APPEARANCE OF DEPOLARIZATION-INDUCED AND MAITOTOXIN-INDUCED [CA2-1 HUMAN NEUROBLASTOMA-CELLS ON EXPOSURE TO RETINOIC ACID(](I) ELEVATION INSINGLE LAN)

LAN-1 is a human neuroblastoma cell line that, in the undifferentiated state, does not respond to membrane depolarization with an elevation of [Ca2+](i), monitored by fura-2 single-cell microfluorimetry. The ex posure of LAN-1 cells to the differentiating agent retinoic acid induc ed the appearance of [Ca2+](i) elevation elicited by 55 mM K+. Maitoto xin, a putative activator of voltage-sensitive Ca2+ channels, did not evoke an elevation of [Ca2+](i) in undifferentiated LAN-1 cells, but p roduced a marked and sustained increase in [Ca2+](i) when superfused i n retinoic acid-treated cells. Both high K+- and maitotoxin-induced [C a2+](i) elevation in retinoic acid-differentiated LAN-1 cells was reve rsed by the lanthanide Gd3+, an inorganic Ca2+-entry blocker, and by t he snail toxin omega-conotoxin GVIA, which interacts with the N subtyp e of voltage-sensitive Ca2+ channels. In contrast, both Bay K 8644 and nimodipine, dihydropyridines that selectively activate or block, resp ectively, the L-channel subtype, were completely ineffective. The tumo r promoter phorbol 12-myristate 13-acetate (100 nM), a protein kinase C activator, inhibited the elevation of [Ca2+](i) due to Ca2+ influx e licited by membrane depolarization. K+-induced [Ca2+](i) elevation app eared 24 h after the addition of retinoic acid and reached the highest magnitude after 72 h. Furthermore, 8 days after the removal of the di fferentiating agent from the culture medium, the high K+-induced incre ase of [Ca2+](i) was still present. In conclusion, the results of the present study demonstrated that retinoic acid-induced differentiation of LAN-1 cells, which lack a high K+-evoked [Ca2+](i) increase in the undifferentiated state, induces the functional expression of an omega- conotoxin GVIA-sensitive, dihydropyridine-insensitive N-type voltage-s ensitive Ca2+ channel that can be activated by maitotoxin and negative ly modulated by protein kinase C.