Roles of Ca2+ in human neutrophil responses to receptor agonists

Abstract

Previous studies have concluded that cytosolic Ca2+ [( Ca2+]i) transients are essential for neutrophils (PMN) to degranulate and make superoxide anion when challenged with the receptor agonists N-formyl-methionyl-leucyl-phenylalanine, platelet-activating factor and leukotriene B4. This view is based on the profound unresponsiveness of PMN that have their [Ca2+]i fixed at resting levels by removing storage Ca2+ and loading the cells with greater than or equal to 20 microM of a Ca2+ chelator, quin2 AM. We too observed this unresponsive state in PMN loaded with 10-32 microM-quin2 AM, fura-2 AM or 1,2-bis-(2-aminophenoxy) ethane-NNN'N'-tetra-acetic acid (BAPTA). When loaded with less than or equal to 1 microM fura-2 AM, however, Ca(2+)-depleted PMN failed to alter [Ca2+]i appreciably, yet still had substantial degranulation and superoxide-anion-generating responses to the receptor agonists. Function thus did not require [Ca2+]i transients. Moreover, Ca(2+)-depleted PMN had 20-35% decreases in receptor numbers for each of the three agonists, and chelator loading of these cells decreased receptor availability by 30-50%. All receptor losses were reversed by incubating PMN with Ca2+ at 37 degrees C, but not at 4 degrees C, and agonist binding at 4 degrees C was not influenced by the presence or absence of extracellular Ca2+. Ca2+ thus caused PMN to up-regulate their agonist receptors at 37 degrees C, and the effect persisted at 4 degrees C regardless of ambient Ca2+. We conclude that Ca2+ acts in at least three ways to regulate responses to receptor agonists. First, some pool of (probably cellular) Ca2+ maintains receptor expression. Second, [Ca2+]i transients potentiate, but are not required for, function. The [Ca2+]i pool may or may not be the same as that influencing receptors. Finally, another pool(s) of Ca2+ signals or permits responses. This last pool, rather than [Ca2+]i transients, appears essential for the bioactions of standard Ca(2+)-mobilizing stimuli.

Protein Kinases C Translocation Responses to Low Concentrations of Arachidonic Acid

“…The label binds to DG receptors. In consequence, its association with cells rises as these receptors move from the cell interior to surface membrane (34,38). The top panels of Fig.…”

“…3 H]PDB has been used to track PKC in astrocytoma cells (38), macrophage tumor cells (29), platelets (39), and PMN (34,40). The label binds to DG receptors.…”

“…34). At 30 M, AA induced leakage of LDH and the two chromophores but only several minutes after the peak in PKC translocation responses; at Ն50 M, it caused leakage by 2 min of PMN challenge.…”

“…Results are expressed as fmol of total recovered radiolabel that are cell-associated. [ 3 H]PDB binding attains equilibrium within 1 min regardless of PDB concentration (125 pM-10 M) or PMN calcium status (34,35).…”

“…Based on the criterion of potency, AA is better suited as a translocating than activating signal for PKC. (33)(34)(35). pPKC␦-EGFP was a gift from Prof. N. Saito (Biosignal Research Center, Kobe University, Kobe, Japan).…”