Secretin release from the isolated, vascularly perfused pig duodenum

Establishment of a model for equine small intestinal disease: effects of extracorporeal blood perfusion of equine ileum on metabolic variables and histological morphology - an experimental ex vivo study

BACKGROUND

In horses a number of small intestinal diseases is potentially life threatening. Among them are Equine Grass Sickness (EGS), which is characterised by enteric neurodegeneration of unknown aetiology, as well as reperfusion injury of ischaemic intestine (I/R), and post-operative ileus (POI), common after colic surgery. The perfusion of isolated organs is successfully used to minimize animal testing for the study of pathophysiology in other scenarios. However, extracorporeal perfusion of equine ileum sourced from horses slaughtered for meat production has not yet been described. Therefore the present study evaluated the potential of such a model for the investigation of small intestinal diseases in an ex vivo and cost-efficient system avoiding experiments in live animals.

RESULT

Nine ileum specimens were sourced from horses aged 1-10 years after routine slaughter at a commercial abattoir. Ileum perfusion with oxygenated autologous blood and plasma was successfully performed for 4 h in a warm isotonic bath (37.0-37.5 °C). Ileum specimens had good motility and overall pink to red mucosa throughout the experiment; blood parameters indicated good tissue vitality: 82 ± 34 mmHg mean arterial partial pressure of oxygen (pO₂) compared to 50 ± 17 mmHg mean venous pO_2, 48 ± 10 mmHg mean arterial partial pressure of carbon dioxide (pCO₂) compared to 66 ± 7 mmHg venous pCO₂ and 9.8 ± 2.8 mmol/L mean arterial lactate compared to 11.6 ± 2.7 mmol/L venous lactate. There was a mild increase in ileum mass reaching 105 ± 7.5% of the pre-perfusion mass after 4 hours. Histology of haematoxylin and eosin stained biopsy samples taken at the end of perfusion showed on average 99% (±1%) histologically normal neurons in the submucosal plexus and 76.1% (±23.9%) histologically normal neurons in the myenteric plexus and were not significantly different to control biopsies.

CONCLUSION

Extracorporeal, normothermic perfusion of equine ileum over 4 h using autologous oxygenated blood/plasma perfusate showed potential as experimental model to test whether haematogenous or intestinal exposure to neurotoxins suspected in the pathogenesis of EGS can induce neuronal damage typical for EGS. Also, this model may allow investigations into the effect of pharmaceuticals on I/R injury, as well as into the pathogenesis of equine POI.

Role of the gut in modulating lipoprotein metabolism

The intestinal production of lipoproteins is one of the key processes by which the body prepares dietary lipid for dissemination to locations throughout the body where they are required. Paramount to this is the relationship between dietary lipid and the enterocytes that line the gut, along with the processes which prepare this lipid for efficient uptake by these cells. These include those which occur in the mouth and stomach along with those which occur within the intestinal lumen itself. Additionally, the interplay between digested lipid, dual avenues for lipid uptake by enterocytes (passive and lipid transporter proteins), a system of intercellular lipid resynthesis and transport, and a complex system of lipoprotein synthesis yield a system open to significant modulation. In this review, we will attempt to outline the processes of lipid digestion, lipoprotein synthesis and the exogenous and endogenous factors which exert their influence.

Physiological parameters governing the action of pancreatic lipase

The most widely used pharmacological therapies for obesity and weight management are based on inhibition of gastrointestinal lipases, resulting in a reduced energy yield of ingested foods by reducing dietary lipid absorption. Colipase-dependent pancreatic lipase is believed to be the major gastrointestinal enzyme involved in catalysis of lipid ester bonds. There is scant literature on the action of pancreatic lipase under the range of physiological conditions that occur within the human small intestine, and the literature that does exist is often contradictory. Due to the importance of pancreatic lipase activity to nutrition and weight management, the present review aims to assess the current body of knowledge with regards to the physiology behind the action of this unique gastrointestinal enzyme system. Existing data would suggest that pancreatic lipase activity is affected by intestinal pH, the presence of colipase and bile salts, but not by the physiological range of Ca ion concentration (as is commonly assumed). The control of secretion of pancreatic lipase and its associated factors appears to be driven by gastrointestinal luminal content, particularly the presence of acid or digested proteins and fats in the duodenal lumen. Secretion of colipase, bile acids and pancreatic lipase is driven by cholecystokinin and secretin release.

Effects of noradrenaline and galanin on duodenal motility in the isolated perfused porcine pancreatico-duodenal block

The influence of neurotransmitters on gastrointestinal motility is different in different segments of the gastrointestinal tract. To clarify the regulation of duodenal motility, the aim of the present study was to investigate the effects of alpha-adrenoceptor agonism and blockade and of galanin on duodenal motility. The study was undertaken in the isolated perfused porcine pancreatico-duodenal block. The agents under investigation were administered arterially. Duodenal motility was measured by means of a low-compliance perfusion system using an intraluminal catheter. In addition the concentration of galanin was measured in the portal effluent. We found that spontaneous motility was abolished by noradrenaline by an effect that was counteracted by the alpha 2-adrenoceptor antagonist idazoxan. In contrast, the selective alpha 1-adrenoceptor antagonist prazosin did not influence the effect of noradrenaline. Galanin, like noradrenaline, abolished duodenal motility. Furthermore, the concentration of galanin in the portal effluent was decreased by noradrenaline by an alpha 2-adrenoceptor mediated mechanism. We conclude that alpha 2-adrenoceptor activation and galanin inhibit duodenal motility and that the release of galanin from the pancreatico-duodenal preparation is reduced by alpha 2-adrenoceptors.

An alpha 1-adrenoceptor-sensitive mechanism is responsible for the adrenergic inhibition of insulin secretion in the pig pancreas

The alpha-adrenoceptor subtype mediating inhibition of insulin secretion in the pig was examined in an isolated pancreas preparation perfused with 5 mM glucose. It was found that noradrenaline and the alpha 1-adrenoceptor agonist phenylephrine, but not the alpha 2-adrenoceptor agonist UK 14,304, inhibited insulin secretion. The effect of noradrenaline was abolished by the alpha 1-adrenoceptor antagonist prazosin but was not affected by the alpha 2-adrenoceptor antagonist idazoxan. The inhibitory effect of phenylephrine was also inhibited by prazosin. Thus, in the pig, the alpha-adrenoceptor subtype inhibiting insulin secretion is of the alpha 1-type, which is different from the subtype involved in other species.

Release of PYY from pig intestinal mucosa; luminal and neural regulation

The localization, molecular nature and secretion of Peptide YY (PYY), a putative gut hormone belonging to the Pancreatic Polypeptide family of peptides, was studied in pigs. Immunoreactive PYY was identified in a population of endocrine cells in the mucosal epithelium of the pig ileum. Release of PYY was observed in isolated perfused pig ileum in response to luminal stimulation with glucose and vascular administration of the neuropeptide gastrin-releasing peptide (GRP). Electrical stimulation of the vagus nerve supply to the distal small intestine in intact anaesthetized pigs resulted in release of PYY into the circulation. Stimulation of the splanchnic nerves did not affect the basal release of PYY. PYY-immunoreactivity extracted from ileal tissue or released to plasma or perfusate from the ileum was indistinguishable from synthetic porcine PYY by gel filtration and reverse phase HPLC. It is concluded that the secretion of PYY in the pig ileum may be regulated not only by nutritional luminal factors, but also by postsynaptic parasympathetic nerves.

Secretion of neurotensin from isolated perfused porcine ileum

The secretion and molecular nature of immunoreactive neurotensin (NT) was studied following stimulation of an isolated perfused porcine ileal segment with glucose, triglyceride and intra-arterial infusion of gastrin-releasing peptide (GRP). Secreted peptides were separated using gel chromatography and analyzed with 3 sequence-specific radioimmunoassays towards NT. Glucose (5%) and GRP both stimulated NT secretion from the ileal segment whereas pure triglyceride did not. Maximal secretion of NT during glucose perfusion was 0.448 nmol/min and 6.9 nmol/min during GRP infusion (medians, n = 5). GRP infused in doses from 10(-10) to 10(-8) M stimulated NT release in a dose-related manner. Following gel chromatography only the intact peptide and no smaller or larger molecular size immunoreactive components were observed. The study showed that both luminal and humoral stimuli release NT from the isolated pig ileum. Apparently no fragments or other NT-related immunoreactive components were cosecreted with the peptide.

Effect of neuroactive agents on cholecystokinin release from the isolated, perfused porcine duodenum

To elucidate the mechanisms involved in the regulation of CCK release from the small intestine, the effect of neuroactive agents on the release of CCK from the isolated, perfused porcine duodenum has been studied. CCK in the venous effluent was measured by three radio-immunoassays specific for the N-terminus, the tyrosine-o-sulphated region and the C-terminus of CCK-33. Gastrin-releasing peptide (GRP) at concentrations of 10(-10) and 10(-9) M in the arterial line increased the CCK output by about 100 and 500%, respectively, as measured with the assay specific for the sulphated region of CCK. Pilocarpine and physostigmine at concentrations of 10(-5) M reduced the CCK output to 24 and 40% of basal, respectively, and also abolished the GRP-stimulated CCK response. Atropine (10(-6) M) increased CCK output about 100%. The results suggest that both stimulatory and inhibitory nervous impulses may be transmitted to the intestinal CCK cell, mediated by intramural GRP neurons and cholinergic pathways, respectively.

The effect of vagal stimulation on the release of cholecystokinin in anaesthetized pigs

The effect of electrical vagal stimulation on the release of cholecystokinin (CCK) was studied in nine anaesthetized pigs. Plasma CCK concentrations were measured radioimmunologically by means of an antiserum specific for the sulphated tyrosine region of CCK. Stimulation of both vagal nerves for 10 min induced an increase in CCK concentrations from 1.9 +/- 0.4 pmol/l to a peak value of 3.6 +/- 0.4 pmol/l in portal vein plasma and from 1.5 +/- 0.3 to 2.7 +/- 0.4 pmol/l in arterial plasma. Mean integrated increments during stimulation were 12.0 +/- 2.5 pmol/l/10 min (p less than 0.01) and 8.3 +/- 1.0 pmol/l/10 min (p less than 0.001), respectively. The results suggest a vagal innervation of the CCK cell in the upper small intestine.

The effect of cyclic nucleotides on secretin secretion in canine duodenal mucosa in vitro

We examined the effects of cyclic nucleotides and calcium on secretin release from canine duodenal mucosal explants incubated in organ culture media. Time course studies revealed that at pH 7.4, 5 and 10 mM dibutyryl cyclic adenosine monophosphate (DBcAMP) increased secretin release progressively, reaching a peak at 2 hours. Two mM of DBcAMP at pH 7.4 did not increase secretin release but at pH 4.5, all 3 doses potentiated secretin release. DBcAMP-stimulated secretin release was not dependent on the influx of extracellular calcium. Graded doses of 3-isobutyl-1-methylxanthine (IBMX) did not stimulate secretin secretion but 1 mM IBMX with 2 mM DBcAMP increased secretin secretion significantly. Dibutyryl cyclic guanosine monophosphate, cholera toxin and 5'-guanylyl-imidodiphosphate (GPP(NH)p) did not stimulate basal secretion release. The release of secretin from our explants incubated at pH 7.4 was not due to specific leakage because all of our viability studies revealed that our explants were functionally intact at the end of 2 hours. Our observations suggest that cyclic nucleotides may participate in the intracellular regulation of secretin secretion.

Neurohormonal regulation of secretin secretion in canine duodenal mucosa in vitro

We examined the effects of cholinergic, peptidergic and GABAergic agents on secretin secretion from canine duodenal mucosal explants incubated in organ culture media. Carbachol (10(-12) to 10(-4) M), atropine (10(-6) to 10(-4) M), hexamethonium (10(-6) to 10(-4) M), and somatostatin did not alter basal secretion of secretin. Somatostatin (10(-7) to 10(-8) M) inhibited secretin secretion stimulated by pH 4.5. Met, Leu and their D-ala2-analogs inhibited both basal and pH 4.5-stimulated secretin. Naloxone reversed the inhibition caused by met-enkephalin at pH 7.4. GABA (10(-9) to 10(-6) M) stimulated both basal and pH 4.5-stimulated secretin secretion. GABA-stimulated secretin secretion was neuronal in nature, bicuculline sensitive and was mediated via post ganglionic cholinergic neurons. GABA-stimulated secretin secretion was inhibited by both somatostatin and metenkephalin, suggesting that GABA-stimulated secretin secretion may be under the inhibitory control of peptidergic agents as well.

The intestinal mucosa preferentially releases somatostatin-28 in pigs

We studied the molecular forms of somatostatin-like immunoreactivity (SLI), newly released from isolated perfused preparations of the porcine antrum, stomach, pancreas and upper small intestine: Perfusion effluents were concentrated by Sep-Pak C-18 adsorption, eluted with ethanol, dessicated, and subjected to gel filtration with subsequent radioimmunoassays for somatostatin-14 and N-terminal somatostatin-28 immunoreactivity. All the SLI newly released from the stomach and antrum eluted at the position of somatostatin-14, and such was also the case for more than 95% of the SLI newly released from the pancreas, while 68 -/+ 7% and 75 -/+ 8% of the SLI newly released from the isolated perfused jejunum and ileum, respectively, corresponded to somatostatin-28. By reverse phase HPLC the identity of these peptides with synhetic somatostatin-14 and -28 was established.

Release of secretin immunoreactivity from the ileum

In the rat, about 80% of the intestinal content of secretin immunoreactivity is found distal to the duodenum with a peak in the ileum. This work therefore studied whether the secretin immunoreactivity in distal ileum could be released by substances normally found in the intestinal lumen. An isolated loop of ileum or duodenum was perfused with 100 mM HCl, 3 mM HCl, 50 mM taurocholate, 50 mM oleate, 308 mM glucose, 273 mM amino acids, or 308 mM NaCl in anesthetized rats. Release of secretin immunoreactivity was estimated by the integrated response above basal in portal blood. Substantial release of secretin immunoreactivity was seen after both ileal and duodenal perfusion with concentrated HCl and taurocholate, whereas perfusion with oleate was followed by a lesser response. The response to taurocholate was slower than that to HCl, but more likely represents a physiologic mechanism. The results show that the distal small intestine is capable of contributing to secretin immunoreactivity in the circulation and it is speculated that bile salts and secretin constitute parts of a negative feedback loop.

Vasoactive intestinal polypeptide (VIP) in the pig pancreas: role of VIPergic nerves in control of fluid and bicarbonate secretion

Vasoactive intestinal polypeptide (VIP) in the pig pancreas is localized to nerves, many of which travel along the pancreatic ducts. VIP stimulates pancreatic fluid and bicarbonate secretion like secretin. Electrical vagal stimulation in the pig causes an atropine-resistant profuse secretion of bicarbonate-rich pancreatic juice. In an isolated perfused preparation of the pig pancreas with intact vagal nerve supply, electrical vagal stimulation caused an atropine-resistant release of VIP, which accurately parallelled the exocrine secretion of juice and bicarbonate. Perfusion of the pancreas with a potent VIP-antiserum inhibited the effect of vagal stimulation on the exocrine secretion. It is concluded, that VIP is responsible for (at least part of) the neurally controlled fluid and bicarbonate secretion from the pig pancreas.

Autonomic nervous control of pancreatic somatostatin secretion

We studied the autonomic nervous control of pancreatic somatostatin secretion using isolated perfused pig pancreases prepared with either intact vagal or splanchnic nerve supply. Electrical stimulation of the vagus nerves increased pancreatic protein output 59-fold, whereas somatostatin output decreased to 57% of prestimulatory secretion. Acetylcholine mimicked the somatostatin response to vagal stimulation, and atropine abolished the inhibition. Splanchnic nerve stimulation increased perfusion pressure up to threefold, whereas somatostatin output decreased to 68%. Phenoxybenzamine abolished the pressure response to splanchnic nerve stimulation and reversed the inhibition to a 20% increase in somatostatin output. Propranolol did not influence the inhibitory effect of splanchnic stimulation but abolished the increase seen after phenoxybenzamine. It is concluded that both divisions of the autonomic nerve supply to the pancreas are inhibitory to somatostatin secretion, but increased secretion may be brought about by a beta-adrenergic mechanism.

The molecular nature of vascularly released cholecystokinin from the isolated perfused porcine duodenum

Using sequence-specific radioimmunoassays, the quantities and molecular nature of cholecystokinin (CCK) have been determined in extracts of porcine duodenal mucosa and in the vascular perfusate from the isolated porcine duodenum. The basal concentration of CCK in the perfusate was 84 pM equiv. CCK-8 (mean; range: 32-173 pM, n = 5). After intraluminal stimulation with amino acids, acidified fat emulsions and hydrochloric acid, the concentrations increased 2--5-fold. Both in the basal and stimulated state the concentrations of the related hormone, gastrin, were below 5 pM equiv. gastrin-17. CCK in the perfusate was concentrated by affinity-chromatography using antibodies directed against the bioactive C-terminus. Subsequent gel chromatography revealed a form with a size like or slightly larger than the C-terminal dodecapeptide (CCK-12), a predominant form resembling the C-terminal octapeptide (CCK-8), and a form resembling the C terminal tetrapeptide (CCK-4). The duodenal mucosa contained in addition CCK-33, -39 and CCK-peptides with further N-terminal extensions. The results suggest that small CCK peptides are the principal circulating forms, while CCK-33 and larger forms are biosynthetic precursors.

Nervous control of gastro-pancreatic somatostatin secretion in pigs

The influence of the autonomic nervous system on the secretion of somatostatin from the antral and the fundic parts of the stomach and from the pancreas of the pig was investigated in experiments involving electrical stimulation of the vagal nerves and the splanchnic nerves in (1) intact, anesthetized pigs and (2) isolated perfused preparations of (a) antrum with intact vagal supply, (b) pancreas with intact vagal supply, (c) pancrease with intact sympathetic supply. The results clearly demonstrated that parasympathetic activity inhibits D-cell function in all gastro-pancreatic tissues; antral gastrin secretion was inversely correlated to somatostatin secretion and it is suggest that antral D-cell secretion participates in the control of gastrin secretion; the inhibitory effect of Gastric Inhibitory Polypeptide (GIP) as well as intraluminal HCl on gastrin secretion may be exerted via the stimulatory effect of both on somatostatin secretion. The sympathetic innervation of the pancreas is also clearly inhibitory to the pancreatic somatostatin secretion, whereas sympathetic nervous activity influences little the gastric somatostatin release.