is a 52 amino acid integral membrane protein that regulates the Ca2 pump in cardiac muscle and skeletal muscle cells. When phospholamban is phosphorylated by protein kinase A (PKA) its ability to to inhibit the sarcoplasmic reticulum calcium pump (SERCA) is lost. Thus, activators of PKA, such as the beta-adrenergic agonist epinephrine (released by sympathetic stimulation) may enhance the rate of cardiac myocyte relaxation.
Additionally, since SERCA is more active, the next action potential will cause an increased release of calcium resulting in increased contraction (positive inotropic effect). When phospholamban is not phosphorylated, such as when PKA is inactive, it can interact with and inhibit SERCA.