Diabetologie und Stoffwechsel 2007; 2 - P241
DOI: 10.1055/s-2007-982336

Drug-induced desensitization of insulin secretion

K Hatlapatka 1, A Wienbergen 1, A Jörns 2, I Rustenbeck 1
  • 1University of Braunschweig, Institute of Pharmacology, Braunschweig, Germany
  • 2Medical School, Institute of Anatomy, Hannover, Germany

Introduction: Desensitization of insulin secretion is a reversible state of decreased secretory responsiveness of the endocrine pancreas after prolonged exposure to nutrient or non-nutrient stimuli. The underlying mechanisms are poorly understood inspite of their obvious relevance.

Methods: Mouse islets were cultured for 20h in RPMI medium containing tolbutamide or the imidazoline efaroxan. Secretion was measured by perifusion and ELISA. KATP channel activity was measured by patch clamping, the cytosolic calcium concentration ([Ca2+]i) by microfluorometry. The insulin granule content was assessed by electron microscopy or by microfluorometry of EGFP-insulin fusion proteins.

Results: 500µM tolbutamide was about equieffective with 100µM efaroxan to block KATP channels and to depolarize the plasma membrane. An overnight culture in RPMI 1640 (5 mM glucose) containing either 500µM tolbutamide or 100µM efaroxan led to a strongly diminished secretory response upon re-exposure to either drug. When the desensitization was followed by a 4h culture in RPMI alone, an overshooting recovery of secretion resulted upon re-exposure to efaroxan, but only a partial recovery upon re-exposure to tolbutamide. A stimulatory glucose concentration (10 mM) during the 20h culture period did not prevent the drug-induced desensitization. The pattern of [Ca2+]i response upon exposure to tolbutamide or efaroxan was not altered during desensitization or recovery. Tolbutamide desensitization degranulated B-cells within islets more strongly than efaroxan desensitization. The recovery of the granule content was more extensive after tolbutamide desensitization. These observations could be confirmed by use of MIN-6 cells expressing EGFP-Insulin.

Conclusion: Drug-induced desensitization is not due to an impaired [Ca2+]i signalling in B-cells. The responsiveness of the exocytotic machinery seems to be reduced after sulfonylurea desensitization and enhanced after imidazoline desensitization as evidenced by the discrepancies between granule content and secretion rate.