Figure 1. Proteins in the cardiomyocyte Ca²⁺ Signaling Pathway.

A typical contraction in a mammalian cardiomyocyte is initiated by Ca²⁺ entry into the cytosol via L-type Ca²⁺ channels on the sarcolemma. This Ca²⁺ influx can directly cause release of Ca²⁺ from the sarcoplasmic reticulum via the ryanodine receptor (RyR2) channel on the sarcoplasmic reticulum (SR) resulting in an increased [Ca²⁺]_i. The high levels of [Ca²⁺]_i, in turn, trigger the myofilaments to contract and result in activation of CamKIIγ which can phosphorylates phospholamban to release its inhibition from Serca2a. Serca2a then pumps Ca²⁺ from the cytosol into the SR for the next cardiac cycle. Ca²⁺ signaling can also be initiated by vasopressin through the G-protein coupled AVP receptor to activate the IP3 pathway. Local production of IP3 results in release of Ca²⁺ form the SR via the IP3 receptor (IP3R) Ca²⁺ channel. The removal of cytosolic Ca²⁺ is also through Serca2a.