Calcium-activated K Channels of Mouse -cells are Controlled by Both Store and Cytoplasmic Ca 2 : Experimental and Theoretical Studies

A novel calcium-dependent potassium current (K slow ) that slowly activates in response to a simulated islet burst was identified recently in mouse pancreatic -cells (Göpel, S.O., T. Kanno, S. Barg, L. Eliasson, J. Galvanovskis, E. Renström, and P. Rorsman. 1999. J. Gen. Physiol. 114:759–769). K slow activation may help terminate the cyclic bursts of Ca 2 -dependent action potentials that drive Ca 2 influx and insulin secretion in -cells. Here, we report that when [Ca 2 ] i handling was disrupted by blocking Ca 2 uptake into the ER with two separate agents reported to block the sarco/endoplasmic calcium ATPase (SERCA), thapsigargin (1–5 M) or insulin (200 nM), K slow was transiently potentiated and then inhibited. K slow amplitude could also be inhibited by increasing extracellular glucose concentration from 5 to 10 mM. The biphasic modulation of K slow by SERCA blockers could not be explained by a minimal mathematical model in which [Ca 2 ] i is divided between two compartments, the cytosol and the ER, and K slow activation mirrors changes in cytosolic calcium induced by the burst protocol. However, the experimental findings were reproduced by a model in which K slow activation is mediated by a localized pool of [Ca 2 ] in a subspace located between the ER and the plasma membrane. In this model, the subspace [Ca 2 ] follows changes in cytosolic [Ca 2 ] but with a gradient that reflects Ca 2 efflux from the ER. Slow modulation of this gradient as the ER empties and fills may enhance the role of K slow and [Ca 2 ] handling in influencing -cell electrical activity and insulin secretion. key words: islets of Langerhans • KCa channels • ER • insulin • intracellular calcium I N T R O D U C T I O N -Cells of the islets of Langerhans control blood glucose levels by secreting insulin in response to an increase in extracellular glucose. At glucose concentrations 7 mM, glucose metabolism triggers rhythmical electrical activity in mouse islets. This activity consists of periodic depolarizing plateaus with superimposed rapid spikes, separated by silent phases of 65 mV (Dean and Matthews, 1968, 1970; Meissner and Schmelz, 1974; Rosario et al., 1993; Satin and Smolen, 1994). Glucoseinduced closure of ATP-dependent potassium (K ATP ) channels depolarizes islets to about 50 mV, where bursting commences. The spikes and plateaus are mediated by voltage-activated Ca 2 channels (Meissner and Schmeer, 1981; Ribalet and Beigelman, 1981) and increasing glucose increases calcium influx, [Ca 2 ] i , and insulin secretion by prolonging the spiking plateau phase and concomitantly shortening the silent phase (Cook, 1984). -Cell electrical activity is tightly coupled to insulin secretion, since the spiking phase duration and the rate of insulin secretion have the same dependence on glucose concentration (Meissner and Schmelz, 1974; Wollheim and Sharp, 1981; Ashcroft and Rorsman, 1989). Despite extensive investigation, the ionic basis of islet pacemaking is still incompletely understood. The cyclic activation of Ca 2 -activated K (K Ca ) current has long been considered a candidate ionic pacemaker mechanism (Atwater et al., 1979; for reviews see Satin and Smolen, 1994; Sherman, 1996), since islet bursting is readily simulated by theoretical models that incorporate the activation and deactivation of K Ca channels by bursting-induced [Ca 2 ] i oscillations (i.e., Chay and Keizer, 1985). However, direct characterization of the rapidly activating, large conductance K Ca channels of -cells that were the first K Ca channels identified in these cells (Cook et al., 1984) raised doubts that they constituted the primary pacemaker mechanism. In particular, charybdotoxin, a selective inhibitor of large conductance K Ca channels, was reported to have no effect on islet electrical activity (Kukuljan et al., 1991), and the voltage dependence of these channels is incompatible with their mediating a sustained repolarization to 65 mV after each burst. In addition, studies using [Ca 2 ]The online version of this article contains supplemental material. Address correspondence to Leslie S. Satin, Department of Pharmacology and Toxicology, Medical College of Virginia Virginia Commonwealth University, P.O. Box 980524, Richmond, VA 23298. Fax: (804) 828-1532; E-mail: [email protected]