Frank K, Tilgmann C, Shannon T R, Bers D M, Kranias E G
Department of Pharmacology and Cell Biophysics, University of Cincinnati College of Medicine, Cincinnati, Ohio 45267, USA.
Biochemistry. 2000 Nov 21;39(46):14176-82. doi: 10.1021/bi001049k.
Phospholamban is an inhibitor of the sarcoplasmic reticulum Ca(2+) transport apparent affinity for Ca(2+) in cardiac muscle. This inhibitory effect of phospholamban can be relieved through its phosphorylation or ablation. To better characterize the regulatory mechanism of phospholamban, we examined the initial rates of Ca(2+)-uptake and Ca(2+)-ATPase activity under identical conditions, using sarcoplasmic reticulum-enriched preparations from phospholamban-deficient and wild-type hearts. The apparent coupling ratio, calculated by dividing the initial rates of Ca(2+) transport by ATP hydrolysis, appeared to increase with increasing [Ca(2+)] in wild-type hearts. However, in the phospholamban-deficient hearts, this ratio was constant, and it was similar to the value obtained at high [Ca(2+)] in wild-type hearts. Phosphorylation of phospholamban by the catalytic subunit of protein kinase A in wild-type sarcoplasmic reticulum also resulted in a constant value of the apparent ratio of Ca(2+) transported per ATP hydrolyzed, which was similar to that present in phospholamban-deficient hearts. Thus, the inhibitory effects of dephosphorylated phospholamban involve decreases in the apparent affinity of sarcoplasmic reticulum Ca(2+) transport for Ca(2+) and the efficiency of this transport system at low [Ca(2+)], both leading to prolonged relaxation in myocytes.
受磷蛋白是肌浆网Ca(2+)转运的抑制剂，对心肌中的Ca(2+)具有明显的亲和力。受磷蛋白的这种抑制作用可通过其磷酸化或缺失来解除。为了更好地表征受磷蛋白的调节机制，我们使用来自受磷蛋白缺陷型和野生型心脏的富含肌浆网的制剂，在相同条件下检测了Ca(2+)摄取的初始速率和Ca(2+) - ATP酶活性。通过将Ca(2+)转运的初始速率除以ATP水解来计算的表观偶联比，在野生型心脏中似乎随着[Ca(2+)]的增加而增加。然而，在受磷蛋白缺陷型心脏中，该比值是恒定的，并且与在野生型心脏中高[Ca(2+)]时获得的值相似。蛋白激酶A的催化亚基对野生型肌浆网中的受磷蛋白进行磷酸化，也导致每水解一个ATP所转运的Ca(2+)的表观比值恒定，这与受磷蛋白缺陷型心脏中的情况相似。因此，去磷酸化的受磷蛋白的抑制作用涉及肌浆网Ca(2+)转运对Ca(2+)的表观亲和力降低以及该转运系统在低[Ca(2+)]时的效率降低，这两者都会导致心肌细胞舒张延长。
