Phospholamban Involvement in the Maintenance of Basal Calcium Transport in Cardiac Sarcoplasmic Reticulum

Abstract

Phosphorylation of cardiac sarcoplasmic reticulum membrane vesicles by exogenous c-AMP and c-AMP-dependent protein kinase stimulates calcium uptake and Ca²⁺-dependent ATP hydrolysis by 40–50% and results in the incorporation of ³²P into a 22-KDa protein, phospholamban. Treatment of the membrane with DOC (0.0002% or 5 × 10⁻⁶ M) solubilizes phospholamban from the membrane and induces a 90% inhibition of basal calcium uptake. This inhibition cannot be attributed to an alteration in vesicle integrity or membrane permeability. The (Ca²⁺ + Mg²⁺)-ATPase remains associated with the membrane fraction and exhibits optimal levels of Ca²⁺-stimulated ATP hydrolysis. Phosphorylation prior to DOC treatment allows retention of the phospholamban in the membrane, concomitant with maintenance of the calcium transport activity. The results presented suggest that phospholamban is involved in the maintenance of basal calcium transport function in cardiac sarcoplasmic reticulum and that its phosphorylation stimulates Ca²⁺ transport.

Key Words:

• membranes
• cell surfaces
• receptors