Final answer:
In vascular smooth muscle, calcium ions enter cells during the contraction phase through ligand-gated and voltage-dependent channels. The sarcoplasmic reticulum releases additional calcium needed for contraction. Muscle relaxation involves active transport of calcium back into the SR and out of the cell by ATP-dependent calcium pumps.
Step-by-step explanation:
Regulation of Calcium in Vascular Smooth Muscle -
In vascular smooth muscle, calcium regulation is essential for the process of muscle contraction and relaxation. During the contraction phase, characterized by the transient calcium peak and sustained calcium plateau, calcium ions enter the smooth muscle cells primarily through ligand-gated calcium ion channels and voltage-dependent Ca²⁺ channels. These calcium channels allow extracellular Ca²⁺ to flow into the cytoplasm, raising the cytoplasmic calcium concentration, which is necessary for muscle contraction.
Once inside the cell, some calcium ions diffuse out of the sarcoplasmic reticulum (SR), which provides the rest of the calcium required for contraction. The SR has a limited storage capacity in smooth muscle fibers, compared to skeletal muscle fibers. The interaction of calcium with calmodulin and other proteins within the cell triggers contraction.
During the phase of muscle relaxation, the clearance of cytosolic calcium is achieved by ATP-dependent calcium pumps, which actively transport Ca²⁺ back into the SR and out of the cell, restoring low cytosolic calcium levels and relaxing the muscle. This ATP-dependent transport is essential for the muscle to return to a resting state after contraction.