Evidence that Ca(2+) cycling by the plasma membrane Ca(2+)-ATPase increases the 'excitability' of the extracellular Ca(2+)-sensing receptor.
The extracellular Ca(2+)-sensing receptor (CaR) is a widely expressed G-protein-coupled receptor that translates information about [Ca(2+)] in the extracellular milieu to the interior of the cell, usually via intracellular Ca(2+) signaling pathways. Using fura-2 imaging of cytoplasmic [Ca(2+)], we observed that HEK293 cells expressing CaR oscillated readily under conditions permissive for CaR activation. Spiking was also triggered in the absence of external Ca(2+) by the CaR agonist spermine (1 mM). Oscillating cells were typically located in clusters of closely apposed cells, but Ca(2+) spiking was insensitive to the gap junction inhibitor 18alpha-glycyrrhetinic acid. We hypothesized that Ca(2+) signals might be amplified, in part, through a positive feedback loop in which Ca(2+) extrusion via the plasma membrane Ca(2+)-ATPase (PMCA) activates CaRs on the same cell or adjacent cells through local increases in [Ca(2+)](out). In support of this idea, addition of exogenous Ca(2+) buffers (keeping free [Ca(2+)](out) constant) attenuated or eliminated Ca(2+) signals (manifested as oscillations), as did PMCA inhibitors (HgCl(2), orthovanadate and Caloxin 2A1). Measurement of extracellular [Ca(2+)] using the near membrane probe fura-C(18) revealed that external [Ca(2+)] rose following receptor activation, sometimes displaying an oscillatory pattern. Our data suggest that PMCA-mediated cycling of Ca(2+) across the plasma membrane leads to localized increases in [Ca(2+)](out) that increase the excitability of CaR.
De Luisi
Annunziata
A
Dipartimento di Fisiologia Generale ed Ambientale, Universitá di Bari, Via Amendola 165/A, I-70126 Bari, Italy.
Hofer
Aldebaran M
AM
D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002460:Cell Line; D002478:Cells, Cultured; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000031:analogs & derivatives* / Q000494:pharmacology; D006801:Humans; D018613:Microscopy, Confocal; D008954:Models, Biological; D044169:Receptors, Calcium-Sensing / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D013997:Time Factors
D002614:Chelating Agents; D044169:Receptors, Calcium-Sensing; D004533:Egtazic Acid; D000252:Calcium-Transporting ATPases; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium