Segmental diversity of electrogenic glucose transport characteristics in the small intestines of weaned pigs

It has been shown in several species that the intestinal Na(+)-dependent glucose co-transporter 1 (SGLT1) is more abundant in the jejunum than in ileum. In contrast, the efficiency of intestinal glucose uptake rates in suckling piglets or weaned pigs is not clearly fitting with this segmental distribution. The aim of this study was to evaluate SGLT1 mediated glucose absorption in the jejunum and ileum of growing pigs (Sus scrofa) in more detail. In Ussing chambers, basal short-circuit currents were significantly more positive in the jejunum. It could be demonstrated that the electrogenic ileal glucose transport was significantly more pronounced in different breeds and occurred at 5 mmol?L(-1) glucose 7 times faster in the ileum, although slightly higher jejunal expression of glycosylated SGLT1 was detected by Western blotting. This expression pattern was connected to significantly lower phlorizin sensitivity in the jejunum. As the more efficient ileal glucose absorption was also observable with glucose uptake studies into isolated brush-border membrane vesicles without differences in abundance and activity of the Na(+)/K(+)-ATPase in both segments, we conclude that the segmental differences in porcine glucose transport characteristics may be based on direct or indirect modulations of SGLT1 activity.     

Proximal-distal absorptive gradients in the in vivo intestine of normal and infected (Hymenolepis diminuta: Cestoda) rats.

Using a single-pass perfusion technique, H2O, Na+, Cl-, HCO3-, and glucose absorption were studied in the jejunum and proximal and distal ileum of rats either uninfected or infected with a tapeworm parasite (Hymenolepis diminuta). The effect of parasitization, region of the intestine, type of buffer, and concentration of glucose in the perfusion fluids on the transport data were analyzed by univariate and multivariate techniques. Proximal-distal flux gradients were observed for water and all the solute species studied, as well as for glucose- and bicarbonate-stimulated salt and water transport; there was a decreasing sensitivity to low pH proceeding distally. The major regional differences occurred between the proximal and distal ileum, with the fluxes in the jejunum being similar to those in the proximal ileum. Na+, H2O, and glucose transport decreased, while Cl- absorption increased, proceeding distally. The parasites diminished the rates of absorption of glucose, salt, and water, and altered the flux gradients, particularly the Na+ and HCO3- transport gradients. The differences in the gradients between control and infected animals were related to differential sensitivity of the different transport systems in the various regions of the gut to parasitism.     

A phosphorylated phloretin derivative. Synthesis and effect on intestinal Na(+)-dependent phosphate absorption

2'-Phosphophloretin (2'-PP), a phosphorylated derivative of the plant chalcone, was synthesized. The effect of 2'-PP, on Na(+)-dependent phosphate uptake into intestinal brush-border membrane vesicles (BBMV) isolated from rabbit and rat duodenum and jejunum was examined. 2'-PP decreased Na(+)-dependent phosphate uptake into rabbit BBMV with an IC(50) of 55 nM and into rat BBMV with an IC(50) of 58 nM. 2'-PP did not affect Na(+)-dependent glucose, Na(+)-dependent sulfate, or Na(+)-dependent alanine uptake by rabbit intestinal BBMVs. 2'-PP inhibition of rabbit intestinal BBMV Na(+)-dependent phosphate uptake was sensitive to external phosphate concentration, suggesting that 2'-PP inhibition of Na(+)-dependent phosphate uptake was competitive with respect to phosphate. Binding of [(3)H]2'-PP to rabbit intestinal BBMV was examined. Binding of [(3)H]2'-PP was Na(+)-dependent with a K(0.5) for Na(+)(Na(+) concentration for 50% 2'-PP binding) of 30 mM. The apparent K(s) for Na(+)-dependent [(3)H]2'-PP binding to rabbit BBMVs was 58 nM in agreement with the IC(50) for 2'-PP inhibition of Na(+)-dependent phosphate uptake. These results indicate that 2'-PP bound to rabbit or rat intestinal BBMV Na(+)-phosphate cotransporter and inhibited Na(+)-dependent phosphate uptake. In rats treated with 2'-PP by daily gavage, the effect of 2'-PP on serum phosphate, serum glucose, and serum calcium was examined. In a concentration-dependent manner, 2'-PP reduced serum phosphate by 45% 1 wk after starting treatment. 2'-PP did not alter serum calcium or serum glucose. The apparent IC(50) for 2'-PP in vivo was 3 microM.     

Phosphate transport in the duodenum and jejunum of goats and its adaptation by dietary phosphate and calcium

Endogenous P(i) recycling is a characteristic feature of the P homeostasis in ruminants. A pronounced salivary P(i) secretion into the rumen is balanced by a high intestinal P(i) absorption and an almost complete renal P(i) reabsorption. In monogastric animals, the major P(i) transport mechanism across the apical membrane of the enterocyte is an Na(+)-dependent transport mediated by NaPi cotransporter type IIb. In ruminants, an Na(+)-, as well as an H(+)-dependent, P(i) transport system seems to exist in the small intestines. Therefore, morphological localization, type of ionic dependence, and ability to adapt to dietary P or Ca restriction of duodenal and jejunal P(i) transport were characterized in goats. In the duodenum, there was an H(+)-dependent, Na(+)-sensitive P(i) transport system that did not belong to the NaPi type II family and was not influenced by dietary P or Ca restriction. In contrast, in the jejunum, there was an Na(+)-dependent, H(+)-sensitive P(i) transport mainly mediated by NaPi IIb. P restriction stimulated the NaPi IIb protein expression, resulting in higher P(i) transport capacity.     

Characterization and chemical modification of the Na(+)-dependent bile-acid transport system in brush-border membrane vesicles from rabbit ileum.

The Na(+)-dependent uptake system for bile acids in the ileum from rabbit small intestine was characterized using brush-border membrane vesicles. The uptake of [3H]taurocholate into vesicles prepared from the terminal ileum showed an overshoot uptake in the presence of an inwardly-directed Na(+)-gradient ([Na+]out > [Na+]in), in contrast to vesicles prepared from the jejunum. The Na(+)-dependent [3H]taurocholate uptake was cis-inhibited by natural bile acid derivatives, whereas cholephilic organic compounds, such as phalloidin, bromosulphophthalein, bilirubin, indocyanine green or DIDS - all interfering with hepatic bile-acid uptake - did not show a significant inhibitory effect. Photoaffinity labeling of ileal membrane vesicles with 3,3-azo- and 7,7-azo-derivatives of taurocholate resulted in specific labeling of a membrane polypeptide with apparent molecular mass 90 kDa. Bile-acid derivatives inhibiting [3H]taurocholate uptake by ileal vesicles also inhibited labeling of the 90 kDa polypeptide, whereas compounds with no inhibitory effect on ileal bile-acid transport failed to show a significant effect on the labeling of the 90 kDa polypeptide. The involvement of functional amino-acid side-chains in Na(+)-dependent taurocholate uptake was investigated by chemical modification of ileal brush-border membrane vesicles with a variety of group-specific agents. It was found that (vicinal) thiol groups and amino groups are involved in active ileal bile-acid uptake, whereas carboxyl- and hydroxyl-containing amino acids, as well as tyrosine, histidine or arginine are not essential for Na(+)-dependent bile-acid transport activity. The irreversible inhibition of [3H]taurocholate transport by DTNB or NBD-chloride could be partially reversed by thiols like 2-mercaptoethanol or DTT. Furthermore, increasing concentrations of taurocholate during chemical modification with NBD-chloride were able to protect the ileal bile-acid transporter from inactivation. These findings suggest that a membrane polypeptide of apparent M(r) 90,000 is a component of the active Na(+)-dependent bile-acid reabsorption system in the terminal ileum from rabbit small intestine. Vicinal thiol groups and amino groups of the transport system are involved in Na(+)-dependent transport activity, whereas other functional amino acids are not essential for transport activity.     

Comparison of VIP-induced electrolyte secretion at three levels in rat small intestine.

Duodenal, jejunal and ileal loops were prepared and an iso-osmotic test solution injected, containing 80 mM Na+, 5-mM K+, 1.2 mM Ca2+, 77 mM Cl-, 10 mM HCO3- and 136 mM mannitol. 14CPEG 4000 was used as a non-absorbable marker and 36Cl was added to measure the bidirectional fluxes. During the 60-min in vivo incubation time, the duodenum actively secreted bicarbonate, a virtually zero flux in the jejunum was observed, whereas the ileum absorbed water and chloride and secreted bicarbonate. The response to the perfused doses of 0.15 to 2.4 nmol.100 g-1.h-1 of VIP (vasoactive intestinal peptide) differed qualitatively and quantitatively in the 3 segments: VIP increased bicarbonate secretion and induced chloride secretion in the duodenum, induced chloride secretion in the jejunum without changing bicarbonate minimal influx, induced bicarbonate secretion and suppressed chloride absorption in the ileum. The minimal dose required was lower in the duodenum (0.3 nmol.100 g-1.h-1) than in the jejunum and ileum (1.2 nmol.100 g-1.h-1). The functional heterogeneity of the small intestine was clearly demonstrated after VIP stimulation.     

Activation of CFTR by ASBT-mediated bile salt absorption

In cholangiocytes, bile salt (BS) uptake via the apical sodium-dependent bile acid transporter (ASBT) may evoke ductular flow by enhancing cAMP-mediated signaling to the cystic fibrosis transmembrane conductance regulator (CFTR) anion channel. We considered that ASBT-mediated BS uptake in the distal ileum might also modulate intestinal fluid secretion. Taurocholate (TC) induced a biphasic rise in the short circuit current across ileal tissue, reflecting transepithelial electrogenic ion transport. This response was sensitive to bumetanide and largely abrogated in Cftr-null mice, indicating that it predominantly reflects CFTR-mediated Cl- secretion. The residual response in Cftr-null mice could be attributed to electrogenic ASBT activity, as it matched the TC-coupled absorptive Na+ flux. TC-evoked Cl- secretion required ASBT-mediated TC uptake, because it was blocked by a selective ASBT inhibitor and was restricted to the distal ileum. Suppression of neurotransmitter or prostaglandin release, blocking of the histamine H1 receptor, or pretreatment with 5-hydroxytryptamine did not abrogate the TC response, suggesting that neurocrine or immune mediators of Cl- secretion are not involved. Responses to TC were retained after carbachol treatment and after permeabilization of the basolateral membrane with nystatin, indicating that BS modulate CFTR channel gating rather than the driving force for Cl- exit. TC-induced Cl- secretion was maintained in cGMP-dependent protein kinase II-deficient mice and only partially inhibited by the cAMP-dependent protein kinase inhibitor H89, suggesting a mechanism of CFTR activation different from cAMP or cGMP signaling. We conclude that active BS absorption in the ileum triggers CFTR activation and, consequently, local salt and water secretion, which may serve to prevent intestinal obstruction in the postprandial state.     

Aboral changes in D-glucose transport by human intestinal brush-border membrane vesicles.

D-Glucose transport was investigated in isolated brush-border membrane vesicles from human small intestine. Characteristics of D-glucose transport from the jejunum were compared with that in the mid and terminal ileum. Jejunal and mid-ileal D-glucose transport was Na+-dependent and electrogenic. The transient overshoot of jejunal D-glucose transport was significantly greater than corresponding values in mid-ileum. The terminal ileum did not exhibit Na+-dependent D-glucose transport, but did exhibit Na+-dependent taurocholate transport. Na+-glucose co-transport activity as measured by tracer-exchange experiments was greatest in the jejunum, and diminished aborally. We conclude that D-glucose transport in man is Na+-dependent and electrogenic in the proximal intestine and directly related to the activity of D-glucose-Na+ transporters present in the brush-border membranes. D-Glucose transport in the terminal ileum resembles colonic transport of D-glucose.     

Renal transport of taurine in luminal membrane vesicles from rabbit proximal tubule

The uptake of taurine by luminal membrane vesicles from pars convoluta and pars recta of rabbit proximal tubule was examined. In pars convoluta, the transport of taurine was characterized by two Na(+)-dependent (Km1 = 0.086 mM, Km2 = 5.41 mM) systems, and one Na(+)-independent (Km = 2.87 mM) system, which in the presence of an inwardly directed H(+)-gradient was able to drive the transport of taurine into these vesicles. By contrast, in luminal membrane vesicles from pars recta, the transport of taurine occurred via a dual transport system (Km1 = 0.012 mM, Km2 = 5.62 mM), which was strictly dependent on Na+. At acidic pH with or without a H(+)-gradient, the Na(+)-dependent flux of taurine was drastically reduced. In both kind of vesicles, competition experiments only showed inhibition of the Na(+)-dependent high-affinity taurine transporter in the presence of beta-alanine, whereas there was no significant inhibition with alpha-amino acids, indicating a beta-amino acid specific transport system. Addition of beta-alanine, L-alanine, L-proline and glycine, but not L-serine reduced the H(+)-dependent uptake of taurine to approx. 50%. Moreover, only the Na(+)-dependent high-affinity transport systems in both segments specifically required Cl-. Investigation of the stoichiometry indicated 1.8 Na+: 1 Cl-: 1 taurine (high affinity), 1 Na+: 1 taurine (low affinity) and 1 H+: 1 taurine in pars convoluta. In pars recta, the data showed 1.8 Na+: 1 Cl-: 1 taurine (high affinity) and 1 Na+: 1 taurine (low affinity).     

The role of chloride in taurine transport across the human placental brush-border membrane.

Taurine, a sulfated beta-amino acid, is conditionally essential during development. A maternal supply of taurine is necessary for normal fetal growth and neurologic development, suggesting the importance of efficient placental transfer. Uptake by the brush-border membrane (BBM) in several other tissues has been shown to be via a selective Na(+)-dependent carrier mechanism which also has a specific anion requirement. Using BBM vesicles purified from the human placenta, we have confirmed the presence of Na(+)-dependent, carrier-mediated taurine transport with an apparent Km of 4.00 +/- 0.22 microM and a Vmax of 11.72-0.36 pmol mg-1 protein 20 s-1. Anion dependence was examined under voltage-clamped conditions, in order to minimize the contribution of membrane potential to transport. Uptake was significantly reduced when anions such as thiocyanate, gluconate, or nitrate were substituted for Cl-. In addition, a Cl(-)-gradient alone (under Na(+)-equilibrated conditions) could energize uphill transport as evidenced by accelerated uptake (3.13 +/- 0.8 pmol mg-1 protein 20 s-1) and an overshoot compared to Na+, Cl- equilibrated conditions (0.60 +/- 0.06 pmol mg-1 protein 20 s-1). A Cl(-)-gradient (Na(+)-equilibrated) also stimulated uptake of [3H]taurine against its concentration gradient. Analysis of uptake in the presence of varying concentrations of external Cl- suggested that 1 Cl- ion is involved in Na+/taurine cotransport. We conclude that Na(+)-dependent taurine uptake in the placental BBM has a selective anion requirement for optimum transport. This process is electrogenic and involves a stoichiometry of 2:1:1 for Na+/Cl-/taurine symport.     

Membrane localization of H+ and HCO3- transporters in the rat pancreatic duct

The pancreatic duct secretes alkaline fluid that is rich in HCO3- and poor in Cl-. The molecular mechanisms that mediate ductal secretion and are responsible for the axial gradients of Cl- and HCO3- along the ductal tree are not well understood because H+ and HCO3- transport by duct cells have not been characterized or localized. To address these questions, we microdissected the intralobular, main, and common segments of the rat pancreatic duct. H+ and HCO3- transporters were characterized and localized by following intracellular pH while perfusing the bath and the lumen of the ducts. In intralobular ducts, Na(+)-dependent and amiloride-sensitive recovery from acid load in the absence of HCO3- was used to localize a Na+/H+ exchanger to the basolateral membrane (BLM). Modification of Cl- gradients across the luminal (LM) and BLM in the presence of HCO3- showed the presence of Cl- /HCO3- exchangers on both membranes of intralobular duct cells. Measurement of the effect of Cl- on one side of the membrane on the rate and extent of pHi changes caused by removal and addition of Cl- to the opposite side suggested that both exchangers are present in the same cell. In the presence of HCO3-, intralobular duct cells used three separate mechanisms to extrude H+: (a) BLM-located Na+/H+ exchange, (b) Na(+)-independent vacuolar-type H+ pump, and (c) BLM-located, Na(+)- dependent, amiloride-insensitive, and 4',4'-diisothiocyanatostilbene- 2,2'-disulfonic acid sensitive mechanism, possibly a Na(+)-dependent HCO3- transporter. The main and common segments of the duct displayed similar mechanisms and localization of H+ and HCO3- transporters to the extent studied in the present work. In addition to the transporters found in intralobular ducts, the main and common ducts showed Na+/H+ exchange activity in the LM. Three tests were used to exclude a significant luminal to basolateral Na+ leak as the cause for an apparent luminal Na+/H+ exchange in an HCO3- secreting cells: (a) addition of amiloride and removal of Na+ from the LM had a profound effect on Na+/H+ exchange activity on the BLM and vice versa; (b) inhibition of all transporters in the BLM by bathing the duct in the inert hydrocarbon Fluorinert FC-75 did not prevent cytosolic acidification caused by removal of luminal Na+; and (c) luminal Na+ did not activate the basolateral Na(+)-dependent HCO3- transporter. An Na(+)-independent, bafilomycin-sensitive H+ pumping activity was marginal in the absence of HCO3-.(ABSTRACT TRUNCATED AT 400 WORDS)     

Forskolin activation of apical Cl- channel and Na+/K+/2Cl- cotransporter via a PTK-dependent pathway in renal epithelium

Forskolin induced the transepithelial Cl- transport (secretion) by activating the apical Cl- channel and basolateral Na+/K+/2Cl- cotransporter in renal epithelial A6 cells via an increase in cytosolic cAMP concentration. The cAMP activation of apical Cl- channel and Na+/K+/2Cl- cotransporter was partially mediated through a protein kinase A (PKA)-dependent pathway, but a PKA-independent pathway was also suggested to be involved in the cAMP activation. Therefore, we assessed a possibility of involvement of protein tyrosine kinase (PTK)-dependent pathway as a PKA-independent pathway in the cAMP activation by applying a PTK inhibitor, tyrphostin A23 (AG18). Tyrphostin A23 abolished the forskolin-induced transepithelial Cl- secretion by partially diminishing the activity of the Cl- channel and completely inhibiting the Na+/K+/2Cl- cotransporter. Further, forskolin increased phosphorylation of protein tyrosine, suggesting that cAMP activates PTK. These observations suggest that cAMP activates the Cl- channel and the Na+/K+/2Cl- cotransporter by activating PTK.     

Purinergic P2Y6 receptors induce Ca2+ and CFTR dependent Cl- secretion in mouse trachea.

In airways Cl- secretion is activated and Na+ absorption is inhibited when P2Y2 receptors are stimulated by ATP or UTP. Both nucleotides are subject to degradation to ADP and UDP by ecto-nucleotidases. Here we show that these metabolites change electrolyte transport by stimulation of P2Y6 receptors in mouse trachea. Immunohistochemistry confirmed luminal and basolateral expression of P2Y6 receptors. In Ussing chamber experiments luminal ADP, UDP or the P2Y6 receptor agonist INS48823 induced both transient and persistent increase in short circuit currents (ISC). Activation of ISC was inhibited by the P2Y6 receptor blocker PPADS. The transient response was inhibited by DIDS, whereas the persistent ISC was inhibited by glibenclamide and by the protein kinase A (PKA) blocker H-89. Moreover, sustained activation of ISC by luminal UDP was inhibited by blocking basolateral K+ channels with 293B. Possible effects of diphosphates on P2Y1 or adenosine receptors were excluded by the inhibitors MRS2179 and 8-SPT, respectively. Inhibition of amiloride sensitive Na+ absorption was only seen after blocking basolateral K+ channels with 293B. In contrast, Cl- secretion activated by basolateral ADP or UDP was only transient and was blocked by the sk4 K+ channel blocker clotrimazole. In summary, activation of luminal P2Y6 receptors in the airways shifts electrolyte transport towards secretion by increasing intracellular Ca+ and activation of PKA.     

Net fluxes of electrolytes in the rat intestine infected with Moniliformis dubius (Acanthocephala).

The effect of Moniliformis dubius on fluxes of Na+, K+, Cl-, and HCO3-in the rat intestine was determined using a conventional in vivo single-pass perfusion technique. Results for ion and water movements in the uninfected gut were in agreement with previous studies. In the parasitized intestine the jejunal pH was significantly lower than that in control animals, matching the restriction of the parasites to this region of the small intestine. While parasitism did not affect Na+ transport in the distal ileum, Na+ absorption was reduced (pH 7.0), or secretion enhanced (pH 6.0), in the two proximal regions. Cl-absorption was reduced in the distal ileum, but secretion was enhanced in the other two segments. Parasitism also enhanced K+ secretion in all segments. Net H2O absorption was reduced at pH 7.0; at pH 7.0; at pH 6.0 net secretion was also reduced. These changes clearly indicate that a parasite restricted to the jejunum may significantly affect the absorptive and secretory activity of the intestine distal to the site of infection. The results are discussed in the light of current concepts of electrolyte transport. The effect of the parasites on mucosal function distal to their site of attachment is discussed in terms of the release by the parasite of toxin-like substances, changes in the physical-chemical characteristics of the intestinal lumen, and interference with neurohormonal control of gastrointestinal function.     

Vanadium-induced Cl(-)-secretion in rabbit descending colon is mediated by prostaglandins.

Vanadium in the 4+ (vanadyl-ion) and 5+ (vanadate-ion) oxidation state stimulates furosemide-sensitive electrogenic Cl- secretion in isolated epithelia of rabbit descending colon. This effect is associated with an increased release of prostaglandin E2 from the tissue. Inhibitors of phospholipase A2 or cyclooxygenase abolish both vanadium-induced release of prostaglandin E2 and Cl- secretion. Neuronal mechanisms are not likely to be involved, as tetrodotoxin does not affect the vanadate induced Cl- secretion. Although vanadate is known to inhibit Na+,K(+)-ATPase activity, no inhibition of active Na+ transport was observed in intact colonic epithelia suggesting a rapid intracellular reduction of vanadate ions to vanadyl ions which have no inhibitory effect on the Na+,K(+)-ATPase. The present findings therefore indicate that vanadate stimulated colonic Cl- secretion involves intracellular conversion of vanadate to vanadyl and release of prostaglandin E2.     

Electrogenic ion transport in the intestine of the Australian common brushtail possum, <Emphasis Type="Italic">Trichosurus vulpecula</Emphasis>: indications of novel transport patterns in a marsupial

In this study, electrogenic ion transport in the intestine of the Australian common brushtail possum, Trichosurus vulpecula was investigated. In the ileum, a Na(+)-dependent, phloridzin- and amiloride-insensitive short-circuit current ( Isc) was present. Mucosal glucose stimulated a further phloridzin-sensitive, dose-dependent increase in Isc. A Na(+)-dependent, ouabain-sensitive Isc was also present in the caecum and colon. In the proximal and distal colon, amiloride (100 micro mol l(-1), mucosal) inhibited this Isc by 81+/-4% and 65+/-3%, respectively and the Ki for amiloride (approximately 1 micro mol l(-1)) was consistent with the inhibition of a classical epithelial Na(+) channel. In the caecum, 50% of the Isc was inhibited by amiloride (100 micro mol l(-1), mucosal). The amiloride-insensitive Isc in the colon was not due to electrogenic Cl(-) secretion, as serosal bumetanide (100 micro mol l(-1)) had no effect on the Isc. Furthermore, the secretagogues forskolin (10 micro mol l(-1)), carbachol (100 micro mol l(-1)) and dibutyryl-cAMP or dibutyryl-cGMP (100 micro mol l(-1)) did not stimulate electrogenic Cl(-) secretion by the colon. These results indicate that the transport properties of the hindgut of the possum differ significantly from those of eutherian mammals and may be associated with different functions of the hindgut of possums when compared to eutherian mammals.     

Role of Ca2(+)-calmodulin and protein kinase C in the secretory action of heat-labile enterotoxin of Escherichia coli in mice

The mucosal-to-serosal and serosal-to-mucosal fluxes of Na+ and Cl- were carried out in control and heat-labile enterotoxin treated mice in the presence or absence of Ca2(+)-ionophore A23187, the activator of Ca2(+)-calmodulin or Phorbol-12-myristate-13-acetate (PMA), the activator of Protein kinase C (PKC) or 1-(5-isoquinolinyl sulphonyl)-2-methyl piperazine (H-7), an inhibitor of PKC. There was net secretion of Na+ and Cl- in experimental group in comparison to net absorption in control group. The addition of ionophore or PMA resulted in net secretion of Na+ and Cl- in control group. In experimental group ionophore increased the net secretion of Na+ and Cl- while, PMA could not cause any change in Na+ and Cl- fluxes in experimental group. Calmodulin activity remained unaltered in heat-labile enterotoxin treated mice as compared to control. H-7, reversed the effects of PMA and heat-labile enterotoxin. These studies demonstrate that heat-labile enterotoxin primarily involves PKC in its action.