The two main sources of calcium in vascular smooth muscles are: Ca2+ Influx through plasma membrane channels and Ca2+ release from intracellular stores. Vascular smooth muscle cells regulate Ca2+ levels by allowing calcium ions to enter the cell through ion-permeable channels located in the plasma membrane. These channels are often voltage-gated or ligand-gated and open in response to various stimuli. The second main source is the release of calcium ions from intracellular Ca2+ stores, particularly the sarcoplasmic reticulum (SR). The SR acts as a reservoir for Ca2+ and releases stored calcium ions.