When stimulation rate of an isolated rabbit heart increases, incomplete relaxation can be observed in its ventricles. What causes this phenomenon?

• A. Inhibition of the sodium–potassium pump
• B. Increased potassium levels in the interstitium
• C. Accumulation of calcium in cardiomyocytes
• D. Increased sodium levels in cardiomyocytes
• E. Increased potassium levels in cardiomyocytes