Countercurrent multiplication of sodium in intestinal villi during absorption of sodium chloride

Inactivation of paracellular cation-selective claudin-2 channels attenuates immune-mediated experimental colitis in mice

The tight junction protein claudin-2 is upregulated in disease. Although many studies have linked intestinal barrier loss to local and systemic disease, these have relied on macromolecular probes. In vitro analyses show, however, that these probes cannot be accommodated by size- and charge-selective claudin-2 channels. We sought to define the impact of claudin-2 channels on disease. Transgenic claudin-2 overexpression or IL-13-induced claudin-2 upregulation increased intestinal small cation permeability in vivo. IL-13 did not, however, affect permeability in claudin-2-knockout mice. Claudin-2 is therefore necessary and sufficient to effect size- and charge-selective permeability increases in vivo. In chronic disease, T cell transfer colitis severity was augmented or diminished in claudin-2-transgenic or -knockout mice, respectively. We translated the in vitro observation that casein kinase-2 (CK2) inhibition blocks claudin-2 channel function to prevent acute, IL-13-induced, claudin-2-mediated permeability increases in vivo. In chronic immune-mediated colitis, CK2 inhibition attenuated progression in claudin-2-sufficient, but not claudin-2-knockout, mice, i.e., the effect was claudin-2 dependent. Paracellular flux mediated by claudin-2 channels can therefore promote immune-mediated colitis progression. Although the mechanisms by which claudin-2 channels intensify disease remain to be defined, these data suggest that claudin-2 may be an accessible target in immune-mediated disorders, including inflammatory bowel disease.

Sodium channels are required during in vivo sodium chloride hyperosmolarity to stimulate increase in intestinal endothelial nitric oxide production

NaCl hyperosmolarity increases intestinal blood flow during food absorption due in large part to increased NO production. We hypothesized that in vivo, sodium ions enter endothelial cells during NaCl hyperosmolarity as the first step to stimulate an increase in intestinal endothelial NO production. Perivascular NO concentration ([NO]) and blood flow were determined in the in vivo rat intestinal microvasculature at rest and under hyperosmotic conditions, 330 and 380 mosM, respectively, before and after application of bumetanide (Na(+)-K(+)-2Cl(-) cotransporter inhibitor) or amiloride (Na(+)/H(+) exchange channel inhibitor). Suppressing amiloride-sensitive Na(+)/H(+) exchange channels diminished hypertonicity-linked increases in vascular [NO], whereas blockade of Na(+)-K(+)-2Cl(-) channels greatly suppressed increases in vascular [NO] and intestinal blood flow. In additional experiments we examined the effect of sodium ion entry into endothelial cells. We proposed that the Na(+)/Ca(2+) exchanger extrudes Na(+) in exchange for Ca(2+), thereby leading to the calcium-dependent activation of endothelial nitric oxide synthase (eNOS). We blocked the activity of the Na(+)/Ca(2+) exchanger during 360 mosM NaCl hyperosmolarity with KB-R7943; complete blockade of increased vascular [NO] and intestinal blood flow to hyperosmolarity occurred. These results indicate that during NaCl hyperosmolarity, sodium ions enter endothelial cells predominantly through Na(+)-K(+)-2Cl(-) channels. The Na(+)/Ca(2+) exchanger then extrudes Na(+) and increases endothelial Ca(2+). The increase in endothelial Ca(2+) causes an increase in eNOS activity, and the resultant increase in NO increases intestinal arteriolar diameter and blood flow during NaCl hyperosmolarity. This appears to be the major mechanism by which intestinal nutrient absorption is coupled to increased blood flow.

Integration of intestinal structure, function, and microvascular regulation

Without an increase in blood flow to provide additional oxygen, intestinal absorption of nutrients cannot proceed. Studies of the intestinal microvascular structure and distribution of resistance indicated that most of the microvascular regulation must occur outside the mucosal tissues. This requires a communication system from the mucosa to resistance vessels unlike that of any other organ. The various mechanisms involved and their communication from mucosal to arteriolar cells has required an integrated study of intestinal structure, physiology, and microvascular regulation. The results of this analysis using diverse approaches have revealed some of the major physical and cellular mechanisms that couple intestinal absorption and microvascular function.

Paracellular transport of water and carbohydrates during intestinal perfusion of protamine in the rat

With these experiments, the authors' purpose was to determine whether the intestinal perfusion of protamine would successfully block paracellular transport without causing significant change in cardiovascular function. In anesthetized (50 mg x kg-1 sodium pentobarbital) rats (n=12), heart rate and mean arterial blood pressure were measured during perfusion (0.5 mL x min-1) of a carbohydrate-electrolyte solution through the small intestine. The carbohydrate-electrolyte solution contained 150 mM glucose, 150 mM fructose, 10 mM lactulose, 17 mEq sodium, 3 mEq potassium, and either 0.0, 0.1, 1.0, or 10 mg x mL-1 protamine. Osmolality of the 4 solutions ranged from 363 +/- 2 to 365 +/- 3 mOsm x kg-1. Core temperature was maintained at 37 degrees C in an environmental chamber. Heart rate and mean arterial blood pressure were constant during all intestinal perfusions. Forty-one percent of the perfused lactulose was absorbed. Absorption of glucose, fructose, and lactulose was significantly inhibited by 0.1 mg x mL-1 protamine, while water absorption was decreased 41 percent by 1.0 mg x mL-1 protamine. Water and lactulose absorption fell 75% with protamine, and glucose and fructose absorption fell 50%. Lactulose and fructose absorption did not decrease further when protamine dose rose to 10 mg x mL-1. These results indicate that 1) perfusion of protamine into the small intestine in doses that significantly affect intestinal transport does not significantly affect heart rate and mean arterial blood pressure; and 2) if the primary effect of protamine is to block paracellular movement of water and solute, the greater protamine inhibition of water and lactulose absorption is consistent with a greater paracellular transport of water and lactulose than for glucose and fructose.

Intestinal absorption of sodium and nitric oxide-dependent vasodilation interact to dominate resting vascular resistance

The villi of the small intestine maintain a hypertonic interstitium at all times, and the submucosal glands constantly secrete ions and accompanying water into the lumen. Generation of the 400- to 600-mOsm interstitial fluid in the villus and secretion by glands may require a large expenditure of energy and, consequently, have major effects on intestinal vascular regulation to supply oxygen and nutrients. Blood flow and oxygen consumption were measured in the ileum of anesthetized rats during natural resting conditions with physiological sodium chloride in the bathing fluid and during isosmotic replacement of sodium chloride with mannitol. Microvascular pressures and blood flow were used to determine the changes in resistance of the major arterioles and the terminal vasculature. When mannitol replaced sodium chloride in contact with the villi, intestinal blood flow decreased to 58.6 +/- 2.8% of control, and oxygen consumption was 54.2 +/- 3.4% of control. Resistance of the major arterioles increased 101.7 +/- 9.9%, and that of the terminal vasculature increased 40.4 +/- 6.2%. The increased resistance appeared to be caused by suppression of a nitric oxide mechanism. Local application of 10(-4) mol/L NG-nitro-L-arginine methyl ester caused about the same reduction in flow and increases in regional vascular resistance as during replacement of sodium but did not alter the oxygen consumption. These data indicate that about half of the intestinal metabolic rate during natural resting conditions is devoted to sodium secretion/absorption. Large resistance vessels are dilated to maintain a high blood flow through release of nitric oxide. We propose that dilation of the terminal vasculature in the metabolically active tissues increased flow velocity sufficiently in the major resistance vessels to cause a flow-mediated release of nitric oxide.

Differential in vivo and in vitro intestinal permeability to lactulose and mannitol in animals and humans: a hypothesis

BACKGROUND/AIMS

Clinical interpretation of urinary recovery ratios of lactulose and mannitol is hampered by incomplete understanding of the mechanisms of transmucosal passage. The aim of this study was to compare in vivo and in vitro probe permeability.

METHODS

Stripped sheets of small intestine from rodents and human biopsy specimens were mounted in Ussing chambers, and mucosa-to-serosa fluxes of lactulose and mannitol were determined. Urinary recovery of orally applied probes was measured in rodents, cats, and humans.

RESULTS

In vitro lactulose/mannitol flux ratios were close to 0.8 in all species. Urinary recovery ratios differed between rodents and cats or humans; low ratios in cats and humans were due to high mannitol recovery.

CONCLUSIONS

Interspecies variation in urinary recovery of mannitol is caused by differences specific for the intact small intestines in vivo. Because hyperosmolality of villus tips in vivo varies, being highest in humans and cats as a result of vascular countercurrent multiplication, it is hypothesized that the high urinary recovery of mannitol in these species is caused by solvent drag through pores that allow the passage of mannitol but not of lactulose. Therefore, the lactulose/mannitol ratio is primarily a standard for the normal functioning of villus epithelial cells in metabolite absorption and for normal villus blood flow.

Transmucosal impedance of small intestine: correlation with transport of sugars and amino acids

Transmucosal impedances of isolated perfused segments of jejunum from mice and hamsters were measured at frequencies from 10-100,000 Hz in the presence and absence of sugars and amino acids. Na-coupled transport of organic substrates caused large decreases of transmucosal impedance, reflecting contraction of cytoskeletal proteins controlling permeability of tight junctions, functional surface of basolateral membranes, and width of extracellular pathways. The observed changes of impedance were closely correlated with molar rates of Na-coupled active transport rather than with molecular species. Thus amino acids and sugars having the same molar rates of active transport also have the same effects on transmucosal impedance. It is proposed that a nonspecific increase of intracellular osmotic pressure during active transport is the first step initiating cytoskeletal contraction. Cell volume regulatory responses, including increased basolateral K+ conductance and Ca2+ influx, may be subsequent steps leading to contraction of perijunctional actomyosin, formation of junctional dilatations, and exposure of lateral membranes. Enhancement of oxygen capacity of perfusion fluids (e.g., with fluorocarbon emulsion) is required to maintain viability of isolated intestinal epithelium; in plain oxygenated Ringer-HCO3 solution, the transmucosal impedance is abnormally low and cytoskeletal contractile responses to Na-coupled transport are attenuated. An electrical circuit analog is presented that simulates almost exactly the observed transmucosal impedances and provides quantitative evaluation of the effects of Na-coupled transport of sugars and amino acids on resistances of tight junctions, capacitance of basolateral membranes, and postjunctional resistances of lateral intercellular spaces and villus cores.

Tissue osmolality in intestinal villi of four mammals in vivo and in vitro

Using a freezing point depression method osmolality in the intestinal tissue of four mammals (gerbils, guinea-pigs, rabbits and rats) was estimated in vivo, during fluid transport from an isotonic electrolyte-glucose solution. Net fluid transport was also measured. In gerbils, guinea-pigs and rabbits tissue osmolality was also estimated during in vitro conditions. A marked hyperosmolality was observed in vivo in the upper parts of the villi of all four mammals studied. The tissue osmolality was significantly higher than that seen in the same species during in vitro conditions. A villus hyperosmolality was observed also in species which exhibited a net fluid secretion (guinea-pig, rabbit ileum), indicating that the fluid secretion emanated from the intestinal crypts. Based on the results of the present experiments and on observations made in earlier experiments performed on the cat, it is proposed that the villus hyperosmolality is created by a countercurrent multiplier present in the intestinal villus. The hyperosmolar compartment in the villus tissue creates the force that drives fluid from lumen to tissue.

No evidence of a countercurrent multiplier in the intestinal villus of the dog

The hypothesis that a countercurrent multiplier within the intestinal villus increases osmolality in the distal villus tip was tested. Total sodium, potassium, and water content of intestinal tissue samples was measured. The results showed no significant difference in the total Na plus K concentration (millimoles per kilogram H2O +/- SEM) between the villi tips (185.1 +/- 7.7), whole villi (179.8 +/- 5.4), or intestine minus villi (177.2 +/- 1.7). In contrast, in the kidney (where the existence of a countercurrent multiplier has been demonstrated), the renal medulla had a total Na plus K concentration of 284 +/- 17.1 that was significantly more than the renal cortical concentration of 163 +/- 3.1. Villous tissue osmolality should be in osmotic equilibrium with intestinal luminal fluid. Sampling of intestinal luminal fluid revealed a total Na plus K concentration of 145-165 mmol/kg H2O, a figure compatible with normal luminal osmolality of 280-320 mosmol/kg H2O. These results deny the existence of a countercurrent multiplier in the intestinal villus of the dog.

Evidence for a countercurrent exchanger in the intestinal villi of suckling swine

The possible existence of a countercurrent exchanger (multiplier) in the intestinal villi of suckling swine was investigated with three different methods: (1) Comparing venous appearance of oxygen and red cells in the mesenteric vein after close i.a. injection of a blood sample equilibrated with pure oxygen and containing methaemoglobinaemic red cells. (2) Determining sodium (micrograms) over protein (mg) ratios along villi. (3) Estimating tissue osmolality in the villus tissue from measurements of freezing-point depression. It was observed that oxygen appeared earlier than red cells in the mesenteric vein after i.a. injection. Furthermore, both the chemical measurements of sodium/protein ratios in the villi and the estimations of tissue osmolality indicate that there exists a gradient of osmolalities along the villus length, the villus tip being hyperosmolar as compared to the villus base. We conclude that a countercurrent exchanger (multiplier) exists in the villi of suckling swine. Its possible pathophysiological significance is discussed.

Villous sodium gradient associated with volume absorption in the feline intestine: an electron-microprobe study on freeze-dried tissue

Water transport in biological tissue is driven by local osmotic gradients created by accumulation of actively transported ions in tissue compartments. To localize and measure such gradients, jejunal segments from the small intestine of anaesthetized cats were perfused with modified isotonic Krebs-Henseleit electrolyte solution, and net fluid transport was measured with a volumetric technique. The segments were then rapidly frozen, freeze-dried, and prepared for X-ray micro-analysis of elemental content. Whenever the lumen perfusate contained sodium, the apical third of the villus was found to have a sodium gradient rising to a tip concentration more than twice that at the base of the villus. This sodium gradient was associated with a chloride gradient and fluid absorption. No similar potassium gradient was found. When choline replaced sodium in the intestinal lumen, no gradient of sodium chloride was found and no net fluid absorption occurred. Absorption of fluid was thus apparently coupled to absorption of sodium through creation of a local osmotic gradient in the tip of the intestinal villus.

Changes in the pattern of intercellular transfer of lucifer yellow between villus epithelial cells during postnatal development

We demonstrate the pattern of intercellular transfer of Lucifer yellow between villus epithelial cells during postnatal development. Epithelial cells in all villi from newborn mice demonstrate high dye transfer efficiency. The efficiency of dye transfer decreases with the age of the mouse and with villus size, until in adult mice, dye transfer is observed only in what are presumably newly forming villi. We suggest that the development of an osmolality gradient in taller villi, a result of sodium pumps and a countercurrent multiplier, may be responsible for the changes in the pattern of dye transfer observed.

Influence of sodium deoxycholate on morphology, net fluid transport and motility in the small intestine of the rat

Intestinal fluid secretion and motility were induced by luminal perfusion of rat small intestine with sodium deoxycholate, a dihydroxy bile salt for 1-3 h. Changes in intestinal morphology were studied simultaneously with the changes in fluid transport and motility. The results suggest that the bile salt causes epithelial lesions which may lead to a reduced fluid absorption in the villi, thereby explaining part of the total change in net fluid transport caused by the bile salt. Pyrilamine and indomethacin did not influence the bile salt-induced secretion. Based on earlier studies, it is proposed that the major part of the bile salt-evoked secretion is mediated via activation of intramural nervous reflex(es), which also stimulate the intestinal smooth muscle cells.

Blood flow distribution, lymph flow, villus tissue osmolality and fluid and electrolyte transport after exposing the cat small intestine to sodium deoxycholate

The effect of luminal perfusion of a dihydroxy bile salt (sodium deoxycholate) on net fluid transport, intestinal haemodynamics, lymph flow, electrolyte transport and villus tissue osmolality was studied in cat jejunum. Furthermore, the effects of hexamethonium and tetrodotoxin, two drugs influencing nervous activity, were investigated. Concomitant to net fluid secretion, the bile salt increased mucosal blood flow whereas capillary filtration coefficient and lymph flow remained unchanged. Net sodium and chloride transport changed from absorption to secretion. The change of sodium transport was due to both an increased flux from tissue to lumen and a reduced flux in the opposite direction. Villus tissue hyperosmolality was reduced. None of the effects on intestinal haemodynamics correlated with the change in net fluid transport. Furthermore, hexamethonium and tetrodotoxin inhibited the secretion of fluid and electrolytes without influencing the induced changes in intestinal haemodynamics. It is concluded that the bile salt induces intestinal fluid secretion by stimulating an active secretory process in the crypts via enteric nerves. A minor part of the total change in net fluid transport may be due to a reduced uptake in the villi.

Developmental changes in intra-enterocyte cation activities in hamster terminal ileum

Intracellular Na+ and K+ activities have been determined under visual control in enterocytes located at known positions along hamster ileal villi in vitro. The intracellular K+ activity found in enterocytes migrating over the basal third of villi was significantly less than that found in older enterocytes (83 and 113 mM respectively). The intracellular Na+ activity remained unaffected by the state of enterocyte development (32 mM throughout). Extracellular K+ and Na+ activities measured close to the luminal surface of the intestine were always similar to those measured in the bulk of the bathing medium. There was, however, a small difference in K+ activity detected between the bulk solution and solution near the luminal surface of enterocytes (3 . 4 and 3 . 6-3 . 7 mM respectively). Na+ activity in the villus core was also similar to that of the bathing medium. The brush border membrane content of sucrase-isomaltase, determined cytochemically, was found to increase rapidly as enterocytes migrated over the basal third of the villus and to plateau at a point roughly equivalent to that where intracellular K+ activities became maximal. The numerous ways by which enterocytes might control their intracellular K+ concentrations during development are emphasized. Attention is nevertheless drawn to the close correspondence seen between different aspects of brush border membrane development and the ability of enterocytes to maintain high internal concentrations of K+.

Intestinal water transport

The current knowledge of the physiology of the transport of sodium and water across the intestinal epithelium is summarized. A brief review of its hormonal and nervous control is also given.

Sympathetic innervation of the jejunal villus

Using fluorescent histochemical methods it has been shown that the noradrenergic nerves in the jejunal villus are associated with the capillaries underlying the basolateral membrane of the epithelium. Noradrenergic fibres were also seen to lie between the epithelial basolateral membrane and the capillaries but were never observed close to the epithelium unless accompanied by an underlying capillary. The distribution of noradrenergic fibres suggests that it is unlikely that noradrenaline diffuses directly from the varicosity to the epithelial basolateral membrane. Noradrenaline may, therefore, act on the capillary itself and in some way affect fluid absorption. However, noradrenaline released adjacent to a capillary might diffuse into the capillary to be distributed at another site along its course.

Sites of noradrenergic innervation of the jejunal villus

1. It has been demonstrated that the upper region of the jejunal villus is supplied with noradrenergic nerves. 2. These nerves are in close proximity to the capillary network underlying the epithelial cells and the lacteal. 3. Hitherto sympathetic innervation of the gut had not been demonstrated to extend further than the crypt region. 4. This evidence allows the proposal that the vasculature and epithelial cells may be involved in the control of absorption from the gastrointestinal tract.

Villous tissue osmolality, water and electrolyte transport in the cat small intestine at varying luminal osmolalities

Villous tissue osmolality and net transport for water, sodium, potassium and chloride were determined in the feline small intestine when exposing the mucosa to solutions with different mannitol concentrations (0, 100, 315 and 600 mmol/l). Tissue osmolality at the villous tip varied with luminal osmolality. At the villous base, on the other hand, tissue osmolality remained around the plasma osmolality regardless of the osmolality of the luminal fluid. Transport rates for water were affected in the way predicted from the lumen to tissue osmolality difference. A net flux from tissue to lumen was always recorded for the studied electrolytes. The hydraulic conductivity (Lp) of the intestinal epithelium with dilated intercellular spaces was estimated from the present results to be around 30 x 10(-12) cm x s-1 x Pa-1. When the intercellular spaces were collapsed Lp was estimated to be 15 x 10(-12 cm x s-1 x Pa-1.

Effects of cholera toxin on villous tissue osmolality and fluid and electrolyte transport in the small intestine of the cat

The effects of cholera toxin on tissue osmolality and on net transport rates of water, sodium, chloride and potassium as well as on unidirectional fluxes of water and sodium were studied in vivo. In all experiments the toxin caused a net secretion of water, sodium, chloride and potassium. The unidirectional sodium transport from tissue to lumen was increased while the flux in the opposite direction was reduced 180 min after cholera toxin instillation. Cholera toxin produced only a small reduction in the villous tissue hyperosmolality, created by the intestinal countercurrent exchanger. This reduction was far too small to explain the observed net secretion of fluid and solutes induced by the cholera toxin. Other mechanisms underlying the cholera secretion are discussed.

Villous tissue osmolality and intestinal transport of water and electrolytes

The villous tissue hyperosmolality created by the intestinal countercurrent multiplier has been proposed to be of importance for fluid transport across the intestinal epithelium in vivo. This study was performed to test this hypothesis. Net transport of fluid and electrolytes (sodium, potassium and chloride), as well as unidirectional fluxes of water and sodium were determined in the small intestine of the cat. The villous osmolality was altered by varying the composition of sodium and glucose in the isotonic solutions perfusing the intestinal lumen. Net transport of fluid was correlated to tissue osmolality mainly due to an increase of the unidirectional flux of water from lumen to tissue with augmented tissue osmolality. The results are thus consistent with the view that the intestinal countercurrent multiplier is of essential importance for net water transport. A correlation was found between net water and net sodium intestinal transport. A similar correlation was also demonstrated between net sodium and net chloride absorption rates in the jejunum while in the ileum net loss of sodium into the intestinal lumen was not accompanied by any corresponding loss of chloride ions. This observation suggests the presence of a sodium independent transport mechanism for chloride in the ilium but not in the jejunum.

Importance of sodium and glucose for the establishment of a villous tissue hyperosmolality by the intestinal countercurrent multiplier

The intestinal countercurrent multiplier has earlier been shown to create an increased tissue osmolality in the villi (Jodal et al. 1978). In the present paper the importance of varying the luminal contents on the creation of the villous hyperosmolality was investigated using the cryoscopic technique described by Jodal et al. (1978). The perfusion solutions used contained 0, 25 or 147 mmol Na/l and were either provided with mannitol or glucose (30 mmol/l). It was demonstrated that sodium was of particular importance for the establishment of the villous hyperosmolality while glucose only contributed significantly at low luminal sodium concentrations. It is therefore proposed that glucose only in the absence of sodium in the luminal perfusate may effectively participate in the generation of the villous tissue hyperosmolality via the countercurrent multiplication mechanism.

The effect of vasodilatation and sympathetic nerve activation on net water absorption in the cat's small intestine

The rate of net water uptake from the feline small intestine has been investigated during control conditions, during graded infusions of the vasodilator drug isopropylnoradrenaline, and during electrical stimulation of the regional sympathetic nerve fibres to the gut. Net water absorption rate was largely unaffected by intestinal vasodilatation. The fraction of the absorbate transported via the lymphatics remained also constrant at 20-40% of the total absorption regardless of blood flow rate. Stimulating the sympathetic nerve fibres to the small intestine increased, however, net water absorption rate. The increase was particularly pronounced when blood pressure was kept constant during the period of stimulation. The absorption rate was on an average almost doubled at a stimulation frequency of 8 Hz during constant pressure conditions. The mechanism(s) explaining this nervous control of water absorption are tentatively discussed.

Vascular anatomy and tissue osmolality in the filiform and fungiform papillae of the cat's tongue

The vascular anatomy of the filiform and fungiform papillae of the feline tongue was studied by i.a. injection of India ink. Vascular loops of various appearances were found in the types of papillae studied, i.e. the large and the small filiform papillae and the fungiform ones. Such hairpin loops may function as countercurrent exchangers and to test this hypothesis tissue osmolality was determined in the papillae, while exposing them to various isotonic electrolyte solutions. The large filiform papillae with a vascular arrangement similar to that of intestinal villi exhibited a marked osmolar gradient from tip to base when exposed to a solution containing both glucose and sodium. If sodium and/or glucose was excluded from the solution, tissue osmolality was significantly decreased. This was also the case when the chloride ions of the solution was substituted with sulphate. The small filiform papillae are only provided with one or a few capillary loops. They exhibited a less marked osmolar gradient than the large ones and one of the different electrolyte solutions decreased the gradient. In the fungiform papillae a tissue hyperosmolality at the tip was also demonstrated. It is proposed that the papillary epithelium is provided with active transport mechanism(s) and that the papillary vessels function as countercurrent multipliers. The functional importance of these mechanisms are tentatively discussed.

Evidence for the existence of a countercurrent exchanger in the small intestine in man

The vasculature in the human villus forms vascular loops by the supplying arterial vessel and the draining capillaries and/or veins. This study reports two experimental observations that strongly suggest that these vascular loops function as countercurrent exchangers. (1) The elimination of intraarterially injected 85Kr from the human small bowel exhibits an initial very rapid component of the type earlier reported in the feline gut. This component in all probability reflects the extravascular "shunting" in the exchanger of the injected radioactive tracer. (2) When exposing the intestinal mucosa to an isotonic electrolyte solution containing glucose, an osmolality gradient from the tip to the base of the human villi was demonstrated, the tips having an osmolality of around 700 milliosmoles per kg H2O. This hyperosmolality is created by the exchanger acting as a countercurrent multiplier.

Effect of ethanol on the morphology of hamster jejunum

In order to study the morphological effects of exposure of the jejunum to low ethanol concentrations, we perfused hamster jejunum with 2.1-4.8% ethanol. Following 45 min exposure, many of the villi developed fluid-filled blisters. To compare these findings to the effect of an inert solute at similar concentrations, we perfused hamster jejuna with mannitol. This caused necrosis of the villus tips but no blister formation. Therefore the blisters were the result of the action of ethanol. The rat jejunum was less resistant to ethanol than that of the hamster. We suggest that the initial insult of the freely permeant ethanol is deep to the epithelium, resulting in accumulation of edema under the epithelium.

The effects of cholera toxin on intramural blood flow distribution and capillary hydraulic conductivity in the cat small intestine

Blood flow distribution to the mucosa-submucosa and to the muscularis in the cat small intestine was investigated with a 85Kr elimination technique before and after exposing the intestinal mucosa for 30 min to cholera enterotoxin. In all experiments the toxin induced an intestinal secretion. Concomitantly, total intestinal blood flow was increased to a level 50 per cent above control 3 h after exposure. This vasodilatation reflected a doubling of mean blood flow in the mucosa--submucosa while muscularis blood flow remained unchanged. In another series of experiments the effect of cholera toxin on intestinal capillary hydraulic conductivity was investigated by determining the capillary filtration coefficient (CFC). A slight increase in CFC was noted during the 3 h observation period but this was not more pronounced than would have been expected from the concomitant vasodilatation. It is concluded that hemodynamic changes in the intestinal mucosa may be one of the several factors that probably are involved in the pathogenesis of cholera.

Tissue osmolality in intestinal villi during luminal perfusion with isotonic electrolyte solutions

A cryoscoptic technique has been developed that makes it possible to determine tissue osmolality in the core of the intestinal villi. During absorption from an isotonic electrolyte solution containing glucose an osmolality gradient was demonstrated from tip to base of the villi in both the jejunum and the ileum. The tissue osmolality at the villous tips was measured to 1 000-1 200 mOsm/kg H2O while the osmolality at the villous base was approximately isotonic with plasma. Increasing intestinal blood flow by i.a. administration of a vasodilator drug, or making the intestine ischemic by clamping the intestinal vascular supply while supplying the mucosa with oxygen, markedly decreased tissue osmolality. Substituting all sodium ions with choline in the luminal perfusate abolished almost completely the tissue hyperosmolality and the intestine became a secretory organ. These observations are consistent with the view that the observed villous tissue hyperosmolality was created by a countercurrent multiplication of sodium chloride. The physiological implications of this mechanism is discussed and it is, among other things, proposed that the hyperosmolar region represents the hyperosmotic compartment necessary for explaining intestinal water absorption.

Transmural ionic fluxes and electrical potential difference in the human jejunum during perfusion with a dihydroxy bile acid

Perfusion studies of the proximal jejunum were performed in healthy volunteers to define the influence of glycochenodeoxycholic acid (GCDC) 2.5 mmol/1 on the net movements of water and electrolytes, the bidirectional fluxes of sodium, potassium, and chloride, and the transmural electrical potential difference (PD). The flux data supported the notion that active sodium transport is inhibited by luminal GCDC, which on the other hand elicits active secretion of chloride. PD was 3 +/- 1 mV, lumen negative, and was not influenced by GCDC. The flux data fit a previously proposed model for the GCDC effect.