Kargacin M E, Ali Z, Kargacin G
Department of Physiology and Biophysics, University of Calgary, 3330 Hospital Drive NW, Calgary, Alberta, T2N 4N1, Canada.
Biochem J. 1998 Apr 1;331 ( Pt 1)(Pt 1):245-9. doi: 10.1042/bj3310245.
The activity of the SERCA2a Ca2+ pump in the sarcoplasmic reticulum (SR) of cardiac muscle is inhibited by phospholamban. When phospholamban is phosphorylated by cyclic-AMP-dependent protein kinase (PKA) this inhibition is relieved. It is generally agreed that this results in an increase in the Ca2+ sensitivity of the SR Ca2+ pump; however, some investigators have also reported an increase in the maximum velocity of the pump. We have used a sensitive fluorescence method to measure net Ca2+ uptake by native cardiac SR vesicles and compared the effects of a constitutively active subunit of PKA (cPKA) with those of a monoclonal antibody (A1) that binds to phospholamban and is thought to mimic the effect of phosphorylation. Both the Ca2+ sensitivity and the maximum velocity of uptake were increased by cPKA and by A1. The effects of cPKA and A1 on uptake velocity were only slightly additive. No changes in uptake were detected with denatured cPKA or denatured A1. These results indicate that the functional effect of phospholamban phosphorylation is to increase both the Ca2+ sensitivity and the maximum velocity of net Ca2+ uptake into the SR.
心肌肌浆网（SR）中SERCA2a钙泵的活性受受磷蛋白抑制。当受磷蛋白被环磷酸腺苷依赖性蛋白激酶（PKA）磷酸化时，这种抑制作用解除。人们普遍认为，这会导致SR钙泵对钙离子的敏感性增加；然而，一些研究人员也报告称泵的最大速度有所增加。我们使用一种灵敏的荧光方法来测量天然心肌SR囊泡的净钙离子摄取量，并比较了PKA组成型活性亚基（cPKA）与一种与受磷蛋白结合且被认为能模拟磷酸化作用的单克隆抗体（A1）的作用效果。cPKA和A1均增加了钙离子敏感性和摄取的最大速度。cPKA和A1对摄取速度的影响只是略有叠加。未变性的cPKA或未变性的A1未检测到摄取变化。这些结果表明，受磷蛋白磷酸化的功能作用是增加SR中净钙离子摄取的钙离子敏感性和最大速度。
