T-Type Calcium Channel α1G and α1H Subunits in Human Retinoblastoma Cells and Their Loss After Differentiation

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Abstract
Human retinoblastoma cells are multipotent retinal precursor cells capable of differentiating into photoreceptors, neurons, and glia. The current-voltage relation of the undifferentiated cells is dominated by a transient inward current that disappears shortly after differentiation. In 20 mM Ba2+-containing bath solutions, the current has an activation midpoint near −25 mV and appears to be fully inactivated at −20 mV. Sr2+and Ca2+ are preferred charge carriers relative to Ba2+, and the current vanishes in the absence of these divalent cations. Cd2+ blocks the current with an IC50 of 160 μM, and Ni2+ blocks in a biphasic manner with IC50s of 22 and 352 μM. The current is unaffected when sodium is replaced with other monovalent cations, and it is insensitive to nifedipine, ω-conotoxin GVIA, ω-agatoxin IVA, and ω-conotoxin MVIIC. RT-PCR revealed the presence of α1G and α1H mRNA in undifferentiated cells, but following differentiation, a striking reduction of both α1G and α1H mRNA was found, and this was paralleled by the loss of T-type Ca channel currents. α1I subunit mRNA levels were low in undifferentiated and differentiated cells. These results suggest that T-type Ca channels could play a role in undifferentiated retinoblastoma cell physiology since α1G and α1H Ca channel subunit expression is reduced in cells that have differentiated and exited the cell cycle.