5'-CMP STIMULATES PHOSPHOLIPASE-A-MEDIATED HYDROLYSIS OF PHOSPHATIDYLINOSITOL IN PERMEABILIZED PITUITARY GH3 CELLS

We showed previously that 5'-CMP activates PtdIns-Ins base exchange and reversal of 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 [H-3]Ins and incubated in buffer with LiCl and a free Ca2+ concentration of 0.1-mu-M but without added Ins, 5'-CMP stimulated formation of glycerophospho[H-3]inositol (GroP[H-3]Ins) after a lag period of at least 5 min. This effect was concentration-dependent; the apparent K(m) 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 [H-3]arachidonic acid, 5'-CMP stimulated release of [H-3]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.