Phospholamban has been suggested to be a key regulator of cardiac sarcoplasmic reticulum (SR) Ca cycling and contractility and a potential therapeutic target in restoring the depressed Ca cycling in failing hearts. Our understanding of the function of
phospholamban stems primarily from studies in genetically altered mouse models. To evaluate the significance of this
protein in larger mammalian species, which exhibit Ca cycling properties similar to humans, we overexpressed
phospholamban in adult rabbit cardiomyocytes. Adenoviral-mediated gene transfer, at high multiplicities of
infection, resulted in an insignificant 1.22-fold overexpression of
phospholamban. There were no effects on twitch Ca-transient amplitude or decay under basal or
isoproterenol-stimulated conditions. Furthermore, the SR Ca load and Na/Ca exchanger function were not altered. These apparent differences between
phospholamban overexpression in rabbit compared with previous findings in the mouse may be due to a significantly higher (1.5-fold) endogenous
phospholamban-to-sarco(endo)plasmic reticulum Ca-
ATPase (SERCA) 2a ratio and potential functional saturation of SERCA2a by
phospholamban in rabbit cardiomyocytes. The findings suggest that important species-dependent differences in
phospholamban regulation of SERCA2a occur. In larger mammals, a higher fraction of SERCA2a pumps are regulated by
phospholamban, and this may influence therapeutic strategies to enhance cardiac contractility and functional cardiac reserve.