Phospholamban has been proposed to coexist as pentamers and monomers in native sarcoplasmic reticulum membranes. To determine its functional unit in vivo, we reintroduced wild-type (pentameric) or monomeric mutant (C41F) phospholamban in the hearts of phospholamban knockout mice. Transgenic lines, expressing similar levels of mutant or wild-type phospholamban, were identified, and their cardiac phenotypes were characterized in parallel. Sarcoplasmic reticulum Ca2+ transport assays indicated similar decreases in SERCA2 Ca2+ affinity by mutant or wild-type phospholamban. However, the time constants of relaxation and Ca2+ transient decline in isolated cardiomyocytes were diminished to a greater extent by wild-type than mutant phospholamban, even without significant differences in the amplitudes of myocyte contraction and Ca2+ transients between the two groups. Langendorff perfusion also indicated that mutant phospholamban was not capable of depressing the enhanced relaxation parameters of the phospholamban knockout hearts to the same extent as wild-type phospholamban. Moreover, in vivo assessment of mouse hemodynamics revealed a greater depression of cardiac function in wild-type than mutant phospholamban hearts. Thus, the mutant or monomeric form of phospholamban was not as effective in slowing Ca2+ decline or relaxation in cardiomyocytes, hearts, or intact animals as wild- type or pentameric phospholamban. These findings suggest that pentameric assembly of phospholamban is necessary for optimal regulation of myocardial contractility in vivo.
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