Inhibitory effects of pirenzepine on cholecystokinin and secretin stimulation on exocrine and endocrine rat pancreas

Control of pancreatic exocrine secretion via muscarinic receptors: which subtype(s) are involved? A review

The present report gives an overview of the experimental, pharmacological and molecular investigations that have been undertaken during the past two decades to characterize and identify the muscarinic receptor subtype(s) involved in the cholinergic control of pancreatic exocrine secretion in humans and different animal species. The results published in the literature clearly indicate that both M1 and M3 receptors contribute to the regulation of pancreatic enzyme secretion, although contradictory conclusions have been drawn from secretory studies using specific M1 and M3 receptor antagonists in vivo and in vitro. Binding studies using specific M1 and M3 receptor antagonists have supported the existence of both M1 and M3 receptors on pancreatic acinar cells, which was confirmed by the demonstration of specific mRNA for both receptor subtypes in rat pancreatic acinar cells. In addition, experimental evidence exists that nonacinar (possibly presynaptic) M1 receptors also contribute to the control of pancreatic enzyme secretion. The role of the different muscarinic receptor subtypes in the control of pancreatic fluid and bicarbonate output, however, still needs to be clarified. Future research should cover the evaluation of the relative contribution of the different receptor subtypes to the regulation of pancreatic exocrine function, the localization of the receptors involved as well as possible species differences.

Significant cholinergic role in secretin-stimulated exocrine secretion in isolated rat pancreas

Effects of intrapancreatic cholinergic activation by electrical field stimulation (EFS) on secretin-stimulated pancreatic exocrine secretion were investigated in the totally isolated perfused rat pancreas. EFS at 15 V, 2 ms, and 8 Hz for 45 min markedly increased spontaneous pancreatic secretion. This increase was completely inhibited by tetrodotoxin (1 microM) but not by hexamethonium (100 microM). Atropine (2 microM) significantly reduced the EFS-evoked volume flow and amylase output by 52% and 80%, respectively. EFS further increased the secretin (12 pM)-stimulated pancreatic secretion of fluid and amylase. The increases of the two parameters were significantly suppressed by atropine by 28% and 72%, respectively. Interestingly, EFS significantly increased concentrations of somatostatin-like immunoreactivity in portal venous effluents. When pertussis toxin (200 ng/ml) or rabbit antisomatostatin serum (0.1 ml/10 ml; titer of 1:50,000) was intra-arterially administered, EFS further increased the secretin-stimulated pancreatic secretion. In conclusion, the activation of intrapancreatic cholinergic neurons potentiated the secretin action on pancreatic exocrine secretion in the rat. This potentiating effect was significantly reduced by local somatostatin released during EFS that activated intrapancreatic cholinergic tone.

Muscarinic receptor subtypes in rat pancreatic islets: binding and functional studies

Cholinergic agents are potent modulators of insulin release that act via muscarinic receptors. We now investigated the muscarinic receptor subtype present in rat pancreatic islets in binding and functional studies. Binding of 5 nM [3H]N-methylscopolamine ([3H]NMS) was half maximal at 30 min. At 60 min, the maximal total binding was 1.29% and the non-specific binding (presence of 100 microM atropine) was 0.18% of the total radioactivity per 10 micrograms islet protein. Unlabelled atropine inhibited [3H]NMS binding with an IC50 of ca. 30 nM. The rank order of antagonist high-affinity binding was atropine greater than sila-hexocyclium methyl sulfate (SiHC; M1 greater than M3 greater than M2) greater than pirenzepine (M1 greater than M2 approximately M3) = methoctramine (M2 greater than M1 greater than M3). The high-affinity Kds were 8.5, 56, 1300 and 1300 nM, respectively. The high affinity Kd of the muscarinic receptor agonist, arecaidine propargyl ester (APE), was 8.1 nM. The EC50 for the biological effects of APE on insulin and glucagon secretion was 3.2 and 2.3 nM. The rank order for the high-affinity biological effects of antagonists (inhibition of APE-mediated insulin/glucagon release) was almost the same as for binding. The data indicate that rat pancreatic islets contain neither an M1 subtype (high-affinity for pirenzepine) nor an M2 subtype (high-affinity for methoctramine) receptor. However, the data evidence an M3 receptor subtype, since SiHC in the absence of the M1 receptor subtype shows a relatively high affinity to the receptors in rat pancreatic islets.