Inhibitory effects of pertussis toxin on a depolarization-evoked Ca²⁺ influx in NG108-15 cells

Depolarized stimulation 1.5-fold increased Ca²⁺ influx which was inhibited by pretreatment with verapamil or LaCl₃. Treatment with pertussis toxin, islet-activating protein (IAP), induced a reduction in 50 mM K⁺- induced Ca²⁺ influx and stimulated adenylate cyclase (AC) activity in NG108-15 cells. However, addition of dibutyryl cAMP or forskolin treatment elevating cAMP level exerted no effects on a depolarization-induced Ca²⁺ influx. Dissociated B-oligomer of IAP after treatment with dithiothreitol and ATP increased a depolarization-evoked Ca²⁺ influx. It is suggested that inhibitory GTP-binding protein (G_i) or other IAP substrate proteins could directly be involved in Ca²⁺ influx via voltage-sensitive Ca²⁺ channel.