Signal transduction in smooth muscle - Invited review: Mechanisms of calcium handling in smooth muscles

The concentration of cytoplasmic Ca2+ regulates the contractile state of sm ooth muscle cells and tissues. Elevations in global cytoplasmic Ca2+ result ing in contraction are accomplished by Ca2+ entry and release from intracel lular stores. Pathways for Ca2+ entry include dihydropyridine-sensitive and -insensitive Ca2+ channels and receptor and store-operated nonselective ch annels permeable to Ca2+. Intracellular release from the sarcoplasmic retic ulum (SR) is accomplished by ryanodine and inositol trisphosphate receptors . The impact of Ca2+ entry and release on cytoplasmic concentration is modu lated by Ca2+ reuptake into the SR, uptake into mitochondria, and extrusion into the extracellular solution. Highly localized Ca2+ transients (i.e., s parks and puffs) regulate ionic conductances in the plasma membrane, which can provide feedback to cell excitability and affect Ca2+ entry. This short review describes the major transport mechanisms and compartments that are utilized for Ca2+ handling in smooth muscles.