Robert W. Freel

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−2 · 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 JNetOxor net Na+ flux (JNetNa). Addition of Na+ to the serosal side of tissues bathed by NMDG+ buffer increased JmsOx40% without altering mucosal to serosal Cl− flux (JmsCl). Serosal amiloride or dimethyl amiloride (10−3 M) abolished JNetOxby decreasing JmsOx, it increased serosal to muscosal Cl− flux (JsmCl) and it gradually inhibited short-circuit current (Isc). Mucosal amiloride (10−4 M) abolished Isc but had no effect on Ox2− or Cl− fluxes. Serosal 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS, 10−6 M) reduced JmsOxby 20% and JNetOxby 43% without affecting JmsClor JNetCl. Dibutyryl cyclic adenosine monophosphate (dB-cAMP, 5×10−4 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 5×10−4 M mucosal NPPB [5-nitro-2-(3-phenylpropylamino)-benzoic acid], a putative Cl− channel blocker. It is proposed that: (1) basal absorption of Ox2−, but not Cl−, is dependent upon a previously undescribed basolateral Na+-H+ exchanger that may be coupled to a DIDS-sensitive, basolateral anion exchange system that mediates Ox2− flux; (2) the DC secretes Ox2− in response to dB-cAMP by a mechanism that is indistinguishable from the pathway for Cl− secretion.

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 μequiv 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 HCO3−-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 JmsNa. 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 μM) stimulated oxalate absorption (21.3±6.3 pmol cm−2 h−1) and sodium absorption (5.71±1.20 μequiv 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 μequiv 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.

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

Abstractu2002The 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 (↓ΔIsc=1.96±0.49 µEq·cm–2·h–1, n=6) and 10–10 M (↓ΔIsc=0.16±0.19 µEq·cm–2·h–1, n=7) ANG II, whereas Isc was increased at the higher concentrations (↑ΔIsc= 3.36±0.52 µEq·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.

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

Abstractu2002The 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.

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

AbstractThe 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 (10–4xa0mmol/l) to the serosal reservoir. Stimulation with dB-cAMP (1.0xa0mmol/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+ (

Regulatory aspects of oxalate secretion in enteric oxalate elimination.

J_{SM}^{Rb}

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

n) in a concentration-dependent manner with a half-maximum concentration ≅5xa0µmol/l. Pretreatment with CCh (100xa0µmol/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 (20xa0µmol/l) also inhibited