Studies of the mechanism of noradrenaline stimulation of fluid absorption by rat jejunum in vitro

Central nervous system control of gastrointestinal motility and secretion and modulation of gastrointestinal functions

Although the gastrointestinal (GI) tract possesses intrinsic neural plexuses that allow a significant degree of autonomy over GI functions, the central nervous system (CNS) provides extrinsic neural inputs that regulate, modulate, and control these functions. While the intestines are capable of functioning in the absence of extrinsic inputs, the stomach and esophagus are much more dependent upon extrinsic neural inputs, particularly from parasympathetic and sympathetic pathways. The sympathetic nervous system exerts a predominantly inhibitory effect upon GI muscle and provides a tonic inhibitory influence over mucosal secretion while, at the same time, regulates GI blood flow via neurally mediated vasoconstriction. The parasympathetic nervous system, in contrast, exerts both excitatory and inhibitory control over gastric and intestinal tone and motility. Although GI functions are controlled by the autonomic nervous system and occur, by and large, independently of conscious perception, it is clear that the higher CNS centers influence homeostatic control as well as cognitive and behavioral functions. This review will describe the basic neural circuitry of extrinsic inputs to the GI tract as well as the major CNS nuclei that innervate and modulate the activity of these pathways. The role of CNS-centered reflexes in the regulation of GI functions will be discussed as will modulation of these reflexes under both physiological and pathophysiological conditions. Finally, future directions within the field will be discussed in terms of important questions that remain to be resolved and advances in technology that may help provide these answers.

Effect of alpha-methylnorepinephrine, an alpha 2-adrenergic agonist, on jejunal absorption in neurally intact conscious dog

Although alpha 2-adrenergic agonists stimulate absorption in the mammalian small and large intestine in vitro, the possibility of central neural effects have confounded interpretation of in vivo studies. Our aim was to assess the effects of intravenous administration of alpha-methylnorepinephrine (MNE), an alpha 2-adrenergic agonist that does not cross the blood-brain barrier, on net jejunal absorption of water and electrolytes in the neurally intact, conscious dog. Absorption from a 30-cm proximal jejunal segment was studied using a triple-lumen perfusion technique in seven dogs. A warmed, isosmolar, balanced electrolyte solution containing [14C]polyethylene glycol was infused at 5 ml/min. Net jejunal fluxes of water and electrolytes were determined before, during, and after a 1.5-hr infusion of MNE (900 nmol/kg/hr). MNE increased net jejunal water absorption (from 12.9 +/- 1.8 to 22.5 +/- 1.5 microliters/cm/min, P < 0.05). Peripheral alpha 2-adrenergic receptors mediate a net proabsorptive response in the neurally intact canine jejunum in vivo independent of direct central neural effects.

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.

Metabolic basis of catecholamine-induced water transport in everted gut sacs of mouse

1. Catecholamine-induced water transport was measured using an everted gut sac technique. Adrenaline, noradrenaline and isoprenaline induce dose-dependent increases in water transport by the proximal intestinal sacs. Use of selective adrenergic agents revealed the possible involvement of alpha 1- and beta 2-receptors in mediation of catecholamine stimulation of water transport in this segment. 2. Inhibition of glycolysis reduced the effect mediated through alpha 1-receptors, while the inhibition of oxidative phosphorylation blocked the beta 2-receptor mediated increase in water transport. 3. Basal transport of water was also significantly reduced by inhibition of glycolysis but was significantly elevated by blockage of oxidative phosphorylation. 4. Suppression or stimulation of glycolysis was paralleled by similar changes in lactic acid release from the gut wall. 5. It is concluded that the energy for the catecholamine-induced water transport is contributed by glycolysis and oxidative phosphorylation coupled to alpha 1- and beta 2-receptors, respectively. Under basal conditions water transport is mainly dependent on glycolysis in the segment of intestine examined.

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.

The effect of calcium channel blockade on basal- and substance P-induced intestinal secretion

Calcium plays a central role in modulating many physiologic events. We have investigated the role of calcium channel blockade in the control of basal (n = 6)- and substance P-stimulated (n = 6) intestinal transport in the isolated perfused rabbit ileum. Twenty-centimeter segments of ileum, harvested from New Zealand rabbits, were arterially perfused at 1.5 ml/min with an oxygenated modified Krebs buffer solution containing washed human red cells (Hct = 15-20%) and 2.5 mM Ca2+. The intestinal lumen was perfused at 2 ml/min with an isotonic solution containing 1.2 mM Ca2+ and [14C]PEG as a nonabsorbable volume marker. The infusion of verapamil (1 microgram/min) significantly reduced (P less than 0.05) the basal secretion of H2O, and Cl-. Verapamil prevented the secretory effect of substance P infused at 0.25 microgram/min. Intraarterial verapamil had no effect on vascular perfusion pressure. These data indicate that calcium channel blockade has significant effects on basal- and substance P-stimulated intestinal secretion and suggest that transmembrane calcium fluxes function as major determinants of basal- and secretagogue-stimulated intestinal transport.

The effect of norepinephrine on intestinal transport and perfusion pressure in the isolated perfused rabbit ileum

Norepinephrine (NE) is an autonomic neurotransmitter and a potent vasoactive agent, with putative effects on intestinal absorption and secretion. To characterize the effects of NE on intestinal water and ion transport, independent of changes in intestinal blood flow, we studied the effects of three doses of NE (0.1, 0.4, and 2.0 microgram/min) on the isolated, vascularly perfused rabbit ileum at a constant vascular perfusion rate. Twenty-centimeter segments of New Zealand white rabbit distal ileum (n = 21) were vascularly perfused at a fixed rate of 1.5 ml/min using a modified Krebs buffer solution (37 degrees C) containing washed human red blood cells with a hematocrit of 15-20%. The intestinal lumen was perfused at 2 ml/min with a warm buffered isotonic electrolyte solution containing 10 microCi [14C]PEG as a nonabsorbable volume marker. Net fluxes of H2O, Na+, and Cl- were calculated during 20-min basal, NE infusion, and recovery periods. Perfusion pressure was monitored continuously. NE caused statistically significant graded increases in vascular resistance, as reflected by perfusion pressure, with increasing doses. NE stimulated absorption of H2O, Na+, and Cl- with a less distinct response to increasing dose. These data suggest that the separate effects of NE on absorption and hemodynamics may be mediated through different pathways or different receptors in the intestine.

Opioid action of the intestine: the importance of the intestinal mucosa

Drug effects on the intestine are traditionally explained in terms of action on the muscle layers and the nerves that control them. This is particularly true in the case of the opioids but research starting two decades ago has identified the intestinal mucosa as the site of action of the antidiarrhoeal opioids. Continued research using the intestinal mucosa offers a fresh approach to solving some old problems. For example it could lead to more confident predictions to be made about the wanted and unwanted effects of opioid drugs on the intestine and may help to find better drug treatments for alleviating withdrawal diarrhoea in addicts. Eventually it may help to explain how the general process of opioid dependence occurs at a cellular level.

Evidence for tryptaminergic and noradrenergic involvement in the antisecretory action of morphine in the rat jejunum

Experiments have been performed to determine whether the antisecretory (antidiarrhoeal) effect of morphine in the intestine is mediated by a direct action of morphine on enteric nerves. Rats were pretreated with 6-hydroxydopamine (6-OHDA) or p-chlorophenylalanine (PCPA) to deplete intestinal stores of noradrenaline and 5-hydroxytryptamine (5-HT). Intraperitoneal injection of 6-OHDA (3 doses at 50 mg kg-1) caused a selective reduction in the level of noradrenaline in the jejunum to 7.3% of control. Intraperitoneal injection of PCPA (200 mg kg-1) selectively reduced the jejunal level of 5-HT to 30.5% of control. Groups of rats that had been treated as described above were anaesthetized and then injected intravenously with saline or with blocking doses of either atropine (0.25 mg kg-1), hexamethonium (20 mg kg-1), ketanserin (30 micrograms kg-1), methysergide (30 micrograms kg-1), phentolamine (2 mg kg-1) or propranolol (1 mg kg-1). Following perfusion of the lumen of the jejunum, the rate of glucose absorption was measured to assess the integrity of the mucosa. Glucose absorption was unaltered in animals pretreated with hexamethonium and propranolol but there was a small enhancement in animals pretreated with atropine, PCPA, methysergide, 6-OHDA and phentolamine. The rate of net water absorption from the lumen of the jejunum and the rate of fluid secretion into the lumen following intra-arterial infusion of vasoactive intestinal peptide (VIP, 0.8 microgram min-1) were unaltered by any of the drug treatments. Intravenous injection of morphine (10 mg kg-1) did not alter the levels of noradrenaline or 5-HT in the whole jejunum. However, this dose of morphine did cause a 63.5% decrease in the VIP-induced change in water transport. This antisecretory effect of morphine was unaltered in animals pretreated with atropine, hexamethonium and propranolol. In contrast, methysergide, ketanserin and 6-OHDA abolished the antisecretory effect of morphine. PCPA and phentolamine produced a partial inhibition of morphine's antisecretory effect. It is concluded that morphine produces its antisecretory effect in the jejunum by activation of noradrenergic and tryptaminergic systems.

The influence of diltiazem and nifedipine on the haemodynamic and tubular responses of the rat kidney to renal nerve stimulation

An investigation was undertaken in the pentobarbitone anaesthetized rat to determine the influence of calcium entry blockade on the haemodynamic and tubular responses of the kidney to renal sympathetic nerve stimulation. Electrical activation of the nerves, at rates causing a 12% reduction in renal blood flow, did not change glomerular filtration rate but significantly reduced urine flow (32%) and absolute (34%) and fractional sodium excretion (33%). Intravenous administration of diltiazem (10 and 20 micrograms/kg/min) and nifedipine (1.0 and 2.0 micrograms/kg/min) caused significant reductions of systemic blood pressure. Stimulation of the renal nerves, to reduce renal blood flow between 15% and 18% in the presence of both low and high doses of diltiazem, caused significant falls in glomerular filtration rate of 9% and 23%, respectively. During the low dose of nifedipine glomerular filtration rate did not change but in animals receiving the higher dose it fell by 17%. The magnitude of the neurally induced changes in urine flow, absolute and fractional sodium excretions were not different at either dose level of diltiazem or nifedipine from that observed in the absence of drugs. Stimulation of the renal nerves at low rates, which did not change renal blood flow, had no effect on glomerular filtration rate but significantly reduced urine flow (38%) and absolute (39%) and fractional sodium excretion (35%). At these low rates of nerve stimulation glomerular filtration rate remained unchanged during the infusion of either dose level of diltiazem. However, during administration of both the low and high doses of nifedipine there were significant reductions of glomerular filtration rate of 20% and 17%, respectively. The magnitude of the neurally induced changes in urine flow, absolute and fractional sodium excretions in the presence of both low and high doses of diltiazem and nifedipine were the same as those observed in the absence of drugs. The results of this study provide no evidence to indicate that the nerve mediated increases in tubular sodium reabsorption, a response involving alpha-adrenoreceptors, is dependent on the movement of calcium into the epithelial cells. The data did not indicate that blockade of calcium entry into cells impaired the ability of the kidney to regulate glomerular filtration rate which appeared to be due to a lack of renal efferent arteriolar vasoconstriction.