Enhancing the activity of the sarco(endo)plasmic reticulum (SR) Ca²⁺ ATPase (SERCA) by increasing its Ca²⁺ affinity is considered as therapy in heart failure. We created mice in which the cardiac SERCA2a isoform is replaced by SERCA2b, a higher Ca²⁺-affinity isoform (SKO). These mice suffered from impaired cardiac contractility and hypertrophy. To evaluate whether this could relate to the increased SERCA inhibition by phospholamban (PLB), SKO were crossed with PLB^-/-, rendering DKO mice with a further increased Ca²⁺ affinity of the pump. This was associated with a shorter life span, aggravated concentric hypertrophy, diastolic dysfunction and increased ventricular stiffness. Strikingly, beta-adrenergic stress induced acute heart failure and death in DKO. Thus, the spontaneously increased PLB inhibition in SKO preserved cardiac function by counteracting the high Ca²⁺-affinity of SERCA2b. Although SERCA expression further reduced in DKO (-65%) than in SKO (-50%), cardiomyocyte SR Ca²⁺-uptake improved in DKO vs. WT. In addition, cross-breeding SKO with SERCA2b-overexpressing mice enhanced cardiac SERCA-expression by 20%, but did not counter hypertrophy. In conclusion, the high Ca²⁺ affinity of SERCA2b, and not reduced SERCA activity, triggered a hypertrophic response which affected cardiac function. This warns for potential negative consequences of a too high Ca²⁺-pump affinity in the heart.