E. Brochiero et al., NA CA EXCHANGE IN THE BASOLATERAL MEMBRANE OF THE A6 CELL MONOLAYER -ROLE IN CA-I HOMEOSTASIS/, Pflugers Archiv, 430(1), 1995, pp. 105-114
The presence of a Na/Ca exchanger in A6 cells was investigated by meas
uring intracellular calcium (Ca-i) fluctuations and the Ca-45 fluxes t
hrough the basolateral membranes (blm) of the cell monolayer. Removal
of Na+ from the medium produced a transient increase in Ca-i followed
by a regulatory phase returning Ca-i to control levels in 3-4 min, thi
s phase being greatly accelerated (< 60 s) by NaCl addition (apparent
K-m of approximately 5 mM Na+). The Ca-i increase was only found with
the Na+-free medium on the basolateral side of the cell monolayer. A t
wofold increase in the Ca-45 influx was observed under these condition
s. In Ca2+- depleted cells, the initial Ca-i increase after Ca2+ addit
ion to the medium was greater when the putative Na/Ca exchanger was no
t functioning (i.e. in a Na+-free medium). Ca-45 effluxes through the
blm of the monolayer were greatly and transiently increased by a Na+-f
ree medium on the serosal side and blocked by orthovanadate (1 mM). Th
e Ca-i increase induced by a hypo-osmotic shock was greater in cells b
athed in a Na+-medium, conditions expected to block the activity of th
e Na/Ca exchanger. These findings support the hypothesis that a Na/Ca
exchanger is present on the blm of A6 cells and affirm its role in Ca-
i homeostasis in steady-state conditions and following osmotic shock.
In addition, a Ca2+ pump also located on the blm and Ca2+ stores sensi
tive to inositol 1,4,5-trisphosphate were found to be implicated in Ca
i homeostasis.