ENHANCED CALCIUM SIGNALING EVENTS IN NEUROBLASTOMA X GLIOMA HYBRID NG108-15 CELLS AFTER TREATMENT WITH DIBUTYRYL-CYCLIC-AMP

The effects of dibutyryl cyclic AMP (dbcAMP) treatment on Ca2+ channel activities, Ca2+ accumulation by intracellular Ca2+ pools, and sizes of IP3- and GTP-releasable pools in neuroblastoma x glioma hybrid NG108-15 cells were studied. High extracellular K+ induced a greater rise in intracellular calcium concentration ([Ca2+](i)) in dbcAMP-treated cells than in control cells. In dbcAMP-treated cells, the initial phase of the high K+-induced [Ca2+](i) rise displayed a much higher sensitivity to omega-conotoxin than it did in control cells, whereas the plateau phase of the [Ca2+](i) rise was sensitive only to nifedipine. These results indicate that predominantly L-type Ca2+ channels exist in control cells, and that N-type channels develop only after dbcAMP treatment. In dbcAMP-treated cells, mitochondria showed an increased Ca2+ uptake capacity (5.3 nmol Ca2+/mg protein) compared with that in control cells (4.2 nmol Ca2+/mg protein). However, dbcAMP treatment did not cause significant change in the affinity for Ca2+. Dibutyryl cAMP treatment enhanced the Ca2+ accumulation activity by nonmitochondrial pools (from 0.84 to 0.97 nmol Ca2+/mg protein) and increased the affinity for Ca2+ (EC(50) for Ca2+ decreased from 0.146 mu M to 0.063 mu M). Our data also indicate that the pool that is sensitive to both IP3 and GTP was enlarged. The affinities for IP3 and GTP in causing Ca2+ release remained the same before or after dbcAMP treatment.