“Consequently, under conditions where breakdown of polyphosphoinositides by … was dependent on PtdIns kinase activity, fluoride inhibited activation …[S] plus carbachol of phospholipase C. When conditions allowed direct breakdown of PtdIns and precluded PtdIns kinase activity, … effects of fluoride and GTP[S] plus carbachol on phospholipase C activity were additive.”