Ca2+ homeostasis in permeabilized human neutrophils. Characterization of Ca2+-sequestering pools and the action of inositol 1,4,5-triphosphate.
The regulation of Ca2+ transport by intracellular compartments was studied in digitonin-permeabilized human neutrophils, using a Ca2+-selective electrode. When incubated in a medium containing ATP and respiratory substrates, the cells lowered within 6 min the ambient [Ca2+] to a steady state of around 0.2 microM. A vesicular ATP-dependent and vanadate-sensitive non-mitochondrial pool maintained this low [Ca2+] level. In the absence of ATP, a higher Ca2+ steady state of 0.6 microM was seen, exhibiting the characteristics of a mitochondrial Ca2+ "set point." Both pools were shown to act in concert to restore the previous ambient [Ca2+] following its elevation. Thus, the mitochondria participate with the other pool(s) in decreasing [Ca2+] to the submicromolar range whereas only the nonmitochondrial pool(s) lowers [Ca2+] to the basal level. The action of inositol 1,4,5-triphosphate (IP3) which has been inferred to mediate Ca2+ mobilization in a few cell types was studied. IP3 released (detectable within 2 s) Ca2+ accumulated in the ATP-dependent pool(s) but had no effect on the mitochondria. The response was transient and resulted in desensitization toward subsequent IP3 additions. Under experimental conditions in which the ATP-dependent Ca2+ influx was blocked, the addition of IP3 resulted in a very large Ca2+ release from nonmitochondrial pool. The results strongly suggest that IP3 is a second messenger mediating intracellular Ca2+ mobilization in human neutrophils. Furthermore, the nonmitochondrial pool appears to have independent influx and efflux pathways for Ca2+ transport, a Ca2+ ATPase (the influx component) and an IP3-sensitive efflux component activated during Ca2+ mobilization.
D000255:Adenosine Triphosphate / Q000097:blood; D002118:Calcium / Q000097:blood*; D002451:Cell Compartmentation; D002463:Cell Membrane Permeability / Q000187:drug effects; D004072:Digitonin / Q000494:pharmacology; D006706:Homeostasis; D006801:Humans; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology*; D009504:Neutrophils / Q000187:drug effects / Q000378:metabolism*; D013403:Sugar Phosphates / Q000494:pharmacology*; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology
D007295:Inositol Phosphates; D013403:Sugar Phosphates; D014639:Vanadium; D014638:Vanadates; D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D004072:Digitonin; D002118:Calcium