Effects of pirenzepine on plasma insulin, glucagon and pancreatic ...

European Journal of Clinical Pharmacology

Eur J Clin Pharmacol (1985) 27:701-705

© Springer-Verlag 1985

Effects of Pirenzepine on Plasma Insulin, Glucagon and Pancreatic Polypeptide Levels in Normal Man M. Zaccaria, G. Giordano, C. Pasquali a, E. Ragazzi, M. Zeviani, P. Valentini, and C. Scandellari Istituto di Semeiotica Medica, III. Cattedra di Patologia Speciale Medica and 1Clinica Chirurgica l., Universit~i di Padova, Padova, Italy

Summary. The secretion of various pancreatic hormones (insulin, glucagon and pancreatic polypeptide) is affected to a different extent by the cholinergic system. In 7healthy subjects the effects of treatment for I week with pirenzepine, an anticholinergic drug selective for muscarinic receptors, on basal secretion of these hormones and on that induced by i. v. glucose (IVGTT) and arginine were evaluated. The drug did not reduce basal levels of insulin and glucagon whereas it caused an appreciable reduction in basal pancreatic polypeptide (PP). The responses of insulin and blood glucose to IVGTF and to arginine were not changed by treatment, nor was that of plasma glucagon to arginine. The infusion of arginine did induce an increase in PP level, which reached a statistically significant maximum at 90 min. This response was not particularly different after administration of pirenzepine. Thus, the results confirm the finding that arginine stimulates PP secretion in vivo and that pirenzepine reduces the basal level of the hormone, whereas it did not appear to affect the response to arginine. The findings exclude any direct action of the drug on insulin or glucagon secretion or on glucose metabolism in general. Key words: pirenzepine, pancreatic hormones; insulin, glucagon, pancreatic polypeptide, cholinergic system

Pirenzepine is commonly used in clinical practice as an anti-ulcer drug. It acts as an anti-cholinergic substance, antagonizing gastrointestinal muscarinic receptors, and showing higher affinity for gastric mucosa receptors than for smooth muscle of the stomach wall, without affecting vagal actions of oth-

er areas of the body [1, 2]. The effects of the drug on the gastrointestinal endocrine system, which is known to be affected by the cholinergic system, have previously been carefully evaluated. For gastrin it has been demonstrated that administration of pirenzepine reduces the response of this hormone to the usual stimuli [3, 4, 5]. E1 Sabbagh et al. [6] showed that acute i.v. administration of pirenzepine did not affec~ either basal or postprandial levels of GIP, VIP, neurotensin, somatostatin, insulin or glucagon, although it did cause a significant increase in the basal levels of enteroglucagon, motilin and PR With regard to the latter, Arnold et al. [3] had previously found a reduced response after pirenzepine administration to normal subjects, and this finding was subsequently confirmed by Konturek et al. [5] in ulcer patients. In these studies, as the effects of the drug were evaluated only after acute i.v. administration, they might not apply to clinical practice, where administration is usually oral and over a long period. Therefore, it was decided to evaluate the possible effect of oral administration of pirenzepine on secretion of 3 pancreatic hormones, insulin, glucagon and PP, in healthy subjects. Since insulin secretion is modulated by a cholinergic system [7], particular care was taken to investigate whether this modulation was affected by pirenzepine. That is the reason why an intravenous glucose load was preferred as the stimulus, in order to avoid any possible interference by an effect of the drug on the gastrointestinal system. Materials and Methods Seven healthy subjects, aged 21-26 years, with no family history of diabetes, were studied. All the subjects gave their informed consent to the trial. Each

M. Zaccaria et al.: Effects of Pirenzepine on Pancreatic Hormones

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During the IVGTT, blood samples for insulin and glucose levels were taken at 0, 3, 10, 20, 25, 30, 35, 40, 45, 50 and 60 rain. During the arginine test, blood samples for insulin, glucagon, pancreatic polypeptide (PP) and glucose assays were collected at - 1 5 , 0, 15, 30, 45, 60 and 90 rain. Blood glucose was measured using the glucoseoxidase enzyme method [8]. Insulin was determined by the RIA method of Herbert et al. [9]; pancreatic glucagon was determined using the specific antibody 30 K, following Aguilar-Parada et al. [10]. PP was determined with an antibody against human PP (kindly provided by Dr. R. E. Chance, Lilly Laboratories, Indianapolis, USA) using Adrian's method as modified by Pasquali et al. [11]; the lower limit of detection was 15 pg/rnl (confidence limit=95%). Coefficient of glucose utilization (Conard's K) was determined after interpolation of experimental values using the least squares method.

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Time (min) Fig.l. Blood glucose and plasma insulin responses during IVGTT in normal subjects (mean_+ SEM), before (0------@) and after (© . . . . . . . O) pirenzepine 100 mg/d for 7 days

subject received oral pirenzepine 100 mg/d, in 3 divided doses, for 7 days. An IVGTT (0.33 g/kg in 3 min) and an i.v. arginine test (25g arginine monohydrochloride over 30 min) were performed in random order before and after treatment with pirenzepine (Gastrozepin, Boehringer Ingelheim s.p.a., Florence, Italy). Both tests were carried out in the morning, after a fast of at least 10h.

The drug was well tolerated by all subjects and there were no complaints of any side-effects. The treatment with pirenzepine, although it did not affect basal levels of blood glucose, insulin or glucagon, did produce a decrease in basal PP values on the borderline of significance (from 37 + 5.1 to 26.2+5.9pg/ml, X + S E M ; p=0.052; 0 . 0 5 < p < 0.06). During the IVGTY the peak insulin response showed a slight but nonsignificant increase after pirenzepine (26.8 + 5.5 vs 44.8 + 7.3 lxU/ml, x + SEM). Both the insulin and glucose curves showed no significant effects (Fig. 1). Peripheral glucose disappearance, evaluated as Conard's K, also was unchanged after treatment (1.92 + 0.09 vs 2.09 + 0.10, X___SEM). The insulin response to arginine appeared unaffected by treatment; the two curves were not significantly different by the Fisher-Pearson test (Fig. 2). During arginine infusion, PP showed a slight but constant increase which reached a significant peak at 90 min. (from 3 7 _ 5 to 61 _ 7 pg/ml, X__+SEM; p < 0.05). The increase was still apparent, although consistently lower, after treatment with pirenzepine. The percentage increase in PP was not significantly different during the two tests (70.4+29 vs 85.8___ 22.9%, x + SEM).


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Fig.2. Effects ofi.v, arginine infusion in normal subjects on blood glucose (BG) and on plasma levels of insulin (IRI), glucagon (IRG) and pancreatic polypeptide (PP) before (O---------O) and after (O . . . . . . . ©) pirenzepine treatment (100 m g / d for 7 days; mean +_SEM). * significant difference (p < 0.05) from basal levels (mean values at - 15 and 0 time)

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Discussion

Arginine infusion was used here since it permitted exploration not only of the separate plasma responses of insulin and glucose but also the response of PP. The effect of arginine on PP has previously been reported: in vitro Loo et al. (Personal communication) and Hermansen et al. [23] observed that arginine did not constitute a very powerful stimulus, but at least it had an effect on PP secretion, whereas in similar experimental conditions Floyd et al. [24] observed a significant increase of PP at a late phase after arginine infusion. In the present study, the maximum PP values were seen at the 90th min. Unfortunately no observations were made subsequent to that time which would have permitted confirmation of this finding, as the arginine stimulus had been chosen mainly in order to study insulin and glucagon secretion. In conclusion, the present results have confirmed stimulation by arginine of PP secretion in vivo and the reduction by pirenzepine of the basal level of the hormone. The drug did not appear to affect the response to arginine. The results exclude any direct action of the drug on insulin or glucagon secretion or on glucose metabolism in general.

Insulin and gtucagon secretion is affected to a different extent by the cholinergic system through muscarinic receptors [•2, 13, 14, 15, •6]. The present study was an evaluation of whether prolonged oral administration of pirenzepine, a drug capable of evoking a variety of anti-muscarinic effects, could produce an alteration in the levels not only of these two hormones, but also of PP which is controlled by the chotinergic system and muscarinic receptors to a greater extent than insulin and glucagon [17, 18, 19,

201. The results obtained show that in normal subjects oral treatment with pirenzepine did not affect insulin or glucagon secretion either in basal conditions or in response to the intravenous stimuli employed. On the other hand, the drug appreciably reduced the basal level of PP, although it did not appear to interfere with PP secretion during the i.v. infusion of arginine. The selective action of pirenzepine at the gastric level has been amply ascertained, so the fact that it did not alter basal levels of insulin and glucagon suggests that it does not interfere with the cholinergic system regulating basal secretion of these two hormones. This view is in agreement with the findings of E1 Sabbagh et al. [6] and shows that the specific direct action of pirenzepine on high affinity muscarinic receptors does not include the receptors on pancreatic alpha and beta cells. Insulin and glucagon secretion were not significantly altered by treatment; this further supports the view that Pirenzepine does not so interfere with cholinergic tone as to affect the response of the two hormones to i.~: glucose a n d / o r arginine. The IVGq~ test results were in agreement with those of Henderson et al. [21] both before and after administration of atropine. Bloom et al. [22] observed an "unexpected" increase in the arginine - induced insulin response after prior treatment with atropine and a decreased glucagon response during treatment with it. The significance of these findings is uncertain; it may be temptatively explained by the different characteristics of atropine and pirenzepine. In agreement with E1 Sabbagh et al. [6], the basal levels of PP were appreciably decreased after pirenzepine. This confirms the importance of cholinergic tone in control of the basal secretion of PP and the presence in PP cells of specific, high affinity muscarinic receptors for pirenzepine as an antagonist. Treatment with the drug did not modify the magnitude of the hormone response to arginine, which appears to exclude involvement of the cholinergic system in arginine-induced secretion of PP.

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Received: November 28, 1983 accepted in revised form: August t 0, 1984 Marco Zaccaria, M.D. Universitg degli Studi di Padova Istituto di Semeiotica Medica III Cattedra di Patologia Speciale Medica e Metodologia Clinica Via Ospedale, 105 1-35100 Padova, Italy