THE EFFECT OF OVEREXPRESSION OF AUXILIARY CA2-15 CELLS( CHANNEL SUBUNITS ON NATIVE CA2+ CHANNEL CURRENTS IN UNDIFFERENTIATED MAMMALIAN NG108)

1. High voltage activated (HVA) Ca2+ channels are composed of a pore-f orming alpha 1 subunit and the accessory beta and alpha 2-delta subuni ts. However, the subunit composition of low voltage activated (LVA), o r T-type, Ca2+ channels has yet to be elucidated. We have examined whe ther native calcium channels in NG108-15 mouse neuroblastoma x rat gli oma hyrbrid cells, which express predominantly LVA currents when undif ferentiated, are modulated by overexpression of accessory calcium chan nel subunits. 2. Endogenous alpha 1A, B, C, D and E, and low levels of beta and alpha 2-delta subunit protein were demonstrated in undiffere ntiated NG108-15 cells. 3. The alpha 2-delta, beta 2a or beta 1b acces sory subunits were overexpressed by transfection of the cDNAs into the se cells, and the effect examined on the endogenous Ca2+ channel curre nts. Heterologous expression, particularly of alpha 2-delta but also o f beta 2a subunits dearly affected the profile of these currents. Both subunits induced a sustained component in the currents evoked by depo larizing voltages above -30 mV, and alpha 2-delta additionally caused a depolarization in the voltage dependence of current activation, sugg esting that it also affected the native T-type currents. In contrast, beta 1b overexpression had no effect on the endogenous Ca2+ currents, despite immunocytochemical evidence for its expression in the transfec ted cells. 4. These results suggest that in NG108-15 cells, overexpres sion of the Ca2+ channel accessory subunits alpha 2-delta and beta 2a induce a sustained component of HVA current, and alpha 2-delta also in fluences the voltage dependence of activation of the LVA current. It i s possible that native T-type alpha 1 subunits are not associated with beta subunits.