Phorbol ester-induced changes in cytoplasmic Ca2+ in human neutrophils. Involvement of a pertussis toxin-sensitive G protein

Activation of neutrophils by most soluble stimuli is associated with a marked increase in intracellular free Ca2+ (Ca2+]i). However, under physiological conditions (Na+-rich media), the potent activator 12-O-tetradecanoylphorbol-13-acetate (TPA) causes no change or a decrease in Ca2+]i. We report here that the Ca2+]i response to phorbol esters varies depending on the ionic composition of the medium. A marked increase in Ca2+]i was detected in Na+-free solutions. Maximal effects were observed when N-methyl-D-glucammonium+ or choline+ were substituted for Na+, whereas an intermediate response was recorded in K+ medium. The increase in Ca2+]i was substantially (approximately 65%) inhibited by removal of external Ca2+. A Ca2+]i increase was also elicited by other beta-phorbol diesters and by diacylglycerol, but not by unesterified phorbol or by alpha-phorbol diesters, indicating involvement of protein kinase C. The increase in Ca2+]i observed in Na+-free media is not due to inhibition of Na+/Ca2+ exchange, since no change in Ca2+]i in response to TPA was observed in: 1) cells suspended in Li+, which is not countertransported for Ca2+; 2) cells preloaded with Na+ to eliminate the driving force for Na+/Ca2+ exchange; and 3) cells treated with 3',4'-dichlorobenzamyl, an inhibitor of Na+/Ca2+ exchange. Similarly, the Ca2+]i increase in Na+-free media is not linked to the absence of Na+/H+ exchange and the associated cytoplasmic acidification since: 1) it was not observed in Na+ media in the presence of inhibitors of the Na+/H+ antiport and 2) it was not mimicked by inducing acidification with nigericin. Pretreatment with pertussis toxin largely inhibited the phorbol ester-induced change in Ca2+]i, while activation of protein kinase C under these conditions was unaffected. It is concluded that in the absence of extracellular Na+ (or Li+), activation of protein kinase C leads to a net Ca2+ influx into the cytoplasm through a process mediated by a GTP-binding or G protein. Opening of a Na+-sensitive Ca2+ channel could partially explain these observations. Alternatively, the nature of the monovalent cation could conceivably affect the conformation of a G protein or of an associated receptor, inducing the appearance of a site susceptible to an activating phosphorylation by protein kinase C.