ON THE ROLE OF AGONIST-EVOKED CA2- A STUDY ON INTACT SK-N-BE(2) NEUROBLASTOMA-CELLS SUBJECTED TO MUSCARINIC STIMULATION( MOBILIZATION IN SUSTAINING THE ONGOING PHOSPHOINOSITIDE HYDROLYSIS )

Stimulation of SK-N-BE(2) cells with 1 mM carbachol (Cch) elicited pho sphoinositide (PPI) hydrolysis and a rapid elevation of cytosolic Ca2 concentration ([Ca2+](i)) from 115 nM to about 500 nM, followed by a plateau around 200 nM. In myo [H-3]inositol-labelled cells, Cch-evoked accumulation of [H-3]inositol phosphates (IPs) was not affected when [Ca2+](i) was clamped at resting by cell loading with 10 mu M BAPTA/AM ; under these conditions, maximal 1,4,5-inositol trisphosphate accumul ation was not reduced either. When [Ca2+](i) was clamped around 700 nM by cell treatment with 600 nM ionomycin, Cch-evoked [H-3]IPs accumula tion was enhanced by less than 20%,but it was impaired by a 30% and a 55% after [Ca2+](i) reduction to about 70 nM and 35-50 nM, by cell loa ding with 20 mu M or 40 mu M BAPTA/AM, respectively. These results sho w that, in SK-N-BE(2) cells, Cch-activated PPI-specific phospholipase C is sensitive to [Ca2+](i) but it already operates under suboptimal c onditions at resting [Ca2+](i).