Abstract

The inotropic/lusitropic effects of β-adrenergic agonists on the heart are mediated largely by protein kinase A (PKA)-catalyzed phosphorylation of phospholamban, the natural protein regulator of the Ca²⁺ pump present in sarcoplasmic reticulum (SR) membranes. Gingerol, a plant derivative, is known to produce similar effects when tested in isolated cardiac muscle. The purpose of the present study was to compare the effects of gingerol and another plant derivative, ellagic acid, on the kinetics of the SR Ca²⁺ pump with those of PKA-catalyzed phospholamban phosphorylation to elucidate their mechanisms of Ca²⁺ pump regulation. As previously demonstrated for PKA, 50 μM gingerol or ellagic acid increasedV_max(Ca) of Ca²⁺ uptake and Ca²⁺-ATPase activity assayed at millimolar ATP concentrations in light cardiac SR vesicles. Unlike PKA, which decreases K_m(Ca), neither compound had a significant effect on K_m(Ca) in unphosphorylated vesicles. However, gingerol increasedK_m(Ca) in phosphorylated vesicles, in which Ca²⁺ uptake was significantly increased further at saturating Ca²⁺ and remained unchanged at subsaturating Ca²⁺. An inhibition of Ca²⁺ uptake by gingerol at micromolar MgATP concentrations was overcome with increasing MgATP concentrations. The stimulation 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 only about 12% in phosphorylated microsomes as well as in rabbit fast skeletal muscle light SR. The present results support the view that an ATP-dependent increase in V_max(Ca)of the SR Ca²⁺ pump plays an important role in mediating cardiac contractile responses to gingerol and phospholamban-dependent β-adrenergic stimulation.