Guanine nucleotide regulation of phospholipase C activity in permeabilized rabbit neutrophils. Inhibition by pertussis toxin and sensitization to submicromolar calcium concentrations

Rabbit neutrophils labelled with [³H]inositol and permeabilized with saponin produced [³H]inositol trisphosphate (InsP₃) when incubated with stable analogues of GTP or millimolar concentrations of Ca²⁺. [³H]InsP₃ production elicited by guanosine 5'-[γ-thio]triphosphate was enhanced by the chemoattractant formylmethionyl-leucyl-phenylalanine and inhibited by pertussis-toxin pretreatment. A pertussis-toxin-sensitive stimulation of [³H]InsP₃ concentration was also observed with guanosine 5'-[βγ-imido]triphosphate, but not with guanosine 5'-[β-thio]diphosphate or GTP. Millimolar Ca²⁺ alone was sufficient to stimulate [³H]InsP₃ production; however, in the presence of guanosine 5'-[γ-thio]triphosphate, the Ca²⁺ dose-response curve was shifted to submicromolar concentrations. These findings directly confirm the role of a pertussis-toxin-sensitive guanine nucleotide regulatory protein (G protein) in chemoattractant-stimulated phospholipase C activity in rabbit neutrophils. Moreover, the ability of guanine nucleotides to sensitize phospholipase C to physiologically relevant Ca²⁺ concentrations suggests that the role of the activated G protein may be to enhance the apparent affinity of phospholipase C for Ca²⁺ and thus to activate the enzyme without an increase in the Ca²⁺ concentration.