Abnormally Expressed Low-Voltage-Activated Calcium Channels in β-Cells From NOD Mice and a Related Clonal Cell Line

A macroscopic low-voltage-activated (LVA) inward current was found in pancreatic β-cells isolated from NOD mice. However, this current was not present in nondiabetic prone mouse (e.g., Swiss-Webster) pancreatic β-cells. We performed pharmacological analyses on this current in NOD insulinoma tumor cells (NIT-1). This cell line was developed from pancreatic β-cells of a transgenic NOD mouse. The sodium-channel blocker, tetrodotoxin (TTX; 2 micromol/l) had no effect on this LVA current. The amplitudes of currents elicited by a –20 mV test pulse retained similarity when the extracellular sodium concentration was increased from 0 to 115 mmol/l; when the extracellular calcium concentration was decreased from 10 to 2 mmol/l, there was an approximate 50% reduction of this current elicited by a –30 mV test pulse. Neither the L-type calcium-channel blocker, nifedipine (3 μmol/l), nor the N-type calcium-channel blocker, ω-CgTx-GVIA (1 μmol/l), at –30 mV produced an appreciable effect. The T-type calcium-channel blockers, nickel (3 μmol/l) and amiloride (250 μmol/l), effectively reduced the peak of this current. In 2 mmol/l calcium external solution, the threshold of voltage-dependent activation of this calcium current was approximately –65 mV, and the peak current occurred at –20 mV. Half-maximum steady-state inactivation was around –43 mV. The mean time constant of slow deactivating tail currents generated by a preceding 20 mV pulse was 2.53 ms. The intracellular free calcium concentration was two- to threefold higher in NOD mouse pancreatic β-cells compared with Swiss-Webster pancreatic β-cells. We concluded that there are LVA calcium channels abnormally expressed in NOD mouse β-cells. This LVA calcium channel may be factorial to the high cytosolic free calcium concentration observed in these cells, and thereby may contribute to the pathogenesis of NOD mouse β-cells.