The mechanisms of sodium absorption in the human small intestine

Abstract

A B S T R A C T The present studies were designed to characterize sodium transport in the jejunum and ileum of humans with respect to the effects of water flow, sodium concentration, addition of glucose and galactose, and variations in anionic composition of luminal fluid. In the ileum, sodium absorption occurred againstvery steep electrochemical gradients (110 mEq/liter, 5-15 mv), was unaffected by the rate or direction of water flow, and was not stimulated by addition of glucose, galactose, or bicarbonate. These findings led to the conclusion that there is an efficiently active sodium transport across a membrane that is relatively impermeable to sodium. In contrast, jejunal sodium (chloride) absorption can take place against only the modest concentration gradient of 13 mEq/ liter, was dramatically influenced by water movement, and was stimulated by addition of glucose, galactose, and bicarbonate. The stimulatory effect of glucose and galactose was evident even when net water movement was inhibited to zero by mannitol. These observations led to the conclusion that a small fraction of jejunal sodium absorption was mediated by active transport coupled either to active absorption of bicarbonate or active secretion of hydrogen ions. The major part of sodium absorption, i.e. sodium chloride absorption, appeared to be mediated by a process of bulk flow of solution along osmotic pressure gradients. The stimulatory effect of glucose and galactose, even at zero water flow, was explained by a model in

Colonic Secretion of Water and Electrolytes Induced by Bile Acids: Perfusion Studies in Man

“…Net absorption of water (ml/min) and electrolytes (,gEq/min) by the whole colon was calculated, relative to changes in the concentration of PEG, using standard formulae (17). Unidirectional movements of sodium and potassium were calculated by disappearance of isotope and changes in specific activity (18).…”

Intestinal fluid and electrolyte transport in human cholera

“…The marker perfusion technique (11)(12)(13)(14), recently utilized for studying absorption from the normal human intestine, has been used in this investigation to define the site, nature, magnitude, and duration of fluid and electrolyte loss in the small intestine during different phases of human cholera. This information is required for a better understanding of the pathogenesis of cholera diarrhea and to validate the animal models of this disease.…”

“…The initial acute jejunal or ileal studies were arbitrarily defined as those performed within 48 hr of admission and additional repeat acute studies were obtained later during the phase of diarrhea (day 2-4) either by withdrawing the tube from the ileum manually by applying gentle traction for 20-40 min to relocate the perfusion system in the jejunum or, alternatively, by allowing the tube to advance with balloon inflated from jejunum to ileum .After deflating the balloon, a 60 min equilibration period was allowed in all studies before sampling intestinal fluid. (b) Early convalescent studies (day [4][5][6][7][8][9][10][11][12][13][14], 15 jejunal and four ileal studies, were performed on 16 patients. In all but four cases who were perfused during day 4-6 diarrhea had ceased at the time the early convalescent study was performed.…”

Abnormal passive chloride absorption in cystic fibrosis jejunum functionally opposes the classic chloride secretory defect

“…After steady-state conditions were obtained, samples of jejunal perfusate were collected at a constant rate (1.5 ml/min) for 1 hour from apertures in the tube located 10 and 40 cm distal to the infusion aperture. Using previously reported analytical methods and standard nonabsorbable marker equations, solute and water absorption rates by the 30-cm segment of proximal jejunum (located between the two sampling sites) were measured (16,17).…”

“…The absence of bicarbonate from these solutions removes a luminal buffer for secreted hydrogen ions, inhibiting Na + /H + exchange (16,22). Furthermore, normal human jejunum does not actively absorb chloride (17,(22)(23)(24). Under these conditions, therefore, net chloride transport represents the balance between gradient-generated passive chloride absorption and CFTR-mediated active chloride secretion.…”