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1 Instituto de Investigación Médica Mercedes y Martín Ferreyra, 5000 Córdoba, Argentina; 2 Centro de Estudios Científicos de Santiago, Santiago 9; and Departamento de Fisiología y Biofísica, Facultad de Medicina, Universidad de Chile, Santiago 7, Chile; and 3 Laboratorio de Permeabilidad Iónica, Instituto Venezolano de Investigaciones Científicas, Caracas 1020-A, Venezuela
In cardiac
sarcolemmal vesicles, MgATP stimulates
Na+/Ca2+
exchange with the following characteristics:
1) increases 10-fold the apparent
affinity for cytosolic Ca2+;
2) a Michaelis constant for ATP of
~500 µM; 3) requires micromolar vanadate while millimolar concentrations are inhibitory;
4) not observed in the presence of
20 µM eosin alone but reinstated when vanadate is added;
5) mimicked by adenosine
5'-O-(3-thiotriphosphate), without the need for vanadate, but not by 
,
-methyleneadenosine 5'-triphosphate; and 6) not
affected by unspecific protein alkaline phosphatase but abolished by a
phosphatidylinositol-specific phospholipase C (PI-PLC). The PI-PLC
effect is counteracted by phosphatidylinositol. In addition, in the
absence of ATP,
L-
-phosphatidylinositol
4,5-bisphosphate (PIP2) was able
to stimulate the exchanger activity in vesicles pretreated with PI-PLC.
This MgATP stimulation is not related to phosphorylation of the
carrier, whereas phosphorylation appeared in the phosphoinositides,
mainly PIP2, that
coimmunoprecipitate with the exchanger. Vesicles incubated with MgATP
and no Ca2+ show a marked
synthesis of
L--phosphatidylinositol
4-monophosphate (PIP) with little production of
PIP2; in the presence of 1 µM Ca2+, the net synthesis of PIP is
smaller, whereas that of PIP2
increases ninefold. These results indicate that
PIP2 is involved in the MgATP
stimulation of the cardiac
Na+/Ca2+
exchanger through a fast phosphorylation chain: a
Ca2+-independent PIP formation
followed by a Ca2+-dependent
synthesis of PIP2.
phosphorylation; phosphoinositides; membrane transport
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