Phospholamban mediates the β-adrenergic-enhanced Ca²⁺ uptake in mammalian ventricular myocytes

To probe the molecular mechanism responsible for the relaxant effect of catecholamines in heart muscle, we studied the effect of a monoclonal antibody (2D12) against phospholamban in intact whole cell clamped guinea pig ventricular myocytes, in which intracellular Ca²⁺ transient and Ca²⁺ current were simultaneously measured. The antibody stimulated Ca²⁺ uptake in guinea pig ventricular sarcoplasmic reticular vesicles, shifting the apparent dissociation constant for activation by Ca²⁺ from 200 to 60 nM. The stimulatory effect of the antibody could be mimicked by the catalytic subunit of adenosine 3',5'-cyclic monophosphate-dependent kinase and could be blocked by phospholamban peptide 2-25. Dialysis of ventricular myocytes with the antibody enhanced the rate of uptake of Ca²⁺ and siguificantly suppressed the ability of isoproterenol to enhance the rate of uptake and release of Ca²⁺ by depolarizing pulses. These data suggest that not only is phosphorylation of phospholamban crucial in sequestration of Ca²⁺ by the sarcoplasmic reticulum, but that this process may account for the catecholamine-enhanced rate of Ca²⁺ uptake release in heart muscle.