Phorbol ester stimulation of active anion secretion in intestine

Optic nerve head blood flow response to reduced ocular perfusion pressure by alteration of either the blood pressure or intraocular pressure

PURPOSE

To test the hypothesis that blood flow autoregulation in the optic nerve head has less reserve to maintain normal blood flow in the face of blood pressure-induced ocular perfusion pressure decrease than a similar magnitude intraocular pressure-induced ocular perfusion pressure decrease.

MATERIALS AND METHODS

Twelve normal non-human primates were anesthetized by continuous intravenous infusion of pentobarbital. Optic nerve blood flow was monitored by laser speckle flowgraphy. In the first group of animals (n = 6), the experimental eye intraocular pressure was maintained at 10 mmHg using a saline reservoir connected to the anterior chamber. The blood pressure was gradually reduced by a slow injection of pentobarbital. In the second group (n = 6), the intraocular pressure was slowly increased from 10 mmHg to 50 mmHg by raising the reservoir. In both experimental groups, optic nerve head blood flow was measured continuously. The blood pressure and intraocular pressure were simultaneously recorded in all experiments.

RESULTS

The optic nerve head blood flow showed significant difference between the two groups (p = 0.021, repeat measures analysis of variance). It declined significantly more in the blood pressure group compared to the intraocular pressure group when the ocular perfusion pressure was reduced to 35 mmHg (p < 0.045) and below. There was also a significant interaction between blood flow changes and the ocular perfusion pressure treatment (p = 0.004, adjusted Greenhouse & Geisser univariate test), indicating the gradually enlarged blood flow difference between the two groups was due to the ocular perfusion pressure decrease.

CONCLUSIONS

The results show that optic nerve head blood flow is more susceptible to an ocular perfusion pressure decrease induced by lowering the blood pressure compared with that induced by increasing the intraocular pressure. This blood flow autoregulation capacity vulnerability to low blood pressure may provide experimental evidence related to the hemodynamic pathophysiology in glaucoma.

Calcium-calmodulin-dependent Activation of Adenylate Cyclase in Prostaglandin-induced Electrically-monitored Intestinal Secretion in the Rat

The calcium-calmodulin antagonist 5-iodo-C8-W7 inhibited the PGE2-induced stimulation of cAMP production by isolated enterocytes from rat small intestine. It also reduced the secretory response of intestinal sheets to PGE2, measured as a rise in short-circuit current. It did not however, inhibit the electrical responses to forskolin and dibutyryl cAMP, nor to acetylcholine, a secretagogue whose effect is not mediated by cAMP. It is concluded that the receptor-mediated activation of adenylate cyclase and the subsequent secretory response are dependent upon calcium-calmodulin.

Modulation of ileal apical Na+-dependent bile acid transporter ASBT by protein kinase C

Ileal apical Na(+)-dependent bile acid transporter (ASBT) is responsible for reabsorbing the majority of bile acids from the intestinal lumen. Rapid adaptation of ASBT function in response to physiological and pathophysiological stimuli is essential for the maintenance of bile acid homeostasis. However, not much is known about molecular mechanisms responsible for acute posttranscriptional regulation of ileal ASBT. The protein kinase C (PKC)-dependent pathway represents a major cell signaling mechanism influencing intestinal epithelial functions. The present studies were, therefore, undertaken to investigate ASBT regulation in intestinal Caco-2 monolayers by the well-known PKC activator phorbol 12-myristate 13-acetate (PMA). Our results showed that Na(+)-dependent [(3)H]taurocholic acid uptake in Caco-2 cells was significantly inhibited in response to 2 h incubation with 100 nM PMA compared with incubation with 4alpha-PMA (inactive form). The inhibitory effect of PMA was blocked in the presence of 5 microM bisindolylmaleimide I (PKC inhibitor) but not 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (Ca(2+) chelator) or LY-294002 (phosphatidylinositol 3-kinase inhibitor). PMA inhibition of ASBT function was also abrogated in the presence of myristoylated PKCzeta pseudosubstrate peptide, indicating involvement of the atypical PKCzeta isoform. The inhibition by PMA was associated with a significant decrease in the maximal velocity of the transporter and a reduction in ASBT plasma membrane content, suggesting a modulation by vesicular recycling. Our novel findings demonstrate a posttranscriptional modulation of ileal ASBT function and membrane expression by phorbol ester via a PKCzeta-dependent pathway.

‘In vivo’ studies on the pathophysiological mechanism of Vibrio parahaemolyticus TDH+—induced secretion

The thermostable direct haemolysin (TDH) is considered to be the major virulence factors of Vibrio parahaemolyticus; however, poor information is available about its mechanism of action. In our study we examined the capacity of two V. parahaemolyticus TDH-producers (strains 2067 and 3305) to induce fluid secretion in rat ileal loop and to reveal the role of calcium ions (Ca(2+)), calmodulin (CaM), and protein kinase C (PKC) in V. parahaemolyticus TDH(+)-induced fluid secretion. The results show that V. parahaemolyticus TDH(+) strains were able to induce secretion in small intestine; on the contrary, this ability was not evidenced in the V. parahaemolyticus TDH(-) strain used as negative control. The data suggest an enterotoxic activity of haemolysin. Calcium ionophore A23187 and 1-verapamil (calcium channel blocker), when injected alone, induced fluid accumulation in the control loops. A further increase in fluid accumulation (P<0.001) was noted when calcium ionophore was injected along with bacterial suspension of both TDH(+) strains and a significant decrease (P<0.001) in experimental loops when 1-verapamil was inoculated along with bacterial suspension. The other modulating agents increased fluid accumulation in both control and experimental loops, without significant differences with respect to the positive control. Our findings suggest that Ca(2+) appears to be an important messenger involved in the stimulation of intestinal secretion, contrary to PKC and calmodulin which do not appear to have any role.

The effect of nephrotomy on renal function and morphology in dogs

OBJECTIVE

To determine whether bisection or intersegmental nephrotomy adversely affects glomerular filtration rate (GFR) and whether bisection nephrotomy has less adverse effect than intersegmental nephrotomy.

STUDY DESIGN

Dogs assigned to 5 groups of 3 dogs each, depending on time of euthanasia at postoperative day 1, 4, 8, 15, or 29.

ANIMALS

Fifteen healthy adult intact female dogs.

METHODS

Within 4 days of total and single kidney GFR measurements using technetium Tc 99m-DTPA, left intersegmental nephrotomy and right bisection nephrotomy were performed. The GFR measurements were repeated the day before dogs were killed except for the day 1 group. Histologic examination was performed on kidneys harvested from day 1, 4, 8, and 29 groups. Severity of lesions was scored on a 0 to 4 scale. Single kidney GFR was compared in each kidney before and after nephrotomy. Histologic and GFR changes were compared between bisection and intersegmental nephrotomy kidneys for each time period.

RESULTS AND CONCLUSIONS

Three days after surgery, total GFR was increased from baseline by 176%, with no difference in percent change in single kidney GFR between intersegmental and bisection nephrotomy. By day 7, total and single kidney GFR were not different than baseline. Bisection nephrotomy caused more intrarenal hemorrhage and cortical infarction and inflammation than intersegmental nephrotomy. By 4 weeks, no significant histologic differences were observed between the 2 techniques.

CLINICAL RELEVANCE

Neither intersegmental nor bisection nephrotomy adversely affected GFR. As intersegmental nephrotomy requires additional surgical manipulation and time, bisection nephrotomy is the technique of choice for nephrotomy in dogs.

Uptake of (3)H-1-methyl-4-phenylpyridinium ((3)H-MPP(+)) by human intestinal Caco-2 cells is regulated by phosphorylation/dephosphorylation mechanisms

Several transmembrane transporters of organic compounds are regulated by phosphorylation/dephosphorylation mechanisms. The aim of this study was to investigate the possible regulation of the intestinal uptake of organic cations by these mechanisms. The intestinal apical uptake of 1-methyl-4-phenylpyridinium (MPP(+)) was studied by incubating Caco-2 cells at 37 degrees for 5 min with 200 nM (3)H-MPP(+). Uptake of (3)H-MPP(+) by Caco-2 cells was not affected by activators of protein kinase G, and was not affected or slightly reduced (by 15-20%) by activators of protein kinase A or protein kinase C. Uptake of (3)H-MPP(+) by Caco-2 cells was reduced in a concentration-dependent manner by non-selective phosphodiesterase inhibitors (3-isobutyl-1-methylxanthine (IBMX), caffeine, teophylline). The IC(50) of IBMX was found to be 119 microM (102-138; n=9). Uptake of (3)H-MPP(+) by Caco-2 cells was not affected by inhibition of protein tyrosine kinase, but it was concentration-dependently reduced in the presence of inhibitors of mitogen-activated protein kinase. Uptake of (3)H-MPP(+) by Caco-2 cells was strongly reduced by Ca(2+)/calmodulin-mediated pathway inhibitors, but it was not dependent on extracellular Ca(2+). Our results suggest that the intestinal apical uptake of MPP(+) is regulated by phosphorylation/dephosphorylation mechanisms, being most probably active in the dephosphorylated state. Moreover, uptake of (3)H-MPP(+) by Caco-2 cells and by the extraneuronal monoamine transporter (EMT) are regulated in a very similar manner, suggesting an important participation of EMT in the intestinal uptake of this compound.

Correlation of hemodynamic impact and morphologic degree of renal artery stenosis in a canine model

In a noninvasive comprehensive magnetic resonance (MR) examination, the morphologic degree of renal artery stenosis was correlated to corresponding changes in renal artery flow dynamics. Different degrees of stenosis were created with the use of a chronically implanted inflatable arterial cuff in seven dogs. For each degree of stenosis, an ultrafast three-dimensional gadolinium MR angiography with high spatial resolution was performed, followed by cardiac-gated MR flow measurements with high temporal resolution for determination of pulsatile flow profiles and mean flow. Flow was also measured by a chronically implanted flow probe. In three of the dogs, trans-stenotic pressure gradients (DeltaP) also were measured via implanted catheters. Five different degrees of stenosis could be differentiated in the MR angiograms (0%, 30%, 50%, 80%, >90%). The MR flow data agreed with the flow probe within +/-20%. Stenoses between 30 and 80% gradually reduced the early systolic peak (Max(1)) of the flow profile but only minimally affected the midsystolic peak (Max(2)) or mean flow. Stenoses of more than 90% significantly depressed mean flow by more than 50%. The ratio between Max(1) and Max(2) (Rmax(1/2)) gradually fell with the degree of stenosis. The onset of significant mean flow reduction and DeltaP was indicated by a drop of Rmax(1/2) below 1 to 1.2. Thus, the analysis of high-resolution flow profiles allows detection of early hemodynamic changes even at degrees of stenoses not associated with a reduction of mean flow. Rmax(1/2) allows differentiation of the grade of hemodynamic compromise for a given morphologic stenosis independent of mean flow in a single comprehensive MR examination.

Murine colonic mucosa hyperproliferation. II. PKC-beta activation and cPKC-mediated cellular CFTR overexpression

In the companion article (Umar S, Scott J, Sellin JH, Dubinsky WP, and Morris AP, Am J Physiol Gastrointest Liver Physiol 278: 753-764, 2000), we have shown that transmissible murine colonic hyperplasia (TMCH) increased cellular cystic fibrosis transmembrane conductance regulator (CFTR) mRNA and protein expression, relocalized CFTR within colonocytes, and enhanced mucosal cAMP-dependent Cl(-) secretion. We show here that these changes were dependent on elevated cellular levels of membrane-bound Ca(2+)- and diacylglycerol-sensitive protein kinase C (PKC) activity (12-fold), induced by selective (3- to 4-fold) rises in conventional PKC (cPKC) isoform expression and membrane translocation. Three cPKC isoforms were detected in isolated crypts: alpha, beta1, and beta2. cPKC-beta1 rises preceded and those of cPKC-alpha and cPKC-beta2 paralleled cellular hyperproliferation and its effects on CFTR expression and cAMP-dependent Cl(-) current secretion. Only cPKC-beta1 and cPKC-beta2 were membrane translocated during TMCH. Furthermore, only cPKC-beta1 trafficked to the nucleus, whereas cPKC-beta2 remained partitioned among cytosolic, membrane, and cytoskeletal subcellular fractions. Modest increases in novel PKC-epsilon (nPKC-epsilon) expression and subcellular membrane partitioning were recorded during TMCH, but no changes were seen for PKC-delta or -eta. No nPKC isoform nuclear partitioning was detected. The orally bioactive cPKC inhibitor Ro-32-0432 reversed both TMCH and elevated cellular CFTR mRNA levels, whereas a pharmacologically inert analog (Ro-31-6045) failed to inhibit either response. On the basis of these facts, we present a new hypothesis whereby PKC-dependent cellular proliferation promotes endogenous cellular CFTR levels. PKC-beta1 was identified as a candidate regulatory PKC isoform.

The regulation of epithelial cell cAMP- and calcium-dependent chloride channels

This chapter has focused on two types of chloride conductance found in epithelial cells. The leap from the Ussing chamber to patch-clamp studies has identified yet other conductances present which have also been electrophysiologically characterized. In the case of the swelling activated wholecell chloride current, a physiological function is apparent and a single-channel basis found, but its genetic identity remains unknown (see reviews by Frizzell and Morris, 1994; and Strange et al., 1996). The outwardly rectified chloride channel has been the subject of considerable electrophysiological interest over the past 10 years and is well characterized at the single-channel level, but its physiological function remains controversial (reviewed by Frizzell and Morris, 1994; Devidas and Guggino, 1997). Yet other conductances related to the CLC gene family also appear to be present in epithelial cells of the kidney (reviewed by Jentsch, 1996; Jentsch and Gunter, 1997) where physiological functions for some isoforms are emerging. Clearly, there remain many unknowns. Chief among these is the molecular basis of GCa2+Cl and many of other the conductances. As sequences become available it is expected that the wealth of information gained by investigation into CFTR function will provide a conceptual blueprint for similar studies in these later channel clones.

Modulation of Ca2+-dependent anion secretion by protein kinase C in normal and cystic fibrosis pancreatic duct cells

The study investigated the role of protein kinase C (PKC) in the modulation of agonist-induced Ca2+-dependent anion secretion by pancreatic duct cells. The short-circuit current (ISC) technique was used to examine the effect of PKC activation and inhibition on subsequent ATP, angiotensin II and ionomycin-activated anion secretion by normal (CAPAN-1) and cystic fibrosis (CFPAC-1) pancreatic duct cells. The ISC responses induced by the Ca2+-mobilizing agents, which had been previously shown to be attributed to anion secretion, were enhanced in both CAPAN-1 and CFPAC-1 cells by PKC inhibitors, staurosporine, calphostin C or chelerythrine. On the contrary, a PKC activator, phorbol 12-myristate 13-acetate (PMA), was found to suppress the agonist-induced ISC in CFPAC-1 cells and the ionomycin-induced ISC in CAPAN-1 cells. An inactive form of PMA, 4alphad-phorbol 12, 13-didecanote (4alphaD), was found to exert insignificant effect on the agonist-induced ISC, indicating a specific effect of PMA. Our data suggest a role of PKC in modulating agonist-induced Ca2+-dependent anion secretion by pancreatic duct cells. Therapeutic strategy to augment Ca2+-activated anion secretion by cystic fibrosis pancreatic duct cells may be achieved by inhibition or down-regulation of PKC.

Vascular endothelial growth factor mRNA in pericytes is upregulated by phorbol myristate acetate

Increased microvascular permeability, which occurs in conditions such as the adult respiratory distress syndrome and diabetes mellitus, is related to physicochemical alterations in the microvascular barrier. We postulate that, in part, capillary pericytes affect microvascular permeability via production of a vasoactive cytokine, viz, vascular endothelial growth factor (VEGF), also known as vascular permeability factor. The goal of the present study was to evaluate the effects of phorbol myristate acetate (PMA), a substance known to produce nonhydrostatic pulmonary edema in intact animals, on VEGF gene expression in pericyte cultures. Microvascular pericytes were isolated from bovine retinas using magnetic microspheres coated with 3G5 monoclonal antibody. Pericyte identity was confirmed both morphologically and by immunostaining for alpha-smooth muscle actin and 3G5 ganglioside. The cultured pericytes were stimulated with N(omega)-nitro-L-arginine methyl ester (L-NAME, 1 x 10(-4) mmol/L), angiotensin II (1 x 10(-6) mmol/L), and PMA (5 x 10(-8) mmol/L), selected because of their ability to upregulate VEGF mRNA expressions in other cell types. Northern blot analysis was performed using [32P]dCTP labeled human VEGF cDNA (Genentech). Lane-loading differences were normalized using mouse GAPDH control cDNA probe. VEGF mRNA expression was upregulated by PMA (10(-9) to 10(-6) mol/L) in a dose-dependent manner, whereas neither L-NAME nor angiotensin II affected VEGF mRNA expression in pericytes. These results support the hypothesis that pericytes increase permeability of the endothelial barrier through increased VEGF production.

Activation of host cell protein kinase C by enteropathogenic Escherichia coli

Enteropathogenic Escherichia coli (EPEC) consists of a group of diarrhea-producing E. coli strains, common in developing countries, which do not produce classical toxins and are not truly invasive. EPEC strains adhere to mammalian cells in an intimate fashion, trigger a localized increase in intracellular calcium levels, and elevate inositol phosphate production. We hypothesized that these mediators could activate host cell protein kinase C (PKC) and tested this idea in vitro with two cultured human cell lines, HeLa cells and T84 cells. Using a recently described subculturing protocol to "induce" or accelerate EPEC adherence, we infected the cells with EPEC at a multiplicity of infection of approximately 100:1 for 30 to 60 min. Under these conditions, EPEC E2348 increased membrane-bound PKC activity 1.5- to 2.3-fold in HeLa cells and T84 cells, respectively. The increase in membrane-bound PKC activity was accompanied by a decrease in cytosolic PKC activity in EPEC-infected HeLa cells. Nonadherent laboratory E. coli strains such as HB101 and H.S. failed to trigger any consistent change in PKC production, similar to the nonadherent mutant strains derived from E2348, JPN15 (plasmid cured) and CVD206 (eaeA). In addition, immunoblots performed on extracts of T84 cells with a monoclonal antibody against PKC-alpha showed an increased PKC content in membranes of EPEC-infected cells. Finally, EPEC-infected T84 cells showed a 60% increase in responsiveness to the E. coli heat-stable toxin. We conclude that mediators produced in response to EPEC adherence activate PKC in intestinal and nonintestinal cells.

Phosphorylation by protein kinase C is required for acute activation of cystic fibrosis transmembrane conductance regulator by protein kinase A

Protein kinase A (PKA) stimulates Cl secretion by activating the cystic fibrosis transmembrane conductance regulator (CFTR), a tightly regulated Cl- channel in the apical membrane of many secretory epithelia. The CFTR channel is also modulated by protein kinase C (PKC), but the regulatory mechanisms are poorly understood. Here we present evidence that PKA-mediated phosphorylation alone is not a sufficient stimulus to open the CFTR chloride channel in the presence of MgATP; constitutive PKC phosphorylation is essential for acute activation of CFTR by PKA. When patches were excised from transfected Chinese hamster ovary cells, CFTR responses to PKA became progressively smaller with time and eventually disappeared. This decline in PKA responsiveness did not occur in the presence of exogenous PKC and was reversed by the addition of PKC to channels that had become refractory to PKA. PKC enhanced PKA stimulation of open probability without increasing the number of functional channels. Short-term pretreatment of cells with the PKC inhibitor chelerythrine (1 microM) reduced the channel activity that could be elicited by forskolin in cell-attached patches. Moreover, in whole cell patches, acute stimulation of CFTR currents by chlorophenylthio-cAMP was abolished by two chemically unrelated PKC inhibitors, although an abrupt, partial activation was observed after a delay of >15 min. Modulation by PKC was most pronounced when basal PKC phosphorylation was reduced by briefly preincubating cells with chelerythrine. Constitutive PKC phosphorylation in unstimulated cells permits the maximum elevation of open probability by PKA to reach a level that is approximately 60% of that attained during in vitro exposure to both kinases. Differences in basal PKC activity may contribute to the variable cAMP responsiveness of CFTR channels in different cell types.

Effect of second messenger systems on oxalate uptake in renal epithelial cells

The oxalate transport system along with protein phosphorylation appears to be deranged in stone formers. This study was undertaken to characterize in LLC-PK1 cells in culture the effect of altering specific intracellular second messenger systems on oxalate uptake. Cellular uptake experiments were performed at 37 degrees C in buffer [265 mM mannitol, 5 mM NaOH, 5 mM KOH, 10 mM Ca-EGTA, 25 mM HEPES/TRIS, pH = 7.4 or in Hank's balanced salt solution (HBSS)] containing 200 microM labeled oxalate (1-14C, 0.3 microCi). Cells were preincubated with DAG (final concentration of 100 microM), phorbol myristate acetate (10 microM), forskolin (50 microM), 8-bromo-cyclic AMP (50 microM), trifluoroperazine (20 microM) and low molecular weight heparin (1 mg/ml) for 10 min in the presence and absence of the anion transport inhibitor DIDS (100 microM) and the effect(s) on oxalate uptake at 10, 25 and 45 min incubation were determined. Chemicals (DAG, forskolin, TPA and 8-bromo-cAMP) which stimulate protein kinase A or C activity resulted in an increased uptake of oxalate while inhibitors of these systems (trifluoroperazine and low molecular weight heparin) resulted in decreased oxalate uptake. The results demonstrate that oxalate uptake in renal tubular cells is modulated by protein kinase C and A dependent mechanisms.

Calcium and protein kinase C play an important role in Campylobacter jejuni-induced changes in Na+ and Cl- transport in rat ileum in vitro

The pathophysiological mechanism of Campylobacter jejuni (enterotoxigenic) induced secretory diarrhoea remains least understood. To investigate the mechanism(s) involved, the unidirectional fluxes of Na+ and Cl- were measured across the C. jejuni live culture infected and control (non infected) rat ileum (unstriped), in vitro by Ussing technique under short circuit conditions, in the presence or absence of: Ca2+ ionophore A23187 (5 microM), 1-verapamil (100 microM), calmodulin (CaM) antagonist W-7 (100 microM), dantrolene (25 microM), protein kinase C (PKC) activator PMA (100 ng/ml) and H-7 (60 microM), selective inhibitor of PKC. There was net absorption of Na+ and enhanced Cl- secretion in infected animals while in control animals there was net absorption of Na+ and marginal secretion Cl-.Ca2+ ionophore A23187 mimicked the effects of C. jejuni infection whereas 1-verapamil had significant antisecretory effect on Na+ and Cl- secretion in infected animals. In vitro measurement of undirectional 45Ca fluxes in Ussing chamber experiments revealed net absorption of Ca2+ in infected rat ileum as compared to net secretion of Ca2+ in control rat ileum. These observations clearly indicate that there is increased stimulation of Ca2+ uptake from extracellular milieu to the enterocytes during C. jejuni-induced diarrhoea. The intracellular calcium levels (Ca2+]i (as measured by fluorescent probe Fura-2AM) were found to be raised significantly (P < 0.0001) in enterocytes isolated from C. jejuni infected ileum as compared to the enterocytes from control ileum. The observed increase in [Ca2+]i in enterocytes isolated from C. jejuni live culture supernatant treated rat ileum further shows the involvement of enterotoxin in diarrhoeal process. Dantrolene decreased significantly C. jejuni-induced net Na+ and Cl- secretion but it could not reverse it to absorption suggesting the partial involvement of Ca2+ mobilised from intracellular stores in mediating secretion. W-7 failed to inhibit the C. jejuni-induced net Na+ and Cl- secretion. In addition the CaM activity estimated in intestinal microvillar core remained same in both the control and C. jejuni infected animals. This indicates that C. jejuni-induced diarrhoea is not mediated through the activation of Ca(2+)-CaM complex pathway of the Ca2+ messenger system. The PKC activator PMA, induced net secretion of Na+ and Cl- in the control animals but it could not enhance further the C. jejuni-induced Na+ and Cl- secretion, suggesting that there is overlapping effect of PMA and C. jejuni live culture infection.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein kinase C regulates prairie dog gallbladder ion transport

BACKGROUND

Gallstone formation is characterized by increased biliary calcium (Ca2+) level and altered gallbladder absorption. Recent studies suggest that luminal Ca2+ regulates gallbladder ion transport via intracellular calcium ([Ca2+]ic). Ca2+-calmodulin and protein kinase C (PKC) are two major systems through which [Ca2+]ic carries out second-messenger functions in many cell types. We have previously shown that Ca2+-calmodulin regulates basal gallbladder ion transport in prairie dog. The present study tests the hypothesis that PKC is also essential in regulation of gallbladder ion transport in this model.

METHODS

The role of PKC in regulation of gallbladder ion transport was determined by studying the effects of phorbol esters, synthetic analogues of diacylglycerol, which directly activates PKC. Gallbladders were mounted in Ussing chambers, and standard electrophysiologic parameters were recorded after exposing tissues to either 10(-5) mol/L of 4-alpha-phorbol 12,13-didecanoate (PDD), 4-beta-phorbol 12-myristate 13-acetate, 4-beta-phorbol 12,13-dibutyrate (PDB), or 10(-4) mol/L serotonin. Unidirectional Na+, Cl-, and H2O fluxes were measured before and after treatment with only inactive PDD and most active PDB.

RESULTS

Mucosal and serosal exposure of tissues to either 4-beta-phorbol 12-myristate 13-acetate or PDB resulted in a decrease in short-circuit current and transepithelial potential difference without any change in tissue resistance. Serotonin induced similar changes in gallbladder electrical properties. PDB caused an inhibition of mucosal to serosal fluxes of Na+, Cl-, and H2O, with a decrease in net Na+ absorption, an increase in net Cl- secretion, and a conversion of net H2O absorption to net H2O secretion. Serosal-to-mucosal fluxes of Na+, Cl-, and H2O did not change. Inactive PDD had no effect on either electrophysiologic parameters or ion and water fluxes. Pretreatment of tissues with PKC antagonist 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine blocked the phorbol ester-induced inhibition of ion transport.

CONCLUSION

PKC regulates gallbladder ion transport in the prairie dog by inhibiting Na+ absorption and stimulating Cl- secretion.

Histamine modulates three types of K+ current in a human intestinal epithelial cell line

K+ conductance species in a human intestinal epithelial cell line (Intestine 407) were studied in connection with their sensitivities to an intestinal secretagogue, histamine, using the tight-seal whole-cell patch-clamp technique. Applications of positive command pulses rapidly induced outward K+ currents. The conductance became progressively larger with increasing command voltages, exhibiting an outwardly rectifying current voltage relation. Inward K+ currents were also rapidly activated upon applications of hyperpolarizing pulses at potentials negative to the equilibrium potential of K+ (EK), and the conductance inwardly rectified. Application of a Ca2+ ionophore, ionomycin, brought about activation of additional K+ currents. An inhibitor of protein kinase C, polymyxin B, did not affect the ionomycin-induced response. Histamine (10-200 microM) also activated a similar K+ current which was abolished by cytosolic Ca2+ chelation. Under conditions where Ca2+ mobilization was minimized, histamine was found to significantly augment inwardly rectifying K+, but suppress outwardly rectifying K+, currents. Polymyxin B blocked these effects of histamine. An activator of protein kinase C, 1-oleoyl-2-acetylglycerol, mimicked the histamine effects. It is concluded that the intestinal epithelial cell has three distinct types of K+ conductance and that histamine modulates not only Ca(2+)-activated K+ conductance via Ca2+ mobilization, but also inward- and outward-rectifier K+ conductances via activation of protein kinase C.

Identification of protein kinase C-alpha, epsilon and zeta in rabbit ileal enterocytes

Activation of protein kinase C (PKC) has been shown to regulate electrolyte transport in rabbit small intestine. We investigated the types of PKC isoforms in rabbit ileal villus and crypt cells and the potential for phorbol 12-myristate 13-acetate (PMA) to induce translocation of the PKC from the cytosol to the plasma membrane. Our results indicate that there are at least three PKC isoforms, alpha, epsilon and zeta, are present in both villus and crypt enterocytes. Acute treatment with PMA resulted in translocation of the PKC-alpha from the cytosol to the membrane fraction in both cell types. Prolonged exposure of the villus cells to PMA resulted in a significant progressive decrement in PKC-alpha, suggesting down regulation. Since PMA treatment results in translocation, this isoform may be involved in the regulation of electrolyte transport in the rabbit ileum.

Na+/H+ exchangers, NHE-1 and NHE-3, of rat intestine. Expression and localization

Na-H exchange (NHE) is one of the major non-nutritive Na absorptive pathways of the intestine and kidney. Of the four NHE isoforms that have been cloned, only one, NHE-3, appears to be epithelial specific. We have examined the regional and cellular expression of NHE-3 in the rat intestine. NHE-3 message in the small intestine was more abundant in the villus fractions of the small intestine than in the crypts. Analysis of NHE-3 mRNA distribution in the gut by in situ hybridization demonstrated epithelial cell specificity, as well as expression preferential to villus cells. NHE-1 message, in contrast, was ubiquitous, with slightly greater expression exhibited in the differentiating crypt and lower villus cells of the small intestine. Isoform-specific NHE-3 fusion protein antibody identified a 97-kD membrane protein in the upper villus cells of the small intestine, which was exclusively localized in the apical membrane. In contrast, antibody previously developed against the COOH-terminal region of human NHE-1 (McSwine, R. L., G. Babnigg, M. W. Musch, E. B. Chang, and M. L. Villereal, manuscript submitted for publication) identified a 110-kD basolateral membrane protein. These data suggest that unlike NHE-1, which probably serves a "housekeeping" function, NHE-3 may be involved in vectorial Na transport by the intestine.

Studies on the pathophysiological mechanism of Campylobacter jejuni-induced fluid secretion in rat ileum

Calcium has been reported to play an important role in regulating the intestinal electrolyte transport via Ca2+/calmodulin (CaM) and/or protein kinase C (PKC) systems. The role of Ca2+, CaM and PKC in the pathogenesis of Campylobacter jejuni-induced fluid accumulation was studied in vivo in ligated rat ileal loops. Calcium ionophore A23187 (5 microM) and PKC activator, phorbol-12-myristate-13-acetate (PMA, 100 micrograms kg-1) when injected alone induced fluid accumulation in the control loops. However, these modulators did not enhance further C. jejuni-induced fluid accumulation when injected along with C. jejuni live culture in the experimental loops. Both 1-verapamil (100 microM) and PKC antagonist, H-7 (15 micrograms/ml-1) significantly reduced C. jejuni-induced fluid accumulation (P < 0.001). The effect of CaM antagonist W-7 (60 microM) on C. jejuni-induced fluid secretion was not significant (P > 0.05). Our findings suggest that both Ca2+ and PKC appear to be the important second messengers involved in the stimulation of intestinal fluid accumulation in C. jejuni infection.

Evidence for protein kinase C stimulation in rat enterocytes pretreated with heat stable enterotoxin of Escherichia coli

Rat intestinal epithelial cells were isolated and the activity of the calcium- and phospholipid-dependent protein kinase C (PKC) was investigated. The stimulation of activity by Escherichia coli heat stable enterotoxin (STa) was about 5-fold compared to control activity (16.91 +/- 1.69 vs 93.56 +/- 10.40 nmol/mg protein/min) and was dose dependent. Maximum enzyme activity was observed after incubation for 1 min with 6 ng of purified STa. The synergistic effects of calcium, phosphatidylserine and diolein on the enzyme activity were noted both in control and STa-treated cells. Staurosporine, a potent PKC inhibitor, significantly reduced the enzyme activity. Autoradiographic analysis of polyacrylamide gel electrophoresis revealed that pretreatment of the cells with STa also resulted in the phosphorylation of specific membrane proteins each with a molecular mass of 37 kDa, 100 kDa and 140 kDa. However, STa had no direct role on the enzyme activity. Our results, therefore, provide evidence for the involvement of PKC in STa-induced signal transduction in rat enterocytes.

Stimulation of enterocyte protein kinase C by laxatives in-vitro

To elucidate the role of protein kinase C in the mechanism of action of stimulatory laxatives, experiments were performed with preparations of rat lysed enterocytes. The phorbol ester 4-beta-phorbol 12-myristate 13-acetate (PMA) concentration-dependently (2-200 micrograms mL-1) stimulated the activity of protein kinase C in this preparation. Ricinoleic acid, the active principle of castor oil, deacetylbisacodyl, the active principle of bisacodyl, and deoxycholic acid exerted the same effect, although less efficiently. This reflects their potency for inducing intestinal fluid secretion and prostaglandin release, effects that are also induced more potently by PMA. Accordingly, the potency of the three C18 fatty acids, ricinoleic acid, stearic acid and oleic acid on protein kinase C activity in-vitro, on prostaglandin E2 release and on net fluid secretion in-vivo runs in parallel. It is therefore concluded that stimulatory laxatives activate protein kinase C, leading to prostaglandin E2 release, thus resulting in net fluid secretion.

Bethanechol inhibition of chicken intestinal brush border Na/H exchange: Role of protein kinase C and other calcium-dependent processes

Bethanechol, a muscarinic agonist, inhibits the initial rate of amiloride-sensitive Na uptake by intact mucosa of avian small intestine as well as by isolated chicken villus enterocytes, an effect that is maximal at 90 seconds and reverses by 6 minutes. Bethanechol similarly decreases intracellular pH in isolated cells suspended in bicarbonate-free buffer in a time course similar to inhibition of enterocyte Na uptake, suggesting inhibition of Na/H exchange. In brush border membrane vesicles rapidly prepared from cells stimulated with bethanechol, proton-dependent 22Na uptake is transiently inhibited in a time course similar to inhibition of cell Na uptake. Bethanechol also stimulates transient translocation of protein kinase C from the cytosol to the particulate fraction, a portion of this activity translocating to the brush border membrane. To determine the calcium dependence of bethanechol's action, enterocytes were loaded with varying concentrations of the calcium buffering agent quin-2. Inhibition of cell Na uptake by the calcium ionophore ionomycin could be completely reversed by quin-2 buffering in a concentration-dependent manner. In contrast, quin-2 buffering had little or no effect on the inhibition of Na uptake caused by the protein kinase C activators phorbol esters and oleoylacetylglycerol. Bethanechol's inhibitory effects were partially, but not completely reversed by quin-2 buffering. These data suggest that the effects of bethanechol on chicken villus enterocyte brush border Na/H exchange are mediated by calcium-dependent process(es) as well as by protein kinase C.

Dual role for protein kinase C alpha as a regulator of ion secretion in the HT29cl.19A human colonic cell line

The involvement of protein kinase C (PKC) in the regulation of intestinal ion secretion was studied in polarized monolayers of the HT29cl.19A human colon carcinoma cell line. Carbachol, phorbol esters [PMA (phorbol 12-myristate 13-acetate) and PDB (phorbol 12,13-dibutyrate)] and 8-bromo cyclic AMP (8-Br-cAMP) induced Cl secretion, as measured by a rise in the short-circuit current (ISC). The electrical response to carbachol coincided with a transient translocation of PKC alpha from the soluble to the particulate fraction. The carbachol-, PDB- and 8-Br-cAMP-induced ISC responses were inhibited by pretreatment of the cells with PMA (0.5 microM) for 2 h, a time period in which PKC alpha, beta 1 and gamma levels were not changed. As shown by 86Rb+ and 125I- efflux studies, the main targets for this inhibition were basolateral K+ transporters rather than apical Cl- channels. Prolonged exposure to PMA (24 h) led to a 60% recovery of the 8-Br-cAMP response, but not of the carbachol- or PDB-provoked secretion. As shown by immunoblotting with PKC-isoenzyme-specific antisera, the recovery of the 8-Br-cAMP response coincided with the down-regulation of PKC alpha, whereas the levels of PKC beta 1 and gamma were unmodified. These results suggest that PKC alpha, but not PKC beta 1 or gamma, is involved in both acute stimulation and chronic inhibition of ion secretion in the HT29cl.19A colonic cell line.

Olsalazine-related diarrhoea: does rat intestine adapt in vivo?

Diarrhoea may occur in up to 10% of patients with ulcerative colitis treated with olsalazine, the azolinked dimer of 5-aminosalicylic acid. However, this symptom often disappears despite continued drug medication. To examine reversibility of and adaptation to olsalazine effects on intestinal absorption, rats were fed olsalazine (4 mg/100 g body weight/day) for 0 (controls), 12, 24, and 32 days. Jejunal, ileal, and colonic loops were perfused in situ with buffer or olsalazine (11.6 mM) in a pendular perfusion system. Water and electrolyte absorption was inhibited in all intestinal segments (p less than 0.001). In the proximal small intestine, however, sodium absorption was inhibited by 61%, whereas chloride and potassium absorptions were turned into net secretion. In contrast, in ileal and colonic segments sodium, chloride, and potassium absorptions were turned into a net secretion. All inhibitory effects were reversible within a short time. Intestinal absorption remained inhibitable compared with controls (p = not significant) after chronic administration of olsalazine even for 1 month. Jejunal monosaccharide absorption was not altered by acute olsalazine perfusion. In the ileum, glucose absorption was significantly inhibited, but the inhibitory capacity of acute olsalazine application decreased significantly (p less than 0.05) depending on duration of olsalazine pretreatment (51% (controls) versus 38% (32 days)). These results point to a complex, acute, but fully reversible effect of olsalazine on intestinal passive and chloride-coupled absorptive processes. Since a mucosal adaptation to these diarrheogenic effects does not occur, the resulting increase in fluid load on the diseased colon may be important in the pathogenesis of olsalazine-related diarrhoea.

Study on the mechanism of Giardia lamblia induced diarrhoea in mice

The transmucosal fluxes of Na+ and Cl- were studied in Giardia lamblia infected mice in the presence or absence of phorbol-12-myristate-13-acetate (PMA), the activator of protein kinase C (PKC) or 1-(5-isoquinolinylsulphonyl)-2-methylpiperazine (H-7), the inhibitor of PKC or Ca(2+)-calmodulin. There was net secretion of Na+ and Cl- in infected animals, while in control animals there was net absorption of these ions. The addition of ionophore or PMA resulted in net secretion of Na+ and Cl- in the control group while in the infected group there was no change in the fluxes of these ions. The selective potent inhibitor of protein kinase C, H-7, reversed the secretion of Na+ and Cl- in infected group to absorption. The addition of PMA and Ca(2+)-ionophore together in the infected group had a partial additive effect. This study suggests that G. lamblia induced fluid secretion involves protein kinase C and further protein kinase C acts in synergism with calcium.

Relative sensitivity to inhibition by cimetidine and clonidine differentiates between the two types of Na(+)-H+ exchangers in cultured cells

Available evidence indicates that there are two types of Na(+)-H+ exchangers, type A (housekeeping type) and type B (epithelial or apical type), in mammalian cells. We have recently reported, using isolated membrane vesicles, that these two types can be differentiated by their relative sensitivities to inhibition by clonidine and cimetidine [Kulanthaivel, Leibach, Mahesh, Cragoe & Ganapathy (1990) J. Biol. Chem. 264, 1249-1252]. The present study was undertaken to determine whether this approach is also effective in cultured cells. The JAR human placental choriocarcinoma cell line and the opossum kidney (OK) cell line, when grown as confluent monolayer cultures on an impermeable plastic support, express Na(+)-H+ exchanger activity which is measurable by determining Na+ uptake into the cells from the culture medium. The JAR cell Na(+)-H+ exchanger was found to be about 100 times more sensitive to inhibition by dimethylamiloride than the OK cell Na(+)-H+ exchanger. Inhibition studies with clonidine and cimetidine were able to differentiate between these two exchangers very clearly. Cimetidine was 18 times more potent than clonidine in inhibiting the JAR cell Na(+)-H+ exchanger. In contrast, clonidine was at least 8 times more potent than cimetidine in inhibiting the Na(+)-H+ exchanger of the OK cell. The results show that the JAR cell expresses the type A Na(+)-H+ exchanger, whereas the OK cell expresses the type B Na(+)-H+ exchanger. This approach also proved to be very effective in correctly identifying the type of Na(+)-H+ exchanger in a third cell line (HeLa). It is concluded that the relative susceptibility to inhibition by clonidine and cimetidine offers an easy and efficient means of differentiating between the two types of Na(+)-H+ exchangers in cultured cells.

Studies on the mechanism of Salmonella typhimurium enterotoxin-induced diarrhoea

The unidirectional fluxes of Na+ and Cl- were studied in Salmonella typhimurium enterotoxin-treated rats. There was net secretion of Na+ and Cl- in toxin-treated animals, while in control animals there was net absorption of these ions. In the presence of the Ca(2+)-ionophore, there was net secretion of Na+ and Cl- in the control group, while the ionophore enhanced the secretion of these ions in experimental animals. The calcium channel blocker, verapamil, decreased the secretion induced by salmonella toxin, but could not reverse the secretion to absorption. There was no difference in the net absorption of Ca2+ in both the control and experimental animals. There was a significant increase in the intracellular free calcium concentrations in enterocytes isolated from toxin-treated rat intestines as compared to that in enterocytes isolated from control animals. In the presence of PMA (phorbol-12-myristated-13-acetate) there was net secretion of Na+ and Cl- in the control group, while in the experimental group there was no change in the fluxes of these ions. The selective, potent inhibitor of protein kinase C, H-7 (1-(5-isoquinolinylsulphonyl)-2-methylpiperazine) reversed the secretion of Na+ and Cl- in the toxin-treated group to absorption. The addition of indomethacin also inhibited the secretion induced by salmonella toxin, but failed to reverse it to absorption. However, the addition both H-7 and indomethacin to the experimental group had a partial additive effect. These studies demonstrate that the Salmonella enterotoxin-mediated fluid secretion involves protein kinase C and the arachidonic acid metabolites and perhaps does not involve the extracellular calcium pools.

Carbachol- and elevated Ca(2+)-induced translocation of functionally active protein kinase C to the brush border of rabbit ileal Na+ absorbing cells

Protein kinase C is involved in mediating the effects of elevated Ca2+ in ileal villus Na+ absorbing cells to inhibit NaCl absorption. The present studies were undertaken to understand the mechanism by which this occurs. The effects of carbachol and the calcium ionophore A23187, agents which elevate intracellular Ca2+ and inhibit NaCl absorption in ileal villus cells, were studied. Carbachol treatment of villus cells caused a rapid decrease in protein kinase C activity in cytosol, with an accompanying increase in microvillus membrane C kinase. Exposure of the villus cells to calcium ionophore also caused a quantitatively similar decrease in cytosol C kinase and increase in C kinase activity in the microvillus membrane. This increase caused by carbachol and Ca2+ ionophore was specific for the microvillus membrane. In fact, 30 s and 10 min after exposure of the cells to carbachol, basolateral membrane protein kinase C decreased, in a time-dependent manner; whereas 10 min of Ca2+ ionophore exposure did not alter basolateral C kinase. Exposure of villus cells to Ca2+ ionophore or carbachol caused similar increases in microvillus membrane diacylglycerol content. As judged by the ability to inhibit Na+/H+ exchange measured in ileal villus cell brush border membrane vesicles, the protein kinase C which translocated to the microvillus membrane was functionally significant. Inhibition of Na+/H+ exchange required ATP and was reversed by the protein kinase C antagonist H-7. In conclusion, the effect of carbachol and Ca2+ ionophore in regulation of ileal NaCl absorption is associated with an increase in microvillus membrane diacylglycerol content and functionally active protein kinase C. The effects of both carbachol and Ca2+ ionophore are different on brush border and basolateral membrane distribution of protein kinase C.

Properties of two calcium transport systems of isolated rat ileal epithelial cells: effects of Ca2+ channel modulators and membrane potential examined with fluorescent dye, fura-2

Calcium transport systems of isolated ileal epithelial cells were investigated. The concentration of cytosolic free calcium ions, [Ca2+]i, was monitored with a fluorescent Ca2+ dye, fura-2. The fluorescence intensity ratio (I340/I380) was used as an index of [Ca2+]i. [Ca2+]i of the cells suspended in the nominally Ca(2+)-free solution was estimated at 52 +/- 3 nM. Ca2+ uptake was followed for as long as 5 min in the presence of 100-1000 microM added CaCl2. Most of the experiments were performed at 200 microM CaCl2. The Ca2+ uptake was abolished by 0.8 mM Ni2+ and 50 microM Mn2+ and partitally antagonized by 50 microM verapamil and 50 microM diltiazem but not affected by 20 microM nifedipine. The Ca2+ entry was reduced by increasing concentrations of extracellular K+ in the presence of valinomycin, suggesting a voltage-dependent nature of the uptake. On the other hand, the Ca2+ transport doubled in the presence of Bay K8644 (8 microM), a Ca2+ channel agonist. The Bay-K-8644-induced uptake was inhibited by either 10 microM nifedipine, 10 microM verapamil or 10 microM diltiazem and was relatively independent of extracellular K+ concentration. These results suggest that there are at least two distinct Ca2+ transport systems in the rat ileal epithelial cells, one resistant to organic Ca2+ channel blockers but relatively sensitive to membrane potential (basal uptake) and another inducible by Bay K 8644 and sensitive to the channel blockers but relatively independent of membrane potential.

Phosphorylation-regulated Cl- channel in CHO cells stably expressing the cystic fibrosis gene

A cyclic AMP-stimulated chloride conductance appears when the cystic fibrosis gene is expressed in non-epithelial cells by infection with recombinant viruses. Cyclic AMP-stimulated conductance in this system is mediated by the same ohmic, low-conductance Cl- channel as in human secretory epithelia, but control of this channel by phosphorylation has not been directly demonstrated. Here we report the appearance of the low-conductance Cl- channel in Chinese hamster ovary cells after stable transfection with the cystic fibrosis gene. The channel is regulated on-cell by membrane-permeant analogues of cAMP and off-cell by protein kinases A and C and by alkaline phosphatase. These results are further evidence that the cystic fibrosis transmembrane regulator is a Cl- channel which can be activated by specific phosphorylation events and inactivated by dephosphorylation; they reveal an unsuspected synergism between converging kinase regulatory pathways.

Effect of 12-O-tetradecanoyl phorbol 13-acetate on solute transport and production of cAMP in isolated frog skin

In the present study we have examined the action of the phorbol diester tetradecanoyl phorbol acetate, an activator of protein kinase C, on the transepithelial transport of sodium, chloride and water and the production of cAMP in the isolated frog skin epithelium (Rana esculenta). Addition of tetradecanoyl phorbol acetate to the mucosal solution resulted initially in an increase in the short-circuit current, which was followed by a progressive decrease. If the short-circuit current was first activated by addition of the antidiuretic hormone, arginine vasotocin, then the addition of tetradecanoyl phorbol acetate resulted only in a pronounced inhibition. The changes in the short-circuit current were the result of changes in the active influx of Na+. The effect of tetradecanoyl phorbol acetate on the intracellular potential measured under short-circuited conditions (Vscc) was time-dependent. Just after addition of tetradecanoyl phorbol acetate to the mucosal solution, Vscc depolarized; this was followed by a slight hyperpolarization, after which Vscc continued to decline. The inhibition of the Na+ transport by tetradecanoyl phorbol acetate was associated with a decline in the response to the antidiuretic hormone (arginine vasotocin), but the ability of arginine vasotocin to increase the cellular level of cAMP and to stimulate the osmotic water flow was not affected by the presence of tetradecanoyl phorbol acetate. In skin halves in which the short-circuit current was stimulated with arginine vasotocin, addition of tetradecanoyl phorbol acetate resulted in a dose-dependent inhibition of the short-circuit current, but only minor changes in Vscc were observed. The results presented suggest that the addition of tetradecanoyl phorbol acetate to the isolated frog skin first increases and then decreases the arginine vasotocin-sensitive sodium permeability of the apical membrane. This might be due to a stimulating effect of tetradecanoyl phorbol acetate on both the activation and deactivation (turnover) of the sodium channels.

Time- and dose-dependent effects of protein kinase C on proximal bicarbonate transport

Activation of protein kinase C has been shown to cause both stimulation and inhibition of transport processes in the brush-border membrane and renal tubule. This study was designed to examine the dose-response nature and time-dependent effect of 4 beta-phorbol-12-myristate-13-acetate (PMA) on the rates of bicarbonate absorption (JHCO3) and fluid absorption (Jv) in the proximal convoluted tubule (PCT) of rat kidney. Bicarbonate flux was determined by total CO2 changes between the collected fluid and the original perfusate as analyzed by microcalorimetry. Luminal perfusion of PMA (10(-10) approximately 10(-5) M) within 10 min caused a significant increase of JHCO3 and Jv. A peaked curve of the dose response was observed with maximal effect at 10(-8) M PMA on both bicarbonate and fluid reabsorption, which could be blocked completely by amiloride (10(3) M) and EIPA (10(-5) M). On the other hand, with an increase of perfusion time beyond 15 min. PMA (10(-8) and 10(-6) M) could inhibit JHCO3 and Jv. Amiloride (10(-3) M) or EIPA (10(-5) M) significantly inhibits JHCO3 and Jv, while there is no additive effect of PMA and amiloride or EIPA on PCT transport. An inactive phorbol-ester, 4 alpha-phorbol, that does not activate protein kinase C, had no effects on JHCO3 and Jv. Capillary perfusion of PMA (10(-8) M) significantly stimulate both JHCO3 and Jv; however, PMA did not affect glucose transport from either the luminal side or basolateral side of the PCT. These results indicate that activation of endogenous protein kinase C by PMA could either stimulate or inhibit both bicarbonate and fluid reabsorption in the PCT dependent on time and dose, and these effects are through the modulation of Na+/H+ exchange mechanism.

Phorbol ester-stimulated secretion in chicken ileum: role of arachidonic acid metabolism

Phorbol 12,13-dibutyrate, an activator of protein kinase C, stimulates electrogenic anion secretion in chicken ileum. This effect can be inhibited by the cyclooxygenase inhibitor piroxicam, suggesting arachidonic acid metabolites as mediators. Phorbol 12,13-dibutyrate stimulates increases in prostaglandin E2 release and mucosal 3',5'-cyclic adenosine monophosphate content in intact mucosa. Stimulation of prostaglandin E2 release appears to come mainly from the subepithelial compartment and not from epithelial cells. Unlike its effect on anion secretion, phorbol 12,13-dibutyrate inhibition of sodium uptake by sodium-hydrogen exchange in isolated mature villus enterocytes is not inhibited by piroxicam. This study concludes that phorbol ester effects on anion secretion in chickens appear to be mediated by arachidonic acid metabolites most likely produced by cells of the lamina propria and submucosa, whereas phorbol ester effects on inhibiting upper villus cell sodium-hydrogen exchange may be a direct effect of phorbol esters.

Protein kinase C and intestinal fluid secretion: involvement of prostaglandin E2 but not of 5-hydroxytryptamine

To determine the role of protein kinase C in the regulation of intestinal fluid transport, experiments were performed with the rat jejunum in vivo, using the active phorbol ester, 4-beta-phorbol 12-myristate 13-acetate (PMA), as stimulator of protein kinase C. Intraluminally administered PMA dose dependently reversed the net fluid absorption to net fluid secretion and significantly increased prostaglandin E2 (PGE2) but not 5-hydroxytryptamine (5-HT) output into the lumen. Mucosal cyclic AMP levels remained unchanged by PMA. Indomethacin inhibited the increase in PGE2 output and partially reduced the secretory response to PMA. Ketanserin was without effect whereas verapamil totally blocked the secretory response to PMA. It is concluded that intestinal fluid secretion, stimulated by activation of protein kinase C is partly mediated by PGE2 release. PGE2 may facilitate calcium entry rather than increase intracellular calcium through activation of cyclic AMP. Protein kinase C appears to play an important role as an intermediate in phosphoinositol hydrolysis, which is initiated by 5-HT, and finally induces fluid secretion via PGE2.

Interleukins 1 and 3 stimulate anion secretion in chicken intestine

Interleukin 1 or 3 added to the serosal side of chicken small intestine transiently increases short-circuit current. Replacement of bathing-medium Cl and HCO3 with gluconate and HEPES abolished the short-circuit current increase, consistent with these cytokines stimulating electrogenic anion secretion. Cytokine-stimulated short-circuit current changes were inhibited by preincubation with piroxicam (10(-5) M), an inhibitor of arachidonic acid cyclooxygenase, suggesting prostaglandin formation as an intermediate step for cytokine stimulation of short-circuit current. In intact mucosal strips, interleukin 1 and 3 stimulated prostaglandin E2 release and elevated tissue 3',5'-cyclic adenosine monophosphate concentration. When prostaglandin E2 release from epithelial and subepithelial fractions of the mucosa by interleukin 1 was determined, increases were found only from the subepithelium. The secretory actions of cytokines appear to be mediated by arachidonic acid metabolites most likely produced by cells of the lamina propria and submucosa and may play a role in inflammatory processes in which intestinal secretion is enhanced.

Neural blockade in basal and postreceptor-stimulated intestinal transport

Postreceptor protein stimulation significantly alters the transport state of the ex vivo small intestine. This study investigated the effects of neural blockade on basal and stimulated ionic transport. Rabbit ileal segments (n = 46) were arterially perfused with an oxygenated sanguinous buffered electrolyte solution. The lumen was perfused with an isotonic solution containing [14C]polyethylene glycol as a nonabsorbable marker. Net fluxes of H2O, Na+, and Cl- were calculated. Tetrodotoxin (TTX) was used to block enteric neural transmission. Forskolin (FOR) was used to activate adenylate cyclase, and phorbol 12,13-dibutyrate (PDB) served to activate protein kinase C. Two groups were studied. Group A preparations had no TTX pretreatment, while group B preparations were pretreated with TTX. In the Group A preparations, TTX at 10(-6) M and PDB at 10(-5) M caused significant proabsorptive effects with a delta FH2O of +20 +/- 7 and +15 +/- 2 microliters/min, respectively (P less than 0.05), while FOR stimulated significant secretion with a delta FH2O of -14 +/- 3 microliter/min (P less than 0.05). In the Group B TTX-pretreated preparations, FOR did not cause secretion and PDB maintained an absorptive state. These results indicate that neural blockade with TTX reverses basal secretion in the ex vivo intestine, suggesting that an intact enteric nervous system maintains the secretory status of the intestine. FOR-induced adenylate cyclase-activated secretion does not occur in the presence of TTX, implying that intact neural transmission is required for the FOR effect. PDB-induced protein kinase C-activated absorption occurs despite neural blockade, suggesting that the PDB-induced proabsorptive effect is mediated without neural intermediaries.

Phorbol esters enhance the cyclic GMP response of T84 cells to the heat-stable enterotoxin of Escherichia coli (STa)

We examined the effect of protein kinase C (PKC) activation on the cyclic GMP response to heat-stable enterotoxin (STa) in a colonic carcinoma intestinal epithelial cell line, T84 cells. Our results demonstrate that the active phorbol ester analog, phorbol dibutyrate, but not the inactive alpha-phorbol dibutyrate, acts synergistically with STa to elevate cyclic GMP in intact T84 cells. The effect is observed in the absence or presence of the phosphodiesterase inhibitor, isobutylmethylxanthine, which suggests that phorbol dibutyrate modifies cyclic GMP synthesis rather than cyclic GMP degradation. In contrast to several systems in which prolonged treatment with phorbol ester desensitizes PKC-mediated responses, the cyclic GMP response in T84 cells is not diminished by prolonged treatment of T84 cells with phorbol dibutyrate. Also, transient treatment of T84 cells with phorbol dibutyrate enhances subsequent STa-stimulated cyclic GMP accumulation. These observations suggest that PKC activation produces a long-lived signal in T84 cells which enhances cyclic GMP accumulation in response to STa. Second messenger "cross talk" [T. Yoshimasa, D. R. Sibley, M. Bouvier, R. J. Lefkowitz, and M. G. Caron, Nature (London) 327:67-70, 1987] may be important in the pathogenesis of diarrheal disease.

Calcium mediated neurohumoral inhibition of chicken enterocyte Na influx: role of phosphatidylinositol metabolites

Carbachol (CCH), serotonin (5HT), divalent ionophore A23187, cAMP, and certain neuropeptides, i.e. substance P (SP), inhibit the initial rate of uptake (influx) of 22Na into isolated chicken villus enterocytes. All these agents also increase cytosolic Ca. However, the increases stimulated by CCH, 5HT, and cAMP are not blocked by chelation of extracellular Ca, whereas those of A23187 and SP are. Only CCH and 5HT stimulate hydrolysis of membrane phosphoinositides to form inositol phosphates. CCH and 5HT also stimulate incorporation of [32P]-PO4 into membrane polyphosphoinositides. These studies suggest that at least three mechanisms exist to increase cytosolic Ca in chicken enterocytes and thereby inhibit Na influx. Certain neurohumoral agents such as SP open a plasma membrane permeability for Ca, permitting extracellular Ca to enter the cell down its electrochemical gradient. These agents do not stimulate phosphatidylinositol breakdown. CCH and 5HT stimulate phosphatidylinositol breakdown and via the formation of inositol trisphosphate release Ca from intracellular stores. A third mechanism exists for cAMP which mobilizes Ca from intracellular stores, but does not involve the metabolism of membrane phosphatidylinositols.

The effect of Escherichia coli heat-stable enterotoxin on protein kinase activity

Isolated rat enterocytes were incubated with E. coli heat-stable enterotoxin or buffer alone and the protein kinase activity and cyclic GMP level determined on the particulate fraction or cytosol, respectively. In the control cells, particulate protein kinase activity and cyclic GMP concentration were at a maximum after 20 sec and 1 min of incubation, respectively. In heat-stable enterotoxin-treated cells the particulate protein kinase activity was significantly increased (P less than 0.05) after 20 sec of incubation, but decreased (P less than 0.05) after 30 sec, 1 min and 2 min, when compared to the control reaction. During this time period the concentration of intracellular cyclic GMP increased 10-fold. The effect of heat-stable enterotoxin on particulate protein kinase activity and cyclic GMP concentration was dose-dependent. Analysis of radioactive membrane phosphorylation products indicate a role for phosphoproteins with a mol. wt of 25,000 and 120,000. These results suggest that the action of heat-stable enterotoxin may involve an effect on protein kinase.

Separate regulatory control of apical and basolateral Na+/H+ exchange in renal epithelial cells

Epithelial layers of LLC-PK1/PKE20 cells, a renal epithelial cell line which expresses Na+/H+ exchange activities in the apical as well as basolateral membrane domains, are examined in the single cell mode by microspectrofluorometry. We provide evidence that basolateral Na+/H+ exchange is more sensitive to amiloride inhibition than is apical Na+/H+ exchange. Furthermore, we demonstrate that the two exchange activities differ in their regulatory control: kinase A activation (forskolin, 8-Br-cAMP) leads to inhibition of both exchange activities, whereas kinase C activation (phorbol ester) stimulates basolateral and inhibits apical Na+/H+ exchange. Thus, renal epithelial cells may contain two Na+/H+ exchange activities: an apical ("epithelial") and basolateral ("housekeeping") which may serve different cellular functions and are under separate regulatory controls.

Effect of auranofin on ion transport by rat small intestine

Auranofin, applied either mucosally or serosally, increased the potential difference and short-circuit current generated by stripped sheets of rat mid-intestine in a concentration-dependent manner. In-vivo auranofin induced a net fluid secretion, suggesting that the rise in electrical activity represented a stimulation of anion secretion. Removal of chloride or addition of frusemide inhibited the auranofin-induced increase in short-circuit current, indicating that chloride was the anion involved and in the case of serosal auranofin this was confirmed by direct measurement of ion fluxes. The effects of both mucosal and serosal auranofin were calcium-dependent. The gold component of the auranofin molecule is probably responsible for its secretory actions as these were mimicked by chlorauric acid. The ability of auranofin to stimulate intestinal secretion may contribute to its diarrhoeagenic action.