Background: Elementary Ca2+ signals, such as 'Ca2+ puffs', that arise from
the activation of clusters of inositol 1,4,5,-trisphosphate (InsP(3)) recep
tors are the building blocks for local and global Ca2+ signalling. We previ
ously found that one, or a few, Ca2+ puff sites within agonist-stimulated c
ells act as 'pacemakers' to initiate global Ca2+ waves. The factors that di
stinguish these pacemaker Ca2+ puff sites from the other Ca2+ release sites
that simply participate in Ca2+ wave propagation are unknown.
Results: The spatiotemporal properties of Ca2+ puffs were investigated usin
g confocal microscopy of fluo3-loaded HeLa cells. The same pacemaker Ca2+ p
uff sites were activated during stimulation of cells with different agonist
s. The majority of agonist-stimulated pacemaker Ca2+ puffs originated in a
perinuclear location. The positions of such Ca2+ puff sites were stable for
up to 2 hours, and were not affected by disruption of the actin cytoskelet
on. A similar perinuclear distribution of Ca2+ puff sites was also observed
when InsP(3) receptors were directly stimulated with thimerosal or membran
e-permeant InsP(3) esters, Immunostaining indicated that the perinuclear po
sition of pacemaker Ca2+ puffs was not due to the localised expression of I
nsP(3) receptors.
Conclusions: The pacemaker Ca2+ puff sites that initiate Ca2+ responses are
temporally and spatially stable within cells. These Ca2+ release sites are
distinguished from their neighbours by an intrinsically higher InsP(3) sen
sitivity.