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

I, 3. The enteric nervous system and infectious diarrhea

This chapter discusses the background knowledge about the enteric nervous system (ENS) as well as the role of ENS in secretory states of the small intestine. The chapter describes the anatomy and physiology of the ENS. A description of the experimental evidence for the involvement of ENS in secretory states of the gut, primarily in cholera toxin-induced secretion that is the most thoroughly investigated secretory state, is presented in the chapter. The chapter focuses on the involvement of ENS in rotavirus (RV) diarrhea. The involvement of the ENS in diarrhea pathophysiology opens up new potential sites of action for drugs in the treatment of intestinal secretory states. The chapter concludes with a discussion of the sites of action for the pharmacological treatment of diarrhea.

Enteric nerves and diarrhoea

This review article discusses the importance of the enteric nervous system for the fluid and electrolyte secretion evoked by luminal secretagogues in the small intestine. The first part of the review summarizes observations on augmented secretion caused by cholera toxin, which has been the subject of extensive studies in the past. The latter part reviews studies of the participation of the enteric nervous system in other secretory states of the gut. The involvement of the enteric nervous system in the pathophysiology of intestinal secretory states opens up potential new sites of actions for drugs in the treatment of diarrhoea. This is discussed in the final part of this review.

Pathogenesis of rotavirus diarrhea

Rotavirus diarrhea is a major cause of infantile gastroenteritis worldwide. This review is mainly devoted to the effects of Rotavirus on intestinal epithelial transport and to the pathophysiological mechanisms proposed to underlie the intestinal fluid secretion caused by the virus.

Scaling of dimensions of small intestines in non-ruminant eutherian mammals and its significance for absorptive mechanisms

The mucosal surface area of small intestines in non-ruminant eutherian mammals increases approximately in proportion to the 0.6 power of body mass, whereas resting metabolic rate (RMR) increases approximately with the 0.74 power of body mass; the mass exponent for field metabolic rates (FMR) may exceed 0.8. These relationships imply that the average rate of absorption of metabolic substrates, expressed per unit area of mucosal surface, is greater in large animals than in small. In the present paper I collate data from the literature relating mucosal surface area, fluid absorption and glucose transport rates to body size. Glucose-stimulated fluid absorption per unit area of mucosal surface increases with body size, whereas transcellular, carrier-mediated glucose transport per unit area decreases with body size. In perfused jejunal segments of normal human subjects the rates of fluid absorption per unit area of mucosa are five to ten times greater than in laboratory rats. The absorbed fluid contains glucose in amounts that may greatly exceed the maximum transport capacity of the apical glucose transporter. It follows that the paracellular component of glucose absorption increases with body size. Scaling of intestinal dimensions and transport therefore provides new information about the relative contributions of transcellular and paracellular pathways to absorption of nutrients.

The enteric nervous system and cholera toxin-induced secretion

This article reviews briefly some general aspects of the enteric nervous system (ENS). Furthermore, the ENS control of epithelial transport is exemplified by a description of the enteric nervous reflexes activated by cholera toxin.

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.

Villus and crypt electrolyte and fluid transport during intestinal secretion

The apical parts of jejunal villi of net-absorbing intestine have been shown to contain sodium chloride concentration gradients which are associated with water absorption (Sjöqvist & Beeuwkes 1989). To determine whether these gradients are different in states of intestinal net secretion, jejunal segments of chloralose-anaesthetized cats were perfused with modified Krebs-Henseleit solutions while secretion was elicited by cholera toxin or vasoactive intestinal polypeptide (VIP). The segments were then rapidly frozen and freeze-dried, and sodium and chloride contents of the lamina propria of single villi were measured by X-ray microanalysis. The apical third of the villus was found to contain a concentration gradient of sodium and chloride when the lumen contained sodium, with no difference between secreting intestine and absorbing control intestine. When the intestine was perfused with hypotonic choline-mannitol solution, no sodium or chloride gradient was found. In this state, treatment of the intestines with secretagogues allowed development of an apical concentration gradient. This demonstrated that the absorptive function of the villus tip was unimpaired during secretion and that secretion from the crypt could supply sufficient electrolyte to allow formation of an apical gradient.

Changes in cyclic 3'5'-adenosine monophosphate tissue concentration and net fluid transport in the cat's small intestine elicited by cholera toxin, arachidonic acid, vasoactive intestinal polypeptide and 5-hydroxytryptamine

We have analysed tissue cyclic 3'5'-adenosine monophosphate (cAMP) concentration in different fractions of the cat's small intestinal mucosa during secretion elicited in vivo by four different secretagogues: cholera toxin (administered intraluminally), vasoactive intestinal polypeptide (VIP; given i.a.), arachidonic acid (AA; administered intraluminally) and 5-hydroxytryptamine (5-HT; given i.a.). Cholera toxin was found to increase cAMP concentration in the villi but not in the crypts. The VIP, AA and 5-HT did not influence tissue cAMP concentration despite a profuse net fluid secretion. Hexamethonium inhibited secretion elicited by cholera toxin and AA but did not significantly influence tissue cAMP concentration. There is strong evidence for the view that villus and crypt regions of the small intestinal mucosa have different functions, secretion taking place in the crypts and absorption in the villi. However, the lack of cAMP increase in the crypts reported in this study suggests that cholera toxin in this model does not reach the crypts. The results are not in agreement with a role for cAMP in mediating secretion from the crypts, but are compatible with a role of cAMP in inhibiting absorption in the villi. It is suggested that the observed fluid secretion from the crypts elicited by cholera toxin, AA and 5-HT is to a major part mediated by intramural enteric reflexes.

The net fluid secretion caused by cyclic 3'5'-guanosine monophosphate in the rat jejunum in vivo is mediated by a local nervous reflex

The tissue concentration of cyclic 3'5'-guanosine monophosphate (cGMP) has been shown to increase in the small intestine when net fluid secretion is evoked by the heat-stable enterotoxine of Escherichia coli. Lipophilic cGMP analogues are also known to elicit intestinal fluid secretion. It is therefore believed that an increase in intracellular cGMP concentration in enterocytes mediates this secretion. The present study reports that the fluid secretion, elicited by placing two different cGMP analogues, dibutyryl-cGMP and 8-Br-cGMP, in the intestinal lumen of anaesthetized rats in vivo, is significantly inhibited by atropine, hexamethonium and lidocaine. It is proposed that cGMP activates a reflex in the enteric nervous system which, in part, explains the observed fluid secretion.

On the mode of action of the sympathetic fibres on intestinal fluid transport: evidence for the existence of a glucose-stimulated secretory nervous pathway in the intestinal wall

The effect of sympathetic nerve stimulation or close i.a. infusion of noradrenaline on net fluid transport was investigated on anesthetized cats. In the presence of glucose in the solution perfusing the intestinal lumen the adrenergic mechanisms increased net fluid absorption in normal intestines. Substituting glucose with mannitol in the perfusate abolished this effect of adrenergic stimulation on the net fluid uptake. Furthermore, the effect of noradrenaline on net fluid transport in normal or choleraic intestines was abolished by tetrodotoxin (TTX), a nerve conductivity blocking agent. This suggests that the sympathetic influence is dependent on intraluminal glucose and that noradrenaline exerts its effect mainly via nerves. TTX significantly increased fluid uptake from normal intestines perfused with an isotonic electrolyte solution containing glucose while a considerably smaller effect was seen in intestinal segments perfused with a solution with mannitol. Based on these findings it is proposed that glucose in some way activates intramural nervous reflex(es) in the intestinal wall. According to this hypothesis the enhancement of fluid transport induced by adrenergic stimuli is explained by an inhibition of the glucose-activated nervous secretion.

The effect of nicotinic and muscarinic receptor blockade on cholera toxin induced intestinal secretion in rats and cats

The effects of hexamethonium (cholinergic nicotinic receptor antagonist) and atropine (cholinergic muscarinic receptor antagonist) on cholera toxin induced secretion were investigated in denervated segments of the small intestine of rats and cats. While there was no effect of atropine, hexamethonium markedly inhibited choleraic secretion and turned it into a net fluid absorption in many animals. This observation further strengthens our hypothesis that the enteric nervous system is involved in cholera secretion.

On the role of intramural nerves in the pathogenesis of cholera toxin-induced intestinal secretion

Intestinal secretion was produced in anesthetized cats and rats by exposing isolated intestinal segments to cholera enterotoxin. Giving, for example, tetrodotoxin, a nerve-conduction-blocking agent, or adding lidocaine, a local anesthetic agent, to the solution in the intestinal segments markedly inhibited the rate of choleraic secretion, and in most experiments a net absorption of fluid was observed. The results suggest that intramural nervous mechanisms are involved in the pathogenesis of choleraic secretion.