Alpha 2-adrenoceptors inhibit the cholera-toxin-induced intestinal fluid accumulation

Cholesterol uptake and efflux are impaired in human trophoblast cells from pregnancies with maternal supraphysiological hypercholesterolemia

Maternal physiological (MPH) or supraphysiological hypercholesterolaemia (MSPH) occurs during pregnancy. Cholesterol trafficking from maternal to foetal circulation requires the uptake of maternal LDL and HDL by syncytiotrophoblast and cholesterol efflux from this multinucleated tissue to ApoA-I and HDL. We aimed to determine the effects of MSPH on placental cholesterol trafficking. Placental tissue and primary human trophoblast (PHT) were isolated from pregnant women with total cholesterol <280 md/dL (MPH, n = 27) or ≥280 md/dL (MSPH, n = 28). The lipid profile in umbilical cord blood from MPH and MSPH neonates was similar. The abundance of LDL receptor (LDLR) and HDL receptor (SR-BI) was comparable between MSPH and MPH placentas. However, LDLR was localized mainly in the syncytiotrophoblast surface and was associated with reduced placental levels of its ligand ApoB. In PHT from MSPH, the uptake of LDL and HDL was lower compared to MPH, without changes in LDLR and reduced levels of SR-BI. Regarding cholesterol efflux, in MSPH placentas, the abundance of cholesterol transporter ABCA1 was increased, while ABCG1 and SR-BI were reduced. In PHT from MSPH, the cholesterol efflux to ApoA-I was increased and to HDL was reduced, along with reduced levels of ABCG1, compared to MPH. Inhibition of SR-BI did not change cholesterol efflux in PHT. The TC content in PHT was comparable in MPH and MSPH cells. However, free cholesterol was increased in MSPH cells. We conclude that MSPH alters the trafficking and content of cholesterol in placental trophoblasts, which could be associated with changes in the placenta-mediated maternal-to-foetal cholesterol trafficking.

Wine Components Inhibit Cholera Toxin-Induced Intestinal Secretion in Rats

Gut bacterial toxins contribute to many diseases, including diarrhea. Cholera represents a life-threatening form of enterotoxin-induced diarrhea caused by infection with the bacterium Vibrio cholerae. Diarrhea results from the prosecretory actions of a bacterial enterotoxin. The aim of this study was to identify dietary compounds that antagonize the intestinal secretory actions of cholera toxin and which might be useful in diarrhea prevention and treatment. Rat small intestine (jejunum) was ligated in vivo and separate segments of small bowel were randomly injected with various dietary compounds in the presence and absence of cholera toxin. The secretory response to cholera toxin was assessed by measuring the amount of fluid secreted into the bowel segments. Compared to controls, red wine and wine components significantly inhibited the small intestinal secretory response to cholera toxin. Grape juice, ethanol (12%), and World Health Organization rehydration fluid had lesser antisecretory effects. These novel results demonstrate that nonethanol components of wine possess antisecretory effects. These substances may be useful in treating gut enterotoxin-mediated diseases such as cholera.

Characterization of alpha-adrenoceptor subtypes in smooth muscle of equine ileum

OBJECTIVE

To determine the concentration and binding characteristics of alpha-adrenoceptor subtypes in smooth muscle cell membranes of equine ileum.

SAMPLE POPULATION

Segments of longitudinal and circular smooth muscle from the ileum of 8 male and 8 female adult horses.

PROCEDURE

Distribution of alpha-adrenoceptor subtypes was assessed by use of radioligand binding assays incorporating [3H]-prazosin and [3H]-rauwolscine, highly selective alpha1- and alpha2-adrenoceptor antagonists, respectively. Characterization of adrenoceptor subtypes was performed by use of binding inhibition assays.

RESULTS

On the basis of binding affinity for specific radioligands, low- and high-affinity alpha1- and alpha2-adrenoceptors were detected. Concentration of low-affinity alpha2-adrenoceptors was significantly greater in male horses, compared with females. Competition studies confirmed the specificity of the radioligands used in the binding assays. Alpha1-adrenoceptors of both subtypes in male and female horses had a higher affinity for prazosin than phentolamine, whereas yohimbine did not compete with the radioligand for binding. For alpha2-adrenoceptors regardless of subtype, potency of inhibition elicited by each drug varied between sexes. In males, yohimbine was a more potent inhibitor than phentolamine, which was more potent than prazosin. In females, yohimbine was more potent than prazosin, which was more potent than phentolamine.

CONCLUSIONS AND CLINICAL RELEVANCE

High- and low-affinity alpha1- and alpha2-adrenoceptors were detected in smooth muscle of equine ileum. Because alpha-adrenoceptor subtypes, particularly alpha2-adrenoceptors, are involved in the regulation of gastrointestinal tract function, characterization of these receptors may represent the basis for development of new therapeutic strategies for the control of gastrointestinal disturbances in horses.

A morpholinocatechol compound (UK42620) with clonidine- and tyramine-like actions

1. The actions of a morpholinocatechol (UK42620) were studied in rat isolated atria preparations consisting of spontaneously beating left and right atrial pairs. 2. UK42620 produced positive inotropic and chronotropic responses and, in atria that were incubated with [(3)H]-noradrenaline, it also produced a massive increase in the release of radioactivity. 3. These actions of UK42620 were similar to those of tyramine and were blocked by the beta-adrenoceptor antagonist propranolol (0.3 microM) and by the neuronal uptake blocker desipramine (1 microM). 4. In the presence of desipramine, UK42620 but not tyramine produced a decrease in the stimulation-induced efflux of radioactivity that was antagonized by idazoxan. 5. Thus, UK42620 had prejunctional alpha(2)-adrenoceptor activity like that of clonidine- and tyramine-like activity releasing large amounts of noradrenaline.

The inhibitory effects of alpha(2)-adrenoceptor agonists on gastrointestinal transit during croton oil-induced intestinal inflammation

1. The peripheral effects of alpha(2)-adrenoceptor agonists were investigated in a model of intestinal inflammation induced by intragastric administration of croton oil (CO). Our hypothesis was that inflammation would 'sensitize' adrenoceptors in peripheral and/or central terminals of myenteric and submucous plexus neurones, and enhance systemic effects of alpha(2)-adrenoceptor agonists. 2. Male swiss CD-1 mice, received intragastrically CO (0.05 ml), castor oil (CA, 0.1 ml) or saline (SS) 3 h before the study: gastrointestinal transit (GIT) was evaluated 20 min afterwards with a charcoal meal. The presence of inflammation was assessed by electron microscopy. 3. The intragastric administration of CA or CO caused an increase in GIT and weight loss, but only CO induced an inflammatory response. Both clonidine (imidazoline1/alpha(2)-agonist) and UK-14304 (alpha(2)-agonist) produced dose-related inhibitions of GIT in all groups. During inflammatory diarrhoea (CO), potencies of systemic (s.c.) clonidine and UK-14304 were significantly increased 3.5 and 2.1 times, respectively, while potencies remained unaltered in the presence of diarrhoea without inflammation (CA). The effects were reversed by administration (s.c.) of receptor-specific adrenoceptor antagonists, but not by naloxone. 4. Clonidine was 8.3 (SS) and 2.8 (CO) times more potent when administered intracerebroventricularly (i.c.v.), than when administered s.c. Inflammation of the gut did not alter the potency of i.c.v. clonidine, demonstrating that enhanced effects of s.c. clonidine are mediated by peripheral receptors. During inflammation, i.c.v. efaroxan did not antagonize low doses of s.c. clonidine (ED20 and ED50S), but partially reversed ED80S, further supporting the peripheral effects of the agonists in CO treated animals. 5. The results demonstrate that inflammation of the gut enhances the potency of alpha(2)-adrenoceptor agonists by a peripheral mechanism. The results also suggest that the inflammatory response induces an up-regulation or sensitization of alpha(2)-adrenoceptors and/or imidazoline receptors.

Alpha-adrenoceptors

Major advances have been made in our understanding of the molecular structure and function of the alpha-adrenoceptors. Many new subtypes of the alpha-adrenoceptor have been identified recently through biochemical and pharmacological techniques and several of these receptors have been cloned and expressed in a variety of vector systems. Currently, at least seven subtypes of the alpha-adrenoceptor have been identified and the molecular structure and biochemical functions of these subtypes are beginning to be understood. The alpha-adrenoceptors belong to the super family of receptors that are coupled to guanine nucleotide regulatory proteins (G-proteins). A variety of G-proteins are involved in the coupling of the various alpha-adrenoceptor subtypes to intracellular second messenger systems, which ultimately produce the end-organ response. The mechanisms by which the alpha-adrenoceptor subtypes recognize different G-proteins, as well as the molecular interactions between receptors and G-proteins, are the topics of current research. Furthermore, the physiological and pathophysiological role that alpha-adrenoceptors play in homeostasis and in a variety of disease states is also being elucidated. These major advances made in alpha-adrenoceptor classification, molecular structure, physiologic function, second messenger systems and therapeutic relevance are the subject of this review.

Review article: adrenergic control of motor and secretory function in the gastrointestinal tract

The role that the sympathetic nervous system plays in modulating physiological processes in the gastrointestinal tract is becoming clearer. It is now known that motor, secretomotor and vasomotor activity are all modulated independently by the system. Adrenoreceptor stimulation appears to reduce intestinal contraction (except at sphincters), both via alpha-receptors which inhibit neurotransmitter release and also by a direct beta-receptor mediated action on smooth muscle. There is also evidence for tonic activity in the beta-adrenergic pathway, since beta-antagonists tend to increase contraction pressures. In animals alpha-receptor-mediated pathways modulate fluid and electrolyte absorption, and alpha-adrenergic agonists enhance net absorption and reduce net secretion. In man there is also evidence for a beta-adrenergic pathway which controls secretomotor function. Carbohydrate absorption appears to be dependent on activity in a beta-adrenergic pathway, although this may be an indirect effect of changes in motor function. The time course of changes of both secretomotor and motor activity, induced by modulating sympathetic or adrenergic input, differ from the vascular changes indicating that the effects occur independently of each other. The gastrointestinal response to stressors is mediated, in part at least, by the sympathetic nervous system. Differences between individuals are likely to prove important. Since the sympathetic nervous system regulates gastrointestinal function both in the basal state and under stressful conditions, it will have effects on pathophysiological responses. Modification of such responses is likely to ameliorate symptoms, as has already been found for alpha-2-adrenergic agonists which have an antidiarrhoeal action.

Antisecretory effect of splanchnic nerve stimulation on choleratoxin-induced secretion in the cat, an effect mediated at the crypts

The experiments were performed on cats anaesthetized with alpha-chloralose. Segments of the small intestine were perfused with sodium-free hypotonic choline-mannitol solution and intestinal net fluid transport was recorded with a volumetric technique. The content of sodium and chloride in the lamina propria of the small intestinal villus was measured with an electron microprobe in freeze-dried paraffin embedded tissue. In absorbing control intestine, there was an even distribution of electrolytes along the villi. Sympathetic nerve stimulation (5 Hz, 5 ms, 5 V) did not significantly affect electrolyte distribution and net fluid transport. Intestinal secretion was elicited by pretreatment of the intestine with cholera toxin. The concentration of sodium and chloride was elevated in the apical third of the villi in intestines during the secretion since secreted sodium from the crypts was reabsorbed into the villi. Sympathetic nerve stimulation decreased the cholera secretion significantly in intestines pretreated with cholera-toxin. Furthermore, the apical gradients of sodium and chloride in the villi, caused by the reabsorbed sodium and chloride, disappeared during sympathetic nerve stimulation. It is concluded that, in the used experimental model, the antisecretory effect of sympathetic nerve stimulation was caused by inhibition of crypt secretion and not by augmented villus absorption.

Regulation of serotonin release from the intestinal mucosa

In the mammalian intestine serotonin (5-hydroxytryptamine, 5-HT) is present in high concentrations in the enterochromaffin cells. The release of 5-HT from the intestinal mucosa is regulated by a complex pattern of neuronal and humoral inputs to the enterochromaffin cells. The enterochromaffin cells appear to be endowed with different inhibitory (alpha 2-adrenoceptors, GABAA- and GABAB-receptors, histamine H3-receptors, receptors for vasoactive intestinal polypeptide and somatostatin) as well as stimulatory receptors (beta-adrenoceptors, muscarine and nicotine receptors). The physiological significance of this complex system of receptors is suggested by experiments which demonstrate that the respective intrinsic neurotransmitters (catecholamines, acetylcholine, GABA and vasoactive intestinal polypeptide) released within the gut are involved in the regulation of the release of 5-HT from the enterochromaffin cells.

Alpha 2-adrenoceptor antisecretory responses in the rat jejunum

1. Addition of noradrenaline (5 microM), UK-14,304 (0.1 microM) or phenylephrine (100 microM) to the serosal surface of sheets of rat jejunum mounted in Ussing type chambers decreased resting transepithelial short circuit current (SSC). These responses were inhibited by the selective alpha 2-adrenoceptor antagonist idazoxan (0.5-5.0 microM) but were not significantly affected by the alpha 1-adrenoceptor antagonist corynanthine (100 microM). 2. All three agonists caused a dose-related reduction of SSC which had been elevated by prior addition of theophylline (4 mM). UK-14,304 was 8.1 times more potent than noradrenaline and 76.2 times more potent than phenylephrine. Idazoxan (10 nM-1 microM) caused rightward, parallel displacements of the concentration-response curves for noradrenaline and UK-14,304; PA2 values were 7.87 and 8.02 respectively. Dose-response curves to noradrenaline were unaffected by prazosin (1 microM). 3. The reduction by noradrenaline (10 microM) of SCC which had been elevated by theophylline (4 mM) was not significantly affected by tetrodotoxin (5 microM), suggesting that noradrenaline reduces SCC in rat jejunum via a direct mucosal mechanism. 4. The resting SCC was markedly reduced in chloride-free PSS and under these conditions, noradrenaline did not elicit any further fall in SCC. In bicarbonate-free physiological salt solution (PSS) the resting SCC and the noradrenaline-induced fall in SCC were not significantly different from those in normal PSS. It is concluded that the decrease in SCC observed with noradrenaline, UK-14,304 and phenylephrine is mediated via stimulation of postjunctional alpha 2-adrenoceptors.

Clinical trial of clonidine hydrochloride as an antisecretory agent in cholera

Clonidine hydrochloride (an alpha 2-adrenoceptor agonist) was tested for antisecretory effects in patients with cholera in a randomized controlled trial. Nineteen adults with diarrhea due to Vibrio cholerae were treated with clonidine (0.9 mg/24 h orally for 72 h) and 18 served as controls. During the first 24 h of treatment and for 24 h afterwards, the mean +/- SD concentrations of sodium (in millimoles per liter) in the stools of clonidine-treated patients were 120.6 +/- 10.9 and 112.3 +/- 11.9, which were significantly lower than 135.5 +/- 17.1 and 125.0 +/- 16.4 in the controls (p less than 0.01). Stool chloride concentrations (in millimoles per liter) were also significantly less in the clonidine group during the same periods: 82.1 +/- 16.8 and 62.4 +/- 19.4 vs. 92.1 +/- 18.3 and 78.0 +/- 23.0, respectively (p less than 0.05). Concentrations of potassium but not bicarbonate were also significantly reduced in the stools of clonidine-treated patients (p less than 0.05). However, there were no significant differences in the mean +/- SD stool volumes (in liters) between the clonidine and the control group in any of the six 12-h periods after treatment or in the cumulative volumes in 72 h (24.2 +/- 10.6 and 22.9 +/- 8.3, respectively). We conclude that clonidine causes modest reduction of stool electrolyte loss but does not significantly reduce fecal fluid loss in patients with cholera.

Adrenergic modulation of the release of 5-hydroxytryptamine from the vascularly perfused ileum of the guinea-pig

1. Isolated segments of the guinea-pig ileum were vascularly perfused and the release of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) into the portal venous effluent was determined by h.p.l.c. with electrochemical detection. Test substances were applied via the arterial perfusion medium. 2. Isoprenaline (0.1 microM) increased the outflow of 5-HT and 5-HIAA maximally by about 75% and this was antagonized by propranolol (0.1 microM). Forskolin (1-10 microM) increased the outflow of 5-HT by approximately 105% and that of 5-HIAA by approximately 55%. The phosphodiesterase inhibitor AH 21-132 (0.1-1 microM) increased the outflow of 5-HT and 5-HIAA by about 70%. Isoprenaline (1 nM) and AH 21-132 (10 nM), which alone had no effect, increased the outflow of 5-HT and 5-HIAA by 75%, when applied in combination. 3. Clonidine (1 microM) reduced the outflow of 5-HT by 45%, an effect blocked by tolazoline (1 microM), but not by prazosin (0.1 microM). 4. The effects of isoprenaline, forskolin and clonidine were also observed in the presence of tetrodotoxin (1 microM) demonstrating a direct modulation of 5-HT release from the enterochromaffin cells. 5. In conclusion, the release of 5-HT from enterochromaffin cells is facilitated by activation of beta-adrenoceptors and inhibited via alpha 2-adrenoceptors. Enhancing intracellular cyclic AMP, by direct stimulation of adenylate cyclase with forskolin or by inhibition of phosphodiesterase, also facilitates the release of 5-HT. The beta-adrenoceptor-mediated effect on 5-HT release appears to involve an increase in cyclic AMP, as the effect of isoprenaline was potentiated after inhibition of phosphodiesterase.

A submucosal mechanism for catecholamine-induced increases in fluid absorption in rabbit ileum in vitro

1. The effects of clonidine and dopamine on water movements across the mucosal and serosal surfaces of rabbit ileum have been investigated using a high-resolution method for monitoring water flows in vitro. 2. Theophylline (10 mM) and carbamyl choline (10 microM) caused a reduction in fluid inflow across the mucosal surface and a smaller decrease in fluid outflow across the serosal surface. Addition of the alpha 2-adrenergic agonist clonidine or dopamine fully reversed the theophylline, or carbamyl choline-induced decrease in mucosal inflow in a dose-related manner. 3. The effects of clonidine on mucosal inflow are blocked by the alpha 2-adrenergic antagonist, yohimbine. Yohimbine was much less effective than pimozide or d-butaclamol in blocking the effect of dopamine on mucosal inflow. These findings support the view that there are separate alpha 2-adrenergic and dopaminergic receptors. 4. The hydraulic conductance (Lp) of the serosal surface was measured directly from the change in serosal exit flow following addition of 2 mosmol kg-1 of polyethylene glycol (molecular mass 20,000 Da) to the serosal bathing solution. Theophylline reduced the Lp by 35%. Clonidine (1 microM) added to theophylline-treated tissues increased the Lp by 66%. This effect was prevented by yohimbine (1 microM). 5. The effects of theophylline, clonidine and dopamine on the permeability of the mucosal and serosal surfaces of the tissue to [3H]mannitol were measured. These showed that theophylline increased the rate of labelled mannitol loss across the mucosal surface but reduced the mannitol permeability across the serosal surface. This latter effect was reversed by clonidine and dopamine. 6. Changes in transepithelial electrical potential difference (PD), short-circuit current and resistance were monitored. Theophylline caused a rapid increase in PD and short-circuit current and a slower increase in resistance. Clonidine (5 microM) reversed the effects on PD and resistance but was without significant effect on short-circuit current. The results suggest that a major component of secretagogue-induced reduction in fluid transport in vitro is due to mechanical changes in the submucosa, probably induced by modulation of neurotransmitter release within the tissue.

Effect of moderate exercise on salt and water transport in the human jejunum

The effect of moderate exercise on jejunal absorption was examined in seven healthy subjects using a triple lumen perfusion technique. Moderate exercise on a bicycle ergometer significantly reduced net absorption of water from 32.0 (4.0) to 16.2 (6.1) ml/30 cm/50 min (p less than 0.02), sodium from 2.4 (0.4) to 0.5 (0.9) mmol/30 cm/50 cm (p less than 0.05), chloride from 2.0 (0.4) to 0.3 (0.7) mmol/30 cm/50 min (p less than 0.05), and potassium from 0.20 (0.02) to 0.01 (0.04) mmol/30 cm/50 min (p less than 0.01). After exercise, water, sodium, and chloride absorption returned towards basal values, but potassium absorption remained significantly decreased. These results suggest that moderate exercise can influence jejunal absorption of salt and water in man. They support the possibility that the autonomic nervous system has a physiological role in the control of intestinal transport, although other mechanisms may be involved.

Central mu, delta- and kappa-opioid influences on intestinal water and electrolyte transport in dogs

The effects of intracerebroventricular (i.c.v.) administration of opioid peptides with mu-(DAGO), mu- and delta-(DALAMIDE, DADLE) and kappa-(dynorphin) properties on normal and stimulated (cholera toxin) net fluxes of water, Na+ and K+ through a jejunal Thiry-Vella loop were investigated in conscious dogs. Basal net water absorption was slightly, but significantly (P less than 0.05) increased during i.c.v. infusion of DALAMIDE or DAGO (0.5 ng/kg/min) but not DADLE and dynorphin-(1-13) at the same rate; DALAMIDE and DAGO also markedly reduced (by 72.3 and 79.5% respectively) the secretory effects of cholera toxin (0.4 micrograms/ml). Similar effects were obtained with DALAMIDE and DAGO when injected i.c.v. as a bolus (100 ng/kg) prior to cholera toxin infusion; they were suppressed after i.v. pretreatment with naltrexone (0.3 mg/kg) but also with propranolol (0.2 mg/kg). In contrast, i.v. phentolamine (0.2 mg/kg) and bilateral truncal vagotomy, were unable to block their effects. These results suggest that Met-enkephalin can act centrally to affect intestinal transport of (i) water and (ii) electrolytes in dogs. They act probably at central mu-receptors which are involved in the regulation of intestinal secretion mediated through a central or peripheral beta-adrenergic pathway.

Effect of cold-induced pain on salt and water transport in the human jejunum

The possibility that the central and autonomic nervous systems might be involved in the control of intestinal absorption was assessed by measuring the effect of physical stress (cold-induced pain) on jejunal transport in 13 healthy subjects. Using a triple-lumen perfusion technique to determine jejunal absorption, cold pain significantly reduced net water absorption from 34.9 to 15.7 ml/30 cm.50 min (p less than 0.005), net sodium absorption from 2.6 to 0.2 mEq/30 cm.50 min (p less than 0.005), and net chloride absorption from 2.2 to 0.6 mEq/30 cm.50 min (p less than 0.02). These changes were associated with an elevation of blood pressure and plasma norepinephrine during cold pain. During the period following cold pain, systolic blood pressure remained significantly elevated and net jejunal water absorption significantly reduced. These results suggest that physical stress can influence jejunal absorption of salt and water in humans and support the possibility that the central and autonomic nervous systems have a physiologic role in the control of intestinal function.

Electrogenic and electroneutral components of the sympathetic effect on fluid absorption in the rat jejunum

The aim of the study was to test the hypothesis that the sympathetic nerves to the jejunum enhance net fluid absorption rate by inhibiting an electrogenic flux of anions into the lumen. The design of the experiments was based on the observation that the effect of catecholamines on electrogenic transport is abolished by yohimbine, an alpha 2 adrenergic antagonist. Net electrogenic transport in jejunal segments of anaesthetized rats was estimated by measurement of the transepithelial potential difference (PD) and short-circuit current (SCC). Net fluid absorption rate was quantified by a gravimetric technique. The sympathetic nerves to the segment were stimulated electrically, both in vitro and in vivo. The effect of sympathetic nerve stimulation on SCC and net fluid transport was determined in the absence and presence of the alpha-adrenergic antagonists phentolamine (a non-selective alpha-adrenergic antagonist), prazosin (a selective alpha 1-antagonist) and yohimbine (a selective alpha 2-antagonist). Sympathetic nerve stimulation decreased PD and SCC and increased net fluid absorption rate. Phentolamine abolished both the electrogenic response and the effect on net fluid absorption rate, without having any significant intrinsic effects on either parameter. Prazosin per se markedly increased net fluid absorption rate, but did not significantly influence the sympathetic effect on SCC or net fluid absorption rate. Yohimbine abolished the electrogenic effect of sympathetic stimulation, and per se increased net fluid absorption rate in innervated but not in denervated segments. In innervated segments, the absorption rate during sympathetic nerve stimulation was similar in controls and yohimbine-treated animals. In denervated segments, yohimbine significantly attenuated the sympathetic response. The results suggest that the sympathetic nerves enhance fluid absorption rate via effects on both electrogenic and electroneutral epithelial transport. These two components of the sympathetic response seem to be mediated by different alpha-receptor mechanisms.

Neural control of electrogenic transport in the rat jejunum; interaction between intramural and adrenergic mechanisms

The aim of the study was to test the hypothesis that extrinsic adrenergic neurons modulate electrogenic ion transport in the jejunum by inhibition of intramural secretory nerve activity. Isolated pieces of rat jejunum were mounted in the Ussing chamber. Noradrenaline (NA) was released from sympathetic nerve endings in the tissue by electrical stimulation of the mesenteric nerves or by the indirect sympathomimetic agent tyramine. Intramural neurons were activated by electrical field stimulation (EFS). Mesenteric nerve stimulation induced a frequency-related decrease in the transepithelial potential difference (PD) and short circuit current (SCC), effects which were abolished by the alpha-adrenergic antagonist phentolamine. A similar response was induced by the serosal application of tyramine and by exogenous NA. Tetrodotoxin (TTX), a nerve conduction inhibitor, had no significant effect on the response to NA, but attenuated the response to tyramine. Both NA and tyramine markedly attenuated the secretory response to EFS, but there was no significant effect of catecholamines on the secretory response to serosal theophylline. The results suggest at least two mechanisms behind the adrenergic modulation of electrogenic chloride secretion: (I) a ganglionic or presynaptic modulation of secretory nerve activity via innervated alpha-receptors, and (2) a modulation at the level of the enterocytes via non-innervated alpha-receptors. The latter effect does not seem to be mediated by the cAMP system.

Effect of psychological stress on salt and water transport in the human jejunum

The possibility that the central and autonomic nervous system might be involved in the control of intestinal absorption was assessed by measuring the effect of psychological stress, induced by dichotomous listening, on jejunal transport in 15 healthy subjects. Using a triple-lumen perfusion technique to determine jejunal absorption, dichotomous listening significantly reduced mean net water absorption from 31.2 to 8.7 ml X 30 cm-1 X h-1 (p less than 0.005) and changed mean net sodium and chloride absorption to secretion (sodium +1.14 to -1.2 mEq X 30 cm-1 X h-1, p less than 0.01; and chloride +1.5 to -0.9 mEq X 30 cm-1 X h-1, p less than 0.02). During the hour following dichotomous listening these changes were maintained. Jejunal water, sodium, and chloride absorption measured in 9 subjects during three consecutive stress-free periods did not change significantly. Mean transit time over the 30-cm test segment, measured by a dye dilution technique, in 7 subjects was not significantly different during control and dichotomous listening periods, being 2.8 and 4.0 min, respectively. During the hour following dichotomous listening there was a significant increase in mean transit time to 6.4 min (p less than 0.02). In 7 subjects who received a constant intravenous infusion of atropine throughout the control, dichotomous listening, and recovery periods, jejunal sodium, chloride, and water absorption did not change significantly. These results suggest that psychological stress can influence jejunal absorption of salt and water and that this effect may be mediated by a cholinergic parasympathetic nervous mechanism.

Pathogenesis and pharmacology of diarrhea

The etiological factors involved in diarrhea are multiple. Also the mechanisms and mediators involved are multiple: intracellular mediators (Ca, cAMP, cGMP, calmodulin, phospholipids), extracellular mediators (hormones, neurotransmitters, prostaglandins, enterotoxins...), intramural blood flow and oxygen, intestinal motility (local- and peristaltic motility). Till now, antidiarrheals are not so versatile that they provide a solution to all types of diarrhea. The mechanisms of action of fluid replacement therapy, loperamide, alpha 2 agonist and some nonsteroidal anti-inflammatory substances are reviewed.

The interaction of alpha- and beta-adrenergic agonists on amylase release from parotid glands of euthyroid and hypothyroid rats

The interaction between alpha- and beta-adrenergic agonists was examined with respect to amylase release from rat parotid glands. The effect of hypothyroid status on this interaction was also compared with that in euthyroid rats. Both methoxamine and clonidine potentiated isoproterenol-induced amylase release from parotid glands of eu- and hypothyroid rats, but the alpha 2-adrenoceptor-mediated response disappeared in the hypothyroid rats. Dibutyryl cyclic AMP, a second messenger of beta-adrenergic agonists, also showed essentially the same results as those of isoproterenol, but potentiation of dibutyryl cyclic AMP-induced amylase release with alpha-adrenergic agonists was mediated through only alpha 1-adrenoceptors in both groups. Calcium ion plays an important role in the interaction between alpha- and beta-adrenergic agonists. These results suggest that the potentiating effect of alpha-adrenergic agonists may be mediated at least in part through an unknown mechanism at the step distal to cyclic AMP formation in both eu- and hypothyroid rats.

Studies of the antidiarrheal action of clonidine. Effects on motility and intestinal absorption

Clonidine, an alpha 2-adrenergic agonist, has been reported to stimulate the rate of electrolyte absorption in vitro, to alter intestinal motility in vivo, and to have antidiarrheal effects in animals. Experiments were performed in 8 healthy volunteers in order to evaluate the antidiarrheal effect of clonidine in humans. When diarrhea was induced by intragastric infusion of 2700 ml of balanced electrolyte solution over 90 min, oral administration of 0.3 mg of clonidine reduced the volume of rectal effluent by 48% (from 1233 +/- 62 to 640 +/- 77 ml, p less than 0.001), a clear-cut antidiarrheal effect. Clonidine increased total gut volume significantly (from 987 +/- 91 to 1830 +/- 142 ml, p less than 0.001), suggesting that clonidine exerted its antidiarrheal effect by altering gut motility, i.e., increasing the capacity of the gut and slowing the transit of fluid through the intestine. In other experiments, the net absorption rate of the whole gut during steady state total gut perfusion was measured. The rate of absorption of fluid was transiently stimulated by clonidine by 15% (from 696 +/- 77 to 799 +/- 55 ml/h, p less than 0.02), indicating an additional effect on mucosal cell function. These studies indicate that in this experimental diarrhea model, clonidine has antidiarrheal properties that are due largely to effects on motility of the gut but that clonidine also modestly stimulates the net rate of absorption by intestinal mucosa.

Inhibition of ADP-ribosyltransferase activity of cholera toxin by MDL 12330A and chlorpromazine

ADP-ribosylation by cholera toxin of the guanine nucleotide binding regulatory protein (Gs) of rat liver membrane adenylate cyclase was inhibited by 0.1-1 mM MDL 12330A or 0.1-1 mM chlorpromazine. Basal as well as cholera toxin activated adenylate cyclase activity in liver membranes was also inhibited by the two drugs. NAD glycohydrolase activity and self-ADP-ribosylation of cholera toxin were also inhibited by MDL 12330A and chlorpromazine. These effects of MDL 12330A and chlorpromazine may be related to their effects on cholera toxin-induced fluid secretion in vivo.

Antisecretory effects of berberine with morphine, clonidine, L-phenylephrine, yohimbine or neostigmine in pig jejunum

The effects of berberine alone or in combination with morphine, clonidine, L-phenylephrine or yohimbine were compared in Escherichia coli heat-stable enterotoxin (ST)-exposed ligated jejunal loops in 2 week old pigs. In addition, net water and electrolyte fluxes in normal jejunal loops were measured in the presence of neostigmine, morphine, clonidine, L-phenylephrine or yohimbine alone or in combination with berberine. Berberine, morphine, clonidine and L-phenylephrine each reduced the net secretion of water and electrolytes induced by ST (P less than 0.05). A significant enhancement of antisecretory effect was observed only with the combination of berberine and L-phenylephrine. Yohimbine or neostigmine augmented the net loss of water and electrolytes produced by ST. Yohimbine did not block the antisecretory action of berberine. In normal jejunum, there was no significant difference in water and ion absorption between adrenergic or opiate agonists alone and their combination with berberine. Neostigmine reversed absorption to net secretion in normal jejunum and this effect was significantly reduced by berberine. The anti-secretory action of berberine appears similar to that of alpha 2-adrenergic agonists, opiates, and anticholinergic agents.

Inhibition of dibutyryl cyclic AMP-induced insulin release by alpha-2 adrenergic stimulation

Effects of alpha adrenergic agonists and antagonists on dibutyryl cAMP (Bt2cAMP)-induced insulin release were investigated with isolated rat pancreatic islets. Bt2cAMP (4 mM) produced 2-fold stimulation of insulin release in all the concentrations of glucose examined (3.3-16.7 mM). Clonidine but not phenylephrine inhibited the Bt2cAMP-stimulated insulin release in a concentration-dependent manner with approximate EC50 of nanomolar range. Yohimbine but not prazosin antagonized the inhibitory effect of clonidine on the Bt2-cAMP-induced insulin release. These results suggest that alpha-2 adrenergic mechanisms are involved in a step distal to cAMP generation.

Enterotoxigenic diarrhea: mechanisms and prospects for therapy

Studies over the past decade have led to a rudimentary but working knowledge of intestinal electrolyte transport. Purification of bacterial exotoxins has allowed an understanding of how these agents stimulate cells. Coupled NaCl influx processes and chloride secretory mechanisms have been shown to be affected by exotoxin-stimulated increases in cyclic nucleotides, as well as by increases in intracellular calcium and arachidonic acid metabolites. Catecholamines, somatostatin, phenothiazines, nonsteroidal anti-inflammatory drugs, and opiates appear to be the most promising of the antisecretory drugs. While the mechanism of action of these agents remains to be determined, there is significant hope that effective antisecretory drugs will emerge in the near future.