Exercise-induced release of pancreatic polypeptide and its inhibition by propranolol: evidence for adrenergic stimulation

Degradation Paradigm of the Gut Hormone, Pancreatic Polypeptide, by Hepatic and Renal Peptidases

Pancreatic polypeptide (PP) is a gut hormone that acts on Y4 receptors to reduce appetite. Obese humans display a reduced postprandial increase in PP and remain fully sensitive to the anorectic effects of exogenous PP. The utility of PP as an anti-obesity treatment is limited by its short circulating half-life. Insight into the mechanisms by which PP is degraded could aid in the design of long-acting PP analogs. We investigated the role of peptidases in PP degradation to determine whether inhibition of these enzymes enhanced PP plasma levels and bioactivity in vivo. Dipeptidyl peptidase IV (DPPIV) and neprilysin (NEP) were two peptidase found to cleave PP. Limiting the effect of both peptidases improved the in vivo anorectic effect of PP and PP-based analogs. These findings suggest that inhibiting the degradation of PP using specific inhibitors and/or the design of analogs resistant to cleavage by DPPIV and NEP might be useful in the development of PP as an anti-obesity pharmacotherapy.

Pancreatic polypeptide regulates glucagon release through PPYR1 receptors expressed in mouse and human alpha-cells

BACKGROUND

Plasma levels of pancreatic polypeptide (PP) rise upon food intake. Although other pancreatic islet hormones, such as insulin and glucagon, have been extensively investigated, PP secretion and actions are still poorly understood.

METHODS

The release of PP upon glucose stimulation and the effects of PP on glucagon and insulin secretion were analyzed in isolated pancreatic islets. Expression of PP receptor (PPYR1) was investigated by immunoblotting, quantitative RT-PCR on sorted pancreatic islet cells, and immunohistochemistry.

RESULTS

In isolated mouse pancreatic islets, glucose stimulation increased PP release, while insulin secretion was up and glucagon release was down. Direct exposure of islets to PP inhibited glucagon release. In mouse islets, PPYR1 protein was observed by immunoblotting and quantitative RT-PCR revealed PPYR1 expression in the FACS-enriched glucagon alpha-cell fraction. Immunohistochemistry on pancreatic sections showed the presence of PPYR1 in alpha-cells of both mouse and human islets, while the receptor was absent in other islet cell types and exocrine pancreas.

CONCLUSIONS

Glucose stimulates PP secretion and PP inhibits glucagon release in mouse pancreatic islets. PP receptors are present in alpha-cells of mouse and human pancreatic islets.

GENERAL SIGNIFICANCE

These data demonstrate glucose-regulated secretion of PP and its effects on glucagon release through PPYR1 receptors expressed by alpha-cells.

Effect of exercise and physical fitness on large intestinal function

The effect of exercise on large intestinal function has been determined in 14 healthy but normally sedentary men and women, aged 22-34 yr while on a constant diet. For an initial 3-5-wk period (control) no activity was allowed. Six subjects then undertook a 9-wk training schedule by the end of which they were capable of jogging for 1 h per day, 5 days a week. A further 6 subjects undertook a similar training schedule that lasted for only 7 wk, at the end of which they were jogging for 45 min per day. Finally, 2 subjects were studied continuously while taking light exercise for 6 wk and then jogging for a further 3 wk. Physical fitness was monitored and showed significant changes with maximum aerobic capacity increasing from 2.4 +/- 0.5 to 3.1 +/- 0.4 L/min, maximum heart rate after a step test falling from 152 +/- 8 to 129 +/- 5 beats per minute, and resting pulse rate also falling from 56 +/- 4 to 50 +/- 5 beats per minute. High-density lipoprotein cholesterol also increased significantly. Colonic function was assessed by measurement of stool weight and transit time, using the continuous radiopaque marker technique, fecal pH, nitrogen excretion, and ammonia concentration. No change was observed overall in mean daily fecal weight [124 +/- 39 (control) and 129 +/- 49 g/day (exercise)], transit time [55 +/- 20 (control), 54 +/- 23 h (exercise)], nor in fecal frequency, dry stool weight, pH, ammonia, or total nitrogen excretion. Significant changes did occur in 5 individuals with significant slowing of transit time in 2 and speeding up in 3. Overall transit time increased in 9 subjects and decreased in 5; hence, when diet is constant, exercise has marked effects on physical fitness but no consistent effect on large bowel function.

Role of circulating catecholamines in the control of pancreatic polypeptide and gastrin release

The influence of circulating catecholamines on the release of pancreatic polypeptide (PP) and gastrin was studied in volunteers. Physical exercise increased plasma epinephrine by 374 +/- 123% and plasma norepinephrine by 167 +/- 30%, but plasma PP concentrations remained unchanged during standardized bicycle ergometry. Immediately after cessation of exercise catecholamine levels decreased rapidly, whereas PP concentrations increased by 55%. In a second series, epinephrine infusion (5, 25, and 75 ng.kg-1.min-1) increased epinephrine levels by 38 +/- 12, 331 +/- 69, and 1229 +/- 131%, respectively, whilst norepinephrine was unaffected. Neither during nor after catecholamine infusion PP secretion was affected. Gastrin release increased by a maximum of 85 +/- 38% (at epinephrine 75 ng.kg-1.min-1). It is concluded, that (1) changes in circulating adrenaline do not significantly influence PP secretion in man; (2) the PP increase immediately following physical exercise cannot be attributed to a rapid fall of catecholamine levels; (3) endogenous catecholamines are of minor importance in the control of gastrin secretion.

The regulation of basal pancreatic polypeptide levels in dogs

In response to various stimuli, pancreatic polypeptide (PP) release is predominantly mediated by cholinergic mechanisms, and may be modulated by sympathetic and opiate (inhibitory) effects. However, the mechanisms regulating basal PP levels remain unclear. We examined the possible role of the sympathetic nervous system and endogenous opiates in the regulation of basal levels of pancreatic polypeptide in trained conscious dogs. During prolonged (150 min) alpha- or beta-adrenergic blockade with phentolamine and propranolol, separately or in combination, there was no change in the basal PP levels of 154 +/- 20 pg/ml. Effective adrenergic modulation of pancreatic hormones was evident since alpha blockade led to a rise in insulin and glucagon, beta blockade led to a fall in insulin and glucagon, while combined alpha- and beta-adrenergic blockade did not affect insulin or glucagon. Opiate blockade with naloxone (1.25 mg followed by 1 microgram/kg/min) led to a delayed fall in PP from 153 +/- 22 to 89 +/- 15 pg/ml at 90 min (no change by 30 min), without a change of insulin or glucagon. Infusion of a potent morphine analogue D-Met2-Pro5-enkephalinamide (0.5 microgram/kg/min) led to a sustained fall in PP to 91 +/- 8 pg/ml by 30 min without a change in insulin or glucagon. Somatostatin infusion (0.2 microgram/kg/min) with insulin and glucagon replacement, led to a similar sustained fall in PP. It is concluded that in dogs: in contrast to insulin and glucagon, at basal conditions the plasma level of PP is not modulated by endogenous alpha- or beta-adrenergic influences.(ABSTRACT TRUNCATED AT 250 WORDS)

Sympathoadrenal and parasympathetic responses to exercise

Exhaustive exercise is associated with a persistent sensation of weakness and sometimes nausea suggesting abdominal vagal activity. We measured plasma indices of sympathoadrenal (adrenaline, noradrenaline, dopamine) and vagal (pancreatic polypeptide) activity before, during and after submaximal and maximal exercise in healthy young subjects. Plasma adrenaline, noradrenaline and dopamine increased to 8.5 (range 7.4-40.5), 48.0 (32.3-100.5) and 1.8 (1.2-6.6) nmol 1-1 respectively (n = 5), during maximal exercise and decreased towards control values within 15 min of rest. Pancreatic polypeptide (n = 10) increased only during maximal exercise and reached its highest value, 48 (21-145) pmol 1-1, after exertion. The results conform to an increase in sympathetic activity during exercise and a persistent vagal activity after intense exercise which could contribute to the sensation of weakness.

Action of neurotensin on size, composition, and growth of pancreas and stomach in the rat

Since the gastrointestinal peptide neurotensin has a stimulatory effect on the secretion of the exocrine pancreas and an inhibitory effect on secretion and motility of the stomach, we investigated whether chronic parenteral administration of neurotensin would affect pancreatic and gastric growth. We therefore infused synthetic neurotensin subcutaneously (dose, 43 and 282 pmol X kg-1 X min-1) in 20 Wistar rats for 2 weeks using Alzet osmotic minipumps and compared pancreatic weight, DNA, RNA, protein, lipase, amylase, pancreatic polypeptide and insulin with these parameters in 10 control rats from the same litter with subcutaneously implanted plastic cylinders approximately the size of the minipumps. In another experiment, synthetic neurotensin (836 pmol X kg-1) was injected intraperitoneally three times a day for 3 days in 12 rats. Thereafter, we measured pancreatic DNA and in vitro incorporation of [3H]thymidine into pancreatic DNA. These effects were compared with the actions of caerulein and normal saline. Long term infusion of the high neurotensin dose induced an increase of pancreatic weight (control: 0.87 g, neurotensin: 1.02 g) and of DNA (control: 2.5 micrograms; neurotensin: 3.5 micrograms) and pancreatic polypeptide (control: 2.4 ng; neurotensin: 7.4 ng) contents, whereas pancreatic protein, RNA, amylase and lipase contents were not stimulated. In relation to DNA, these parameters even were significantly depressed. Insulin remained unchanged. Intraperitoneal injection of neurotensin induced an increase of pancreatic DNA content and stimulated [3H]thymidine incorporation into DNA (control: 11 000 dpm/g; neurotensin: 15 800 dpm/g pancreas). Moreover, long-term neurotensin infusion with the high dose led to a rise in protein concentration and an increase in the thickness of the gastric antrum; antral DNA concentration was insignificantly stimulated. Parenteral neurotensin in the doses and at the times administered, led therefore, to hyperplasia of the pancreas and induced growth of the gastric antrum. It is concluded that neurotensin can act as a trophic factor on pancreas and gastric antrum of the rat. It remains to be determined whether this represents a physiological effect of neurotensin.

Pharmacological evidence for opioid and adrenergic mechanisms controlling growth hormone, prolactin, pancreatic polypeptide, and catecholamine levels in humans

A group of 14 healthy subjects received 50 mg/kg body weight of 2 deoxy-D-glucose (2DG) IV in a 20-minute infusion to induce glucoprivation and stimulate the release of growth hormone (GH), prolactin (PRL), pancreatic polypeptide (hPP), and catecholamines. Six subjects having spontaneously high GH baseline levels (greater than 8 ng/mL) failed to mount a GH response to 2DG-induced glucoprivation while eight subjects having low GH baseline levels (less than 8 ng/mL) all had increases (greater than 10 ng/mL) of GH levels after 2DG (P less than 0.05). Baseline level of GH was a reliable predictor of subsequent GH response to 2DG. Administration of the alpha 2-adrenoreceptor agonist clonidine (0.5 mg po) reliably increased GH levels (P less than 0.05). Elevated GH levels following clonidine administration abolished GH responses to subsequently infused 2DG (P less than 0.05). While these data do not exclude the possibility of a short loop feedback control of GH secretion, they strongly suggest that the direction of the GH response to a provocative stimulus is determined by the antecedent GH level and that an alpha-adrenoreceptor mechanism is involved in such a biphasic modulation of GH levels. Clonidine administration significantly reduced total catecholamine, pancreatic polypeptide, and prolactin response to 2DG while opiate receptor blockade with naloxone (10 mg IV bolus followed by 2 mg/hr) did not affect catecholamine and pancreatic polypeptide response but did slightly attenuate the GH and PRL response to glucoprivation. We conclude that alpha adrenoreceptor mechanisms are of major importance while opiate receptor mechanisms are of relatively minor importance in modulating the effects of glucoprivation on sympathetic outflow and hPP, GH, and PRL levels.

Gastric and enteral pancreatic polypeptide (PP) immunoreactivity in the dog

Extrapancreatic pancreatic polypeptide (PP) was quantified by determination of PP-cell density using antibodies to human PP (hPP) and bovine PP (bPP) in stomach, duodenum, jejunum, ileum, and colon of the dog. Further, these organs of 3 beagles were extracted and subjected to radioimmunoassay again using 2 different antisera (one directed against bPP, the other against hPP). Purification of these extracts were performed by Sephadex chromatography. We found bPP immunoreactive cells in stomach, small and large bowel, but hPP immunoreactive cells, besides in the pancreas, only in stomach and duodenum, thus demonstrating heterogeneity of extrapancreatic PP. Extraction and chromatography confirm the presence of PP immunoreactivity in the pancreas, stomach, small and large intestine. The observation of a postprandial rise of serum PP levels in a man with total duodenopancreatectomy suggests that the extrapancreatic PP cell may release PP.

A multihormonal tumor of the pancreas producing neurotensin

In a pancreatic adenoma approximately 78.7% of the endocrine cells reacted specifically with antisera to neurotensin, 17.5% to gastrin, 2.8% to pancreatic polypeptide, and 1% to glucagon. The electron microscope revealed that the majority of the endocrine cells were N-cells--morphologically similar to the ileal N-cells which are known to represent the neurotensin-producing cells. Neurotensin was extracted from the tumor and identified by Sephadex, ion-exchange, and high-pressure liquid chromatography. Gastrin, pancreatic polypeptide, and glucagon cells were also identified by the electron microscope; the peptides were extracted and demonstrated by chromatography. The serum concentrations of these hormones were elevated. After total gastrectomy which was necessary because of Zollinger-Ellison syndrome, a jejunoesophageal alkaline reflux, reaching the upper esophagus appeared. As intravenous infusion of synthetic neurotensin in rats caused an increase of luminal enteric pressure, it is suggested that severe jejunoesophageal reflux after gastrectomy may be a clinical feature of a neurotensinoma.

Adrenergic modulation of the release of pancreatic polypeptide after intraduodenal and oral glucose in man

Intraduodenal infusion of glucose increased the concentration of pancreatic polypeptide (PP) in serum from 13.4 (4.0-20.4) to 36.9 (20.7-81.3) pM. Alpha blockade with phentolamine increased the PP concentration from 15.0 (4.0-23.7) to 24.9 (4.6-50.2) pM, and after intraduodenal glucose to 46.8 (23.6-132.8) pM. The PP release after intraduodenal glucose was small, transient, and significantly reduced when beta blockade with propranolol was administered. Oral glucose increased the concentration of PP from 19.3 (4.2-37.0) to 61.1 (14.1-141.7) pM. Isoprenaline increased the PP concentration from 13.5 (4.6-33.8) to 56.0 (5.7-137.3) pM, and after oral glucose to 77.5 (25.3-134.7) pM. The increase in PP concentration was eliminated when propranolol was added to isoprenaline. We conclude that an intestinal phase of PP release exists after intraduodenal glucose in healthy humans, and that the PP release after intraduodenal and oral glucose can be modified by the adrenergic nervous system. Alpha blockade stimulates the PP cell; beta blockade or stimulation respectively inhibits or stimulates the PP cell.