Oxalobacter sp. reduces urinary oxalate excretion by promoting enteric oxalate secretion.

The primary goal of this study was to test the hypothesis that Oxalobacter colonization alters colonic oxalate transport thereby reducing urinary oxalate excretion. In addition, we examined the effects of intraluminal calcium on Oxalobacter colonization and tested the hypothesis that endogenously derived colonic oxalate could be degraded by lyophilized Oxalobacter enzymes targeted to this segment of the alimentary tract. Oxalate fluxes were measured across short-circuited, in vitro preparations of proximal and distal colon removed from Sprague-Dawley rats and placed in Ussing chambers. For these studies, rats were colonized with Oxalobacter either artificially or naturally, and urinary oxalate, creatinine and calcium excretions were determined. Colonized rats placed on various dietary treatment regimens were used to evaluate the impact of calcium on Oxalobacter colonization and whether exogenous or endogenous oxalate influenced colonization. Hyperoxaluric rats with some degree of renal insufficiency were also used to determine the effects of administering encapsulated Oxalobacter lysate on colonic oxalate transport and urinary oxalate excretion. We conclude that in addition to its intraluminal oxalate-degrading capacity, Oxalobacter interacts physiologically with colonic mucosa by inducing enteric oxalate secretion/excretion leading to reduced urinary excretion. Whether Oxalobacter, or products of Oxalobacter, can therapeutically reduce urinary oxalate excretion and influence stone disease warrants further investigation in long-term studies in various patient populations.

Muscarinic down-regulation of cAMP-stimulated potassium ion secretion by rabbit distal colon.

The sustained effects of the cholinergic agonist carbachol (CCh) on electrolyte transport across the isolated, short-circuited rabbit distal colon were examined in the absence and presence of secretagogue (di-butyryl cyclic-adenosine monophosphate, dB-cAMP). Steady-state, basal absorption of 22Na+, 42K+ (86Rb+), and 36Cl- were not significantly altered by addition of the CCh (10(-4) mmol/l) to the serosal reservoir. Stimulation with dB-cAMP (1.0 mmol/l, serosal) promoted K+ (or Rb+) and Cl- secretion across the colon, without significantly affecting the unidirectional or net fluxes of Na+. Serosal (but not mucosal) addition of CCh to dB-cAMP-stimulated tissues reduced the serosal to mucosal flux of Rb+ (J(Rb)SM) in a concentration-dependent manner with a half-maximum concentration approximately equal 5 micromol/l. Pretreatment with CCh (100 micromol/l, serosal) inhibited dB-cAMP-induced K+ secretion, but had no significant effect on the steady-state unidirectional fluxes of Na+ or Cl-. Serosal histamine (20 micromol/l) also inhibited J(Rb)SM in dB-cAMP-stimulated tissues. Serosal epinephrine (10 micromol/l) promoted a decrease in short-circuit current (Isc) and transepithelial potential (VT) that was mirrored by increases in J(Rb)SM. Both Isc, and VT became more positive and J(Rb)SM was reduced when CCh was added to the epinephrine-stimulated tissues. Serosal muscarine (50 micromol/l) mimicked the CCh-induced inhibition of J(Rb)SM, but serosal nicotine (50 micromol/l) had no effect. In atropine-treated tissues (1 micromol/l, serosal), CCh failed to block dB-cAMP-stimulated increases in J(Rb)SM. The inhibitory action of CCh was observed in tissues that had been pretreated with 50 micromol/l serosal hexamethonium (a ganglionic transmission blocker) or 2 micromol/l serosal tetrodotoxin (a voltage-gated Na+ channel blocker), indicating that the inhibitory action of this cholinergic agonist does not depend on remnant enteric neural pathways. Rubidium ion transport across confluent monolayers of T84 cells was similarly affected by dB-cAMP and CCh, supporting the notion that enteric neural pathways are not required. Serosal charybdotoxin (20 nmol/l) mitigated the inhibitory action of CCh on J(Rb)SM in dB-cAMP-stimulated tissues, suggesting a role for basolateral, Ca2+-dependent K+ channels in the actions of CCh. It is concluded that basolateral muscarinic receptors (and possibly other Ca2+-dependent receptor pathways) of secretory colonocytes mediate the down-regulation of potassium ion secretion by rabbit distal colon, possibly by increasing basolateral membrane K+ conductance.

Regulatory aspects of oxalate secretion in enteric oxalate elimination.

Enteric excretion of oxalate was studied in rats with chronic renal failure (CRF) by measuring the magnitude and direction of oxalate fluxes in vitro across short-circuited preparations of distal colon in Ussing chambers. The net absorptive flux of oxalate that is characteristic of colonic tissues removed from control rats was significantly changed to a net secretory flux in CRF rats. Injecting CRF rats with a specific angiotensin II (AngII) receptor (AT1) antagonist, losartan, results in a reversal of the CRF-induced net secretory flux (-13.87+/-0.08 pmol x cm(-2) x h(-1)) to an absorptive flux (+7.32+/-3.68 pmol x cm(-2) x h(-1)) by normalizing the unidirectional fluxes of oxalate. Similarly, oxalate fluxes were normalized across CRF colonic tissues by acute, in vitro application of losartan to the serosal bathing solution. It was also possible to simulate the CRF-induced secretory flux of oxalate (Jsm) in vitro across colonic tissues removed from control rats. Serosal application of AngII at 10(-6), 10(-5), and 10(-4) M resulted in significant increases in the s-->m flux of oxalate (increasing deltaJsm = 4.06+/-1.2, 8.41+/-0.94, and 13.8+/-3.8 pmol x cm(-2) x h(-1), respectively). Taken together, these results suggest that CRF-induced oxalate secretion is at least partly mediated by AngII, which is consistent with previous findings of a twofold upregulation of AT1 receptors in CRF colonic mucosa.

AT1 receptor up-regulation in intestine in chronic renal failure is segment specific.

The role of the AT1 receptor in stimulating colonic K+ secretion was investigated in rats with chronic renal failure (CRF) induced by 5/6 nephrectomy. Compared to control rats, CRF rats up-regulate mucosal AT1 receptors approximately two-fold in the proximal (PC) and distal (DC) colonic segments. In contrast, there was no alteration in AT1 receptor protein mass in jejunal or ileal mucosa. Using 86Rb+ as a tracer for transmural K+ fluxes, a significant stimulation of the basal K+ secretory flux across both PC and DC was observed in vitro and this alteration in K+ transport was temporally correlated with the increase in angiotensin II (ANG II) receptors. In both PC and DC the significant increases in the receptor protein were evident 48 h after partial nephrectomy and they were sustained through 6 weeks. These studies support the hypothesis that CRF-induced secretion of K+ is mediated by an up-regulation of AT1 receptors exclusively in the large intestinal segments.

Conductive pathways for chloride and oxalate in rabbit ileal brush-border membrane vesicles.

To evaluate the possibility that an apical membrane conductive pathway for oxalate is present in the rabbit distal ileum, we studied oxalate ([14C]oxalate) and chloride (36Cl) uptake into brush-border membrane vesicles enriched 15- to 18-fold in sucrase activity. Voltage-sensitive pathways for oxalate and chloride were identified by the stimulation of uptake provided by an inwardly directed potassium diffusion potential in the presence of valinomycin. Additionally, outwardly directed oxalate (or chloride) gradients stimulated [14C]oxalate (or 36Cl) uptake to a greater degree in the absence of valinomycin (when intracellular and extracellular potassium are equal) than in the presence of valinomycin. Voltage-dependent anion uptake was poorly saturable: apparent affinity constants were 141 +/- 17 and 126 +/- 8 mM for chloride and oxalate, respectively. Activation energies for the voltage-dependent uptake processes were low: 4.7 and 6.3 kcal/mol for chloride and oxalate, respectively. Sensitivity profiles of voltage-dependent chloride and oxalate uptake to anion transport inhibitors were similar. We conclude that an anion conductance is present in the apical membranes of ileal enterocytes and that this conductance is a candidate pathway for oxalate efflux from the enterocyte during transepithelial oxalate secretion.

Losartan antagonism of angiotensin-II-induced potassium secretion across rat colon.

The effect of angiotensin II (ANG II) on potassium transport across the short-circuited rat distal colon was investigated using 86Rb+ as a tracer for unidirectional K+ fluxes. The addition of high concentrations of ANG II (>/=10(-6) M) to the serosal bathing solution had no effect on the mucosal to serosal flux of Rb+, but significantly increased the serosal to mucosal flux and abolished the basal net absorptive Rb+ flux. These ANG-II-induced alterations in Rb+ transport were blocked by the AT1 receptor antagonist losartan and its metabolite EXP3174, which is also known to have AT1 receptor antagonistic activity. In contrast, an AT2 receptor antagonist, PD123319, did not prevent the alterations in colonic Rb+ transport induced by ANG II in vitro. At lower bath concentrations (10(-7) M to 10(-10) M ), ANG II had no measurable effects on Rb+ transport across this tissue but ANG II did have a bimodal effect on short-circuit current (Isc), indicating additional effects on the electrogenic transport of other ions. Dose-dependent reductions in Isc were observed between 10(-7) M (DeltaIsc=1.96+/-0.49 microEq.cm-2.h-1, n=6) and 10(-10) M (DeltaIsc=0.16+/-0.19 microEq.cm-2.h-1, n=7) ANG II, whereas Isc was increased at the higher concentrations (DeltaIsc= 3.36+/-0.52 microEq.cm-2.h-1, n=7, at 10(-4) M). The ANG-II-induced increases and decreases in Isc were both blocked by losartan but not by PD123319. These studies are the first to demonstrate an effect of ANG II on K+ transport across rat colon that is independent of aldosterone and mediated by AT1 receptors.

Local upregulation of colonic angiotensin II receptors enhances potassium excretion in chronic renal failure.

The role of angiotensin II (ANG II) in colonic secretion of K+ was examined in rats with chronic renal failure (CRF). The basal net secretory flux of 86Rb+ (as a tracer for K+) across the CRF distal colon (-0.20 +/- 0.04 mu eq.cm-2.h-1) was reversed to an absorptive flux (0.35 +/- 0.05 mu eq.cm-2.h-1) by injecting the rats with the AT1 receptor antagonist, losartan. A similar result was observed when losartan was added to the CRF colonic tissue in vitro. In contrast, an AT2 receptor antagonist, PD-123319, did not reverse the CRF-induced alterations in Rb+ transport across the short-circuited colonic tissue. Plasma concentrations of ANG II, aldosterone, and K+, as well as the ANG II content of colonic tissues from CRF and normal rats, were similar. However, specific 125I-labeled ANG II binding sites in rat distal colon increased twofold in CRF [maximal specific binding (Bmax) = 28.6 +/- 1.6 fmol/mg protein] compared with normal (Bmax = 15.2 +/- 0.4 fmol/mg protein). These studies suggest that CRF-induced secretion of K+ by the colon is mediated by an upregulation of AT1 receptors present in CRF.

cAMP-dependent sulfate secretion by the rabbit distal colon: a comparison with electrogenic chloride secretion.

The ability of a Cl-secreting epithelium to support net secretion of an anion other than a halide was investigated with 35SO4 flux measurements across the isolated, short-circuited rabbit distal colon. In most experiments, 36Cl fluxes were simultaneously measured to validate the secretory capacity of the tissues. Serosal addition of dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP, 0.5 mM) stimulated a sustained net secretion of SO4 (about -3.0 nmol.cm-2.h-1 from a 0.20 mM solution) via an increase in the serosal-to-mucosal unidirectional flux, whereas Ca ionophore A-23187 (1 microM, serosal) produced a more transient stimulation of SO4 and Cl secretion. Net adenosine 3',5'-cyclic monophosphate (cAMP)-dependent SO4 and Cl secretion were strongly voltage sensitive, principally through the potential dependence of the serosal-to-mucosal fluxes, indicating an electrogenic transport process. Symmetrical replacement of either Na, K, or Cl inhibited cAMP-dependent SO4 secretion, whereas HCO3-free buffers had no effect on SO4 secretion. Serosal bumetanide (50 microM) or furosemide (100 microM) reduced DBcAMP-stimulated SO4 and Cl secretion, whereas serosal 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid or 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (50 microM) blocked DBcAMP-induced SO4 secretion while enhancing net Cl secretion and short-circuit current. Mucosal 5-nitro-2-(3-phenylpropylamino)benzoic acid partially inhibited SO4 secretion and completely inhibited Cl secretion. It is concluded that secretagogue-stimulated SO4 secretion, like Cl secretion, may be an electrogenic process mediated by diffusive efflux through an apical anion conductance. Cellular accumulation of SO4 across the basolateral membrane appears to be achieved by a mechanism that is distinct from that employed by Cl.

Effects of the specific angiotensin II receptor antagonist losartan on urate homeostasis and intestinal urate transport.

Possible mechanisms for the hypouricemic effects of the angiotensin II receptor antagonist losartan were examined using rats with experimental chronic renal failure (CRF) and control animals. The results show that losartan has a uricosuric effect in rats with normal or decreased renal function. Renal clearance of urate was increased 3-fold in CRF rats and 2-fold in control rats after 7 days of intraperitoneal losartan administration. Although the results show that losartan and its metabolite EXP-3174 alter urate and Cl- transport across isolated short-circuited intestine, these agents do not promote urate secretion into the intestinal lumen. Unidirectional urate and Cl- fluxes were reduced across normal rat colon and unaltered in the small intestine in the presence of losartan. In CRF rat colon, net secretion of urate and Cl- was abolished after losartan addition at 10(-5) M. Transport across the small intestine of CRF rats did not change in the presence of a similar concentration of drug. Losartan treatment of CRF rats before the removal of colonic tissues reversed the basal net secretion of urate to net absorption. These results suggest that the changes in intestinal transport observed in the presence of losartan appear to be mediated via the angiotensin II receptor antagonistic action of this drug. Direct determination of the effects of angiotensin II on urate and Cl- transport across colonic tissues from control animals revealed a significant angiotensin II stimulation of urate secretion. These angiotensin II-induced alterations in transport were inhibitable by losartan.

Effect of chronic experimental renal insufficiency on urate metabolism.

The rise in plasma uric acid (UA) in chronic renal failure (CRF) is quite limited. This may be due to either increased extrarenal excretion, diminished biosynthesis, and/or enhanced degradation of uric acid. The intestinal flux studies revealed a striking modification of urate transport from no net flux to a net secretory flux in the jejunum and from a basal net absorptive to a net secretory flux in the colon of CRF animals. In addition, CRF animals showed a marked reduction in hepatic, renal, and enteric tissue xanthine oxidase activity and no significant change in tissue uricase activity. The correction of anemia with erythropoietin did not significantly alter the plasma concentration or urinary excretion of urate. Thus, enhanced enteric excretion and depressed production of uric acid (reduced xanthine oxidase activity) may account for the lack of significant hyperuricemia in CRF.

Alterations in intestinal transport of oxalate in disease states.

Normally, absorption of oxalate from dietary sources can occur in all segments of the intestinal tract. However, alterations in both the magnitude and direction of oxalate fluxes across the intestine can occur in disease states. In enteric hyperoxaluria, enhanced absorption of oxalate by the large intestine is caused by increased permeability of a shunt conductance induced by malabsorbed bile salts and fatty acids. In this condition, the contribution of a paracellular passive flux of oxalate moving along its electrochemical gradient will predominate when intraluminal concentrations of free oxalate are high. In contrast, in chronic renal failure, secretion of oxalate can occur across both small and large intestine thereby facilitating extrarenal elimination with subsequent degradation by mucosal substrate-specific microorganisms. Clearly, in recent studies of oxalate transport, the intestine has emerged with an integral role in mass balance of oxalate in health and disease.

Intestinal excretion of oxalate in chronic renal failure.

The extrarenal elimination of oxalate via the intestine was studied in rats with chronic renal failure by measuring the magnitude and direction of oxalate fluxes across the small and large intestine. Oxalate transport was determined in vitro across short-circuited sheets of jejunum, ileum, and colon that were placed in Ussing chambers. The concentration of oxalate in plasma and urine was measured immediately before the transport studies. The results show that, 6 wk after 5/6 nephrectomy, rats with chronic renal failure have lower (decreases 50%) renal clearance of oxalate and a higher mean plasma oxalate concentration (increases 80%) than controls. The basal absorption of oxalate across the colon was changed to secretion in animals with chronic renal failure (from 12.81 +/- 2.22 (N = 9) to -14.96 +/- 2.57 (N = 11) pmol/cm2 per hour). Both the jejunum and the ileum supported a basal net secretory flux of oxalate (-19.71 +/- 2.39 (N = 13) and -30.06 +/- 2.80 (N = 16) pmol/cm2 per hour) that was unaffected by renal insufficiency. These studies demonstrate that intestinal transport systems for oxalate are altered in experimental chronic renal failure, and the distal colon is identified as the primary site for this adaptive response. In chronic renal failure, the entire intestinal tract can potentially excrete oxalate.

Mechanisms of oxalate absorption and secretion across the rabbit distal colon.

To further evaluate the mechanisms of oxalate (Ox2-) transport in the intestine the following studies were performed using isolated, short-circuited segments of the rabbit distal colon (DC). In control buffer, the DC absorbed Ox2- (net Ox2- flux, JNetOx = 5.4 +/- 0.7 pmol.cm-1.h-1). Replacement of Na+ with N-methyl-D-glucamine (NMDG+) abolished Ox2- absorption by decreasing mucosal to serosal Ox2- flux (JmsOx), without affecting Cl- transport, while gluconate substitution for Cl- did not affect JNetOx or net Na+ flux (JNetNa). Addition of Na+ to the serosal side of tissues bathed by NMDG+ buffer increased JmsOx 40% without altering mucosal to serosal Cl- flux (JmsCl). Serosal amiloride or dimethyl amiloride (10(-3) M) abolished JNetOx by decreasing JmsOx, it increased serosal to muscosal Cl- flux (JsmCl) and it gradually inhibited short-circuit current (Isc). Mucosal amiloride (10(-4) M) abolished Ise but had no effect on Ox2- or Cl- fluxes. Serosal 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS, 10(-6) M) reduced JmsOx by 20% and JNetOx by 43% without affecting JmsCl or JNetCl. Dibutyryl cyclic adenosine monophosphate (dB-cAMP, 5 x 10(-4) M, both sides) stimulated Ox2- secretion (JNetOx = -12.6 +/- 3.3 pmol.cm-2.h-1). The dB-cAMP-induced secretion of Ox2- and Cl- were fully abolished by serosal furosemide (10(-4) M) and partially inhibited (35%) by 5 x 10(-4) M mucosal NPPB [5-nitro-2-(3-phenylpropylamino)-benzoic acid], a putative Cl- channel blocker.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of the transport of oxalate and other ions across rabbit proximal colon.

In order to characterize oxalate handling by the P2 segment of the rabbit proximal colon, the fluxes of [14C]oxalate, 22Na+, and 36Cl- were measured in vitro using conventional short-circuiting techniques. In standard buffer the proximal colon exhibited net secretion of Na+ (-2.31 +/- 0.64 mu equiv cm-2 h-1), negligible net Cl- transport, and net secretion of oxalate (-12.7 +/- 1.6 pmol cm-2 h-1). Replacement of buffer Na+ or Cl- abolished net oxalate secretion, while HCO(3-)-free media revealed a net absorption of oxalate (19.3 +/- 4.2 pmol cm-2 h-1) and stimulated NaCl absorption. Mucosal amiloride and dimethylamiloride (1 mM) significantly reduced the unidirectional fluxes of oxalate and enhanced sodium secretion by decreasing JNams. The anion exchange inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS; 0.1 mM, both sides) reduced the unidirectional fluxes of oxalate and chloride. Serosal epinephrine (50 microM) stimulated oxalate absorption (21.3 +/- 6.3 pmol cm-2 h-1) and sodium absorption (5.71 +/- 1.20 mu equiv cm-2 h-1), whereas dibutyryl-cAMP enhanced oxalate secretion (-43.4 +/- 6.9 pmol cm-2 h-1) and stimulated chloride secretion (-7.27 +/- 0.64 mu equiv cm-2 h-1). These results indicate that the P2 segment of the proximal colon possesses (a) secretory as well as absorptive capacities, (b) oxalate fluxes that are mediated by pathways involving Na+, Cl-, HCO3- transport and (c) a net oxalate flux that is sensitive to absorptive and secretory stimuli.

Electrolyte transport across the rabbit caecum in vitro.

Electrolyte transport across rabbit caecal epithelium was investigated in vitro using conventional short-circuiting and radioisotope techniques. In standard saline the caecum exhibited a relatively high short-circuit current (Isc = 4.4 microEq.cm-2.h-1) and conductance (6.43 mS.cm-2). Both sodium and chloride were absorbed (JNa(net) = 6.40 and JCl(net) = 3.40 microEq.cm-2.h-1) and potassium was secreted (JK(net) = -0.5 microEq.cm-2.h-1). Removal of Na+ abolished Isc and JCl(net) whereas removal of Cl- reduced JNa(net) to 2.92 microEq.cm-2.h-1 but did not alter Isc. In HCO3(-)-free salines containing 10(-4) M acetazolamide JCl(net) was abolished and JNa(net) and Isc were reduced to 2.3 and 2.5 microEq.cm-2.h-1 respectively. A positive residual ion flux (approximately 1 microEq.cm-2.h-1) was detected in standard and Cl(-)-free salines but not in Na+-free or HCO3- buffers. Mucosal amiloride (10(-3) M) decreased net Na+ and Cl- absorption but did not decrease Isc. Mucosal DIDS (10(-4) M) decreased JCl(net) while mucosal bumetanide (10(-4) M) did not affect any of the measured parameters. Finally, addition of theophylline (8 mM) stimulated Cl- secretion and increased Isc. It is concluded that net sodium absorption by caecal epithelia occurs by both electrogenic and electroneutral mechanisms whereas net chloride absorption occurs only by an electroneutral process. Coupling of the absorptive fluxes of Na+ and Cl- may result from Na+/H+ and Cl-/HCO3- antiport systems in this tissue. Finally, it is proposed that up to half of the Isc is due to a Na+-dependent secretion of bicarbonate ion.

Dihydroxy bile salt-induced secretion of rubidium ion across the rabbit distal colon.

Possible mechanisms of dihydroxy bile salt-induced K+ secretion by the mammalian colon were evaluated by studying the effects of taurochenodeoxycholate (TCDC) on 86Rb+ transport across the isolated, short-circuited rabbit distal colon. Simultaneous measurements of 86Rb+ and 42K+ unidirectional fluxes were highly correlated [r = 0.964 for serosal (s) to mucosal (m) and 0.765 for m to s], indicating that Rb+ is a suitable tracer for K+ transport across the colon. Furthermore, mucosal Ba2+ (4 mM) or serosal ouabain (0.1 mM) decreased serosal to mucosal rubidium flux (JRbs----m) (from 0.24 +/- 0.02 to 0.09 +/- 0.02, and 0.08 +/- 0.01 mu eq X h-1 X cm-2, respectively) without affecting JRbm----s. Dibutyryl cyclic adenosine monophosphate (dBcAMP, 0.5 mM serosal) specifically increased JRbs----m of controls (from 0.21 +/- 0.05 to 0.67 +/- 0.09 mu eq X h-1 X cm-2) through a barium- (4 mM, mucosal) sensitive pathway without affecting JRbs----m. Mucosal addition of 2 mM TCDC increased tissue conductance (GT), reduced short-circuit (Isc) slightly, and reversed JRbnet (from 0.13 +/- 0.05 to -0.29 +/- 0.08 mu eq X h-1 X cm-2) principally by increasing JRbs----m. The TCDC-induced increases in JRbs----m were reduced by 0.1 mM serosal ouabain (from 0.53 +/- 0.03 to 0.11 +/- 0.02 mu eq X h-1 X cm-2) or 4 mM mucosal Ba2+ (from 0.76 +/- 0.07 to 0.32 +/- 0.04 mu eq X h-1 X cm-2).(ABSTRACT TRUNCATED AT 250 WORDS)

Oxalate and chloride absorption by the rabbit colon: sensitivity to metabolic and anion transport inhibitors.

The effects of transport inhibitors on the movements of oxalate and chloride across the isolated short circuited rabbit colon were studied. Net oxalate absorption was shown in this species and was shown to be an energy dependent process as indicated by its sensitivity to 2-4 dinitrophenol (DNP) 10(-4)M. Mucosal addition of 4-acetamido-4-isothiocyano-2,-stilbene-2,2'-disulfonic acid (SITS) 10(-4)M abolished the net flux of both oxalate and chloride. Acetazolamide (8 mM) in bicarbonate free buffer significantly reduced the mucosal to serosal flux of both anions. These results suggest that in rabbit colon, oxalate and chloride share a common transport pathway and implicate the chloride bicarbonate exchange system. This study also confirms that chloride absorption by the short circuited rabbit colon is an electrically silent process and presents evidence that suggests that chloride absorption is mediated by a chloride bicarbonate exchange system located in the apical membrane of absorbing colonic epithelial cells.

Role of tight-junctional pathways in bile salt-induced increases in colonic permeability.

The effects of a dihydroxy bile salt, taurochenodeoxycholate (TCDC), on the permeability and conductance of isolated, short-circuited segments of the rabbit descending colon were examined using conventional Ussing chamber techniques. Increasing concentrations of TCDC (1-4 mM) produced dose-dependent increases in sodium backflux (JNas leads to m) and tissue conductance (Gt) when applied to either the mucosal or serosal salines. However, mucosal addition was twice as potent in increasing JNas leads to m and Gt at 4 mM. Tracer experiments indicated that the transepithelial serosal-to-mucosal fluxes of sodium and mannitol are via an aqueous, unrestricted, free-solution pathway, while albumin movements are restricted through this pathway both in the absence and presence of mucosal TCDC. The changes in JNas leads to m, JMans leads to m, and Gt caused by 4 mM mucosal TCDC were largely reversed by rinsing the mucosal chamber with fresh buffer. It was also observed that osmotically induced volume flows in the serosal-to-mucosal direction could offset or reverse the changes in Gt produced by 2 mM mucosal TCDC, suggesting that the enhanced conductance pathway is in series with the lateral intercellular spaces. Taken together, these results suggest that low concentrations of TCDC alter the integrity of tight-junctional complexes between the epithelial cells of the rabbit colon.

Dihydroxy bile salt-induced alterations in NaCl transport across the rabbit colon.

The effects of increasing mucosal or serosal concentrations (1-4 mM) of taurochenodeoxycholate (TCDC) on sodium chloride transport across the isolated, short-circuited rabbit colon were examined. Mucosal TCDC produced dose-related increases in tissue conductance (Gt) and the unidirectional fluxes of Na+ and Cl- and dose-related decreases in net NaCl absorption. At 4 mM mucosal TCDC, Gt was increased fivefold, net sodium flux (JNanet) was reduced 50%, and JClnet was abolished. Serosal TCDC also produced dose-dependent changes in permeability that were quantitatively different. Four millimolar serosal TCDC produced a 2.7-fold increase in Gt, abolished JNanet, and stimulated electrogenic Cl- secretion. TCDC-induced Cl- secretion was stimulated by 10(-5) M serosal TCDC, inhibited by serosal furosemide or ouabain, did not alter theophylline-induced secretion (and vice versa), and occurred in the absence of serosal Ca2+. It is suggested that 1) TCDC inhibition of sodium absorption is indirect (i.e., not simply due to a reduction in the activity of the Na "pump," since Cl- secretion persists during conditions that abolish JNanet) and 2) TCDC induces Cl- secretion by enhancing the activity of basolateral membrane adenylate cyclase.

Specificity of response of intestinal ion transport systems to a pair of natural peptide hormone analogs: somatostatin and urotensin II.

Specificity of two intestinal ion transport systems toward the natural peptide hormone analogs somatostatin and urotensin II has been demonstrated by electrophysiological and radiotracer studies in vitro. Somatostatin inhibits active C1 secretion across the theophylline-treated rat colon but urotensin II, a dodecapeptide somatostatin analog from the teleost caudal neurosecretory system, is without effect. Conversely, urotensin II stimulates active Na and C1 absorption across the posterior intestine of the 5% seawater-adapted goby, Gillichthys mirabilis, but somatostatin is ineffective. From these and others' studies, it appears that in both systems, increased net absorption results from increased mucosal-to-serosal unidirectional ion fluxes. Based on structure-activity relationships in these and other systems, it is speculated that the difference in amino acid residues at position 4 (somatostatin-Lys, urotensin II-Ala) may contribute to the observed specificity.