Characterization of Low–Voltage–Activated T–Type Calcium Currents in Retinal Cone Bipolar Cells in Rats

Abstract

Abstract: : Purpose: Previous studies have reported the heterogeneous properties of low–voltage–activated T–type calcium currents and the expression of three T–type calcium channel subunits, α1G, α1H, and α1I, in retinal bipolar cells in rats (Pan, 2000; Pan et al., 2001). Calcium currents of different T–type channel subunits have been reported to show different activation and inactivation kinetics, permeability to Ca²⁺ / Ba²⁺, sensitivity to Ni²⁺, and modulation by dithiothreitol (DTT) (McRory et al., 2001; Todorovic et al., 2001). The purpose of this study is to characterize T–type calcium currents in retinal cone bipolar cells. Methods: Whole–cell patch–clamp recordings were performed on acutely isolated cone bipolar cells from rat retinas and cultured human embryonic kidney (HEK) cells transfected with α1G, α1H, and α1I. Results: Distinct properties of calcium currents were observed among three groups of cone bipolar cells based on their activation and inactivation kinetics, permeability to Ca²⁺ / Ba²⁺, modulation by DTT, and sensitivity to Ni²⁺. The calcium currents in the first group were more permeable to Ba²⁺ than Ca²⁺ (45.2 ± 10.6 %, mean ± SE, n = 29). The currents were enhanced by DTT (2 mM) (32.2 ± 3.2%, n = 58) and were highly sensitive to Ni²⁺ (20 µM) inhibition (74 ± 5%, n = 15). These properties were similar to that of the calcium channel current of α1H. The calcium currents in the second group showed less permeable to Ba²⁺ than Ca²⁺ (24.5 ± 2.7%, n = 44), no modulation by DTT (n = 44) and lower sensitivity to Ni²⁺ (20 µM) inhibition (25.6 ± 6%, n = 26). The calcium currents in the third group showed equally permeable to Ba²⁺ than Ca²⁺, no modulation by DTT (n = 20), and lower sensitivity to Ni²⁺ (20 µM) inhibition (22 ± 8%, n = 20). The calcium currents properties of the second and third groups were similar to that of the calcium channel currents of α1G and α1I, respectively. In addition, the calcium currents in the third group of cone bipolar cells showed slow activation and inaction, consistent with the properties of the calcium channel current of α1I. Conclusions: Our results suggest a differential expression of three T–type calcium channels in retinal cone bipolar cells in rats, implying a potential role of different T–type calcium channels in bipolar cell processing.

Keywords: bipolar cells • ion channels • electrophysiology: non-clinical