Signal flows from two phospholipase C-linked receptors are independent in PC12 cells

Bradykinin (BK) receptor and P2-purinergic receptor are known to be coupled to phospholipase C (PLC) in PC12 cells. To study the interaction between these two PLC-linked receptors, the presence of both receptors on individual cells was demonstrated by sequential Ca2+ spikes caused by BK and ATP in a single fura-2-loaded cell. BK- and ATP-induced catecholamine (CA) secretions were desensitized within 5 min. However, in the sequential experiment, the BK-induced homologous desensitization of CA secretion did not block the ATP-induced secretion, and vice versa. Each agonist-induced an increase in inositol 1,4,5-trisphosphate (IP3) production and intracellular free Ca2+ concentration also led to homologous desensitization. However, there was no heterologous desensitization between the two agonists. When the cells were treated with both BK and ATP simultaneously, the amounts of CA secretion, IP3 production, internal Ca2+ mobilization, and Ca2+ influx were all additive. We also found that both IP3-induced Ca2+ release from intracellular Ca2+ stores and Ca2+ influx from extracellular space were able to release [3H]norepinephrine, and the secretion induced by both agonists was exactly additive in the absence or presence of extracellular Ca2+. The data suggest that the CA secretions caused by BK or ATP may have separate secretory pathways even though they activate identical second messenger pathways.