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Departments of 1 Molecular and Cellular Physiology and 2 Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267
Phospholamban
(PLB), a protein localized in the sarcoplasmic reticulum (SR), inhibits
the SR Ca2+-ATPase;
phosphorylation of PLB relieves this inhibition. We previously reported
significant differences in contractility in aorta from mice in which
the gene for PLB was ablated
(PLB
). In this study, we
measured intracellular Ca2+
concentration
([Ca2+]i)
with fura 2 in the intact mouse aorta to more directly test the
hypothesis that these changes are ascribable to altered SR function in
vivo. Ten micromoles per liter of the 
-agonist phenylephrine (PE)
increased
[Ca2+]i
monotonically to a steady state in the wild-type aorta. In contrast, in
PLB aorta there was an
initial rapid increase to a peak
[Ca2+]i,
which then decreased to a steady state that was lower than that in the
wild type. Upon removal of the stimulus (either PE or KCl), the
decrease in
[Ca2+]i
was two times as fast in the
PLB as in the wild-type
aorta. There were no significant differences between
PLB and wild-type aortas in
the concentration vs. force relations or the time courses of relaxation
in response to forskolin or sodium nitroprusside. Interestingly,
stimulation of the cAMP pathway before cGMP pathway activation resulted
in a significant increase in sensitivity and a difference in relaxation
parameters between PLB and
wild-type aortas. Western blot analysis indicated that the PLB-to-sarcoendoplasmic reticulum Ca2+ATPase ratio in the
mouse aorta was similar to that in the heart; 20-fold more aortic than
heart homogenate was required to achieve a similar level of
immunoreactivity. Our data indicate that PLB can play a major role in
modulating smooth muscle
[Ca2+]i
but only a minor role, if any, in cyclic nucleotide-mediated relaxation.
calcium; contractility; sarcoplasmic reticulum; smooth muscle; intracellular calcium concentration
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