Desensitization of insulin secretory response to imidazolines, tolbutamide, and quinine I. Secretory and morphological studies Ingo Rustenbeck a, *, Monika Winkler c , Anne Jo¨rns b a Institute of Clinical Biochemistry and b Institute of Anatomy, Hannover Medical School, D-30623 Hannover, Germany c Institute of Pharmacology, University of Go¨ttingen; D-37075 Go¨ttingen, Germany Received 28 March 2001; accepted 29 July 2001 Abstract The desensitization of pancreatic B-cells against stimulation by insulin secretagogues that inhibit ATP-dependent K  channels (K ATP channels) was investigated by measuring insulin secretion of perifused pancreatic islets. Additionally, the islet insulin content and the number of secretory granules per B-cell were determined. Prior to the measurement of secretion, islets were cultured for 18 h in the presence or absence of the test agents in a cell-culture medium containing 5 mM glucose. The effects of three imidazolines, phentolamine, alinidine, and idazoxan (100 M each) were compared with those of the well-characterized sulfonylurea, tolbutamide (500 M), and those of the ion channel-blocking alkaloid, quinine (100 M). Insulin secretion was strongly reduced upon re-exposure to phentolamine, alinidine, tolbutamide, and quinine, whereas idazoxan, which stimulated secretion only weakly, had no significant effect. The imidazoline secreta- gogues phentolamine and alinidine induced a cross-desensitization against the stimulatory effect of tolbutamide and quinine. A long-term depolarization with 40 mM KCl was also able to induce a significant reduction of the secretory response to all of the above secretagogues. The insulin content of cultured islets was moderately, but significantly reduced by alinidine, whereas the reduction by phentolamine, tolbutamide, and quinine was not significant. In contrast to these observations, the ultrastructural examination revealed that tolbutamide- treated B-cells had a high degree of degranulation, whereas the other test agents and 40 mM KCl produced only a partial degranulation, except for phentolamine, which produced no significant degranulation at all. These results suggest that the desensitization of insulin secretion is a common property of all agents that stimulate insulin secretion by depolarisation of the plasma membrane. Depending on the specific secretagogue, additional mechanisms, proximal and distal to Ca 2 influx, appear to contribute to the desensitization (see Rustenbeck et al., pages 1695–1703, this issue). © 2001 Elsevier Science Inc. All rights reserved. Keywords: Insulin secretion; Pancreatic islets; Desensitization; Imidazolines; Tolbutamide 1. Introduction It is a well-known feature in pharmacology that a response to a stimulatory agent may decrease in magnitude or may even cease to occur when the agent is continually present or repeat- edly applied. When the response can still be elicited by other stimuli, it is assumed that the desensitization to the first stim- ulus is due to a dissociation between receptor occupancy and subsequent response-generating elements in the signal trans- duction pathway [1,2]. Such a homologous desensitization has to be distinguished from a heterologous desensitization where application of one type of stimulus decreases also the response to other stimuli. This latter phenomenon is often due to effects at more distal steps in signal transduction, but may also involve early steps like changes in the G-protein subunit expression [3] proximal events like where signal pathways converge to elicit cellular responses such as secretion or contraction. Here, we studied the desensitization of insulin secretion by chronic exposure to pharmacological insulin secretagogues acting at B-cell ATP-dependent K  channels (K ATP channels), comparing the effect of imidazolines with those of the sulfonylurea, tolbutamide and quinine. Imidazolines are of interest as potential oral antidiabetic drugs because they enhance insulin secretion only in the presence of a stimulatory glucose concentration [4 – 6]. Like sulfonylureas, imidazolines inhibit the activity of the B-cell ATP-dependent K  channels (K ATP channels) [7–9]. It is * Corresponding author: Dr. I. Rustenbeck, Institute of Pharmacology and Toxicology, Technical University of Braunschweig, Mendelssohnstr. 1, D-38106 Braunschweig, Germany. Fax: 49-531-391-8182. E-mail address: i.rustenbeck@tu-bs.de (I. Rustenbeck). Biochemical Pharmacology 62 (2001) 1685–1694 0006-2952/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S0006-2952(01)00792-4