Effect of guanine nucleotides on phospholipase C activity in permeabilized pituitary cells: Possible involvement of an inhibitory GTP-binding protein

Cultured pituitary cells prelabeled with myo-[2-³H] inositol were permeabilized by ATP^4-, exposed to guanine nucleotides and resealed by Mg²⁺. Addition of guanosine 5′-0-(3-thio triphosphate) (GTPγS) to permeabilized cells, or gonadotropin releasing hormone (GnRH) to resealed cells, resulted in enhanced phospholipase C activity as determined by [³H] inositol phosphate (Ins-P) production. The effect was not additive, but the combined effect was partially inhibited by guanosine 5′-0-(2-thiodiphospate) (GDPβS) or by neomycin. Surprisingly, addition of GDPβS (100-600 uM) on its own resulted in a dose-related increase in [³H]Ins-P accumulation. Several nucleoside triphosphates stimulated phospholipase C activity in permeabilized pituitary cells with the following order: UTP>GTPγ S>ATP>CTP. The stimulatory effect of UTP, ATP and CTP, but not GTPγS or GDPβS, could also be demonstrated in normal pituitary cells suggesting a receptor-activated mechanism. GTP and GTPγS decreased the affinity of GnRH binding to pituitary membranes and stimulated LH secretion in permeabilized cells. These results suggest the existence of at least two G-proteins (stimulatory and inhibitory) which are involved in phospholipase C activation and GnRH action in pituitary cells.