Expression of Phospholamban

In cardiac muscle, the increased activity of the Ca + pump of the SR as a consequence of the phosphorylation of phospholamban plays a key role in the positive chronotropic and inotropic action of -receptor agonists (Tada and Kadoma, 1989 Luo et al., 1994). The highest levels of expression of phospholamban are found in cardiac and in slow skeletal muscle, the two muscle types that express SERCA2a as the major Ca2+-pump isoform. East skeletal muscle, which expresses SERCAl, does not contain phospholamban. Coexpression studies have shown, however, that phospholamban is able to interact with the SERCAl pump (Fujii et al, 1990). 

Phospholamban was initially detected in smooth muscle in purified ER preparations from pig stomach and rabbit aorta as electrophoretic bands comigrating with the 25-kDa pentameric and 5-kDa monomeric forms of cardiac phospholamban. The dissociation of the pentamer depends on the conditions of pretreatment of the sample with the denaturing electrophoresis buffer. Phospholamban protein has also been detected in many other vascular and gastrointestinal smooth muscles, including rat aortic cells (Sarcevic et al, 1989 Karczewski et ai, 1992), canine ileum and iliac artery (Ferguson et al., 1988), and bovine aorta (L. Raeymaekers, unpublished observations Watras, 

However, phospholamban protein was undetectable in pig aorta (Raeymaekers and Jones, 1986 Eggermont etfl/., 1990b). To our knowledge, published data on myometrium are lacking. The amino acid sequence of phospholamban of pig stomach smooth muscle determined from cDNA sequencing is identi- 

The level of phospholamban protein relative to that of SERCA Ca2+-ATPase in isolated ER vesicles from smooth muscle has been compared in bovine aorta (Watras, 1988), in bovine main pulmonary artery, and in several smooth muscles of the pig (Eggermont etal, 1990b). Watras (1988) and Raeymaekers et al. (1990) found a ratio of approximately 1. A possible drawback of these experiments is that they depended on the preservation of the pump s activity during isolation of the membranes. In the porcine smooth muscles investigated by Eggermont et al. (1990b), the level of phospholamban protein relative to that of SERCA2 protein was smaller than in cardiac muscle (Fig. 2). It is not clear whether this different result is due to the tissue difference or to the use of a more reliable method. 

The expression of phospholamban at the mRNA level was examined by Eggermont et al. (1990b) on total RNA extracted from several smooth muscle tissues of the pig using RNAse protection assay. The highest phospholamban mRNA level was observed in the pig ileum, followed by the pig stomach. In these tissues, the amount of both mRNAs was about 10- to 

---