Vascular endothelial cells regulate vascular smooth muscle tone through 
Ca2+-dependent production and release of vasoactive molecules. Phospholamban 
(PLB) is a 24- to 27-kDa phosphoprotein that modulates activity of the 
sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA). Expression of PLB is 
reportedly limited to cardiac, slow-twitch skeletal and smooth muscle in which 
PLB is an important regulator of [Ca2+]i and contractility in these muscles. In 
the present study, we report the existence of PLB in the vascular endothelium, a 
nonmuscle tissue, and provide functional data on PLB regulation of vascular 
contractility through its actions in the endothelium. Endothelium-dependent 
relaxation to acetylcholine was attenuated in aorta of PLB-deficient (PLB-KO) 
mice compared with wild-type (WT) controls. This effect was not due to actions 
of nitric oxide on the smooth muscle, because sodium nitroprusside-mediated 
relaxation in either denuded or endothelium-intact aortas was unaffected by PLB 
ablation. Relative to denuded vessels, relaxation to forskolin was enhanced in 
WT endothelium-intact aortas. The endothelium-dependent component of this 
relaxation was attenuated in PLB-KO aortas. To investigate whether these changes 
were due to PLB, WT mouse aorta endothelial cells were isolated. Both reverse 
transcriptase-polymerase chain reaction and Western blot analyses revealed the 
presence of PLB in endothelial cells, which were shown to be >98% pure by 
diI-acetylated LDL uptake and nuclear counterstaining. These data indicate that 
PLB is present and modulates vascular function as a result of its actions in 
endothelial cells. The presence of PLB in endothelial cells opens new fields for 
investigation of Ca2+ regulatory pathways in nonmuscle cells and for modulation 
of endothelial-vascular interactions.