Intestinal electrolyte absorption by parallel determination of unidirectional sodium and water transfers

A reconsideration of the evidence for Escherichia coli STa (heat stable) enterotoxin-driven fluid secretion: a new view of STa action and a new paradigm for fluid absorption

A review of the evidence for Escherichia coli STa causing fluid secretion in vito leads to the conclusion that the concept of STa acting through enhanced chloride secretion in order to derange intestinal function is unproven. However, a consistent effect of STa in the small intestine is on Na+/H+ exchange, leading to interruption of luminal acidification. A model for the action of STa, involving inhibition of Na+/H+ exchange, is proposed which explains the ability of STa to reduce absorption in vito but its inability to cause secretion in vito in contrast to its apparent secretory effect in vitro. The apparent ability to demonstrate secretion in vitro is shown to derive from methodologies which do not involve measurement of mass transport of water but instead, infer it from in vitro and in vivo proxy measurements. The in vitro demonstration of notional secretion after STa exposure can be reconciled with the proposed new model for fluid absorption in that cell swelling is argued to arise as a transient consequence of STa challenge followed by regulatory volume decrease. Evidence for this derangement model is presented in the form of observations derived from acute in vivo physiological studies and clinical studies on patients without the exchanger. This process of appraisal of the evidence for the mechanism of action of STa has led to a new model for fluid absorption. This is based on the formation of hypotonicity at the brush border luminal surface rather than hypertonicity within the lateral spaces as required by the present standing gradient model of fluid absorption. Evidence from the literature is presented for this new paradigm of water absorption, which may only be relevant for small intestine and other tissues that have Na+/H+ exchangers in contact with HCO-3-containing solutions but which may also be generalizable to all mammalian absorbing epithelial membranes.

Renojejunal reflex controlling jejunal absorption of fluid and NaCl

The aim of this study was to elucidate renojejunal reflex controlling the net jejunal fluid, Na+, and Cl- absorption in response to an increased ureteropelvic pressure (UPP) in anesthetized dogs. Unilateral UPP was increased under hydrostatic pressure from 0 to 80 mmHg by a step of 20 mmHg in random order, while the contralateral UPP was maintained at around 0 mmHg. The net absorption of fluid, Na+, and Cl- decreased in proportion to increased UPP. At a UPP of 0 mmHg, the net jejunal fluid, Na+, and Cl- absorption were 11.1 +/- 0.4 ml, 2.48 +/- 0.07, and 1.98 +/- 0.05 mEq, and significantly decreased to 7.38 +/- 0.6 ml, 2.11 +/- 0.15, and 1.68 +/- 0.13 mEq by an increase of UPP to 40 mmHg, respectively. An increase of bilateral UPP up to 40 mmHg or more occlusively depressed the absorption. These responses were blocked by renal denervation and by administration of atropine but not by yohimbine. These results indicate that the increase of UPP decreases jejunal fluid, Na+, and Cl- absorption (renojejunal reflex), and that the afferent and efferent limbs of this reflex are the renal nerves and cholinergic fibers. This renojejunal reflex might play an important role in body fluid homeostasis under the pathological condition.

The mechanism of salt and water absorption in the intestine of the eel (Anguilla anguilla) adapted to waters of various salinities

1. The absorption of NaCl and water was studied by intraluminal in vivo perfusion of the intestine of the yellow European eel (Anguilla anguilla) adapted to fresh water (FW), to sea water (SW), and to double strength SW (DSW).2. The net lumen to plasma NaCl transport from diluted SW perfusion fluids was independent of the NaCl concentration in the Na(+) concentration range tested. The NaCl absorption (expressed as mu-equiv/100 g.hr.) increased from FW (mean +/- S.E.): Na(+) 166 +/- 17, Cl(-) 205 +/- 24 to SW: Na(+) 363 +/- 33, Cl(-) 423 +/- 37, and again in DSW: Na(+) 640 +/- 110, Cl(-) 676 +/- 149.3. The osmolality of the perfusion fluid which resulted in zero net water transport was higher than plasma osmolality by 73 +/- 3 m-osmole in FW, 126 +/- 5 m-osmole in SW, and 244 +/- 32 m-osmole in DSW (mean +/- S.E.). A fairly constant ratio between net NaCl transport and this osmolality difference prevailed.4. The general osmotic permeability to water in the serosa-mucosa direction (expressed as mul./100 g. hr. m-osmole) measured from experiments with impermeant solute increased from FW: 3.7 +/- 0.5 to SW: 7.2 +/- 1.0 (mean +/- S.E.).5. These results are compatible with the interpretation that the water flow occurring in the absence of a general transmural osmotic gradient, the ;solute-linked water flow', is linearly related both to net NaCl transport and to the osmotic permeability to water. The findings support the view that the ;solute-linked water flow' is, indeed, secondary to the salt movement and is due to osmotic force.6. The amount of water absorbed from dilute SW perfusion fluids isosmotic with plasma was larger than in most other intestinal epithelia. FW: 650, SW: 1620 mul./100 g. hr. The NaCl concentration of the absorbate was hypertonic to plasma.7. The passive permeability of the intestine to NaCl was very low, and the reflexion coefficient was close to unity. Therefore metabolic energy will be used to absorb NaCl, even when the NaCl concentration in the gut is higher than that of plasma due to ingestion of SW. There appears to be a limited interaction in the intestinal wall between passive salt and water flow.8. In DSW the Na(+) ingestion with the oral intake of the surrounding fluid matched the gut absorption capacity. Since DSW is close to the tolerance limit, it is concluded that the gut NaCl transport capacity may be one of the factors limiting the tolerance to water or higher salinity.

Effects of sodium concentration and osmolality on water and electrolyte absorption form the intact human colon

The influence of sodium concentration and osmolality on net water and monovalent electrolyte absorption from or secretion into the intact human colon was studied in healthy volunteers. WHEN ISOTONIC SOLUTIONS CONTAINING NACL AND/OR MANNITOL WERE INFUSED INTO THE COLON: (a) a direct linear relationship between luminal sodium concentration (in the range of 23-150 mEq/liter) and rate of net water, sodium, and chloride absorption was found. No water absorption was found when sodium concentration in the luminal fluid was below 20 mEq/liter; (b) water and sodium absorption from the isotonic test solutions was not enhanced by addition of 80-250 mg/100 ml of glucose; and (c) the rate of water and sodium absorption was decreased markedly when chloride was replaced by bicarbonate in the test solution. WHEN THE COLON WAS PERFUSED WITH HYPERTONIC TEST SOLUTIONS CONTAINING NACL AND MANNITOL OR UREA: (a) water was absorbed from hypertonic NaCl solutions against a lumen-to-blood osmotic gradient of 50 mOsm/kg; (b) when the osmolality of the mannitol solution was increased, water entered the colonic lumen at a more rapid rate. The relationship between the rate of water entering the colon and the osmolality of the test solution was a parabolic one; (c) sodium and chloride entered the colonic lumen at a rate that was lineraly related to that of water entrance when the lumen-to-blood osmotic gradient exceeded 150 mOsm/kg; (d) water flow into the colonic lumen was identical when equimolar urea or mannitol solutions were infused; (e) neither urea nor mannitol was absorbed in significant amounts from the hypertonic solutions; and (f) our results suggest that the equivalent pore radius of the human colon is smaller than the molecular radius of urea (2.3 A).

Water and sodium absorption in the human intestine

1. Studies are reported of total intestinal perfusion in man in which data relating to the absorptive capacity for water and sodium, flow rate transit time and intestinal volume have been obtained.2. A 'bolus' of radiosodium added to the steady-state perfusion has allowed measurement of bidirectional fluxes of sodium ion across the mucosa to be determined.3. The values obtained agree closely with those that can be predicted from small segmental studies reported in the literature.4. Increasing absorption capacity with increasing flow rate does not seem to reflect a true increase in absorption per unit of intestine but rather a progression to more nearly continuous flow conditions.

Mineral composition and rates of flow of effluent from the distal ileum of liquid-fed calves

1. Liquid-fed calves (aged 1(1/2)-4 months) examined more than five weeks after inserting a re-entrant fistula into the distal ileum, of normal sodium and potassium status and without abnormal gut infection, showed mean emergence rates from the ileum for sodium, potassium and water of 2.3 m-mole/hr, 0.38 m-mole/hr and 21 g/hr respectively after 16 hr fasting.2. Sodium and potassium emergence rates changed little when the residues from a milk or glucose-solution feed arrived at the distal ileum. When magnesium chloride was added to a glucose-solution feed an increase sometimes occurred but only in association with decreased small-intestine transit time.3. Widely differing sodium and potassium intakes had no appreciable direct effect on their emergence rates. Continued feeding of a diet deficient in either ion, however, altered the calf's metabolism and led to appropriate changes in the sodium/potassium ratio of ileal effluent. These changes were not simulated by injecting adrenal cortex hormones. The ratio also decreased when ileal effluent was allowed to discharge for several weeks without being returned to the colon. It was abnormally high in samples obtained less than five weeks after inserting cannulae.4. An increase in sodium and potassium emergence rates, which often occurred spontaneously at about 3 months of age, appeared to be due to infection and was usually prevented by giving aureomycin orally.5. Water emergence rate reflected changes in the emergence rates of osmotically effective constituents and isotonicity was maintained. In effluent after fasting, the cations involved were mainly sodium and potassium, and [Na] + [K] was approximately constant (mean 132 m-mole/l.). In effluent following feeds of milk or glucose, magnesium chloride solution, [Na] + [K] was depressed and [Na] + [K] + 1.5 [Mg] was approximately constant (mean 139 m-mole/l.). Magnesium behaved as it were mainly ionic. Calcium had no apparent osmotic effect and was probably insoluble.6. Bicarbonate was the major anion in ileal effluent after a milk feed with smaller amounts of chloride, phosphate and some other unknown anion(s).