Significance of enzyme release from ischemic isolated rat heart

Whole-heart ischemia has been induced in isolated working rat heart. The distribution of the reduced coronary flow was even, as judged by 3H-antipyrine autoradiographs. Reducing the coronary flow resulted in myocardial ischemia, as indicated by a lowered tissue content of glycogen, ATP and creatine phosphate and accumulation of lactate. After a reperfusion period of 30 min there was a restoration of glycogen, ATP and creatine phosphate for hearts that were ischemic for 5 and 10 min, with a concomitant normalization of tissue lactate. Hearts that were ischemic for 30 min did not show restoration of high energy phosphates and glycogen. There was a leakage of ASAT, CK and LD in all groups of hearts, suggesting that a release of these enzymes does not necessarily indicate an irreversibly damaged myocardial cell.

Effect of cholera toxin on passive transepithelial transport of 51 Cr-ethylenediaminetetraacetic acid and 14 C-mannitol in rat jejunum

Intestinal fluid secretion, mainly derived from the crypts, induced, for example, by cholera toxin, decreases the passive transport of small hydrophilic molecules into the lumen. However, the effect of the fluid secretion on the passive absorption of these substances and thus on the permeability of the villus absorptive area is not known. Therefore, the transport rates of 51Cr-ethylenediaminetetraacetic acid (EDTA) and 14C-mannitol from lumen to plasma and from plasma to lumen were recorded in jejunal loops of anaesthetized rats during cholera toxin-induced fluid secretion in the absence and presence of glucose in the intestinal lumen and expressed as clearance (microL (min g)(-1)). The results showed that the cholera toxin induced fluid secretion and abolished the passive absorption of 51Cr-EDTA both in the absence and presence of luminal glucose during a high perfusion rate (0.5 mL min(-1)). The clearance of mannitol was also inhibited at the low perfusion rate (0.2 mL min-1) with the glucose-free perfusate but only reduced with the glucose perfusate. The results show that mechanisms activated by cholera toxin inhibit the passive absorption of inert hydrophilic substances. This is proposed to be mainly caused by a reduction in the accessibility of the villus epithelium to the luminal content. Furthermore, the secretion seems predominantly to inhibit the passive absorption at the basal parts of the villus while the absorption rate at the villus tips is better preserved. The results also show that the intestinal absorption and secretion of fluid takes place at different locations (villus and crypts, respectively).

Inhibition of nitric oxide synthesis potentiates the colonic permeability increase triggered by luminal bile acids

AIM

Experiments were performed in anaesthetized rats to clarify the role of nitric oxide (NO) in the control of colonic permeability.

METHODS

Colonic luminal pressure, the transmucosal potential difference (PD) and the clearance of [3H] mannitol and [14C] urea from blood to lumen were measured. NO synthesis was blocked with Nomega-nitro-L-arginine (L-NNA) i.v. and mucosal permeability was increased by deoxycholic acid (DCA, 4 mm). The involvement of histamine in the response was studied by giving the histamine H1 receptor blocker pyrilamine.

RESULTS

In proximal colon, L-NNA per se increased luminal pressure and PD but had no significant effect on clearance. DCA per se increased luminal pressure, had no significant effect on PD, but increased mannitol and urea clearance and the clearance ratio. L-NNA and pyrilamine both blocked the luminal pressure effect of DCA but L-NNA had no significant effect on the clearance response to DCA. In distal colon, L-NNA per se had no significant effect on pressure and clearance, but increased PD like in proximal colon. DCA had no significant effect on luminal pressure, but markedly reduced PD and increased both clearance and clearance ratio. In this segment, L-NNA significantly potentiated the clearance response to DCA, and further increased clearance ratio to a value not significantly different from unity (1.00 +/- 0.05).

CONCLUSION

The data suggest that in vivo, moderate concentrations of bile acids increase colonic permeability in rats via a mechanism that is inhibited by NO in distal but not in proximal colon. In distal colon, NO may contribute to the maintenance of epithelial barrier function.

Enteric neurones modulate the colonic permeability response to luminal bile acids in rat colon in vivo

BACKGROUND

The mechanisms behind microscopic colitis and exacerbations of ulcerative colitis are incompletely understood. It seems highly likely that both luminal antigens and bile are involved. The aim of this study was to test the hypothesis that bile acids increase colonic mucosal permeability by activating enteric neurones.

METHOD

The effect of 4 mM deoxycholic acid (DCA) on the appearance rate of intravenously administered (3)H-mannitol and (14)C-urea into the lumen of the proximal and distal rat colon was measured in vivo and expressed as clearance. The nerve blocking agents atropine and hexamethonium were given intravenously, and lidocaine was applied onto the serosal surface of the colon, before and after DCA exposure

RESULTS

DCA markedly increased clearance of the permeability probes into the lumen in both colonic segments and also the ratio of mannitol/urea clearance, particularly in the distal colon. Pretreatment with atropine, hexamethonium, and lidocaine significantly inhibited the increase in clearance by approximately 65-80% but did not affect the clearance ratio. In the distal colon, the inhibitory effect of lidocaine was not statistically significant. Also, administration of atropine and hexamethonium after DCA exposure significantly inhibited the DCA effect on clearance of the probes.

CONCLUSION

The results suggest that in vivo, the permeability increase induced by a moderate concentration of bile acid is to a large extent mediated by a neural mechanism involving muscarinic and nicotinic receptors. This mechanism may be a link between the central nervous system and colonic mucosal barrier function, and may be a new target for treatment.

Involvement of enteric nerves in permeability changes due to deoxycholic acid in rat jejunum in vivo

AIM

Stress and Clostridium difficile toxin A increase epithelial permeability in the small intestine via vagus and visceral afferents, in turn activating mucosal mast cells. Bile acids also increase epithelial permeability but it is not known if nerves or mast cells are involved in this effect in the small intestine.

METHOD

In jejunum of anesthetized rats, the effects of hexamethonium and atropine on deoxycholic acid (DCA) induced fluid secretion and increase in epithelial permeability was therefore studied by determining the appearance and disappearance rates of 14C-mannitol and 51Cr-EDTA into and from a perfusion system containing 4 or 8 mm DCA and expressed as clearance.

RESULTS

DCA increased net fluid transport and appearance and to a less extent disappearance rates of the probes. Hexamethonium but not atropine, chronic denervation or the NO synthase inhibitor L-NNA did significantly decrease the appearance rate and net fluid secretion. The levels of the mast cell protease II (RMCP II) in perfusate and plasma were not increased by DCA. The clearance ratio Cr-EDTA/mannitol indicates that the plasma clearance of the permeability probes is partly secondary to net fluid transport only at higher DCA concentrations.

CONCLUSION

We conclude that the DCA effect on epithelial permeability is to a large part induced by intramural reflex(es) containing nicotinic receptors. The results also suggest that mast cell degranulation and NO release are not involved in the mechanism. This indicates that the nerve effect on intestinal paracellular permeability is not mediated by the mechanisms described for stress or Clostridium difficile toxin A.

Water absorption enhances the uptake of mannitol and decreases Cr-EDTA/mannitol permeability ratios in cat small intestine in situ

BACKGROUND

Recently, we hypothesized that mannitol absorption in human intestinal permeability tests is a reflection of small intestinal water absorption and is dependent mainly on the efficiency of the countercurrent multiplier in the villi. This may affect the outcome of clinical double-sugar permeability tests. We tested the hypothesis in cats, another species with an efficient countercurrent multiplier.

METHODS

The lumen-to-tissue transport of [14C]mannitol and [51Cr]EDTA was studied in in situ perfused jejunum of eight anaesthetized cats using four isotonic perfusion solutions with varying sodium and glucose content. The transport of water was monitored, and the absorption rate of the probes was calculated by their disappearance from the perfusate.

RESULTS

There was a significant positive correlation between water absorption and [14C]mannitol clearance from the different perfusates (r = 0.99; P < 0.01), whereas this correlation was absent for [51Cr]EDTA clearance (r = 0.05; P = 0.95). There was also a significant negative correlation between water absorption and [51Cr]EDTA/[14C]mannitol clearance ratios (r = 0.98; P < 0.02).

CONCLUSIONS

The results show a prominent effect of water absorption on mannitol uptake through pores which, also during glucose transport, exclude Cr-EDTA. The difference in water absorption from the solutions used in cat small intestine is dependent on the effectiveness of the countercurrent multiplier; we conclude that the capability of this mechanism influences mannitol absorption in vivo. Qualitatively comparable results were obtained using oral test solutions with varying NaCl and glucose concentrations in human volunteers. We propose that the functioning of the countercurrent multiplier is essential for the interpretation of double-sugar tests in clinical studies.

Involvement of nerves and calcium channels in the intestinal response to Clostridium difficile toxin A: an experimental study in rats in vivo

BACKGROUND

The involvement of nerves and calcium channels in the intestinal response to Clostridium difficile toxin A (luminal concentration 1 or 15 microg/ml) was studied in the small intestine of rats in vivo.

METHODS

Inflammation was quantified by estimating myeloperoxidase (MPO) activity in the intestinal lumen, extravascular accumulation of Evan's blue (EB) in the intestine, and number of red blood cells (RBCs) in veins in histological sections. Intestinal damage was estimated using a histological grading system. In some experiments net fluid transport was recorded using a gravimetric technique.

RESULTS

In acutely denervated intestines, toxin A caused marked destruction of the villi, increased luminal release of MPO activity, and augmentation of intestinal content of EB and venous RBCs. Denervating the intestine 3-4 weeks prior to the actual experiment prevented the development of villus damage and significantly decreased the number of RBCs in intestinal veins in experiments with a low toxin concentration, whereas no effect was demonstrated on luminal MPO activity. Using a high toxin concentration, chronic denervation decreased only the number of RBCs. Pretreatment with hexamethonium (low toxin concentration; acute denervation) attenuated the effect of toxin A on morphology, luminal MPO activity, and number of RBCs. Pretreatment with nifedipine (low toxin concentration; acute denervation) significantly decreased intestinal MPO activity and number of RBCs. Tissue accumulation of EB was not influenced by experimental manipulation. Net fluid transport was measured in experiments exposing the intestinal mucosa to a high toxin concentration. Fluid secretion caused by the toxin was significantly attenuated by intravenous hexamethonium whereas no effect was observed after administration of nifedipine or granisetron.

CONCLUSIONS

At a low toxin concentration, intramural reflexes are involved in the inflammatory response whereas axon reflexes contribute to tissue damage. At a high toxin concentration no nervous involvement in the toxin A response was demonstrated except for fluid secretion evoked by the toxin.

Permeability of the proximal and distal rat colon crypt and surface epithelium to hydrophilic molecules

Our knowledge of the epithelial permeability of different sections of the colon as well as of the surface and crypt epithelium is patchy and contradictory. Therefore, movement of radiolabelled urea, mannitol and Cr-EDTA between the lumen and the plasma of rats was studied, and expressed as clearance. In experiments studying movement from the lumen to the plasma, only the clearance of urea was significant. In experiments on the movement from plasma to the lumen, all three permeability probes exhibited significant clearance in the proximal colon, while in the distal colon the clearance of Cr-EDTA was not significant and the other clearance values were lower than in the proximal colon. Thus, the two methods are proposed to mainly reflect the permeability of two different parts of the epithelium, i.e. the surface and the crypt epithelium. Furthermore, it is proposed that the rat surface epithelium only allows passage of hydrophilic substances smaller than monosaccharides [radius below 0.35 nm (3.5 A] while the crypt epithelium, particularly in the proximal colon, is a heteroporous membrane of higher permeability containing pores corresponding to radii of >3.5-4.0 nm (35-40 A) and 0.4-0.5 nm (4-5 A). Moreover, the results indicate that in vivo luminal fluid solution has no access to the crypt epithelium, a conclusion strengthened by the observation that Evans-blue-labelled albumin and FITC-dextran 4000 do not seem to reach the crypt lumina.

Permeability of the rat small intestinal epithelium along the villus-crypt axis: effects of glucose transport

BACKGROUND & AIMS

The aim of this study was to elucidate the permeability characteristics of the epithelium along the villus-crypt axis and investigate the effect of glucose transport on these characteristics along this axis.

METHODS

The disappearance rates of (14)C-mannitol and (51)Cr-EDTA or (3)H-inulin were determined as clearance (Cl(x)) from a recirculating perfusion system of the jejunal lumen in anesthetized rats. Net fluid transport was varied over a large range by exchanging mannitol with glucose in the perfusate solution and by inhibition of nervously mediated secretory processes with hexamethonium. The perfusion rate was 0.5 or 0.2 mL/min.

RESULTS

Cl(Man) enhanced significantly with increasing net fluid transport (secretion 8.50+/-1.88, to absorption 16.72+/-1.75 microL x min(-1) x g(-1)) and with glucose perfusates. Cl(Cr-EDTA) was constant irrespective of net fluid transport and was reduced to insignificant values at a perfusion rate of 0.2 mL/min. Cl(In) was not different from zero.

CONCLUSIONS

The absorbing apical part of the villus contains small pores (radius, <6 A) allowing passive transport via solvent drag of, e.g., monosaccharides, whereas the pores in the crypts are large (50-60 A) and inaccessible to the luminal content. The basal part of the villus contains medium-sized pores (10-15 A) through which no solvent drag occurs. Active glucose transport in the rat mainly increases the number of small pores accessible for passive transport, whereas the size of these pores seems to stay constant.

Effects of neural blocking agents on motor activity and secretion in the proximal and distal rat colon: evidence of marked segmental differences in nicotinic receptor activity

BACKGROUND

Neuromodulation may be a new therapeutic approach in inflammatory bowel disease, but very little is known about neural control of colonic secretion in vivo. We therefore determined the effects of neural blockade on colonic motor activity and mucosal secretion in anaesthetized rats.

METHODS

A proximal and a distal colonic segment were isolated in four groups of chloralose-anaesthetized rats (n = 8 in each group), and we measured luminal pressure and transmural potential difference (PD) as a marker of electrogenic chloride secretion. Recordings were made from proximal and distal segments simultaneously, which made it possible to directly compare response patterns.

RESULTS

Under control conditions luminal pressure waves were associated with phasic, lumen-negative increases in PD which had a significantly greater magnitude and longer duration in the distal colon. Atropine blocked both pressure waves and PD waves in the proximal colon, but some PD waves, although of lower magnitude, remained in the distal colon. Hexamethonium abolished pressure waves in both segments and induced a marked reduction in PD in the distal but not in the proximal colon. Lidocaine also reduced PD, more so in the distal colon, and dissociated the pressure-PD linkage.

CONCLUSION

In the distal but not in the proximal colon, there is a strong nicotinic, neurogenic 'tone' that maintains a high basal secretory activity. The results encourage the search for neuromodulatory agents in the treatment of colonic secretory disease.

Involvement of serotonin and calcium channels in the intestinal fluid secretion evoked by bile salt and cholera toxin

1. The enteric nervous system (ENS) is activated when exposing the intestinal mucosa to cholera toxin or certain bile salts. Cholera toxin stimulates ENS, at least in part, by the release of 5-hydroxytryptamine (5-HT) from the enterochromaffin cells. Calcium channel blockers of the L-type markedly attenuate the fluid secretion and the luminal release of 5-HT caused by cholera toxin. 2. The objective of the present study was to elucidate if sodium deoxycholate activated ENS in a similar manner as cholera toxin. Furthermore, the effect of several calcium channel blockers was tested on the fluid secretion caused by cholera toxin or bile salt. 3. Sodium deoxycholate (4 mM) caused a release of 5-HT into the intestinal lumen, which was inhibited by calcium channel blockade. Granisetron, a 5-HT3 receptor blocker, partly inhibited the fluid secretion caused by bile salt. 4. The effects of nifedipine, felodipine, R-felodipine, H186/86 (t-butyl analogue of felodipine) on the fluid secretion caused by cholera toxin or sodium deoxycholate were studied. Both secretory states were markedly attenuated in a dose dependent manner by all calcium channel blockers tested regardless of their effects on arterial pressure. 5. It is concluded that both cholera toxin and bile salt activate ENS, at least in part, via a release of 5-HT from the enterochromaffin cells. The antisecretory effect calcium channel blockers is partly explained by an inhibition of this release of 5-HT.

The effects on net fluid transport of noxious stimulation of jejunal mucosa in anaesthetized rats

A major aim of the present study was to investigate whether exposing the jejunal mucosa to a noxious stimulus induces a net fluid secretion by activating the enteric nervous system (ENS) and, if so, to what extent an axon reflex was involved. Net fluid transport was measured in vivo with a gravimetric method. The intestinal mucosa was exposed to an isotonic solution with an unphysiologically low pH (1.0). This evoked a fluid secretion, which was markedly attenuated by giving hexamethonium (nicotinic receptor antagonist) i.v. or exposing the intestinal serosa to lidocaine (local anaesthetic). Atropine (muscarinic receptor antagonist) had no effect. Luminal acid evoked a fluid secretion of the same magnitude in acutely denervated segments and in segments denervated about 3 weeks prior to the experiments. Luminal capsaicin (1.6-16 mM) did not influence jejunal net fluid transport. A second aim of the study is to investigate the effect of nifedipine (Ca channel blocker of L-type) on the acid-induced fluid secretion. Nifedipine markedly attenuated acid-induced fluid secretion. In contrast to cholera toxin-evoked secretion, the nifedipine effect was not mediated via 5 hydroxytryptamine (5-HT) as judged by measurements of 5-HT release into the intestinal lumen and the lack of effect of granisetron (5-HT3 receptor antagonist). It is concluded that the net fluid secretion evoked by hydrochloric acid in the small intestine is mainly mediated via an intramural reflex in the ENS. No experimental evidence was obtained for the involvement of an axon reflex. The site of action of the calcium channel blocker is tentatively discussed.

Nervous control of alkaline secretion in the duodenum as studied by the use of cholera toxin in the anaesthetized rat

There is experimental evidence for an axon reflex control of alkaline secretion in the rat duodenum. We have investigated if there is also an intramural reflex control of alkaline secretion similar to that demonstrated with regard to the control of the fluid transport in the rat jejunum. Alkaline secretion in the duodenum of an anesthetized rat was continuously monitored using an in situ titration technique. The segment was extrinsically denervated. Exposing the duodenal segment to 80 microg cholera toxin markedly increased alkaline secretion. This response was abolished by hexamethonium (28 micromol (10 mg) kg(-1) body wt), a nicotinic receptor blocker, lidocaine (0.5 mL of a 1% solution on the serosal surface), a local anaesthetic, and nifedipine (5.75 micromol (2 mg) kg(-1) body wt i.v.), a calcium channel blocker. The response to cholera toxin was partially abolished by granisetron (0.11 micromol (40 microg) kg(-1) body wt i.v.), a 5-HT3 receptor blocker. Atropine (1.7 micromol (0.5 mg) kg(-1) body wt i.v.), a muscarinic receptor blocker, had no effect. We therefore conclude that the alkaline secretion in the rat jejunum evoked by cholera toxin exhibits the same pharmacological properties as the fluid secretion caused by the toxin in the jejunum. This suggests that the alkaline secretion in the rat duodenum is controlled not only by an axon reflex but also by an intramural secretory reflex similar to that controlling fluid transport in the rat jejunum.

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.

Effects of calcium channel blockade on intestinal fluid secretion: sites of action

Most intestinal secretagogues, including cholera toxin, evoke fluid secretion in part by activating the enteric nervous system (ENS). The enterotoxins that, due to size, cannot pass the intestinal epithelial lining have been proposed to activate the ENS via the release of amines/peptides from the intestinal endocrine cells. It has been shown that calcium channel blockers of the L-type attenuate intestinal fluid secretion. This study was performed on rat jejunal segments to elucidate where calcium channel antagonists interact with the secretory nervous reflex(es) of the ENS. In vivo, net fluid transport, transmural potential difference (PD) and luminal release of serotonin from the enterochromaffin cells were monitored before and after exposing the intestinal mucosa to cholera toxin (20 microg/mL) or the calcium ionophore A23187 (0.5 mM). In vitro, the effects of transmural electrical field stimulation (EFS) on short circuit current (SCC) was investigated using the Ussing chamber method. Cholera toxin and A23187 evoked a net fluid secretion, an increased PD and an augmented luminal release of 5-HT. These effects were markedly attenuated by giving the calcium channel blocker nifedipine i.v. (5.75 micromol kg(-1) body wt). On the other hand, nifedipine (0.02 mM) had no significant effect on the increased SCC caused by EFS in vitro. The results obtained in the in vivo experiments suggest that the nifedipine markedly attenuates the initial event in cholera toxin- and A23187-induced secretion, the release of amines and probably also of peptides from the intestinal endocrine cells. The in vitro experiments seem to exclude an effect of the calcium channel blockade on the efferent part of the secretory nervous reflex.

Calcium channels and intestinal fluid secretion: an experimental study in vivo in rats

Several mechanisms involved in nervous secretory reflex(es) of the enteric nervous system may be dependent on the flux of calcium across the plasma membrane, which may be controlled by voltage-gated calcium channels. In this study, we investigated the importance of plasma membrane calcium channels for intestinal fluid secretion. Two types of studies were performed, in which intestinal net fluid transport in anaesthetized rats was followed with a gravimetric method. First, the effects on intestinal fluid transport of placing A23187, a calcium ionophore, in the intestinal lumen was studied. A23187 induced in a dose-dependent manner a net fluid secretion, which was abolished by giving hexamethonium (10 mg kg(-1) body wt) i.v. or placing lidocaine (1% solution) on the intestinal serosa. Nifedipine (5.75 micromol kg(-1) body wt i.v.) also abolished the fluid secretion caused by the ionophore. In the second study, the effects of various calcium channel blockers (gadolinium chloride, nifedipine, verapamil) were tested on the cholera toxin-induced secretion. It was attenuated by luminal application of gadolinium chloride (1-10 mM) or nifedipine (10-200 microM). Intravenously administered nifedipine (2.5-5.75 micromol kg(-1) body wt) abolished cholera toxin-evoked secretion dose-dependently, whereas verapamil (0.05-1 micromol kg(-1) body wt) was without consistent effect. It is concluded that the fluid secretion evoked by placing A23187 in the intestinal lumen in vivo was induced via an activation of the enteric nervous system. Cholera secretion was attenuated or abolished by calcium channel blockers of the L- or N-type.

Substance P effects on blood flow, fluid transport and vasoactive intestinal polypeptide release in the feline small intestine

1. Substance P (SP) infusions were given close I.A. to the feline small intestine in vivo in a dose that produced plasma concentrations of 1-5 microM. This infusion regularly evoked a net fluid secretion measured with a gravimetric technique. Concomitantly, the release into blood of vasoactive intestinal polypeptide (VIP), a putative neurotransmitter of the enteric nervous system, increased. 2. The SP-induced fluid secretion was blocked by tetrodotoxin (7 micrograms close I.A.), a blocker of fast sodium channels in excitable tissues, and hexamethonium (10 mg (kg body wt)-1, I.V.), a nicotinic receptor antagonist, suggesting that the SP effect was mediated by the enteric nervous system. In line with this it was shown that the SP-evoked release of VIP was also significantly diminished by hexamethonium. 3. Close I.A. infusions of methionine enkephalin (Met-enkephalin; 7-23 nmol min-1) or electrical stimulation of the sympathetic nerve fibres (6 Hz) to the intestine markedly diminished net fluid secretion and the release of VIP caused by SP given close I.A. 4. The cyclo-oxygenase inhibitor diclofenac (5 mg (kg body wt)-1, I.V.) or the histamine-1 receptor antagonist pyrilamine (10 mg (kg body wt)-1, I.V.) did not influence the fluid secretion caused by SP, indicating that the effects of SP were not due to the actions of prostaglandins or histamine. 5. It is proposed that SP activates a nervous reflex arch that we have shown to be activated by various luminal stimuli, including cholera toxin.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of the myenteric plexus in the cholera toxin-induced net fluid secretion in the rat small intestine

BACKGROUND

The enteric nervous system is responsible in vivo for most of the change in fluid transport induced by cholera toxin. The aim of the present study was to investigate the importance of the myenteric plexus in the Intramural reflex responsible for this secretion.

METHODS

Long-term ablation of the myenteric plexus was achieved by serosal application of benzalkonium chloride on jejunal segments in rats.

RESULTS

The treated segments without functioning myenteric plexus showed a normal net fluid absorption. Cholera toxin in this segment only induced a reduction of fluid absorption, whereas in a nontreated ileal segment it concomitantly induced a conspicuous net fluid secretion. Intravenous hexamethonium did not change the cholera toxin response in the treated jejunal segments, whereas vasoactive intestinal polypeptide elicited a marked secretion.

CONCLUSIONS

Benzalkonium chloride treatment eliminated the ability of cholera toxin to induce intestinal secretion. Thus, all afferent fibers in the intramural secretory reflex activated by cholera toxin are probably conveyed via the myenteric plexus, which functions as the integrating center in the enteric nervous system. The Ussing chamber technique using stripped intestinal preparations cannot be used when studying effects of luminal secretagogues.

Nerve involvement in fluid transport in the inflamed rat jejunum

Net fluid transport was measured in denervated jejunal segments of rats infected with larvae of Nippostrongylus brasiliensis. On days 6-9 after nematode inoculation, when the jejunal segment exhibited macroscopic and microscopic signs of inflammation, net fluid absorption was noticeably attenuated compared with control, and in eight of 26 experiments a net fluid secretion was seen. To determine whether enteric nerves participated in the response, intravenous hexamethonium (10 mg/kg body weight) was given or lidocaine (1% solution) was placed on the serosa of the intestinal segment. Both drugs significantly reduced fluid secretion or increased fluid absorption. The effect was more pronounced the lower the rate of fluid absorption or the higher the rate of fluid secretion. The inflammatory response influenced intestinal fluid transport partly via activation of the enteric nervous system. It was estimated that 50-60% of the change in fluid transport caused by the parasite could be ascribed to activation of intramural nervous reflexes. The effect of hexamethonium indicates that a cholinergic synapse is present in the secretory nervous reflux activated by inflammation. Experiments were also performed on animals on days 11-14 after infection when the nematodes had been expelled from the animal. A large net fluid absorption was then recorded.

On the involvement of tachykinin neurons in the secretory nervous reflex elicited by cholera toxin in the small intestine

The possible involvement of tachykinins in the nervous reflex activated by exposing the intestinal mucosa to cholera toxin was investigated in cats and rats. Three types of experiments were performed. In cats the release of tachykinins into blood was followed after placing cholera toxin in the intestinal lumen. In rat experiments a tachykinin receptor antagonist (Spantide II) was given close i.a. and its effect on cholera toxin-evoked fluid secretion was studied. Finally, in rats the effect of cholera toxin on the SP contents in the intestinal mucosa was studied. No release of tachykinins could be demonstrated. Spantide II did not change the rate of cholera toxin induced secretion. The SP content in the intestinal mucosa was not influenced by placing the toxin in the intestinal lumen. Hence, no experimental evidence was obtained for the involvement of a tachykinin neuron in the intestinal secretory nervous reflex activated by cholera toxin. Based on observations reported in the literature the involvement of an acetylcholine/tachykinin neuron in the reflex is tentatively discussed.

Actions of serotonin antagonists on cholera-toxininduced intestinal fluid secretion

The effects of several 5-hydroxytryptamine (5-HT) receptor antagonists were tested in rats in vivo on the intestinal fluid secretion evoked by cholera toxin. Five receptor antagonists were used, namely 2-bromolysergic acid diethylamine (2-bromo-LSD), granisetron, ketanserin, methysergide and ondansetron. The drugs were used in doses that inhibited the arterial hypertension and/or bradycardia evoked by 5-HT given i.v. Granisetron and ondansetron markedly diminished cholera-toxin-evoked secretion, whereas ketanserin was without any effect. Methysergide also diminished cholera-toxin-induced fluid secretion particularly when the drug was given as an i.v. infusion. The results are considered in relation to the pathophysiology of cholera secretion and to the current views of receptor subtypes for 5-HT. It is proposed that the receptor involved is a 5-HT3 receptor, possibly also a receptor of the 5-HT1 type. Results from experiments in which 5-HT (20 mM) was placed in the intestinal lumen to evoke an intestinal secretion suggest that the 5-HT3 receptor is located in the villus tissue. It was also demonstrated that zimeldine, an inhibitor of presynaptic 5-HT reuptake, diminished choleraic secretion, an effect that may be ascribed to a 5-HT tachyphylaxis caused by an accumulation of 5-HT in a synaptic cleft.

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.

On the mechanisms of the basal alkaline secretion in the rat ileum in vivo

Basal alkaline secretion of the denervated rat ileum was monitored by a pH-stat method. Changes of transepithelial electrical potential difference (PD) were also continuously registered. In other experiments net fluid transport was measured with a gravimetric method. The importance of the enteric nervous system for the recorded variables was investigated by giving i.v. hexamethonium, neuropeptide Y (NPY) or methionine-enkephalin or by stimulating electrically the mesenteric nerves surrounding the superior mesenteric artery. Alkaline secretion was inhibited by about 20% by mesenteric nerve stimulation or by neuropeptide Y (NPY) or met-enkephalin i.v. A somewhat greater inhibition (approximately 30%) of transepithelial electrical potential difference was elicited by the mesenteric nerve stimulation and NPY whereas met-enkephalin did not cause any transepithelial electrical potential difference change. Net fluid absorption was markedly diminished (by approximately 65-75%) by met-enkephalin but not by NPY. The cellular mechanisms underlying alkaline secretion were investigated by means of amiloride, SITS and acetazolamide. The basal alkaline secretion and transepithelial electrical potential difference were not influenced by 10(-3)M or 10(-4) SITS. In contrast 10(-3) M amiloride caused a significant increase of alkaline secretion but not of transepithelial electrical potential difference. A 35% reduction in the alkaline secretion but not transepithelial electrical potential difference was observed after acetazolamide had been given intravenously. A similar decrease was observed after giving hexamethonium. We conclude: (1) Enteric nerves are of comparatively small importance in controlling the ileal alkaline secretion recorded during basal conditions; (2) About 35% of the basal ileal alkaline secretion is carbonic anhydrase dependent. This mechanism is not influenced by nicotinic receptor blockade; (3) Under the present experimental conditions there may be an alkaline secretion which is concealed by a simultaneously operating Na+/H+ exchanger and; (4) No consistent quantitative correlation exists between alkaline secretion, transepithelial electrical potential difference and net field transport in the denervated rat ileum.

Further studies of the changes in alkaline secretion, transepithelial potential difference and net fluid transport induced by the heat-stable enterotoxin of Escherichia coli (STa) in the rat jejunum in vivo

A pH-stat technique was used to study the mechanisms underlying the intestinal alkalinization evoked by the heat-stable enterotoxin of Escherichia coli (STa) in the rat denervated jejunum in vivo. In addition, concomitant changes of transepithelial potential difference and fluid transport were also investigated. To test the possible involvement of the enteric nervous system in the STa-stimulated alkaline secretion and potential difference, the mesenteric nerves were electrically stimulated or neuropeptide Y or methionine-enkephalin was infused intravenously. None of these interventions inhibited to any large extent the STa-stimulated alkaline secretion, whereas a greater suppression was noted on the concomitantly increased potential difference. Furthermore, neuropeptide Y but not methionine-enkephalin significantly inhibited STa-induced jejunal fluid secretion although neuropeptide Y was without effect on basal fluid transport. It is concluded that the enteric nervous reflex(es) which are of significant importance in explaining STa-evoked fluid secretion plays a minor role in controlling alkaline secretion. Furthermore, alkaline secretion may not contribute to the increase in potential difference caused by STa Amiloride (10(-4) or 10(-3) M) had no effect on the STa-stimulated alkaline secretion, implying that some cellular mechanism other than an inhibition of Na+/H+ exchanger explains the observed response. Similarly, acetazolamide had no effect on the STa-stimulated alkaline secretion or potential difference, suggesting that the secreted alkaline is of extracellular origin rather than from the cellular metabolism in the enterocytes.

Neuronal influence on intestinal transport

Reflex activation of the enteric nervous system (ENS) from the intestinal lumen and also from the serosa induces intestinal secretion. Thus mechanical distention, cholera toxin, heat-stable enterotoxin from E. coli, bile acids, mucosal inflammation and chemical peritonitis all induce an intestinal secretion that is inhibited by 60-100% by nerve-blocking agents. As a result of a large number of in vitro and in vivo studies, a picture of the organization of the secretory enteric nervous reflexes is now emerging. In secretory states with preserved intact intestinal epithelium, it is proposed that the reflex activation occurs via stimulation of receptor cells, i.e. epithelial endocrine cells such as EC and N-cells, which release peptides/amines into the interstitial space and thereby activate nerves close to the epithelium. The afferent neurones appear to transfer the reflex to the myenteric plexus, probably by using tachykinins as transmitters. This is in agreement with a superior and co-ordinating role for the myenteric plexus in the control of intestinal function by the ENS. Interneurones in turn mediate the transmission of the nerve signal to the submucosal plexus and the efferent neurones via cholinergic, nicotinic postganglionic receptors. The transmitters at the effector cells are acetylcholine and probably VIP.

On the role of vasoactive intestinal polypeptide and tachykinins in the secretory reflex elicited by chemical peritonitis in the cat small intestine

Peritonitis induced by serosal application of 0.1 M hydrochloric acid causes net fluid secretion via the enteric nervous system. The aim of the present study was to investigate the roles of vasoactive intestinal peptide (VIP) and tachykinins in this reflex(es). The release of tachykinins (substance P [SP], neurokinin A [NKA], neuropeptide K [NPK]) and VIP into the mesenteric circulation, net fluid transport, intestinal blood flow and sometimes motility were recorded simultaneously in extrinsically denervated jejunal segments of the cat in vivo. The release of both VIP and NKA was increased upon application of HCl to the cat jejunal serosa. Tetrodotoxin, hexamethonium and methionine enkephalin inhibited both the induced VIP release and the secretory response. The increased release of NKA was unaffected by hexamethonium. We propose that the intramural secretory reflex evoked by acid application of the serosa consists of an 'afferent' tachykinin neuron, a cholinergic interneuron and an 'efferent' VIPergic neuron innervating the secretory enterocytes.

Intestinal fluid transport in the small intestine of normotensive and spontaneously hypertensive rats: the importance of enteric nerves, chloride and bicarbonate secretion

Fluid transport was studied in periarterially denervated jejunal segments of spontaneously hypertensive rats (SHR) of the Okamoto strain and as a control also in Wistar-Kyoto rats (WKR). In agreement with the findings of an earlier report a 'spontaneous' fluid secretion was observed in SHR whereas the intestinal segments of WKR absorbed fluid. The fluid secretion in SHR was inhibited by tetrodotoxin or lidocaine placed on the serosal surface of the intestinal segment under study. These observations confirm our earlier proposal that secretory nervous pathways in the enteric nervous system evoke the fluid secretion in SHR. In an attempt to analyse the cellular mechanisms that underlie the fluid secretion in SHR the animals were given loop diuretics in doses that evoked diuresis. No effect on intestinal fluid transport was seen in SHR or WKR. Furthermore, to study the importance of bicarbonate transport alkaline secretion was monitored with a pH-stat technique. In the initial part of the experiments the alkaline secretion in SHR and WKR was similar. In half of the SHR experiments alkaline secretion increased with time. This increase could be completely reversed with hexamethonium and atropine (only tested in three experiments). The time course of the alkaline and fluid secretion in SHR did not coincide, indicating that bicarbonate ion transport was not the major cause of fluid secretion in SHR. In agreement with this conclusion it was observed that acetazolamide (a blocker of carbonic anhydrase) did not influence rate of fluid transport in SHR or WKR.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies of cholera toxin-induced changes of alkaline secretion and transepithelial potential difference in the rat intestine in vivo

A pH-stat technique was used to investigate the effects of cholera toxin (CT) on alkaline secretion from denervated intestines (jejunum, ileum, colon) in anaesthetized rats. Transepithelial potential difference (PD) was also followed in some experiments. CT, given intraluminally, caused a marked increase in jejunal alkaline secretion, whereas only a small effect was observed in the ileum and no apparent effect was noted in the proximal colon. The pronounced increase in jejunal alkaline secretion was found to be inhibited by 10-25% by hexamethonium (10 mg kg-1 body wt i.v.) and similarly by serosal application of lidocaine, whereas atropine (0.25 mg kg-1 body wt i.v.) had no effect. Thus the cholera toxin-induced alkaline secretion in the jejunum is attributed mainly to a non-nervous mechanism. The small effect of CT on ileal alkaline secretion observed in this study contrasts with the high ileal bicarbonate concentration reported in cholera by authors who estimated the concentration from the total carbon dioxide/bicarbonate contents. This discrepancy may be explained by a CT-evoked increased transport of the coupled Na+/H+ and Cl-/HCO3- exchangers, which cannot be measured with the pH-stat technique used in this study.

Effects of heat-stable Escherichia coli enterotoxin on intestinal alkaline secretion and transepithelial potential difference in the rat intestines in vivo

The effects of the heat-stable enterotoxin of Escherichia coli (STa) on intestinal alkaline secretion and transepithelial electric potential difference (PD) were investigated in vivo in denervated segments of rat jejunum, ileum, and proximal colon. STa caused a significant increase in alkaline secretion in the jejunum but not in the ileum or colon. The jejunal effect of STa may be ascribed to a stimulation of bicarbonate secretion and/or an inhibition of Na+/H+ exchange. With regard to PD, STa caused a marked rise in colonic PD, whereas only a small response was found in the jejunum. No effect on PD was seen in the ileum. Hexamethonium (10 mg/kg intravenously) significantly diminished the effects of STa on PD, whereas only a small inhibition of the STa-induced alkaline secretion was observed. The effect of lidocaine on PD and alkaline secretion was found to be similar to that of hexamethonium. Atropine had no effect on any of the studied variables. These findings suggest that STa exerts, via nerves, a profound influence on the jejunal transport mechanisms responsible for the changes in PD, whereas the influence on alkaline secretion is to a large extent not mediated via enteric nerves. Thus, the extent of enteric nervous control of epithelial function differs for different transport functions. The findings also indicate that the bicarbonate ion is not the anion mainly responsible for the fluid secretion elicited by STa.

Effects of cholera toxin, Escherichia coli heat stable toxin and sodium deoxycholate on neurotensin release from the ileum in vivo

Neurotensin (NT) is a biologically active peptide found in specialized epithelial cells (N-cells) in the distal small intestine. In this study we tested the hypothesis that NT may be released by luminal secretagogues, i.e., cholera toxin, Escherichia coli heat-stable toxin and sodium deoxycholate. Cholera toxin elicited net fluid secretion in anesthetized cats. This secretion was accompanied by an increased release of NT-like immunoreactivity (NTLI) into the mesenteric vein when NTLI was measured with either a C-terminally or a N-terminally directed antibody. An increasing plasma NTLI concentration (N-terminally directed antibody) was recorded in the mesenteric vein and femoral artery in cholera experiments. These results indicate that cholera toxin releases NT from the small intestine. Since neurotensin causes intestinal fluid secretion at least in part via an activation of enteric nerves we propose that the N-cell functions as a 'receptor cell' which activates an intramural secretory reflex upon luminal stimulation by cholera toxin. This study does not support a similar role for NT in the secretion elicited by the heat stable toxin of Escherichia coli or by sodium deoxycholate since we were unable to demonstrate any intestinal release of NTLI after exposing the intestine to these secretory agents.

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.

The importance of the subepithelial resistance for the electrical properties of the rat jejunum in vitro

The aim of the study was to evaluate full-thickness and partially stripped jejunum as a model for neurogenic control of electrogenic ion transport. The electrical properties of full-thickness and partially stripped segments were studied in Ussing chambers. Using square-pulse analysis, subepithelial and epithelial resistances (Rs and Rp) were determined, and by compensating for the potential fall across Rs, the current generated by the epithelium could be measured. In full-thickness tissue, Rs was approximately 80% of total tissue resistance, and the current measured during short-circuiting of the whole tissue (SCC) was therefore only 20-25% of the current generated by the epithelium (Im). Surgical stripping of the tissue decreased Rs by 10-20%. This means that in full thickness as well as in stripped tissue, 70-80% of the potential difference across the epithelial layer remains after traditional 'short circuiting'. Over a 25-min period, none of the electrical parameters changed significantly in the full-thickness tissues. In the stripped group PD, SCC and Im fell significantly, and in parallel during the same period of time. Neither glucose, noradrenaline, met-enkephalin or carbachol had any significant effect on Rs, Rp or the Rs/Rp ratio. The relative effects of these agents on Im and SCC were therefore similar. Substance P and VIP increased the Rs/Rp ratio significantly and, therefore, the effect of these drugs on Im was significantly more pronounced than the effect on SCC. The results show that the subepithelial resistance must be taken into account when the electrogenic activity in the epithelium is to be determined correctly. Conventionally measured SCC reflects the electrogenic effect of the tested putative neurotransmitters, but the magnitude of the responses is grossly underestimated, particularly for substance P and VIP.

Somatostatin and methionine-enkephalin inhibit cholera toxin-induced jejunal net fluid secretion and release of vasoactive intestinal polypeptide in the cat in vivo

A major part of the net fluid secretion that is elicited by cholera toxin in the small intestine of the cat has been shown to be mediated by intramural nervous reflex(es). The release of vasoactive intestinal polypeptide (VIP) from the small intestine is increased by cholera toxin. We report that close intra-arterial infusions of methionine-enkephalin (met-enk) and somatostatin cause a parallel reduction in cholera toxin-induced net fluid secretion and in VIP release from the small intestine of the cat. Intestinal blood flow was slightly, but significantly increased by met-enk and not influenced by somatostatin. These results strengthen the hypothesis that VIP is involved as a neurotransmitter in the nervous reflex mediating cholera toxin-induced secretion.

The effect of splanchnic nerve stimulation and neuropeptide Y on cholera secretion and release of vasoactive intestinal polypeptide in the feline small intestine

The effect of sympathetic nerve stimulation and intra-arterial infusion of neuropeptide Y (NPY) on net fluid secretion and release of vasoactive intestinal polypeptide (VIP) was studied in the cat small intestine during a secretion due to cholera toxin. Activation of the splanchnic nerves (4 Hz, 5 ms, 5 V) decreased net fluid secretion to 57 +/- 10% of control. Concomitantly, the release of VIP was reduced to less than 50%. Furthermore, close i.a. infusion of NPY (estimated increase in plasma concentration 75 nmol l-1) reduced the net fluid secretion and VIP release to 27 +/- 5 and 28 +/- 4% of the pre-stimulatory value. The correlation between the decrease in net fluid secretion and reduction in VIP release showed a strong positive correlation (r = 0.83). These results strongly indicate that the antisecretory effect of sympathetic nerve stimulation during cholera diarrhoea is mediated by inhibition of secretory VIP neurons in the intestinal mucosa. A similar mechanism is also proposed for the intravascularly administered NPY.

Sympathetic Control of Intestinal Fluid and Electrolyte Transport

The sympathetic efferent innervation of the small intestine contains fibres that increase intestinal net fluid absorption. This pathway seems to be part of the regulatory system for maintaining fluid balance. Knowledge of the mechanism and type of receptors involved may be of interest in the development of new drugs against diarrheal disease.

Mechanisms underlying the small intestinal fluid secretion caused by arachidonic acid, prostaglandin E1 and prostaglandin E2 in the rat in vivo

Prostanoids were given intraluminally (PGE2) or infused close intra-arterially (PGE1 and PGE2) or arachidonic acid was administered intraluminally to denervated jejunal segments of the rat in vivo. These experimental manoeuvres caused a net fluid secretion, although a 1,000-fold higher concentration of the prostanoids was needed from the luminal than from the vascular side. I.v. hexamethonium or serosally applied lidocaine diminished the induced fluid secretion suggesting that the prostanoids act mainly by eliciting local secretory reflexes in the enteric nervous system. This nerve-mediated secretion is not accompanied by any increase in tissue cAMP. However, at higher i.a. concentrations of PGE2 there seems to be a non-neurogenic effect on the enterocytes associated with an increase in tissue cAMP.

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.

Autoradiographic location of beta-adrenoceptor subtypes in cat colon smooth muscle

In order to localize beta-adrenoceptors 125I-(-)pindolol (IPIN) was used in binding to sections from cat colon. The binding characteristics for IPIN to beta-adrenoceptors on colon sections were estimated by demonstrating reversible binding in the presence of isoprenaline and by steroselective binding to the isomers of propranolol. The binding of IPIN to both beta 1- and beta 2-adrenoceptors was shown by biphasic displacement curves in the presence of the selective beta-adrenoceptor compounds betaxolol, ICI 118.551 and procaterol. The colon sections were found to contain proportions of beta 1-adrenoceptors (30-50%) and beta 2-adrenoceptors (50-70%). In the autoradiographic studies, 100% of the developed grains after exposure of IPIN to the photographic emulsion were displaced by 50 microM of isoprenaline. By microscopic counting at autoradiographic grains, 30-40% of the grains were found in the circular smooth muscle, while 60-70% of the grains were found in the longitudinal smooth muscle. A concentration of 2 nM ICI 118.551 completely displaced all grains in the circular smooth muscle and partly displaced those found in the longitudinal smooth muscle. A high concentration of ICI 118.551 (1 microM) displaced all grains above background from the smooth muscle. It is concluded that the circular smooth muscle only contains beta 2-adrenoceptors, while longitudinal smooth muscle may contain a proportion of beta 1-adrenoceptors. Whether such a location of beta adrenoceptors can be related to the beta 1-adrenoceptor-mediated inhibition of colon motility can not be clarified from these studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Intraluminal bile salt increases rate of firing in afferent fibers from the small intestine of the rat

Perfusion of a rat intestinal segment with a solution containing sodium deoxycholate (8 mM) increases the rate of firing in periarterial afferent nerves from the gut. This observation indirectly supports our earlier proposal that bile salt evokes a net fluid secretion in the small intestine via an activation of the enteric nervous system.

Mechanisms of neurotensin-induced fluid secretion in the cat ileum in vivo

Neurotensin (NT) is released from N cells in the small intestinal epithelium. Among other effects NT is known to elicit fluid secretion in the small intestine. This study was carried out in order to elucidate the mechanism by which NT elicits this secretion. Neurotensin infusions at two rates (4.5 and 45 pmol min-1 kg-1 body wt) to isolated segments of cat ileum in vivo, caused a steady rate of net fluid secretion and a release of vasoactive intestinal polypeptide into the mesenteric vein. The secretion was totally inhibited by tetrodotoxin. Hexamethonium, a nicotinic receptor antagonist, inhibited the secretion elicited by the lower but not by the higher dose of NT. Met-enkephalin also inhibited the induced secretion while pyrilamine, a histamine-I receptor antagonist had no effect. No significant change in enteric blood flow was caused by the NT infusion. These results indicate that NT elicits a nervous reflex in the enteric nervous system which, accordingly, turns the transport of the enterocytes into net fluid secretion.

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.

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.

Intestinal fluid and electrolyte transport in man during reduced circulating blood volume

The effect on intestinal net transport of fluid and electrolytes of a reduced circulating blood volume was studied in the human jejunum with the triple lumen perfusion technique. The blood volume was reduced by changing the lower extremities from an elevated to a dependent position combined with a venous stasis. The tilting manoeuvre, probably resembling a bleeding of about 600-800 ml, significantly increased net absorption of fluid, sodium and chloride while glucose transport was unaffected. Concomitantly the blood flow decreased and vascular resistance increased in the forearm vascular bed. The results are consistent with the hypothesis that activity in the sympathetic nervous system initiated from unloading of the cardiopulmonary volume receptors enhances intestinal absorption of fluid and electrolytes. The results also indicate that the human intestines are an important target organ in the compensatory mechanisms activated during hypovolaemia due to - for example, haemorrhage.

The adrenergic nervous control of fluid transport in the small intestine of normotensive and spontaneously hypertensive rats

Intestinal net fluid transport in normotensive Wistar Kyoto rats (WKR) and spontaneously hypertensive rats of the Okamoto strain (SHR) were studied during 'rest', during electrical stimulation of the regional sympathetic fibres as well as after acute denervation and alpha-adrenergic receptor blockade (phentolamine). During 'rest' no statistically significant difference in fluid transport rate could be demonstrated between WKR and SHR. Cutting the left splanchnic nerve, severing the periarterial nerves or giving phentolamine turned net fluid absorption to net fluid secretion in most SHR, whereas fluid absorption was little influenced in WKR by these procedures. Stimulating the left splanchnic nerve (2, 4, 8 Hz) markedly increased net fluid uptake or decreased net fluid secretion in SHR in a frequency-dependent manner. A small effect was seen in WKR at a stimulation rate of 4 Hz. The 'spontaneous' fluid secretion in denervated intestinal segments of SHR was accompanied by a net chloride secretion. Giving hexamethonium i.v. turned net fluid and chloride secretion into water and ion absorption, suggesting that the secretion was evoked by secretory nervous pathways in the enteric nervous system. It is concluded that the 'spontaneous' fluid and electrolyte secretion seen in denervated intestines of SHR is normally 'concealed' by an augmented rate of firing in the regional adrenergic nerve fibres controlling fluid and electrolyte transport. The possible importance of the 'spontaneous' intestinal secretion in SHR in the pathophysiology of arterial hypertension is tentatively discussed.