Regulation of Ca2+-pump from cardiac sarcoplasmic reticulum

Calcium uptake by sarcoplasmic reticulum and its modulation by cAMP-dependent phosphorylation in normal and failing human myocardium

ATP-dependent, oxalate-supported Ca2+ uptake by cardiac sarcoplasmic reticulum was examined in microsomes prepared from left-ventricular free wall myocardium obtained from the explanted failing hearts of transplant recipients with idiopathic dilated cardiomyopathy and the non-failing hearts of kidney donors for whose hearts no suitable recipients were available. There were no significant differences between the two groups with respect to values for Vmax, K0.5 (for Ca2+) or nHill of basal Ca2+ uptake. The stimulation of Ca2+ uptake associated with cAMP-dependent phosphorylation of phospholamban could be reproduced by incubation of microsomes with a monoclonal antibody to phospholamban. Stimulation resulted from a decrease in K0.5, with no changes in Vmax or nHill. The magnitude of stimulation of Ca2+ uptake following incubation with anti-phospholamban monoclonal antibody was identical in preparations from normal and failing hearts. Finally, sarcoplasmic reticulum-associated cGMP-inhibited cAMP phosphodiesterase activity in these preparations was characterised. Measurement of steady-state kinetics and pharmacologic sensitivity indicated that this activity was functionally homogeneous. Preparations from failing and non-failing hearts did not differ with respect to either values for Vmax and Km or susceptibility to inhibition by the cilostamide derivative OPC 3911. These observations indicate that abnormalities in the regulation of intracellular [Ca2+] in failing human myocardium cannot be ascribed to changes in the level or function of the Ca(2+)-transporting ATPase, phospholamban or cGMP-inhibited cAMP phosphodiesterase in the sarcoplasmic reticulum.

Effects of monoclonal antibody against phospholamban on calcium pump ATPase of cardiac sarcoplasmic reticulum

A monoclonal antibody against phospholamban has been reported to increase Ca2+ uptake by cardiac sarcoplasmic reticulum. We compared the effect of this antibody on Ca2+ pump ATPase activity of cardiac sarcoplasmic reticulum vesicles to the effect of cAMP-dependent phosphorylation of phospholamban. The antibody markedly stimulated the Ca(2+)-dependent ATPase activity in parallel to the increase in Ca2+ uptake by cardiac sarcoplasmic reticulum. When the Ca(2+)-dependent profile of the ATPase activity was compared, the KCa was shifted from 1.24 to 0.62 microM by the antibody, whereas cAMP-dependent phosphorylation of phospholamban shifted the KCa to 0.84 microM. When cardiac sarcoplasmic reticulum vesicles were treated with both cAMP-dependent protein kinase and the antibody, the stimulation was the same as that with the antibody alone. Thus, the Ca2+ pump ATPase seems to be fully activated by the antibody. The stoichiometry between Ca2+ uptake and ATPase rate was around 1 and no significant change was observed by the treatment with the antibody. Therefore, the stimulation of Ca2+ uptake of cardiac sarcoplasmic reticulum by the antibody occurred by the stimulation of Ca2+ pump ATPase, not by other mechanisms such as channel activity of phospholamban. These results indicate that the binding of the antibody to phospholamban produces essentially the same mode of action on Ca2+ pump ATPase as that of phospholamban phosphorylation. The antibody and phospholamban phosphorylation appear to release the inhibitory action of phospholamban on Ca2+ pump ATPase, resulting in the stimulation of Ca2+ pump.

Identification, kinetic properties and intracellular localization of the (Ca(2+)-Mg2+)-ATPase from the intracellular stores of chicken cerebellum

The microsomal fraction of chicken cerebellum expresses a large amount of Ca(2+)-ATPase (105 kDa), which is phosphorylated by ATP in the presence of Ca2+. The Ca(2+)-ATPase activity is highly sensitive to temperature and to the presence of detergents. This ATPase has kinetic properties similar to those of chicken skeletal-muscle sarcoplasmic reticulum, as (i) it is activated by low (microM) and inhibited by high (mM) Ca2+ concentrations, (ii) it shows biphasic activation with ATP and (iii) it is inhibited by vanadate. However, the vanadate-sensitivity is at least 10 times greater than that observed in chicken skeletal or cardiac sarcoplasmic-reticulum Ca(2+)-ATPases. Thus, despite cross-reacting with antibodies against the cardiac and skeletal isoforms, the cerebellar microsomal Ca(2+)-ATPase appears to be distinct from both muscle enzymes. The Ca(2+)-ATPase is concentrated in, but not exclusive to, Purkinje neurons. In Purkinje neurons the Ca(2+)-ATPase appears to be expressed throughout the cell body, the dendritic tree (and the spines) and the axons. At the electron-microscope level the Ca(2+)-ATPase is found in smooth and rough endoplasmic-reticulum cisternae as well as in other, yet unidentified, smooth-surfaced structures.

Regulation of the Ca2+ pump ATPase by cAMP-dependent phosphorylation of phospholamban

Ca2+ transients in myocardial cells are modulated by cyclic AMP-dependent phosphorylation of a protein in the sarcoplasmic reticulum. This protein, termed phospholamban, serves to regulate the Ca2+ pump ATPase of this membrane, thus altering the mode of Ca2+ transients and the myocardial contractile response. Elucidating the structure of phospholamban and its intimate interaction with the Ca2+ pump ATPase should provide the basis for understanding, at the molecular level, how the cAMP system contributes to excitation-contraction coupling in muscle cells.