<prompt> Answer this question truthfullyWhat are the mechanisms that cause relaxation of a myocardial cell?  <response>: <reason> Relaxation of a myocardial cell occurs when Ca2 is reaccumulated in the sarcoplasmic reticulum by the action of Ca2 ATPase SERCA and when Ca2 is pumped out of the cell by the NaCa2 exchanger NCX. 