Involvement of electrostatic phenomena in phospholamban‐induced stimulation of Ca uptake into cardiac sarcoplasmic reticulum

The activity of the Ca-pumping ATPase of cardiac sarcoplasmic reticulum (SR) is controlled by the phosphorylation of the intrinsic regulatory protein phospholamban (PLB), which affects both the apparent Km(Ca) and the Vmax of the transport process. We have investigated the correlation between phosphorylation of PLB and the surface potential of the SR membrane. This latter influences the local concentration of relevant ionic species near biological membranes and thus modulates the activity of ion pumps and channels. The partitioning of the anionic probe 8-anilino-1-naphthalenesulfonate (ANS-) into the SR membrane was found to be dependent on the phosphorylation level of PLB. Changes of the surface membrane potential up to 7 mV could be obtained by phosphorylation. The increase in the apparent affinity of the Ca pump for Ca2+ induced by PLB phosphorylation was clearly reduced at high ionic strength, i.e. under conditions known to reduce the surface membrane potential and all processes dependent on it. The results show that electrostatic phenomena can account, in good part, for the modulation of the Ca pump by PLB in cardiac SR.