We showed previously that 5′-CMP activates PtdIns-Ins base exchange and reversal PtdIns synthase in permeabilized rat pituitary GH3 cells. Here we report another effect of 5′-CMP on PtdIns metabolism in these cells. In permeabilized GH3 cells prelabelled with [3H]Ins and incubated in buffer with LiCl and a free Ca2+ concentration of 0.1 microM but without added Ins, 5′-CMP stimulated formation of glycerophospho[3H]inositol (GroP[3H]Ins) after a lag period of at least 5 min. This effect was concentration-dependent; the apparent Km was 0.30 +/- 0.02 mM. CDP and CTP stimulated GroPIns formation less effectively than did 5′-CMP, but cytidine, 2′-CMP, 3′-CMP, 5′-AMP and 5′-GMP had no effect. 5′-CMP stimulated formation of lysoPtdIns also. In permeabilized GH3 cells prelabelled with [3H]arachidonic acid, 5′-CMP stimulated release of [3H]arachidonic acid without a measurable lag period. These data show that 5′-CMP stimulates a phospholipase A activity in permeabilized GH3 cells that hydrolyses PtdIns.
