HCO3- secretion by rat distal colon: effects of inhibitors and extracellular Na+.
OBJECTIVE
The large intestine secretes HCO3- via a Cl-/HCO3- exchange mechanism located in the apical membrane of colonocytes. However, an additional transport system(s) must facilitate HCO3- (OH-) entry or H+ exit across the basolateral cell surface. The aim of this study was to determine that mechanism(s).


METHODS
A modified Ussing apparatus was used to measure net HCO3- secretion in segments of rat distal colon.


RESULTS
When added to the serosal solution, 10 mmol/L 4-acetamido-4'-isothiocyano-2,2'-disulfonic acid stilbene (SITS), 1 mmol/L SITS and 0.1 mmol/L diisothiocyanostilbene-2,2'-disulfonic acid, inhibited HCO3- secretion by 88%, 51%, and 30%, respectively. However, the Na+/H+ exchange inhibitors, amiloride (1 mmol/L), dimethylamiloride (0.1 mmol/L), ethylisopropylamiloride (0.1 mmol/L), failed to affect HCO3- secretion. Acetazolamide (1 mmol/L) blocked HCO3- secretion by approximately 60% when in the serosal solution but had little effect when in the mucosal solution. Ion substitution studies showed that HCO3- secretion required Na+ in the serosal solution (K0.5 approximately 12 mmol/L). HCO3- secretion was unaffected by depolarizing the basolateral membrane potential with K(+)-rich medium.


CONCLUSIONS
These data are consistent with Na+ linked HCO3- transport across the colonocyte basolateral membrane, which appears to be electroneutral.
Department of Medicine, McGuire Veterans Affairs Medical Center, Richmond, Virginia.
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Feldman
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology*; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid / Q000494:pharmacology*; D000086:Acetazolamide / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D003106:Colon / Q000187:drug effects / Q000378:metabolism*; D003864:Depression, Chemical; D005110:Extracellular Space / Q000378:metabolism; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D008297:Male; D011188:Potassium / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000502:physiology*
D001639:Bicarbonates; D012856:4-Acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic Acid; D000584:Amiloride; D012964:Sodium; D000086:Acetazolamide; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; D011188:Potassium