PROTEIN PHOSPHATASE INHIBITORS EXERT SPECIFIC AND NONSPECIFIC EFFECTSON CALCIUM INFLUX IN THAPSIGARGIN-TREATED HUMAN NEUTROPHILS

C2-ceramide but not inhibitors of phosphatase types 1 and 2A (okadaic acid, calyculin A, tautomycin) blocked store-regulated Ca2+ entry indu ced in human neutrophils by thapsigargin. This contrasts with previous results showing that both types of compounds inhibit Ca2+ influx in f met-leu-phe-treated cells. In present studies, phosphatase inhibitors increased the rate of secondary Ca2+ influx in a temperature-dependent manner. Their mechanism of action appeared to be independent of phosp hatase inhibition since the inactive congeners, norokadaone and tetraa cetyl okadaic acid, also potentiated Ca2+ influx at similar concentrat ions. When Ca2+ stores were predischarged by thapsigargin, okadaic aci d but not norokadaone acted synergistically with fMLP to inhibit subse quent Ca2+ entry. Results suggest that blockade of Ca2+ influx in neut rophils is mediated by a phosphorylation reaction that is prolonged by phosphatase inhibitors. The requisite phosphorylation occurs in fMLP- activated cells but may be absent in cells incubated with thapsigargin .