Relaxation

For myocardial relaxation to occur, cytosolic Ca²⁺ must fall to promote the removal of Ca²⁺ from binding sites of Tn-C, enabling Tn-I, Tn-T, and tropomyosin to take up their inhibitor roles hiding action binding sites to prevent the interaction of actin and myosin heads.

Cytosolic Ca²⁺ falls as it is taken up mainly into the SR where it is stored for the next contractile cycle.

• A transmembrane protein in the SR called SERCA (Sarcoendoplasmic Reticulum Ca ATPase) is a Ca²⁺ pump that moves Ca²⁺ into the SR.
• At rest the SERCA pump is inhibited by the protein phospholamban, thus Ca²⁺ movement into the SR is impaired. When phospholamban is phosphorylated by protein kinase A, the inhibition of SERCA is removed, allowing it to remove Ca²⁺ from the cytoplasm.

Beta adrenergic stimulation aids both contraction and relaxation. Relaxation is promoted by cAMP-mediated protein kinase A activation that promotes the activity of the SERCA pump reducing intracellular Ca²⁺. In addition, the increased Ca²⁺ entry into the cell activates calmodulin which in turn enhances the activity of phosphodiesterase. Phosphodiesterase increases the rate of breakdown of cAMP. This in turn reduces Ca²⁺ entry into the cell promoting relaxation.