A calcium-dependent chloride current in insulin-secreting beta TC-3 cells

Abstract

Ca(2+)-dependent conductances have been hypothesized to play a role in the bursting pattern of electrical activity of insulin-secreting beta cells in response to high plasma glucose. A Maxi K+ channel has received the most attention, while a low-conductance Ca(2+)-activated K+ current has also been identified. We used an increasingly popular beta cell model system, the beta TC-3 cell line, and the perforated-patch technique to describe the properties of a novel Ca(2+)-dependent Cl- current [ICl(Ca)] in insulin-secreting pancreatic beta cells. The reported ICl(Ca) could be activated under physiological Ca2+ concentrations and is the first of its kind to be described in pancreatic insulin-secreting cells. We found that long depolarizing steps above -20 mV elicited an outward current which showed slow inward relaxation upon repolarization to negative membrane potentials. Both the outward currents and the inward tails showed dependence on Ca2+ influx: their current/voltage (I/V) relations followed that of the "L-like" Ca2+ current (ICa) present in these cells; they were blocked completely by the removal of external Ca2+ or application of Cd2+ at concentrations sufficient for complete block of ICa; and their magnitude increased with the depolarizing step duration. Moreover, the inward tail decayed monoexponentially with a time constant which at voltages negative to activation of ICa showed a weak linear voltage dependence, while at voltages positive to activation of ICa it followed the voltage dependence of ICa. This Ca(2+)-dependent current reversed at -21.5 mV and when the external Cl- concentration was reduced from 159 mM to 62 mM the reversal potential shifted by approximately +20 mV as predicted by the Nernst relation for a Cl(-)-selective current. Cl- channel blockers such as DIDS (100 microM) and niflumic acid (100 microM) blocked this current. We concluded that this current was a Ca(2+)-dependent Cl- current [ICl(Ca)]. From substitution of the external Cl- with various monovalent anions and from the reversal potentials we obtained the following permeability sequence for ICl(Ca): I- > NO3- > Br- > Cl- > Acetate.