Opposing actions of PLB and PLM on EC coupling. A, PLB-KO myocytes also exhibit Ca2+ adaptation in spite of elevated SR load. SR load decreases with sustained β-AR stimulation. B, PLB-KO, PLM-KO myocytes do not exhibit Ca2+ exhibition. SR load does not decrease with sustained β-AR stimulation. C, PLB phosphorylation and PLM phosphorylation exert opposite effects on SR load. D, PLB phosphorylation and PLM phosphorylation exert opposite effects on cytosolic Ca2+ transients.