Increase in cytosolic calcium from intracellular storage, during smooth muscle contraction, is/are due to -

The release of Ca++ from intracellular source, that is sarcoplasmic reticulum, is by IP3-DAG second messenger system, whereas the pumping of Ca++ from extracellular space to cytosol is by Ca++ channels. IP3 when bound to its receptors on sarcoplasmic reticulum, causes the release of calcium into sarcoplasm.