[Ca]i homeostasis and cyclic nucleotide relaxation in aorta of phospholamban-deficient mice

M. Lalli, Jane, Shunichi Shimizu, Roy L. Sutliff, Evangelia G. Kranias, and Richard J. Paul. [Ca]i homeostasis and cyclic nucleotide relaxation in aorta of phospholamban-deficient mice. Am. J. Physiol. 277 (Heart Circ. Physiol. 46): H963–H970, 1999.—Phospholamban (PLB), a protein localized in the sarcoplasmic reticulum (SR), inhibits the SR Ca21-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 (PLB2). In this study, we measured intracellular Ca21 concentration ([Ca]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 a-agonist phenylephrine (PE) increased [Ca]i monotonically to a steady state in the wild-type aorta. In contrast, in PLB2 aorta there was an initial rapid increase to a peak [Ca]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 [Ca]i was two times as fast in the PLB2 as in the wild-type aorta. There were no significant differences between PLB2 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 PLB2 and wild-type aortas. Western blot analysis indicated that the PLB-to-sarcoendoplasmic reticulum Ca21ATPase 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 [Ca]i but only a minor role, if any, in cyclic nucleotide-mediated relaxation.