In the membrane of the sarcoplasmic reticulum, the specialized type of endoplasm reticulum is found inside our muscle cells. Within the membrane we find these calcium ATPases. In fact, calcium Atpas makes up up to 80% of the protein content of the sarcoplasm reticulum. So membrane. So SR basically stands for sarcoplasm reticulum. It's the specialized type of er which is basically used to store the calcium ions found within that muscle cell.