On the role of agonist-evoked Ca2+ mobilization in sustaining the ongoing phosphoinositide hydrolysis.

Stimulation of SK-N-BE(2) cells with 1 mM carbachol (Cch) elicited phosphoinositide (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 [3H]inositol-labelled cells, Cch-evoked accumulation of [3H]inositol phosphate (IPs) was not affected when [Ca2+]i was clamped at resting by cell loading with 10 microM BAPTA/AM; under these conditions, maximal 1,4,5-inositol trisphosphate accumulation was not reduced either. When [Ca2+]i was clamped around 700 nM by cell treatment with 600 nM ionomycin, Cch-evoked [3H]IPs accumulation 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 loading with 20 microM or 40 microM BAPTA/AM, respectively. These results show that, in SK-N-BE(2) cells, Cch-activated PPI-specific phospholipase C is sensitive to [Ca2+]i but it already operates under suboptimal conditions at resting [Ca2+]i.