T-type calcium channel α1G and α1H subunits in human retinoblastoma cells and their loss after differentiation

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.