Calcium enters the smooth muscle cell through voltage-gated, ligand-gated, stretch-activated, and “leak” calcium channels.
Calcium can also be released into the cell from the sarcoplasmic reticulum (SR) by inositol 1,4,5-triphosphate (IP3) and Ca2+-stimulated calcium release. The processes on the left side increase cytoplasmic calcium and promote contraction; those on the right side decrease internal calcium and promote relaxation. DAG, diacylglycerol; IP3, inositol 1,4,5-trisphosphate; PIP2, phosphatidylinositol 4,5-bisphosphate.
Medical Physiology: Principles of Clinical Medicine