Intracellular calcium rise produced by platelet-activating factor is deactivated by fMet-Leu-Phe and this requires uninterrupted activation sequence: role of protein kinase C.

Stimulation of the neutrophils with fMet-Leu-Phe inhibits the rise in intracellular concentration of free calcium produced by the subsequent addition of platelet-activating factor. This deactivation is not observed in pertussis toxin treated cells. In addition, preincubation of the cells with the protein kinase C activator phorbol 12-myristate 13-acetate for three minutes abolishes completely the rise in calcium produced by platelet-activating factor. This inhibition is prevented by the addition of the protein kinase C inhibitor 1-(5-isoquinoline-sulfonyl)-2-methyl piperazine prior to the addition of the phorbol ester. Phorbol 12-myristate 13-acetate, at a concentration that does not produce significant inhibition, accelerates the rate of calcium removal from the cytoplasm, and this is abolished by the protein kinase C inhibitor. In contrast, the deactivation by fMet-Leu-Phe is not prevented by the protein kinase C inhibitor. The results presented here suggest that the protein kinase C system may regulate the opening by platelet-activating factor of possible plasma membrane associated pertussis toxin independent calcium channels and/or the binding of platelet-activating factor to the receptors. In addition, protein kinase C activation increases the rates of the calcium efflux pump and/or calcium sequestering by intracellular organelles. The most simple and straightforward explanation of the observed deactivation by fMet-Leu-Phe is that the addition of fMet-Leu-Phe to neutrophils stimulates the production of platelet-activating factor which then binds to and deactivates the receptors.