Final answer:
Inactivation of phospholamban increases the amount of calcium ions released from the sarcoplasmic reticulum during cardiac muscle contraction, which results in stronger contractions.
Step-by-step explanation:
Inactivation of phospholamban in cardiac muscle affects the calcium ions dynamics and ultimately the force of cardiac contractions. Phospholamban, when active, inhibits the calcium pump (SERCA) in the sarcoplasmic reticulum (SR). Therefore, inactivation of phospholamban relieves this inhibition, leading to an increased rate of calcium uptake back into the SR during muscle relaxation.
With more calcium being stored in the SR, during the next contraction, there is a larger amount of calcium released. This increase in calcium release enhances the interaction between actin and myosin, resulting in a stronger contraction of the cardiac muscle. Thus, inactivation of phospholamban doesn't decrease the amount of calcium released from the sarcoplasmic reticulum; instead, it increases it, contributing to stronger contractions.