1 and provides a tool for future development of animal models based on manipulation of the Atp6v1b1 genomic locus.

Extracellular polyamines regulate fluid secretion in rat colonic crypts via the extracellular calcium-sensing receptor

BACKGROUND AND AIMS

Polyamines are essential for the normal postnatal development, maintenance, and function of gastrointestinal epithelia. The extracellular Ca(2+) (Ca(2+)(o)/nutrient)-sensing receptor is expressed on both luminal and basolateral membranes of colonocytes, and, in other cell systems, this receptor has been shown to respond to polyamines. Thus, the Ca(2+)-sensing receptor could provide a mechanism for modulation of colonocyte function by dietary and systemic extracellular polyamines. In the present study, we investigated the interaction of polyamines, particularly spermine, and extracellular Ca(2+) on second messenger generation by, and on function of, rat distal colonic crypts.

METHODS

Calcium-sensing receptor activation was assessed in colonic epithelial cells and intact crypts freshly isolated from distal colon by monitoring intracellular IP(3) and Ca(2+) accumulation using radioimmunoassay and Fluo-3 fluorometry, respectively. Interactions of extracellular Ca(2+) and spermine on regulation of both basal and forskolin-stimulated fluid transport were measured in crypts microperfused in vitro.

RESULTS

Polyamine (spermine > spermidine > putrescine)-mediated enhancement of intracellular D-myo-inositol 1,4,5-trisphosphate (IP(3)) and Ca(2+) accumulation required extracellular Ca(2+), and the EC(50) for extracellular Ca(2+)-mediated activation of the calcium-sensing receptor was reduced by polyamines. Extracellular spermine modulated both basal and forskolin-stimulated fluid secretion in perfused colonic crypts, and the EC(50) for spermine-induced reduction in forskolin-stimulated fluid secretion was inversely dependent on extracellular Ca(2+) (Ca(2+)(o)).

CONCLUSIONS

The interactions of extracellular Ca(2+) and polyamines on second messenger accumulation and fluid secretion support a role for the luminal and basolateral calcium-sensing receptors in mediating some of the effects of polyamines on distal colonic epithelial cells.

Demonstration of a functional apical sodium hydrogen exchanger in isolated rat gastric glands

Previous studies have shown that gastric glands express at least sodium-hydrogen exchanger (NHE) isoforms 1-4. Our aim was to study NHE-3 localization in rat parietal cells and to investigate the functional activity of an apical membrane NHE-3 isoform in parietal cells of rats. Western blot analysis and immunohistochemistry showed expression of NHE-3 in rat stomach colocalizing the protein in parietal cells together with the beta-subunit of the H(+)-K(+)-ATPase. Functional studies in luminally perfused gastric glands demonstrated the presence of an apical NHE isoform sensitive to low concentrations of 5-ethylisopropyl amiloride (EIPA). Intracellular pH measurements in parietal cells conducted in omeprazole-pretreated superfused gastric glands showed an Na+-dependent proton extrusion pathway that was inhibited both by low concentrations of EIPA and by the NHE-3 specific inhibitor S3226. This pathway for proton extrusion had a higher activity in resting glands and was inhibited on stimulation of histamine-induced H(+)-K(+)-ATPase proton extrusion. We conclude that the NHE-3 isoform located on the apical membrane of parietal cells offers an additional pathway for proton secretion under resting conditions. Furthermore, the gastric NHE-3 appears to work under resting conditions and inactivates during periods of H(+)-K(+)-ATPase activity.

Localization and Regulation of the ATP6V0A4 (a4) Vacuolar H+-ATPase Subunit Defective in an Inherited Form of Distal Renal Tubular Acidosis

ABSTRACT. Vacuolar-type H+-ATPases (V-H+-ATPases) are the major H+-secreting protein in the distal portion of the nephron and are involved in net H+secretion (bicarbonate generation) or H+reabsorption (net bicarbonate secretion). In addition, V-H+-ATPases are involved in HCO3−reabsorption in the proximal tubule and distal tubule. V-H+-ATPases consist of at least 13 subunits, the functions of which have not all been elucidated. Mutations in the accessory ATP6V0A4 (a4 isoform) subunit have recently been shown to cause an inherited form of distal renal tubular acidosis in humans. Here, the localization of this subunit in human and mouse kidney was studied and the regulation of expression and localization of this subunit in mouse kidney in response to acid-base and electrolyte intake was investigated. Reverse transcription-PCR on dissected mouse nephron segments amplified a4-specific transcripts in proximal tubule, loop of Henle, distal convoluted tubule, and cortical and medullary collecting duct. a4 protein was localized by immunohistochemistry to the apical compartment of the proximal tubule (S1/S2 segment), the loop of Henle, the intercalated cells of the distal convoluted tubule, the connecting segment, and all intercalated cells of the entire collecting duct in human and mouse kidney. All types of intercalated cells expressed a4. NH4Cl or NaHCO3loading for 24 h, 48 h, or 7 d as well as K+depletion for 7 and 14 d had no influence on a4 protein expression levels in either cortex or medulla as determined by Western blotting. Immunohistochemistry, however, demonstrated a subcellular redistribution of a4 in response to the different stimuli. NH4Cl and K+depletion led to a pronounced apical staining in the connecting segment, cortical collecting duct, and outer medullary collecting duct, whereas NaHCO3loading caused a stronger bipolar staining in the cortical collecting duct. Taken together, these results demonstrate a4 expression in the proximal tubule, loop of Henle, distal tubule, and collecting duct and suggest that under conditions in which increased V-H+-ATPase activity is required, a4 is regulated by trafficking but not protein expression. This may allow for the rapid adaptation of V-H+-ATPase activity to altered acid-base intake to achieve systemic pH homeostasis. The significance of a4 expression in the proximal tubule in the context of distal renal tubular acidosis will require further clarification.

A rapid enzymatic method for the isolation of defined kidney tubule fragments from mouse

The increasing number of available genetically manipulated mice makes it necessary to develop tools and techniques for examining the phenotypes of these animals. We have developed a straightforward and rapid method for the isolation of large quantities of single tubule fragments from the mouse kidney. Immunohistochemistry, electron microscopy, and fluorescence microscopy were used to evaluate the viability, functional characteristics, and morphology of proximal tubules (PT), and collecting ducts from cortex (CCD) and inner stripe of the outer medulla (ISOMCD). Tubules were isolated using a modified collagenase digestion technique, and selected under light microscopy for experimentation. Electron microscopy and trypan blue exclusion showed that a large portion of unselected proximal tubules were damaged by the digestion procedure. The selected tubules, however, all excluded trypan blue, indicating that the plasma membrane had remained intact. Immunocytochemistry on isolated CCD showed normal distribution of H(+)-ATPase, pendrin, and anion exchanger-1 (AE-1) staining. The pH-sensitive dye 2',7'-bis(2-carboxylethyl)-5(6)-carboxyfluorescein (BCECF) was used to measure Na(+)-dependent and -independent intracellular pH (pH(i)) recovery rates in PT, and in single intercalated cells of CCD and ISOMCD fragments. Na(+)-dependent pH(i)-recovery was 0.144+/-0.008 (PT), 0.182+/-0.013 (CCD), and 0.112+/-0.010 pH units/min. (ISOMCD). Na(+)-independent pH(i) recovery was found in all three segments (PT: 0.021+/-0.002, CCD: 0.037+/-0.002, ISOMCD: 0.033+/-0.002 pH units/min) and was sensitive to concanamycin. In summary, we have developed a new technique for rapid and straightforward preparation of large quantities of defined tubule fragments from mouse kidney. Using this technique, the first measurements of plasma membrane vacuolar H(+)-ATPase activities in mouse PT and collecting duct were made. This technique will facilitate further characterization of kidney function in normal and genetically manipulated animals.

Regulation of the expression of the Cl-/anion exchanger pendrin in mouse kidney by acid-base status

BACKGROUND

Pendrin belongs to a superfamily of Cl-/anion exchangers and is expressed in the inner ear, the thyroid gland, and the kidney. In humans, mutations in pendrin cause Pendred syndrome characterized by sensorineural deafness and goiter. Recently pendrin has been localized to the apical side of non-type A intercalated cells of the cortical collecting duct, and reduced bicarbonate secretion was demonstrated in a pendrin knockout mouse model. To investigate a possible role of pendrin in modulating acid-base transport in the cortical collecting duct, we examined the regulation of expression of pendrin by acid-base status in mouse kidney.

METHODS

Mice were treated orally either with an acid or bicarbonate load (0.28 mol/L NH4Cl or NaHCO3) or received a K+-deficient diet for one week. Immunohistochemistry and Western blotting was performed.

RESULTS

Acid-loading caused a reduction in pendrin protein expression levels within one day and decreased expression to 23% of control levels after one week. Concomitantly, pendrin protein was shifted from the apical membrane to the cytosol, and the relative abundance of pendrin positive cells declined. Similarly, in chronic K+-depletion, known to elicit a metabolic alkalosis, pendrin protein levels decreased and pendrin expression was shifted to an intracellular pool with the relative number of pendrin positive cells reduced. In contrast, following oral bicarbonate loading pendrin was found exclusively in the apical membrane and the relative number of pendrin positive cells increased.

CONCLUSIONS

These results are in agreement with a potential role of pendrin in bicarbonate secretion and regulation of acid-base transport in the cortical collecting duct.

Expression of calcium-sensing receptor in rat colonic epithelium: evidence for modulation of fluid secretion

The calcium-sensing receptor (CaSR) is activated by extracellular calcium (Ca2+(o)) and mediates many of the known effects of extracellular divalent minerals on body cells. Both surface and crypt cells express CaSR transcripts and protein on both apical and basolateral surfaces. Raising Ca2+(o) elicited increases in intracellular calcium (Ca2+(o)) in both surface and crypt cells with an EC50 of 2 mM. The Ca-induced increase in Ca2+(i) was associated with increases in inositol 1,4,5-trisphosphate and eliminated by U-73129, an inhibitor of phosphatidylinositol-phospholipase C, as well as by thapsigargin. Other CaSR agonists, Gd3+ and neomycin, mimicked these Ca2+(o)-induced responses. Both luminal and bath Ca2+(o), Gd3+, and neomycin induced increases in Ca2+(i) in isolated perfused crypts. The stimulatory effect of forskolin on net fluid secretion in perfused crypts was abolished by increasing Ca2+(o) in either luminal or bath perfusates. Thus both apical and basolateral CaSR on crypt cells are functional and provide pathways modulating net intestinal fluid transport that may have important implications for the prevention and treatment of certain diarrheal diseases associated with elevated cAMP.

Cloning and expression of a chloride-dependent Na+-H+ exchanger

Electroneutral Na(+)-H(+) exchange is present in virtually all cells, mediating the exchange of extracellular Na(+) for intracellular H(+) and, thus, plays an important role in the regulation of intracellular pH, cell volume, and transepithelial Na(+) absorption. Recent transport studies demonstrated the presence of a novel chloride-dependent Na(+)-H(+) exchange in the apical membrane of crypt cells of rat distal colon. We describe the cloning of a 2.5-kb full-length cDNA from rat distal colon that encodes 438 amino acids and has six putative transmembrane spanning domains. Of the 438 amino acids 375 amino acids at the N-terminal region are identical to Na(+)-H(+) exchange (NHE)-1 isoform with the remaining 63 amino acids comprising a completely novel C terminus. In situ hybridization revealed that this transcript is expressed in colonic crypt cells, whereas Northern blot analysis established the presence of its 2.5-kb mRNA in multiple tissues. Despite its much smaller size compared with all other known Na(+)-H(+) exchange isoforms, NHE-deficient PS120 fibroblasts stably transfected with this cDNA exhibited Na(+)-dependent intracellular pH recovery to an acid load that was chloride-dependent and inhibited both by 5-ethylisopropylamiloride, an amiloride analogue, and by 5'-nitro-2-(3-phenylproplyamino)benzoic acid, a Cl(-) channel blocker, but only minimally affected by 25 microm 3-methylsulfonyl-4piperidonbenzoylguanidine, an NHE-1 and NHE-2 isoform inhibitor. In contrast to other Na(+)-H(+) exchange isoforms in colonic epithelial cells, chloride-dependent Na(+)-H(+) exchange mRNA abundance was increased by dietary sodium depletion. Based on these results we predict that chloride-dependent Na(+)-H(+) exchange represents a new class of Na(+)-H(+) exchangers that may regulate ion transport in several organs.

Microscopy as a tool for renal research

Microscopy has always played an important role in renal research as a tool to visualize events occurring at the surface of the kidney and as a method to investigate intracellular ion activity and membrane transport. Thanks to the recent advances in fluorochromes as ionic detector agents and as a means to label antibodies, microscopy will maintain its role at the forefront of renal research in the future. This article presents an overview of fluorescence microscopy and then focuses on some of the present applications of microscopy to exam renal epithelial tissues. Other areas covered are the pros and cons of various techniques and their applications to renal research.

Acid-base transport in the collecting duct

The collecting duct is responsible for the final secretion or reabsorption of protons and bicarbonate, respectively. Approximately 2% of the total filtered bicarbonate is reabsorbed in this segment. At least two different types of cells are involved in the transport of protons and bicarbonate, namely, type A and B intercalated cells. Their relative abundance differs in zonal distribution in the collecting duct. A variety of transport proteins and enzymes take part in the transcellular movement of protons and bicarbonate, some of them identified on a molecular level. These proteins are tightly regulated by hormones such as angiotensin II, aldosterone or endothelin or by the metabolic status of the organism. Alterations in acid-base intake or electrolyte status have profound effects on the collecting duct. Moreover, drugs interacting with renal electrolyte or bicarbonate handling (i.e. diuretics, glucocorticoids) also often affect acid-base transport in the collecting duct. For some of the identified proteins mutations have been found in various genetic diseases and syndromes of distal renal tubular acidosis leading to metabolic acidosis and an increased risk of nephrocalcinosis or -lithiasis.

The stomach divalent ion-sensing receptor scar is a modulator of gastric acid secretion

Divalent cation receptors have recently been identified in a wide variety of tissues and organs, yet their exact function remains controversial. We have previously identified a member of this receptor family in the stomach and have demonstrated that it is localized to the parietal cell, the acid secretory cell of the gastric gland. The activation of acid secretion has been classically defined as being regulated by two pathways: a neuronal pathway (mediated by acetylcholine) and an endocrine pathway (mediated by gastrin and histamine). Here, we identified a novel pathway modulating gastric acid secretion through the stomach calcium-sensing receptor (SCAR) located on the basolateral membrane of gastric parietal cells. Activation of SCAR in the intact rat gastric gland by divalent cations (Ca(2+) or Mg(2+)) or by the potent stimulator gadolinium (Gd(3+)) led to an increase in the rate of acid secretion through the apical H+,K+ -ATPase. Gd(3+) was able to activate acid secretion through the omeprazole-sensitive H+,K+ -ATPase even in the absence of the classical stimulator histamine. In contrast, inhibition of SCAR by reduction of extracellular cations abolished the stimulatory effect of histamine on gastric acid secretion, providing evidence for the regulation of the proton secretory transport protein by the receptor. These studies present the first example of a member of the divalent cation receptors modulating a plasma membrane transport protein and may lead to new insights into the regulation of gastric acid secretion.

Effects of the serine/threonine kinase SGK1 on the epithelial Na(+) channel (ENaC) and CFTR: implications for cystic fibrosis

Cystic fibrosis (CF) is characterized by impaired Cl(-) secretion and increased Na(+) reabsorption in several tissues including respiratory epithelium. Many CFTR mutations have been identified over the past years. However, only a poor correlation between the genotype and lung phenotype was found suggesting additional factors influencing the phenotype and course of the disease. The serine/threonine kinase SGK1 has recently been shown to stimulate the activity of the epithelial Na(+) channel ENaC. A variety of stimuli such as aldosterone, cell shrinkage, insulin or TGF-beta1 stimulate transcription and activate the SGK1 kinase. Here we further examined the effects of SGK1 on ENaC and CFTR which have mutual interactions and we analyzed sgk1 mRNA abundance in lung tissue from CF patients. Coexpression of CFTR and h-SGK1 in Xenopus oocytes increased ENaC currents as previously described. In addition CFTR mediated currents were also stimulated. h-SGK1 accelerated the expression of the amiloride sensitive Na(+)- current in Xenopus oocytes paralleled by increased ENaC-protein abundance in the oocyte membrane, an effect which was reversed by a h-SGK1(K127R) mutation lacking the ATP-binding site. The cation selectivity or Na(+) affinity were not affected. However, coexpression of h-SGK1 with ENaC altered the sensitivity of the Na(+)-channel to the inhibitors amiloride and triamterene. The inhibitory effect of CFTR expression on ENaC current was not affected by coexpression of h-SGK1 in Xenopus oocytes. Lung tissue from CF patients strongly expressed the serine/threonine kinase h-sgk1 which was not the case for non-CF lung tissue. Loss of CFTR function itself in a CF lung epithelial cell line did not increase SGK1 expression. In summary, enhanced expression of h-SGK1 in epithelial cells of CF-lung tissue may be a novel pathophysiological factor contributing to increased Na(+) channel activity and thus to increased Na(+) transport in CF.

Gap junctional channels regulate acid secretion in the mammalian gastric gland

Gap junction channels are regarded as a primary pathway for intercellular message transfer, including calcium wave propagation. Our study identified two gap junctional proteins, connexin26 and connexin32, in rat gastric glands by RT-PCR, Western blot analysis, and immunofluorescence. We demonstrated a potential physiological role of the gap junctional channels in the acid secretory process using the calcium indicator fluo-3, and microinjection of Lucifer Yellow. Application of gastrin (10-7 m) to the basolateral membrane resulted in the induction of uniphasic calcium signals in adjacent parietal cells. In addition, single parietal cell microinjections in intact glands with the cell-impermeant dye Lucifer Yellow resulted in a transfer of dye from the injected cell to the adjacent parietal cell following gastrin stimulation, demonstrating gastrin-induced cell-to-cell communication. Both calcium wave propagation and Lucifer Yellow transfer were blocked by the gap junction inhibitor 18alpha-glycyrrhetinic acid. Our studies demonstrate that functional gap junction channels in gastric glands provide an effective means for rapid cell-to-cell communication and allow for the rapid onset of acid secretion.

Impaired hepatocyte glucose transport protein (GLUT2) internalization in chronic pancreatitis

Chronic pancreatitis (CP) is associated with impaired glucose tolerance and with reduced hepatic sensitivity to insulin. We have previously shown that in normal and sham-operated rats, insulin suppresses hepatic glucose production, and this suppression is associated with a decrease in the hepatocyte plasma membrane-bound quantity of the facilitative glucose transport protein GLUT2. The insulin-mediated reduction in membrane-bound GLUT2 is impaired in CP, and may play a role in the glucose intolerance associated with CP. To determine whether GLUT2 is actively internalized and whether this mechanism is disordered in CP, livers from fed and fasting rats in whom CP had been induced 2-3 months earlier by pancreatic duct oleic acid infusion, and in sham-operated (sham) rats, were fractionated to yield endosome (E)- and plasma membrane (PM)-enriched fractions. Forty-five minutes after duodenal intubation alone (fasting) or intubation plus duodenal feeding, livers were removed, homogenized and ultracentrifuged, and microsomal pellets were separated by sucrose density gradient ultracentrifugation. GLUT2 content of fractions was determined by Western blotting and scanning densitometry. The E:PM ratio of GLUT2 increased from 0.68 +/- 0.11 (mean +/- SEM) in fasting sham livers (n = 8) to 1.04 +/- 0.09 in fed sham livers (n = 8; p < 0.05). However, there was no change in the E:PM ratio of GLUT2 in CP livers after duodenal feeding (0.90 +/- 0.12 vs. 0.86 +/- 0.10; n = 8,8; p = NS). To test our findings using confocal laser scanning microscopy, liver specimens from fed and fasting CP and sham rats were minced, fixed in 4% paraformaldehyde, sectioned, and stained with rabbit antirat GLUT2 antibody followed by rhodamine-labeled secondary antibody. GLUT2 was quantified by mean pixel intensity in an 8 x 16-pixel area of PM and a 16 x 16-pixel area of cytosol (CYT) in each of 30 random cells/field (400x) in each of three rats per group. As in the fractionation study, duodenal feeding increased the CYT:PM ratio of GLUT2 from 0.75 +/- 0.01 in fasting sham liver to 0.86 +/- 0.01 in fed sham liver (p < 0.0001), while the CYT:PM ratio in CP remained unchanged. We conclude that feeding induces a shift in GLUT2 from the plasma membrane to the endosomal pool. The feeding-induced internalization of GLUT2 is absent in livers from rats with CP and may play a role in the glucose intolerance associated with CP.

Cl-dependent Na-H exchange. A novel colonic crypt transport mechanism

This communication summaries a series of observations of the transport function of the crypt of the rat distal colon. Development of methods to study both 22Na uptake by apical membrane vesicles prepared from crypt cells and intracellular pHi (pHi), fluid movement (Jv), and bicarbonate secretion during microperfusion of the crypt has led to the identification of (1) a novel Cl-dependent Na-H exchange (Cl-NHE) that most likely represents the coupling of a Cl channel to a Na-H exchange isoform that has not as yet been identified and (2) bicarbonate secretion that appears to be most consistent with HCO3 uptake across the basolateral membrane by a mechanism that is closely linked to Cl transport and its movement across the apical membrane via an anion channel. Na-dependent fluid absorption is the constitutive transport process in the crypt, while fluid secretion is regulated by one or more neurohumoral agonists. Cl-NHE is responsible for both the recovery/regulation of pHi in crypt cells to an acid load and fluid absorption.

Na(+)-dependent fluid absorption in intact perfused rat colonic crypts

BACKGROUND & AIMS

The traditional paradigm of fluid movement in the mammalian colon is that fluid absorption and secretion are present in surface and crypt cells, respectively. We have recently demonstrated Na(+)-dependent fluid absorption in isolated crypts that are devoid of neurohumoral stimulation. We now explore the mechanism of Na(+)-dependent fluid absorption in isolated rat colonic crypts.

METHODS

Net fluid absorption was determined using microperfusion techniques and methoxy[(3)H]inulin with ion substitutions and transport inhibitors.

RESULTS

Net fluid absorption was reduced but not abolished by substitution of either N-methyl-D-glucamine- Cl(-) or tetramethylammonium for Na(+) and by lumen addition of 5-ethylisopropyl amiloride, an amiloride analogue that selectively inhibits Na(+)-H(+) exchange. Net fluid absorption was also dependent on lumen Cl(-) because removal of lumen Cl(-) significantly (P < 0.001) reduced net fluid absorption. DIDS at 100 micromol/L, a concentration at which DIDS is an anion exchange inhibitor, minimally reduced net fluid absorption (P < 0.05). In contrast, either 500 micromol/L DIDS, a concentration at which DIDS is known to act as a Cl(-) channel blocker, or 10 micromol/L NPPB, a Cl(-) channel blocker, both substantially inhibited net fluid absorption (P < 0.001). Finally, both the removal of bath Cl(-) and addition of bath bumetanide, an inhibitor of Na-K-2Cl cotransport and Cl(-) secretion, resulted in a significant increase in net fluid absorption.

CONCLUSIONS

(1) Net Na(+)-dependent net fluid absorption in the isolated colonic crypt represents both a larger Na(+)-dependent absorptive process and a smaller secretory process; and (2) the absorptive process consists of a Na(+)-dependent, HCO(3)(-)-independent process and a Na(+)-independent, Cl(-)-dependent, HCO(3)(-)-dependent process. Fluid movement in situ represents these transport processes plus fluid secretion induced by neurohumoral stimulation.

Cell pH and H(+) secretion by S3 segment of mammalian kidney: role of H(+)-ATPase and Cl(-)

The role of H(+)-ATPase in proximal tubule cell pH regulation was studied by microperfusion techniques and by confocal microscopy. In a first series of experiments, proximal S3 segments of rabbit kidney were perfused "in vitro" while their cell pH was measured by fluorescence microscopy after loading with BCECF. In Na(+)- and Cl(-)-free medium, cell pH fell by a mean of 0.37+/-0.051 pH units, but after a few minutes started to rise again slowly. This rise was of 0.17 +/-0.022 pH units per min, and was significantly reduced by bafilomycin and by the Cl(-) channel blocker NPPB, but not by DIDS. In a second series of experiments, subcellular vesicles of proximal tubule cells of S3 segments of mouse kidney were studied by confocal microscopy after visualization by acridine orange or by Lucifer yellow. After superfusion with low Na(+) solution, which is expected to cause cell acidification, vesicles originally disposed in the basolateral and perinuclear cell areas, moved toward the apical area, as detected by changes in fluorescence density measured by the NIH Image program. The variation of apical to basolateral fluorescence ratios during superfusion with NaCl Ringer with time was 0.0018+/- 0.0021 min(-1), not significantly different from zero (P>0.42). For superfusion with Na(+)0 Ringer, this variation was 0.081+/-0.015 min(-1), P<0.001 against 0. These slopes were markedly reduced by the Cl(-) channel blocker NPPB, and by vanadate at a concentration that has been shown to disrupt cytoskeleton function. These data show that the delayed alkalinization of proximal tubule cells in Na(+)-free medium is probably due to a vacuolar H(+)-ATPase, whose activity is stimulated in the presence of Cl(-), and dependent on apical insertion of subcellular vesicles. The movement of these vesicles is also dependent on Cl(-) and on the integrity of the cytoskeleton.

Dendrimer-assisted patch-clamp sizing of nuclear pores

Macromolecular translocation (MMT) across the nuclear envelope (NE) occurs exclusively through the nuclear pore complex (NPC). Therefore, the diameter of the NPC aqueous/electrolytic channel (NPCC) is important for cellular structure and function. The NPCC diameter was previously determined to be approximately equal to 10 nm with electron microscopy (EM) using the translocation of colloidal gold particles. Here we present patch-clamp and fluorescence microscopy data from adult cardiomyocyte nuclei that demonstrate the use of patch-clamp for assessing NPCC diameter. Fluorescence microscopy with B-phycoerythrin (BPE, 240 kDa) conjugated to a nuclear localization signal (NLS) demonstrated that these nuclei were competent for NPC-mediated MMT (NPC-MMT). Furthermore, when exposed to an appropriate cell lysate, the nuclei expressed enhanced green fluorescence protein (EGFP) after 5-10 h of incubation with the plasmid for this protein (pEGFP, 3.1 MDa). Nucleus-attached patch-clamp showed that colloidal gold particles were not useful probes; they modified NPCC gating. As a result of this finding, we searched for an inert class of particles that could be used without irreversibly affecting NPCC gating and found that fluorescently labeled Starburst dendrimers, a distinct class of polymers, were useful. Our patch-clamp and fluorescence microscopy data with calibrated dendrimers indicate that the cardiomyocyte NPCC diameter varies between 8 and 9 nm. These studies open a new direction in the investigation of live, continuous NPC dynamics under physiological conditions.

Calcium, ATP and nuclear pore channel gating

Nuclear envelope (NE) cisternal Ca2+ and cytosolic ATP are required for nuclear-pore-complex-(NPC-) mediated transport of DNAs, RNAs, transcription factors and other large molecules. Isolated cardiomyocyte nuclei, capable of macromolecular transport (MMT), have intrinsic NPC ion channel behavior. The large ion conductance (gamma) activity of the NPC channel (NPCC) is blocked by the NPC monoclonal antibody mAb414, known to block MMT, and is also silenced during periods of MMT. In cardiomyocytes, neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. To test the role of Ca2+ and ATP in NPCC activity, we carried out the present patch-clamp study with the pipette attached to the outer NE membrane of nuclei isolated from cultured Dunning G prostate cancer cells. Our investigations demonstrate that in these isolated nuclei neither cytosolic Ca2+ nor ATP alone directly affects NPCC gating. However, when simultaneously applied to the bath and pipette, they transiently silence NPCC activity through stimulation of MMT by raising the Ca2+ concentration in the NE cisterna ([Ca2+]NE). Our fluorescence microscopy observations with nuclear-targeted macromolecular fluorochromes (B-phycoerythrin and plasmid for the enhanced green fluorescence protein EGFP, pEGFP-C1) and with FITC-labeled RNA support the view that channel silence accompanies MMT. Repeated Ca2+ loading of the NE with Ca2+ and ATP, after unloading with 1-5 microM inositol 1,4,5-trisphosphate (IP3), thapsigargin (TSG) or 5 mM BAPTA or EGTA, failed to affect channel gating. This result indicates that other factors are involved in this phenomenon and that they are exhausted during the first cycle of NE Ca2+ loading/unloading--in agreement with current theories of NPC-mediated MMT. The results explain how Ca2+ and IP3 waves may convert the NE into an effective Ca2+ barrier and, consequently, affect the regulation of gene activity and expression through their feedback on MMT and NPCC gating. Thus, [Ca2+]NE regulation by intracellular messengers is an effective mechanism for synchronizing gene activity and expression to the cellular rhythm.

HCO(3)(-) secretion in the rat colonic crypt is closely linked to Cl(-) secretion

BACKGROUND & AIMS

The mechanism of colonic HCO(3)(-) secretion has not been established largely because of a lack of experimental methods for its detailed study. The present studies were designed to establish whether the isolated, perfused crypt of the rat distal colon is an excellent model to study HCO(3)(-) movement and the mechanism of colonic HCO(3)(-) secretion.

METHODS

HCO(3)(-) secretion was determined in isolated, microperfused crypts by measuring [HCO(3)(-)] by microcalorimetry on nanoliter samples.

RESULTS

Net HCO(3)(-) absorption was observed during lumen and bath perfusion with an HCO(3)(-)-Ringer solution. Vasoactive intestinal polypeptide (60 nmol/L), acetylcholine (100 nmol/L), or dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP, 0.5 mmol/L) induced active HCO(3)(-) secretion that required bath but not lumen HCO(3)(-)/CO(2). DBcAMP-stimulated HCO(3)(-) secretion was not affected by acetazolamide, an inhibitor of carbonic anhydrase. Removal of lumen Cl(-) did not alter DBcAMP-stimulated HCO(3)(-) secretion but reduced fluid secretion. DBcAMP-stimulated HCO(3)(-) secretion was closely linked to active Cl(-) secretion because HCO(3)(-) secretion was substantially reduced by removal of bath Cl(-), by addition of bath bumetanide, an inhibitor of Na-K-2Cl cotransport and Cl(-) secretion, and by addition of lumen NPPB, a Cl(-) channel inhibitor.

CONCLUSIONS

These studies establish that colonic crypt HCO(3)(-) secretion (1) is not a result of an apical membrane Cl(-)-HCO(3)(-) exchange, (2) is tightly associated with Cl(-) secretion, and (3) primarily occurs via an apical membrane Cl(-) channel.

Continuous detection of extracellular ATP on living cells by using atomic force microscopy

Atomic force microscopy is a powerful technique used to investigate the surface of living cells under physiological conditions. The resolution of the instrument is mainly limited by the softness of living cells and the interactions with the scanning tip (cantilever). Atomic force microscopy, in combination with myosin-functionalized cantilevers, was used in the detection of ATP concentrations in solution and on living cells. Functionally active tips were used to scan the surface of cells in culture and to show that the CFTR+ cell line (S9) had a basal surface ATP concentration that could be detected with atomic force microscopy (n = 10). ATP-dependent signals were not detectable in cells scanned with noncoated or heat-inactivated enzyme-coated tips (n = 9). Enzymatically active tips may serve as a model for future development of atomic force microscopy biosensors that can simultaneously detect topographical and biologically important compounds at the surface microenvironment of living cells.

Rapid effects of aldosterone on sodium-hydrogen exchange in isolated colonic crypts

Aldosterone plays a central role in the homeostatic regulation of extracellular fluid volume by stimulating transepithelial electrolyte transport. These effects involve binding to an intracellular receptor, modification of genomic events and protein synthesis. Rapid cellular responses to steroid hormones have been observed in a variety of nonepithelial tissues. The term "nongenomic" has been proposed for these fast steroid responses since they are unaffected by inhibitors of protein synthesis. We hypothesized that colonic crypts, recently demonstrated to absorb fluid, would respond rapidly to aldosterone. Cytoplasmic pH changes in crypts loaded with a pH-sensitive, fluorescent dye (BCECF) were recorded with confocal laser imaging. An intracellular alkalization of colonic crypts was observed within one minute of aldosterone application that was inhibited by ethylisopropylamiloride or the absence of extracellular sodium, yet unaffected by inhibitors of protein synthesis. The genesis of this rapid and distinct steroid action involves a signal transduction pathway that involves G proteins, protein kinase C, and prostaglandins. We have identified, by real-time imaging, a nongenomic upregulation of sodium-hydrogen exchange in colonic crypts by aldosterone that occurs independent of the traditional receptor. This distinct, rapid onset effect of aldosterone on epithelial ion transport has major implications for our understanding of fluid and electrolyte homeostasis in health and disease.

Direct effects of dopamine on colonic mucosal pH: implications for tonometry

Tonometric measurements of colonic and gastric mucosa pH are used as indirect determinants of splanchnic perfusion in shocked patients or those undergoing aortic cross-clamp. Mucosal acidification in response to splanchnic vasodilators such as dopamine has been assumed to signify ischemia. However, cellular acidification may occur independent of oxygenation and the direct effects of dopamine on mucosal acid-base are unknown. We examined the effects of dopamine on cellular pH (independent of oxygenation) of intestinal mucosa in vitro. Crypts isolated from the distal colon of Sprague-Dawley rats were loaded with a pH-sensitive fluorescent probe, perfused with a Hepes-buffered Ringers solution, and imaged with confocal laser scanning microscopy. In separate experiments, crypts were loaded with a calcium-sensitive probe (Fura-2) and concentrations of free cytosolic calcium were measured with fluorescence imaging. Dopamine perfusion produced a reversible cytosolic acidification of crypts which was not significantly affected by (i) the nominal absence of bicarbonate, (ii) alpha- and beta-adrenergic receptor blockade, or (iii) protein kinase C inhibition. Dopamine did not significantly affect intracellular calcium concentrations. However, dopamine-induced acidification was inhibited by (a) blocking sodium-hydrogen exchange with amiloride, (b) prior exposure to adenosine 3', 5'-cyclic monophosphate (cAMP), or (c) protein kinase A blockade (all P < 0.01). Dopamine directly acidifies mucosal crypt cells in a mechanism that involves a cAMP-mediated inhibition of sodium-hydrogen exchange. This finding accounts for the acidification of intestinal mucosa during low-dose dopamine infusion despite a demonstrable improvement in splanchnic perfusion. Direct mucosal effects of pharmacological agents must be considered in the evaluation of perfusion parameters based on tonometric data.

Expression of an extracellular calcium-sensing receptor in rat stomach

BACKGROUND & AIMS

Circulating levels of Ca2+ can influence secretory functions and myoelectrical properties of the stomach. A Ca2+-sensing receptor (CaR) has recently been identified in tissues that regulate systemic Ca2+ homeostasis. The aim of this study was to evaluate expression of CaR in the stomach of the rat.

METHODS

In forestomach and glandular stomach, reverse-transcription polymerase chain reaction was used to amplify a 380-base pair product, which is 99% homologous with transcripts obtained in parathyroid and kidney.

RESULTS

Northern analysis of gastric mucosal polyA+ RNA revealed 7. 5- and 4.1-kilobase transcripts, similar to those obtained in rat parathyroid and kidney. Immunohistochemistry revealed CaR expression in regions of the submucosal plexus and myenteric neurons. In sections of intact tissue, preparations of primary culture surface cells and surgically dissected gastric glands, staining was observed consistently in epithelial cells of the gastric glands and in gastric surface cells. In parietal cells in isolated gastric glands, intracellular levels of Ca2+ responded to conditions that are known to activate CaR.

CONCLUSIONS

These are the first reported observations that CaR is expressed in different epithelial cells of mammalian gastric mucosa and its enteric nerve regions. The effects of extracellular Ca2+ on gastric function may be attributable to activation of CaR.

Angiotensin II stimulates vesicular H+-ATPase in rat proximal tubular cells

Two mechanisms of H+ ion secretion in the proximal tubule that mediate bicarbonate reabsorption have been identified: the brush border Na/H exchanger and electrogenic H+ ion secretion. Angiotensin II (AII) has been shown to be a regulator of the luminal Na+/H+ exchanger and the basolateral Na+/HCO3- cotransporter. In the present study, we examined the effects of AII on H+-ATPase activity in isolated proximal tubule fragments. H+-ATPase activity was assessed by monitoring intracellular pH after Na+ removal from the bath. In addition, we investigated the effects on pH recovery of the proton pump inhibitor bafilomycin A1, removal of Cl-, and of colchicine. pH was continuously measured with the pH-sensitive fluorescent dye 2', 7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF). Recovery of cell pH was observed in the absence of external Na+ and was significantly accelerated by AII. The AII-stimulated pH recovery was completely abolished by bafilomycin A1, by removal of Cl-, by NPPB [5-nitro-2-(3-phenylpropylamino)-benzoate; a potent Cl- channel blocker], and by colchicine. We conclude from these studies that AII stimulates proton extrusion via H+-ATPase by a Cl--dependent process involving brush border insertion of vesicles. This process may contribute to up-regulation of HCO3- reabsorption along the proximal tubule when tubules are exposed to AII.

Gi regulation of secretory vesicle swelling examined by atomic force microscopy

In the last decade, several monomeric and heterotrimeric guanine nucleotide binding proteins have been identified to associate with secretory vesicles and to be implicated in exocytosis. Vesicle volume also has been proposed to play a regulatory role in secretory vesicle fusion at the plasma membrane. However, the molecular mechanism of function of the guanine nucleotide binding proteins and of the regulation of secretory vesicle volume in the exocytotic process remains unclear. In this study, we report association of the secretory vesicle membrane with the alpha subunit of a heterotrimeric GTP binding protein G(alpha i3) and implicate its involvement in vesicle swelling. Using an atomic force microscope in combination with confocal microscopy, we were able to study the dynamics of isolated zymogen granules, the secretory vesicles in exocrine pancreas. Exposure of zymogen granules to GTP resulted in a 15-25% increase in vesicle height as measured by the atomic force microscope and a similar increase in vesicle diameter as determined by confocal microscopy. Mas7, an active mastoparan analog known to stimulate Gi proteins, was found to stimulate the GTPase activity of isolated zymogen granules and cause swelling. Increase in vesicle size in the presence of GTP, NaF, and Mas7 were irreversible and KCl-sensitive. Ca2+ had no effect on zymogen granule size. Taken together, the results indicate that G(alpha i3) protein localized in the secretory vesicle membrane mediates vesicle swelling, a potentially important prerequisite for vesicle fusion at the cell plasma membrane.

Rapid aldosterone-induced cell volume increase of endothelial cells measured by the atomic force microscope

Atomic force microscopy (AFM) is a useful technique for imaging the surface of living cells in three dimensions. The authors applied AFM to obtain morphological information of individual cultured endothelial cells of bovine aorta under stationary and strain conditions and to simultaneously measure changes in cell volume in response to aldosterone. This mineralocorticoid hormone is known to have acute, non-genomic effects on intracellular pH, intracellular electrolytes and inositol-1,4,5-triphosphate production. In this study whether endothelial cells under tension change their volume in response to aldosterone was tested. Such changes were already shown in human leukocytes measured by Coulter counter. In contrast to leukocytes that are more or less spherical and live in suspension, endothelial cells exhibit a complex morphology and adhere to a substrate. Thus, measurements of discrete cell volume changes in endothelial cells under physiological condition is only feasible with more sophisticated techniques. By using AFM we could precisely measure the absolute cell volume of individual living endothelial cells. Before the addition of aldosterone the cell volume of mechanically stressed endothelial cells mimicking arterial blood pressure was 1827 +/- 172 fl. Cell volume was found to increase by 28% 5 min after hormone exposure. Twenty-five minutes later cell volume was back to normal despite the continuous presence of aldosterone in the medium. Amiloride, a blocker of the plasma membrane Na+/H+ exchanger prevented the initial aldosterone-induced volume increase. Taken together, AFM disclosed a transient swelling of endothelial cells induced by the activation of an aldosterone sensitive plasma membrane Na+/H+ exchanger.

Novel transport properties of colonic crypt cells: fluid absorption and Cl-dependent Na-H exchange

Colonic ion transport is heterogeneous including the long-accepted spatial separation of absorptive and secretory processes between surface and crypt cells. We recently described the isolation of individual crypts from the rat distal colon that were studied using microperfusion technology. Na-dependent fluid absorption was consistently demonstrated in these crypts during perfusion with a Ringer-like solution; dibutyryl cyclic AMP, VIP and acetylcholine, when added to the bath solution, all induced net fluid secretion. As several morphologic techniques, including immunocytochemistry, failed to provide evidence for the presence of myofibroblasts in the isolated crypt preparation, we propose that a Na-dependent absorptive process is a constitutive transport mechanism in crypt cells, while secretory processes are regulated by the release of one or more neurohumoral agonists from lamina propria cells including myofibroblasts. The mechanism of Na-dependent fluid movement was also studied by determining [H] gradient stimulation of 22Na uptake in isolated apical membrane vesicles (AMV) from crypt cells. In contrast to Na-H exchange in surface cell AMV, Na-H exchange in crypt cells is Cl-dependent. Intracellular pH determined in crypt cells using video-imaging fluorescence microscopy established that the response to an acid load requires both lumen Na and Cl. As a result, these studies have identified a novel Cl-dependent Na-H exchange in crypt AMV that may mediate apical membrane Na uptake and regulate pHi.

Differential activities of H+ extrusion systems in MDCK cells due to extracellular osmolality and pH

The aim of the present study was to obtain detailed information on MDCK cell proton secretion characteristics under various growth conditions. Confluent monolayers cultured on glass coverslips were adapted over 48 h to media with different osmolality and pH (200 mosmol/kgH2O, pH 7.4; 300 mosmol/kgH2O, pH 7.4; and 600 mosmol/kgH2O, pH 6.8) corresponding to the luminal fluid composition of the collecting duct segments found in the in renal cortex, the outer stripe of outer medulla and inner medulla. Proton fluxes were determined from the recovery of intracellular pH following an acid load induced by an NH4Cl pulse times the corresponding intrinsic buffering power (beta(i)). The intracellular buffering power was found to change only with culture medium osmolality but not with culture medium pH. In addition to an amiloride and Hoe-694-sensitive Na+/H+ exchange, Madin-Darby canine kidney (MDCK) cells possess a Sch-28080-sensitive, K+-dependent H+ extrusion mechanism that is increased upon adaptation of monolayers to hyperosmotic-acidic culture conditions. A significant contribution of the bafilomycin A1-sensitive vacuolar H+-ATPase could be found only in cells adapted to hyposmotic culture conditions. Exposure of MDCK cells to 10(-5) or 10(-7) M aldosterone for either 1 or 18 h did not alter the H+ extrusion characteristics significantly. The results obtained show that different extracellular osmolality and pH induce different MDCK phenotypes with respect to their H+-secreting systems.

Localization of SH-PTP1 to synaptic vesicles: a possible role in neurotransmission

The tyrosine kinase pp60src is known to phosphorylate synaptophysin and in doing so may regulate neurotransmitter release. The tyrosyl phosphorylated state of synaptophysin is dependent on pp60src kinase and the unknown protein tyrosine phosphate phosphohydrolase (PTPase, EC 3.1.3.48). Here we report the protein tyrosine phosphate phosphohydrolase SH-PTP1, to associate with synaptic vesicles and interact with synaptophysin. These studies identify SH-PTP1 as a new member of the secretory machinery at the nerve terminal and suggest its involvement in neurotransmission.

Localization of amiloride-sensitive sodium channels in A6 cells by atomic force microscopy

Atomic force microscopy (AFM) was used for high-resolution imaging of the apical distribution of epithelial Na+ channels in A6 renal epithelial cells. A6 cells grown on coverslips were labeled with antibodies generated against an amiloride-sensitive epithelial Na+ channel complex purified from bovine renal medulla that had been conjugated to 8-nm colloidal gold particles before preparation for AFM. AFM revealed that there was a marked increase in the height of the microvilli in cells labeled with the anti-epithelial Na+ channel antibodies compared with unlabeled cells or cells labeled with rabbit nonimmune immunoglobulin G conjugated to colloidal gold particles. We interpret this apparent increase in microvillar height to be due to anti-epithelial Na+ channel antibody binding to the apical microvilli. These data demonstrate that epithelial Na+ channels are restricted to the apical microvilli in Na+-transporting renal epithelial cells. Furthermore, they demonstrate the applicability of using AFM for high-resolution imaging of the cell surface distribution of epithelial ion channels.

Surface dynamics in living acinar cells imaged by atomic force microscopy: identification of plasma membrane structures involved in exocytosis

The dynamics at the plasma membrane resulting from secretory vesicle docking and fusion and compensatory endocytosis has been difficult to observe in living cells primarily due to limited resolution at the light microscopic level. Using the atomic force microscope, we have been able to image and record changes in plasma membrane structure at ultrahigh resolution after stimulation of secretion from isolated pancreatic acinar cells. "Pits" measuring 500-2000 nm and containing 3-20 depressions measuring 100-180 nm in diameter were observed only at the apical region of acinar cells. The time course of an increase and decrease in "depression" size correlated with an increase and decrease of amylase secretion from live acinar cells. Depression dynamics and amylase release were found to be regulated in part by actin. No structural changes were identified at the basolateral region of these cells. Our results suggest depressions to be the fusion pores identified earlier in mast cells by freeze-fracture electron microscopy and by electrophysiological measurements. The atomic force microscope has enabled us to observe plasma membrane dynamics of the exocytic process in living cells in real time.

An apical permeability barrier to NH3/NH4+ in isolated, perfused colonic crypts

Fermentation of nonabsorbed nutrients in the colon generates high concentrations of NH3/NH4+ in the colonic lumen. NH3 is a small, lipophilic neutral weak base that readily permeates almost all cell membranes, whereas its conjugate weak acid NH4+ generally crosses membranes much more slowly. It is not known how colonocytes maintain intracellular pH in the unusual acid-base environment of the colon, where permeant acid-base products of fermentation exist in high concentration. To address this issue, we hand dissected and perfused single, isolated crypts from rabbit proximal colon, adapting techniques from renal-tubule microperfusion. Crypt perfusion permits control of solutions at the apical (luminal) and basolateral (serosal) surfaces of crypt cells. We assessed apical- vs. basolateral-membrane transport of NH3/NH4+ by using fluorescent dyes and digital imaging to monitor intracellular pH of microvacuolated crypt cells as well as luminal pH. We found that, although the basolateral membranes have normal NH3/NH4+ permeability properties, there is no evidence for transport of either NH3 or NH4+ across the apical borders of these crypt cells. Disaggregating luminal mucus did not increase the transport of NH3/NH4+ across the apical border. We conclude that, compared to the basolateral membrane, the apical border of crypt colonocytes has a very low permeability-area product for NH3/NH4+. This barrier may represent an important adaptation for the survival of crypt cells in the environment of the colon.

Fluid absorption in isolated perfused colonic crypts

A spatial segregation of ion transport processes between crypt and surface epithelial cells is well-accepted and integrated into physiological and pathophysiological paradigms of small and large intestinal function: Absorptive processes are believed to be located in surface (and villous) cells, whereas secretory processes are believed to be present in crypt cells. Validation of this model requires direct determination of fluid movement in intestinal crypts. This study describes the adaptation of techniques from renal tubule microperfusion to hand-dissect and perfuse single, isolated crypts from rat distal colon to measure directly fluid movement. Morphologic analyses of the isolated crypt preparation revealed no extraepithelial cellular elements derived from the lamina propria, including myofibroblasts. In the basal state, crypts exhibited net fluid absorption (mean net fluid movement = 0.34 +/- 0.01 nl.mm-1.min-1), which was Na+ and partially HCO3- dependent. Addition of 1 mM dibutyryl-cyclic AMP, 60 nM vasoactive intestinal peptide, or 0.1 mM acetylcholine to the bath (serosal) solution reversibly induced net fluid secretion (net fluid movement approximately -0.35 +/- 0.01 nl.mm-1.min-1). These observations permit speculation that absorption is a constitutive transport function in crypt cells and that secretion by crypt cells is regulated by one or more neurohumoral agonists that are released in situ from lamina propria cells. The functional, intact polarized crypt described here that both absorbs and secretes will permit future studies that dissect the mechanisms that govern fluid and electrolyte movement in the colonic crypt.

Unique permeability barrier of the apical surface of parietal and chief cells in isolated perfused gastric glands

Although many factors can influence intracellular pH (pHi), some of the most important are those that involve the movement of acids and bases across the cell membrane. We will discuss recent results concerning barriers to the movement of H+, NH3 and CO2 across the apical cell membranes of gastric gland cells. Cell membranes are generally highly permeable to small, lipophilic molecules such as NH3 and CO2. In fact, only two examples are known of membranes relatively impermeable to NH3 and none membranes permeable to CO2. We recently developed a technique for perfusing the lumen of a single hand-dissected gastric gland on the stage of a microscope, while monitoring pHi with a fluorescent dye. We observed the expected pHi changes when we exposed the basolateral (i.e. blood-side) membrane to a pH 6.4 solution (a large, rapid pHi decrease), to a pH 7.4 solution containing approximately 0.3 mmoll-1 NH3 (a large and rapid pHi increase) or to a pH7.4 solution equilibrated with 1% CO2 (a rapid pHi decrease of -0.08). However, pHi was not significantly affected by perfusing the lumen with a pH 1.4 solution, with a pH 7.4 solution containing as much as 2.7 mmoll-1 NH3 or with a pH 6.1 solution equilibrated with 100% CO2. These data indicate that a barrier at or near the apical membrane has a uniquely low permeability to H+, NH3 and CO2.

Unusual permeability properties of gastric gland cells

Physiologists have long pondered the riddle of why the stomach is itself not digested by the very juice it secretes. One explanation is that a mucus-bicarbonate barrier, coating the stomach lumen as well as superficial portions of gastric glands, prevents autodigestion. However, this leaves unanswered the question of what protects cells deeper in the glands, which seem to lack a mucus barrier. These are the parietal and chief cells, which secrete acid and pepsin. Using perfused single gastric glands from rabbit, we recently found that intracellular pH is uniquely resistant to extreme degrees of luminal acidification, suggesting that the apical (luminal) barrier might also exclude ammonia and carbon dioxide, to which cell membranes are generally highly permeable. We now show that this is indeed the case. There are three reports of membranes with very low permeabilities to NH3 (refs 5-7), and none of membranes impermeable to CO2.