Interactions of 6-gingerol and ellagic acid with the cardiac sarcoplasmic reticulum Ca2+-ATPase

The inotropic/lusitropic effects of p-adrenergic agonists on the heart are mediated largely by protein kinase A (PKA)-catalyzed phosphorylation of pho spholamban, the natural protein regulator of the Ca2+ pump present in sarco plasmic reticulum (SR) membranes. Gingerol, a plant derivative, is known to produce similar effects when tested in isolated cardiac muscle. The purpos e of the present study was to compare the effects of gingerol and another p lant derivative, ellagic acid, on the kinetics of the SR Ca2+ pump with tho se of PKA-catalyzed phospholamban phosphorylation to elucidate their mechan isms of Ca2+ pump regulation. As previously demonstrated for PKA, 50 mu M g ingerol or ellagic acid increased V-max(Ca) of Ca2+ uptake and Ca2+-ATPase activity assayed at millimolar ATP concentrations in light cardiac SR vesic les. Unlike PKA, which decreases K-m(Ca), neither compound had a significan t effect on K-m(Ca) in unphosphorylated vesicles. However, gingerol increas ed K-m(Ca) in phosphorylated vesicles, in which Ca2+ uptake was significant ly increased further at saturating Ca2+ and remained unchanged at subsatura ting Ca2+. An inhibition of Ca2+ uptake by gingerol at micromolar MgATP con centrations was overcome with increasing MgATP concentrations. The stimulat ion of Ca2+ uptake attributable to gingerol in unphosphorylated microsomes at saturating Ca2+ was 30% to 40% when assayed at 0.05 to 2 mM MgATP and on ly about 12% in phosphorylated microsomes as well as in rabbit fast skeleta l muscle light SR. The present results support the view that an ATP-depende nt increase in V-max(Ca) of the SR Ca2+ pump plays an important role in med iating cardiac contractile responses to gingerol and phospholamban-dependen t beta-adrenergic stimulation.