Use of manganese to discriminate between calcium influx and mobilization from internal stores in stimulated human neutrophils

Stimulation of human neutrophils with f-met-leu-phe, platelet-activating factor, or leukotriene B4 resulted in an increase in Ca2+]i. The Ca2+]i rise was greater in the presence than absence of external Ca2+; the component that was dependent on external Ca2+ was blocked by Ni2+, or could be reconstituted by addition of external Ca2+ following discharge of the internal Ca2+ store. These measurements of Ca2+]i responses provide only indirect evidence for agonist-stimulated Ca2+ entry, and here we have used an alternative approach to demonstrate directly agonist-stimulated divalent cation entry. In the presence of extracellular Mn2+, f-met-leu-phe, leukotriene B4, and platelet-activating factor stimulate a quench in fluorescence of fura-2-loaded human neutrophils. This quench was due to stimulated Mn2+ influx and was blocked by Ni2+. When Mn2+ was added in the continued presence of agonist, after discharge of the internal store of Ca2+, a stimulated quench was seen; this result shows that an elevated Ca2+]i is not needed for the stimulation of Mn2+ entry. Depolarization by high K+] or addition of the L-type Ca2+ channel agonist, BAY-R-5417, had little or no effect on either Ca2+]i or Mn2+ entry. These results show that agonists stimulate divalent cation entry (Ca2+ or Mn2+) by a mechanism independent of changes in Ca2+]i and unrelated to voltage-dependent Ca2+ channels.