Effects of acid-base variables and the role of carbonic anhydrase on oxalate secretion by the mouse intestine in vitro

Abstract

Hyperoxaluria is a major risk factor for calcium oxalate kidney stones and the intestine is recognized as an important extra-renal pathway for eliminating oxalate. The membrane-bound chloride/bicarbonate (Cl−/) exchangers are involved in the transcellular movement of oxalate, but little is understood about how they might be regulated. , CO2, and pH are established modulators of intestinal NaCl cotransport, involving Na+/H+ and Cl−/ exchange, but their influence on oxalate transport is unknown. Measuring 14C-oxalate and 36Cl fluxes across isolated, short-circuited segments of the mouse distal ileum and distal colon we examined the role of these acid-base variables and carbonic anhydrase (CA) in oxalate and Cl− transport. In standard buffer both segments performed net oxalate secretion (and Cl− absorption), but only the colon, and the secretory pathway were responsive to and CO2. Ethoxzolamide abolished net oxalate secretion by the distal colon, and when used in tandem with an impermeant CA inhibitor, signaled an intracellular CA isozyme was required for secretion. There was a clear dependence on as their removal eliminated secretion, while at 42 mmol/L was also decreased and eradicated. Independent of pH, raising Pco2 from 28 to 64 mmHg acutely stimulated net oxalate secretion 41%. In summary, oxalate secretion by the distal colon was dependent on , CA and specifically modulated by CO2, whereas the ileum was remarkably unresponsive. These findings highlight the distinct segmental heterogeneity along the intestine, providing new insights into the oxalate transport mechanism and how it might be regulated.

The role of intestinal oxalate transport in hyperoxaluria and the formation of kidney stones in animals and man

“…Nevertheless, the hypothesis that ‘the basolateral step for oxalate secretion is mediated via exchange with SO 4 2− and HCO 3 − on SAT1′ [44] is intriguing. In the presence of SO 4 2− , we found oxalate transport by the distal ileum to be fully independent of extracellular HCO 3 − and carbonic anhydrase (CA) [45]. For the mouse, this appears to rule out a SO 4 2− /HCO 3 − (oxalate) exchange mechanism, at least in this segment of the small intestine.…”

“…In the mouse distal colon, $J_{\mathrm{sm}}^{\mathrm{Ox}}$ is dependent on extracellular HCO 3 − and requires CA activity [45]. This might be suggestive of HCO 3 − /oxalate exchange at either the apical or basolateral membrane, although Whittamore et al [45] did not probe this possibility any further with anion exchange inhibitors such as DIDS or SITS. It is uncertain how CA is involved in oxalate secretion, but the mechanism does not appear to be related to a non-catalytic role for this enzyme [45].…”

“…This might be suggestive of HCO 3 − /oxalate exchange at either the apical or basolateral membrane, although Whittamore et al [45] did not probe this possibility any further with anion exchange inhibitors such as DIDS or SITS. It is uncertain how CA is involved in oxalate secretion, but the mechanism does not appear to be related to a non-catalytic role for this enzyme [45]. We considered whether the extracellular, membrane-bound isoform CA-IV might be supplying HCO 3 − for exchange with intracellular oxalate at the apical membrane.…”

“…We considered whether the extracellular, membrane-bound isoform CA-IV might be supplying HCO 3 − for exchange with intracellular oxalate at the apical membrane. However, $J_{\mathrm{sm}}^{\mathrm{Ox}}$ was unaffected when CA-IV was targeted with a specific inhibitor, possibly implicating one of the cytosolic isoforms such as CAI or CAII [45]. …”

“…Raising the pH from 7.5 to 8.2 enhanced oxalate/OH − exchange by mouse DTDST >2.5-fold [70], although recent work found $J_{\mathrm{sm}}^{\mathrm{Ox}}$ by the mouse distal ileum and distal colon to be unresponsive between pH 6.9 and 7.9 [45]. Furthermore, oxalate transport by mouse, but not human DTDST [69], also displayed characteristics of inverse regulation by extracellular Cl − [70].…”