Interrelationships of chloride, bicarbonate, sodium, and hydrogen transport in the human ileum

Abstract

A B S T R A C T Using a triple-lumen constant perfusion system, the following observations were made in normal subjects. First, chloride, bicarbonate, and sodium were found to exhibit net movement across ileal mucosa against electrochemical gradients. Second, during perfusion with a balanced electrolyte solution simulating plasma, the ileum generally absorbed, but sometimes secreted fluid. A reciprocal net movement of chloride and bicarbonate was noted when sodium movement was zero. Increasing rates of sodium absorption were associated with decreasing bicarbonate secretion rates and finally bicarbonate absorption. Even when bicarbonate was absorbed ileal contents were alkalinized (by contraction of luminal volume). Third, net chloride movement was found to be sensitive to bicarbonate concentration in ileal fluid. For instance, chloride was absorbed from solutions containing 14 or 44 mEq/liter of bicarbonate, but was secreted when ileal fluid contained 87 mEq/liter of bicarbonate. Fourth, when chloridefree (sulfate) solutions were infused, the ileum absorbed sodium bicarbonate and the ileal contents were acidified. Fifth, when plasma-like solutions were infused, the potential difference (PD) between skin and ileal lumen was near zero and did not change when chloride was replaced by sulfate in the perfusion solution.These results suggest that ileal electrolyte transport occurs via a simultaneous double exchange, Cl/HCOs and Na/H. In this model neither the anion nor the cation exchange causes net ion movement; net movement results from the chemical reaction between hydrogen and bicarbonate. No other unitary model explains all of the following observations: (a) human ileal transport in vivo is essentially nonelectrogenic even though Na, Cl, and HCO3 are transported against electrochemical Dr. Turnberg's present address is Manchester Royal Infirmary, Oxford Road, Manchester 13, England.

Acid-Base Disturbances in Gastrointestinal Disease

“…Absorption is driven by sodium and chloride uptake (driving water uptake) and occurs via two linked transporters, the Na ϩ /H ϩ exchanger and the Cl Ϫ /HCO 3 Ϫ exchanger ( Figure 1) (7,8). These two transporters take up Na ϩ and Cl Ϫ from the gut lumen and secrete H ϩ and HCO 3 Ϫ into it.…”

Intestinal ion transport and the pathophysiology of diarrhea

“…The second key set of observations arose from studies, performed in the late 1960s on human subjects, of intestinal salt absorption in the absence of nutrients (3,4). Segments of jejunum and ileum were perfused with solutions of varied electrolyte composition, and the appearance or disappearance of ions and water was recorded.…”

“…Osmotic gap (OG) is defined as follows: OG = 290 -2{[Na + ] + [K + ]}, where 290 is the assumed osmolarity of blood plasma. A gap greater than 50 mM is considered abnormal (S51); the normal gap is made up of Mg 2+ , Ca 2+ , NH 4 + , and perhaps organic cations. The pattern of stool electrolytes in patients with acute cholera shows Na + , K + , and Clconcentrations not very different from those in plasma and an HCO 3 -concentration somewhat higher then in plasma.…”

The role of buffer anions and protons in secretion by the rabbit mandibular salivary gland.

“…One model ('electrogenic chloride secretion ': Frizzel et al 1979) involves a tightly coupled, electroneutral co-transport system. The other involves a double counter-transport system (Turnberg, Bieberdorf, Morawski & Fordtran, 1970) in which a Na+/H+ exchanger leads to HCO3-accumulation in the cell. This HCO3-then exchanges with extracellular Cl-(or another anion) across the same membrane, by means of a HCO3-/Cl-counter-transport mechanism, thus leading to active accumulation of Cl-in the cytosol (Liedtke & Hopfer, 1980;Heintze & Petersen, 1980).…”