Mechanisms involved in the cellular calcium homeostasis in vascular smooth muscle: Calcium pumps

Abstract

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