The Ca2+ Homeostasis

The passive permeability of nonexcitable cells to Ca²⁺ is thought to be very low, but it seems to rely upon the metabolic condition of the cell. There are several mechanisms that have been proposed for the movement of Ca²⁺ across the plasma membrane of resting cells. The rate of the exchange is drastically lowered by removal of ATP and increased by electrical depolarization. Although there is no experimental evidence, it is simple to think that a single Ca2i+-Ca20+ exchange mechanism operates during passive Ca²⁺ inflow and passive Ca²⁺ outflow in red blood cells. Excitable cells which allow penetration of extracellular Ca²⁺ after stimulation, in addition to the Ca²⁺ pump, possess transporters that catalyze the rapid outflow of Ca²⁺ through the plasma membrane to restore the initial concentration of Ca²⁺ in the cytosol. The role of the Ca²⁺ pump of plasma membrane is therefore fundamental in determining the concentration of Ca²⁺ in cytosol and hence in the whole of the cell.