CHOLINERGIC MODULATION OF THE CA2-CELLS( RESPONSE TO BRADYKININ IN NEUROBLASTOMA)

Fura 2 imaging was used to measure intracellular Ca2+ signals in N1E-1 15 mouse neuroblastoma cells during combined activation of bradykinin (BK) and cholinergic receptors. BK and carbachol (CCh) both activate p hospholipase C (PLC) and cause Ca2+ release from inositoi 1,4,5-trisph osphate (IP3)-sensitive Ca2+ stores. The Ca2+ signal in response to CC h is prolonged by the activation of Ca2+ influx, but BK does not appea r to activate the influx pathway. When BK and CCh are applied together (BK + CCh), the Ca2+ response is composed of both Ca2+ release and Ca 2+ influx. Ca2+ influx is also activated by BK + CCh in a subset of ce lls that does not respond with a intracellular Ca2+ concentration incr ease when CCh is presented by itself. This suggests that CCh stimulate s a Ca2+-silent cholinergic receptor that is not coupled to Ca2+ relea se but acts synergistically with BK receptors to activate Ca2+ influx. Pertussis toxin reduces influx without affecting release, indicating that the G protein that modulates the influx pathway is different from the CT protein responsible for activating PLC. Cholinergic stimulatio n also causes progressive heterologous desensitization of BK-evoked Ca 2+ release. Desensitization has the unique property of continuing to d evelop after the cholinergic agonist is removed and the cholinergic Ca 2+ response has fully recovered. Heterologous desensitization is not t he result of Ca2+ store depletion or a long-lasting inhibition of PLC or IP3-dependent Ca2+ release. Instead, it appears to involve an early step in the BK-signaling cascade, possibly at the level of the B-2 re ceptor or associated G proteins.