Short-term cholinergic desensitization of rat pancreatic secretory response

Abstract

Dispersed pancreatic acini were first exposed to carbamylcholine (10(-7)-10(-4) M) for 60 min, washed, and reexposed to this same agonist (10(-8)-10(-3) M) for 15 min. During this second incubation, the functional secretory capacity of these acini was evaluated by measuring amylase release. Acini preexposed to concentrations of carbamylcholine of 10(-6) M or greater showed shifts to the right in the subsequent carbamylcholine dose-response curves of amylase release. A 3-h recovery period (without carbamylcholine) did not restore the altered carbamylcholine dose-response curve. Ca2+ concentrations of 10(-7) M or 2.5 X 10(-3) M instead of 0.5 X 10(-3) M during the 60-min preincubation did not affect the desensitization process. With use of N-[3H]methylscopolamine to evaluate muscarinic receptors, the only changes observed after desensitization were a significant decrease in the high-affinity and an equivalent increase in that of the low-affinity receptors. After cholinergic exposure amylase release stimulated by caerulein was only slightly modified, whereas amylase release in response to a phorbol ester 12-O-tetradecanoylphorbol-13-acetate and to the ionophore A23187 was not altered. These data indicate that short-term desensitization with a cholinergic agent is relatively specific to muscarinic agonists, causes changes in the muscarinic receptor high- and low-affinity concentration but does not alter intracellular steps after calcium mobilization or protein kinase C activation known to be involved in the secretion process.

Carbachol-Induced Down-Regulation of Receptors for Pancreatic Secretagogues

“…However, our re sults differ from those of other studies using dispersed pancreatic acini [3][4][5]. In a pre vious study by Asselin et al [5] using dis persed acini from rat pancreas, first incubat ing acini with carbachol not only caused a 40% down-regulation of the high affinity muscarinic, cholinergic receptors but also caused 30% up-regulation of the low affinity muscarinic, cholinergic receptors with no re sultant change in the total number of musca rinic, cholinergic receptors. Furthermore, there was no change in the affinity of either class of receptors.…”

Specificity of Phospholipase D Activation by Cholecystokinin and Phorbol Myristate Acetate but Not by Carbamylcholine and A23187 in Rat Pancreatic Acini

“…a cholecystokinin analog, by carbamylcholine (CBC), a muscarinic agonist, by phorbol myristate acetate (PMA), and by a calcium ionophore causes digestive enzyme secretion [1][2][3]. Several of the elements that transduce the signal from the external environment into a functional…”

Alterations of the Pancreatic Secretory Responses to Secretin and to the lonophore A23187 by Reserpine: A Calcium-Mediated Phenomenon?

“…Indeed, as indicated in table 6, comparisons of secretory dose-response curves to carba mylcholine, to caerulein and to the phorbol ester TPA fol lowing pancreatic acini pre-exposure to a supramaximal concentration of carbamylcholine [8] or reserpine treat ment [7] indicate identical modifications. Also presented in table 6, the present study reveals that the secretory response to secretin is significantly reduced after reser pine treatment as it was after pre-exposure to a high con centration of carbamylcholine [9].…”

“…Dispersed pancreatic acini were prepared as reported by Peikin and coworkers [11] and resuspended in the incubation buffer (one pancreas per 100 ml of incubation medium). The incubation me dium (pH 7.4) for acini was prepared as previously described [8] and contained 24.1m M Hepes, 98 mA/ NaCl, 6 mA/ KC1, 2.1mA/ KH2PO4, 5 m M sodium pyruvate, 5 mM sodium fumarate, 5 mM sodium glutamate, 11.4 mM glucose, 0.01% (w/v) soybean trypsin inhibitor, 2.5% (v/v) amino acid mixture, 2% (v/v) glutamine, 1% (v/v) essential vitamin mixture, 1 % (w/v) bovine serum albumin, 0.5, 1.25 or 2.5 mM CaCL according to the protocol and 1.2 m M MgCL and was equilibrated with 100% CL. Viability of the acini isolated from C, PF and R rats was determined by trypan blue exclusion, and, in all groups, 90% of the cells excluded the dye.…”

“…Short-term exposure of pancreatic acini to carbamylcholine, a muscarinic agonist, reduced the subsequent stimulation of amylase release caused by this same agonist and induced secretin subsensitivity without altering the subsequent secretory responses to TPA, to caerulein and to the ionophore A23187 [8,9], Because of the remark able similarity existing between secretory responses of pancreatic acini to secretagogues following in vivo reser pine treatment and desensitization of the pancreatic aci nar cell by a cholinergic muscarinic agonist, this study was undertaken to (1) characterize the alterations caused by reserpine on the secretory responses of the pancreatic acini to secretin and to the ionophore A23187 and (2) to substantiate the hypothesis that reserpine alters pancreat ic secretory responses to secretagogues through the release of endogenous acetylcholine [7], Since reserpine has been shown to reduce food consumption in rats [10], the secre tory response of pair-fed (PF) animals to reserpine was also examined to distinguish between a true reserpine effect and that of reduced food intake.…”