The Journal of Neurophysiology Vol. 88 No. 1 July 2002, pp. 196-205
Copyright ©2002 by the American Physiological Society

T-Type Calcium Channel 1G and 1H Subunits in Human Retinoblastoma Cells and Their Loss After Differentiation

 ¹Department of Physiology and Biophysics,  ²Department of Ophthalmology, and  ³Department of Pharmacology, Dalhousie University, Halifax, Nova Scotia B3H 4H7; and  ⁴Neuroscience Research Group, University of Calgary, Calgary, Alberta T2N 4N1, Canada

Hirooka, Kazuyuki, Gabriel E. Bertolesi, Melanie E. M. Kelly, Eileen M. Denovan-Wright, Xiaolu Sun, Jawed Hamid, Gerald W. Zamponi, Alexander E. Juhasz, Lawrence W. Haynes, and Steven Barnes. T-Type Calcium Channel 1G and 1H Subunits in Human Retinoblastoma Cells and Their Loss After Differentiation. J. Neurophysiol. 88: 196-205, 2002. 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, -conotoxin GVIA, -agatoxin IVA, and -conotoxin MVIIC. RT-PCR revealed the presence of 1G and 1H mRNA in undifferentiated cells, but following differentiation, a striking reduction of both 1G and 1H mRNA was found, and this was paralleled by the loss of T-type Ca channel currents. 1I 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 1G and 1H Ca channel subunit expression is reduced in cells that have differentiated and exited the cell cycle.