Читать книгу Vitamin D in Clinical Medicine - Группа авторов - Страница 13

The concentration of Ca in the cellular cytoplasm is about 10^–6 M, whereas the ECF Ca concentration is approximately 10^–3 M. This creates a 1,000-fold concentration gradient across the plasma membrane that favors Ca entry into the cell. Additionally, there is an electrical charge of about 50 mV across the plasma membrane, with the cell interior negative. The cell must, therefore, defend against these chemical and electrical gradients across the plasma membrane, which greatly favor Ca entry in order to preserve viability. Several mechanisms are utilized to prevent Ca-induced cell death. These include extrusion of Ca from the cell by ATP-dependent energy driven Ca pumps and Ca channels, and by Na-Ca exchangers. In addition, intracellular Ca may be bound to proteins in the cytoplasm, endoplasmic reticulum (ER), and mitochondria. The Ca bound in these sites can not only buffer intracellular Ca, but also maintain cytoplasmic Ca levels and create pulsatile peaks of Ca to mediate membrane receptor signaling that regulate a variety of biologic systems.