The nature of the modulation of Ca2+ transport as studied by reconstitution of cardiac sarcoplasmic reticulum.

Membrane vesicles capable of energized Ca2+ pumping have been reconstituted from cardiac sarcoplasmic reticulum (SR). Cardiac SR was solubilized with Triton X-100 in a detergent to protein weight ratio of 0.8, and membranous vesicles were reconstituted by removal of detergent with Bio-Beads SM-2 (a neutral porous styrene-divinylbenzene copolymer). The reconstituted vesicles exhibited ATP-dependent oxalate-facilitated Ca2+ accumulation with rates and efficiency comparable to the best reconstituted skeletal muscle preparation (Ca2+-loading rate = 1.65 +/- 0.31 mumol mg-1 min-1, Ca2+-activated ATPase activity = 2.39 +/- 0.25 mumol mg-1 min-1, efficiency (Ca2+/ATP) = 0.69 +/- 0.09). Phospholamban in the reconstituted vesicles was phosphorylated with added catalytic subunit of cAMP-dependent protein kinase to almost the same extent as that in original vesicles. However, phosphorylation of phospholamban had no effect on the Ca2+ accumulation of the reconstituted vesicles. This is to be contrasted with a decrease in the half-maximal concentration of Ca2+ for Ca2+ accumulation (KCa) in the original vesicles from 1.35 +/- 0.08 microM to 0.75 +/- 0.12 microM by cAMP-dependent phosphorylation of phospholamban. On the other hand KCa for the reconstituted vesicles was about 0.5 microM and remained unchanged by phosphorylation, indicating that the Ca2+ pump in the reconstituted vesicles is already fully activated. These results suggest that in normal cardiac SR, phospholamban in the dephosphorylated state acts as a suppressor of the Ca2+ pump and that phosphorylation of phospholamban serves to reverse the suppression.