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Abnormally expressed low-voltage-activated calcium channels in beta-cells from NOD mice and a related clonal cell
L Wang1, A Bhattacharjee, J Fu
1Department of Pharmacology, University of South Alabama, College of Medicine, Mobile 36688, USA.
Abstract:
A macroscopic low-voltage-activated (LVA) inward current was found in pancreatic beta-cells isolated from NOD mice. However, this current was not present in nondiabetic prone mouse (e.g., Swiss-Webster) pancreatic beta-cells. We performed pharmacological analyses on this current in NOD insulinoma tumor cells (NIT-1). This cell line was developed from pancreatic beta-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 micromol/l), nor the N-type calcium-channel blocker, omega-CgTx-GVIA (1 micromol/l), at -30 mV produced an appreciable effect. The T-type calcium-channel blockers, nickel (3 micromol/l) and amiloride (250 micromol/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 beta-cells compared with Swiss-Webster pancreatic beta-cells. We concluded that there are LVA calcium channels abnormally expressed in NOD mouse beta-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 beta-cells.
Insights
Low-voltage-activated calcium channels are abnormally expressed in NOD mouse pancreatic beta-cells, leading to increased intracellular calcium. This may contribute to the development of diabetes in these mice.
Area of Science:
- Endocrinology
- Cell Physiology
- Molecular Biology
Background:
- Pancreatic beta-cells regulate insulin secretion.
- Non-obese diabetic (NOD) mice are a model for type 1 diabetes.
- Abnormal ion channel function can impair beta-cell function.
Purpose of the Study:
- To investigate the nature of a low-voltage-activated (LVA) inward current in NOD mouse pancreatic beta-cells.
- To characterize the ion channel responsible for this LVA current.
- To determine the potential role of this current in beta-cell dysfunction in NOD mice.
Main Methods:
- Electrophysiological recordings of LVA currents in pancreatic beta-cells and NIT-1 cells.
- Pharmacological profiling using channel blockers (TTX, nifedipine, omega-CgTx-GVIA, nickel, amiloride).
- Ionic substitution experiments to assess ion dependency (sodium, calcium).
Main Results:
- An LVA inward current was identified in NOD mouse beta-cells, absent in control mice.
- The current was insensitive to sodium channel blockers but reduced by T-type calcium channel blockers (nickel, amiloride).
- The current showed voltage dependence characteristic of calcium channels and was associated with elevated intracellular calcium in NOD beta-cells.
Conclusions:
- NOD mouse pancreatic beta-cells exhibit abnormal expression of LVA calcium channels.
- These channels contribute to elevated cytosolic free calcium in NOD beta-cells.
- The aberrant LVA calcium channel activity may play a role in the pathogenesis of NOD mouse beta-cells.