Mechanisms involved in the cellular calcium homeostasis in vascular smoothmuscle: Calcium pumps

The regulation of cytosolic Ca2+ homeostasis is essential for cells, and pa rticularly for vascular smooth muscle cells. In this regulation, there is a participation of different factors and mechanisms situated at different le vels in the cell, among them Ca2+ pumps play an important role. Thus, Ca2pump, to extrude Ca2+; Na+/Ca2+ exchanger; and different Ca2+ channels for Ca2+ entry are placed in the plasma membrane. In addition, the inner and ou ter surfaces of the plasmalemma possess the ability to bind Ca2+ that can b e released by different agonists. The sarcoplasmic reticulum has an active role in this Ca(2+)regulation; its membrane has a Ca2+ pump that facilitate s luminal Ca2+ accumulation, thus reducing the cytosolic free Ca2+ concentr ation. This pump can be inhibited by different agents. Physiologically, its activity is regulated by the protein phospholamban; thus, when it is in it s unphosphorylated state such a Ca2+ pump is inhibited. The sarcoplasmic re ticulum membrane also possesses receptors for 1,4,5-inositol trisphosphate and ryanodine, which upon activation facilitates Ca2+ release from this sto re. The sarcoplasmic reticulum and the plasmalemma form the superficial buf fer barrier that is considered as an effective barrier for Ca2+ influx. The cytosol possesses different proteins and several inorganic compounds with a Ca2+ buffering capacity. The hypothesis of capacitative Ca2+ entry into s mooth muscle across the plasma membrane after intracellular store depletion and its mechanisms of inhibition and activation is also commented.