Sarcoplasmic/endoplasmic-reticulum-Ca2+ -ATPase-mediated Ca2+ reuptake, and not Ins(1,4,5)P-3 receptor inactivation, prevents the activation of macroscopic Ca2+ release-activated Ca2+ current in the presence of physiologicalCa2+ buffer in rat basophilic leukaemia-1 cells
D. Bakowski et Ab. Parekh, Sarcoplasmic/endoplasmic-reticulum-Ca2+ -ATPase-mediated Ca2+ reuptake, and not Ins(1,4,5)P-3 receptor inactivation, prevents the activation of macroscopic Ca2+ release-activated Ca2+ current in the presence of physiologicalCa2+ buffer in rat basophilic leukaemia-1 cells, BIOCHEM J, 353, 2001, pp. 561-567
Whole-cell patch-clamp experiments were performed to examine the mechanism
underlying the inability of intracellular Ins(1,4,5)P-3 to activate the Ca2
+ release-activated Ca2+ current (I-CRAC) in rat basophilic leukaemia (RBL)
-1 cells under conditions of weak cytoplasmic Ca2+ buffering. Dialysis with
Ins(1,4,5)P-3 in weak Ca2+ buffer did not activate any macroscopic I-CRAC,
even after precautions had been taken to minimize the extent of Ca2+ entry
during the experiment. Following intracellular dialysis with Ins(1,4,5)P-3
for > 150 s in weak buffer, external application of the sarcoplasmic/endop
lasmic-reticulum Ca2+-ATPase (SERCA) pump blocker thapsigargin activated I-
CRAC and the current developed much more quickly than when thapsigargin was
applied in the absence of Ins(1,4,5)P-3. This indicates that the Ins(1,4,5
)P-3 receptors had not inactivated much over this timecourse. When external
Ca2+ was replaced by Ba2+, Ins(1,4,5)P-3 still failed to generate any dete
ctable I-CRAC even though Ba2+ permeates CRAC channels and is not taken up
into the intra- cellular Ca2+ stores. In strong Ca2+ buffer, I-CRAC could b
e activated by muscarinic-receptor stimulation, provided protein kinase C (
PKC) was blocked. In weak buffer, however, as with Ins(1,4,5)P-3, stimulati
on of these receptors with carbachol did not activate I-CRAC even after inh
ibition of PKC. The inability of Ins(1.4,5)P-3 to activate macroscopic I-CR
AC in weak Ca2+ buffer was not altered by inhibition of Ca2+-dependent phos
phorylation/dephosphorylation reactions. Our results suggest that the inabi
lity of Ins(1,4,5)P-3 to activate I-CRAC. under conditions of weak intracei
lular Ca2+ buffering is not due to strong inactivation of the Ins(1,4,5)P-3
receptors. Instead, a futile Ca2+ cycle across the stores seems to be occu
rring and SERCA pumps resequester sufficient Ca2+ to ensure that the thresh
old for activation of macroscopic I-CRAC has not been exceeded.