A CALCIUM-DEPENDENT CHLORIDE CURRENT IN INSULIN-SECRETING BETA-TC-3 CELLS

Ca2+-dependent conductances have been hypothesized to play a role in t he bursting pattern of electrical activity of insulin-secreting beta c ells in response to high plasma glucose. A Maxi K+ channel has receive d the most attention, while a low-conductance Ca2+-activated K+ curren t has also been identified. We used an increasingly popular beta cell model system, the beta TC-3 cell line, and the perforated-patch techni que to describe the properties of a novel Ca2+-dependent Cl- current [ I-Cl(Ca)] in insulin-secreting pancreatic beta cells. The reported I-C l(Ca) could be activated under physiological Ca2+ concentrations and i s the first of its kind to be described in pancreatic insulin-secretin g cells. We found that long depolarizing steps above -20 mV elicited a n outward current which showed slow inward relaxation upon repolarizat ion 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 (I-Ca) pr esent in these cells; they were blocked completely by the removal of e xternal Ca2+ or application of Ca2+ at concentrations sufficient for c omplete block of I-Ca and their magnitude increased with the depolariz ing step duration. Moreover, the inward tail decayed monoexponentially with a time constant which at voltages negative to activation of I-Ca showed a weak linear voltage dependence, while at volt ages positive to activation of I-Ca it followed the voltage dependence of I-Ca. This Ca2+-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 approximate to +20 mV as predicted by the Nernst relation for a Cl--selective current. Cl- channel blockers such as DIDS (100 mu M) and niflumic acid (100 mu M) blocked this current. We concluded th at this current was a Ca2+-dependent Cl- current [I-Cl(Ca)]. From subs titution of the external Cl- with various monovalent anions and from t he reversal potentials we obtained the following permeability sequence for I-Cl(Ca):I->NO3->Br->Cl->Acetate(-).