Vol. 187, No. 2, 1992 September 16, 1992 BIOCHEMICAL AND BIOPHYSICAL RESEARCH COMMUNICATIONS Pages 972-87 9 HIGH EXTERNAL Ca2+ LEVELS TRIGGER MEMBRANE POTENTIAL OSCILLATIONS IN MOUSE PANCREATIC n-CELLS DURING BLOCKADE OF K(ATP) CHANNELS Rosa M. Santos, Rui M. Barbosa, Amelia M. Silva, CClia M. Antunes and Luis M. Rosario Center for Neurosciences of Coimbra, Department of Zoology, University of Coimbra, 3049 Coimbra CODEX, Portugal Received July 24, 1992 Glucose depolarizes the pancreatic R-cell and induces membrane potential oscillations, but the nature of the underlying oscillatory conductance remains unknown. We have now investigated the effects of the Ca’+ ionophore ionomycin and high external Ca’+ concentration ([Caz+],) on glucose-induced electrical activity and whole islet intracellular free Ca”+ concentration ([Ca’+]i), under conditions where the K(ATP) channel was blocked (100 PM tolbutamide or 4 PM glibenclamide). Raising [Ca’+], to 10.2 or 12.8 mM, but not to 5.1 or 7.7 mM, turned continuous electrical activity into bursting activity. High [Ca2+], (12.8 mM) regenerated a pattern of fast [Ca”+]; oscillations overshooting the levels recorded in tolbutamide. Ionomycin (10 PM) raised the [Cal+], and synergized with 5.1 mM Ca2+ to hyperpolarize the O-cell membrane. The data indicate that a [Ca”+];-sensitive and sulphonylurea-insensitive oscillatory conductance underlyes the R-cell bursting activity. 0 1992 Academic Press, Inc. Glucose metabolism is thought to raise cytoplasmatic ATP levels of the pancreatic B- cell, thereby closing a class of K+ channels sensitive to sulphonylurea drugs (K(ATP) channels) and raising the membrane potential to a threshold level where voltage-sensitive Ca2+ channels become activated (1). This triggers electrical activity in the form of Ca2+ action potentials (2). In the presence of 7-17 mM glucose, the electrical activity consists of alternating depolarized (active) and hyperpolarized (silent) phases commonly known as bursts. This bursting pattern underlyes the [Ca”+], oscillations that have been recorded from glucose- stimulated whole islets of Langerhans (3.4). Both the Cal+-dependent K+ conductance (5,6), the ATP-sensitive K+ conductance (7) and the voltage-sensitive Ca’+ conductance (8,9) have been hypothesized to serve as generators of the pacemaker current required to hyperpolarize/depolarize the B-cell membrane during the bursts. In the present study, we report that raising external Ca2+ concentration to appropriate levels in the presence of K(ATP) channel blockers regenerated the D-cell bursting activity. Furthermore, the Ca’+ lonophore ionomycin raised the [Ca’+]i and synergized with sub-threshold external Caz+ levels to hyp erp olarize the &cell membrane. Our data support the 0006-29 I X/92 $4.00 Copyright 0 I992 hi Acndernic~ Press. Inc. All rights of reproduction iii on! form reserved. 872