<prompt> Answer this question truthfullyWhat is the mechanism by which Ca2 is released from myocardial cells during the action potential?  <response>: <reason> The action potential is a series of electrical events that occur in myocardial cells, leading to the contraction of the heart muscle. One important aspect of the action potential is the entry of Ca2 into the myocardial cell, which triggers further release of Ca2 through ryanodine receptors Ca2 release channels. These channels are located on the sarcoplasmic reticulum, a specialized organelle within the myocardial cell that stores and releases Ca2. When Ca2 enters the cell during the action potential, it binds to the ryanodine receptors and causes them to open, leading to the release of even more Ca2 from the sarcoplasmic reticulum. This increase in intracellular Ca2 concentration triggers the contraction of the myocardial cell, which ultimately leads to the pumping action of the heart. 