L. Khiroug et al., ROLE OF INTRACELLULAR CALCIUM IN FAST AND SLOW DESENSITIZATION OF P-2-RECEPTORS IN PC12 CELLS, British Journal of Pharmacology, 120(8), 1997, pp. 1552-1560
1 Combined whole-cell patch clamp recording and confocal laser scannin
g microscopy of [Ca2+](i) transients were performed on single PC12 cel
ls to study any correlation between membrane currents induced by ATP a
nd elevation in [Ca2+](i). ATP was applied by pressure from micropipet
tes near the recorded PC12 cells continuously superfused at a fast rat
e, 2 Brief (20 ms) pulses of ATP elicited monophasic inward currents a
nd [Ca2+](i) increases. Long applications (2 s) of ATP (5 mM) evoked p
eak currents which rapidly faded during the pulse and were followed by
a large rebound current, interpreted as due to rapid desensitization
and recovery of P-2-receptors. The associated [Ca2+](i) increase grew
monotonically to a peak reached only after the occurrence of the curre
nt rebound, indicating that it is unlikely this cation has a role in f
ast desensitization. 3 Both membrane currents and [Ca2+](i) transients
were dependent on holding membrane potential, suggesting that Ca2+ in
flux is the predominant cause of [Ca2+](i) elevation. Tills view was s
upported by experiments carried out in Ca2+-free solution. 4 Brief pul
ses of ATP applied after a desensitizing pulse (2 s) of the same elici
ted smaller inward currents and [Ca2+](i) rises indicating a role for
[Ca2+](i) in controlling slow desensitization of P-2-receptors. 5 This
notion was confirmed in experiments with various [Ca2+](i) chelators
which differentially affected slow desensitization in relation to thei
r buffering capacity, while sparing fast receptor desensitization, 6 T
hese results suggest a role for [Ca2+](i) in slow rather than fast des
ensitization of P-2-receptors, thus proposing this divalent cation as
an intracellular factor able to provide an efficient and reversible co
ntrol over receptor activity induced by ATP.