Jejunal Sodium Absorption Research Articles

Effects of vasoactive intestinal polypeptide microinjected into the nucleus tractus solitarius on jejunal electrolytes absorption in rats

It has been shown that vasoactive intestinal polypeptide (VIP) injected into the nucleus tractus solitarius inhibits alanine absorption across the jejunum. The aim of the present study was to investigate the effects of VIP injection into the nucleus tractus solitarius on jejunal absorption of electrolytes in the rat. Fifty-three Wistar rats were submitted to midline laparotomy to expose and isolate 20 cm of jejunal loop and to perform a subdiaphragmatic troncular vagotomy. Saline or VIP (10 pg 100 nl(-1)) was injected into the rostral nucleus tractus solitarius using a stereotaxic instrument. Tyrode solution, pH 8, containing twice glucose, sodium and potassium concentration was infused (0.5 ml min(-1)) into the jejunal loop. Samples were taken at 10-min intervals during the 40-min-experiment. Injection of VIP into the nucleus tractus solitarius increased jejunal potassium absorption. Moreover, VIP associated with vagotomy resulted in inhibition of jejunal potassium absorption by VIP alone at 40 min after perfusion (5.99 +/- 0.74 vs. 9.83 +/- 0.57 microM). There was no change in jejunal sodium absorption in any of the experimental groups. VIP had a modulatory action on jejunal potassium absorption when injected into the nucleus tractus solitarius.

Concerted action of dopamine on renal and intestinal Na(+)-K(+)-ATPase in the rat remnant kidney.

The present study evaluated renal and intestinal adaptations in sodium handling in uninephrectomized (Unx) rats and the role of dopamine. Two weeks after uninephrectomy, the remnant kidney in Unx rats weighed 33 +/- 2% more than the corresponding kidney in sham-operated (Sham) animals. This was accompanied by increases in urinary levels of dopamine and major metabolites [3, 4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid] and increases in maximal velocity values (169 vs. 115 nmol. mg protein(-1). 15 min(-1)) for renal aromatic L-amino acid decarboxylase, the enzyme responsible for the synthesis of renal dopamine. High salt (HS) intake increased (P < 0.05) the urinary excretion of dopamine and DOPAC in Unx and Sham rats. However, the urinary levels of L-3,4-dihydroxyphenylalanine, dopamine, and DOPAC in Sham rats during HS intake were lower than in Unx rats. Blockade of dopamine D(1) receptors (Sch-23390, 2 x 30 microg/kg) reduced the urinary excretion of sodium in Unx (31% decrease) more pronouncedly than in Sham (19% decrease) rats. However, inhibition of renal Na(+)-K(+)-ATPase activity by dopamine was of similar magnitude in Unx and Sham rats. In parallel, it was observed that uninephrectomy resulted in a significant reduction in jejunal sodium absorption and Na(+)-K(+)-ATPase activity in jejunal epithelial cells. In jejunal epithelial cells from Sham rats, dopamine (1 microM) failed to inhibit Na(+)-K(+)-ATPase activity, whereas in Unx rats it produced a significant reduction. It is concluded that uninephrectomy results in increased renal dopaminergic activity and dopamine-sensitive enhanced natriuresis. Furthermore, it is suggested that decreased jejunal absorption of sodium may take place in response to partial renal ablation, as an example of renal-intestinal cross talk.

Endogenous dopamine modulates jejunal sodium absorption during high-salt diet in young but not in adult rats

Background/Aims : This study was designed to investigate the contribution of endogenous catecholamines to the regulation of small intestinal sodium transport during postnatal development. Methods : Jejunal permeability was determined by a constant perfusion, nonabsorbable marker technique in weanling, adolescent, and adult rats fed either a high-salt diet or normal-salt diet. Tissue catecholamine levels were determined by high-performance liquid chromatography with electrochemical detection. Results : In 20-day-old but not in 40-day-old rats, a significantly lower net sodium absorption was observed during high-salt diet compared with age-matched controls on normal-salt diet. Inhibition of dopamine synthesis significantly increased the net sodium absorption in 20-day-old rats on high-salt diet compared with untreated 20-day-old rats on high-salt diet. The basal levels of dopamine in 20-day-old rats were twofold higher than in 40-day-old rats. During high-salt diet, both age groups responded with an increase in dopamine production. Norepinephrine levels were significantly higher (30-fold) in 20-day-old rats than in 40-day-old rats, but norepinephrine content was not significantly changed during high-salt diet in either groups. Conclusions : The results indicate that weanling animals have a greater jejunal sodium absorption than older animals, probably because of higher noradrenergic tonus. A challenge with a high-salt diet results in a decrease of the intestinal sodium absorption in weanling rats but not in adult rats; endogenous dopamine appears to play an important role in this regulation.

Alanine enhances jejunal sodium absorption in the presence of glucose: studies in piglet viral diarrhea.

We measured the response of jejunal sodium (Na) absorption to neutral amino acid (L-alanine) and to dipeptides (L-alanyl-L-alanine, glycylsarcosine) in normal piglets and in piglets with acute viral diarrhea after experimental infection with transmissible gastroenteritis (TGE) virus. In the TGE jejunum villi were blunted, crypts were deepened, and the epithelium was composed of relatively undifferentiated cells with reduced disaccharidase, decreased sodium-potassium-stimulated ATPase, and elevated thymidine kinase activities. The response of Na absorption to a maximal concentration of L-alanine (20 mM) or D-glucose (30 mM) was significantly blunted in TGE jejunum in Ussing chambers. However, the addition of L-alanine together with D-glucose caused a significantly greater increment of Na absorption than either L-alanine or D-glucose alone in control and TGE tissue. The effect of Na absorption of the dipeptide L-alanyl-L-alanine (10 mM), which was rapidly hydrolyzed by control and TGE mucosa, was similar to that of L-alanine (20 mM), while glycylsarcosine, a poorly hydrolyzed dipeptide, did not change net Na absorption in the jejunum. Our data support the concept of separate carrier systems for neutral amino acid and hexose in the crypt-type intestinal epithelium characterizing viral enteritis. We speculate that a sodium-cotransporting amino acid, if added to oral glucose-electrolyte solutions, could benefit oral rehydration therapy in acute viral diarrhea; neither of the dipeptides tested here can be expected to enhance absorption to any greater extent than its constituent amino acids.

Impact of chronic protein-calorie malnutrition on small intestinal repair after acute viral enteritis: a study in gnotobiotic piglets.

To investigate the effect of chronic protein-calorie malnutrition on intestinal repair after an enteric infection, we examined small intestinal structure, enzyme activity, and sodium transport in undernourished piglets during the acute and convalescent phases of a viral enteritis, transmissible gastroenteritis (TGE). Gnotobiotic pigs, nutritionally deprived from the age of 7 days, gained less weight than dietary controls from 14 days of age until the end of the study. Animals from malnourished and control diet groups were inoculated with TGE virus at 22-23 days and studied during the acute (40 h) and convalescent (4, 10, and 15 days) stages of this experimental enteritis along with noninfected dietary controls. After TGE infection, we observed a further decrease in weight gain and an increased mortality only in undernourished pigs. In jejunum and ileum of both dietary groups at 40 h after TGE infection, we observed comparable structural lesions, similar decreased activities of mucosal enzymes (sucrase, lactase, sodium-potassium-dependent ATPase), and increased thymidine kinase activities. Also we noted comparable diminution of glucose-stimulated jejunal sodium absorption in both dietary groups at 40 h. In control diet pigs, transport abnormalities recovered by 4 days after TGE infection and normal mucosal structure and enzyme activity returned over 4-15 days. In undernourished piglets, structural repair and enzyme abnormalities were prolonged when compared with the control diet group; glucose-stimulated sodium transport did not recover until 10 days after infection and never regained the enhanced activity seen in noninfected undernourished controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Intestinal perfusion studies in tropical sprue. 2. Movement of water and electrolytes.

Jejunal absorption of sodium and water has been investigated in 10 normal Indians and 11 patients with chronic tropical sprue. Normal saline and solutions containing amino acids and dipeptides were studied. In both groups little absorption of sodium and water from normal saline was seen. Mean water and sodium absorption from the free amino acid or dipeptide solutions showed no significant difference between the groups and was similar to absorption from normal saline. These results differ from data obtained in normal English subjects where at these concentrations significant stimulation of sodium and water absorption was seen. In a proportion of subjects in both groups net secretion was observed. However, no correlation was seen between mucosal histology or luminal bacteriology and sodium or water movement. These findings in patients with chronic tropical sprue are similar to findings in normal Indians and suggest that jejunal handling of sodium and water is abnormal when compared with normal English subjects, but that the mucosa is not in a secretory phase as seen in certain other diarrhoeal states or in the acute early phase of sprue.

Effect of bicarbonate on sodium absorption by the human jejunum.

THERE is much evidence to suggest that the absorption of sodium and water by the small intestine depends on concomitant absorption of non-electrolytes. Thus, in man, jejunal absorption from isotonic saline is very slow, but the rate of absorption of both sodium and water is greatly increased by the addition of glucose to the intestinal lumen1,2. Galactose, but not fructose, has a similar effect on water absorption in man3. During the course of perfusion studies in man, we have found that bicarbonate ions have a stimulant effect on jejunal absorption of sodium and water and, furthermore, the magnitude of the effect on sodium is comparable with that of glucose.

The mechanisms of sodium absorption in the human small intestine.

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 aniomic composition of luminal fluid. In the ileum, sodium absorption occurred against very 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 which the active transport of monosaccharide generates a local osmotic force for the absorption of solution (NaCl and water) from the jejunal lumen, which, in the presence of mannitol, is counterbalanced by a reverse flow of pure solvent (H(2)O) through a parallel set of channels which are impermeable to sodium. Support for the model was obtained by the demonstration that glucose and bicarbonate stimulated the absorption of the nonactively transported solute urea even when net water flow was maintained at zero by addition of mannitol to luminal contents.