U73122 INHIBITS PHOSPHOLIPASE C-DEPENDENT CALCIUM MOBILIZATION IN NEURONAL CELLS

The aminosteroid U73122 inhibited phospholipase C (PLC)-mediated intra cellular Ca2+ release in differentiated and undifferentiated NG108-15 cells, as well as rat dorsal root ganglion (DRG) neurons grown in prim ary culture. 1 mu M U73122 blocked bradykinin (BK)-induced increases i n the intracellular free Ca2+ concentration ([Ca2+](i)) measured in si ngle cells with indo-1-based dual emission microfluorimetry. A close s tructural analog, U73343, was without effect. The effects of U73122 we re time and concentration-dependent. 1 mu M drug produced half maximal inhibition in approximately 3 min. The IC50 for a 20-min exposure was approximately 200 nM. The effects of the compound were irreversible f or the duration of experiments as long as 1 h. Treatment with 1 mu M U 73122, but not U73343 produced a small but significant increase in [Ca 2+](i) which resulted from Ca2+ release from an intracellular store. I t is not clear whether this [Ca2+](i) increase resulted from inhibitio n of PLC or an action on the store directly. In differentiated NG108-1 5 cells U73122 blocked completely depolarization-induced Ca2+ influx. In contrast, in DRG neurons U73122 inhibited only slightly voltage-sen sitive Ca2+ channels. Thus, we caution that U73122 may not be selectiv e at concentrations required for maximal block of PLC and that the sel ectivity of U73122 is dependent on cell type. Overall, our results are consistent with U73122 inhibiting PLC in neuronal cells and indicate that under the appropriate conditions, this compound is a useful tool for studying inositol 1,4,5-trisphosphate (IP3)-mediated Ca2+ mobiliza tion.