Ion channels in smooth muscle: Regulation by the sarcoplasmic reticulum and mitochondria

In smooth muscle, Ca2+ regulates cell division, growth and cell death as well as providing the main trigger for contraction. Ion channels provide the major access route to elevate the cytoplasmic Ca2+ concentration ([Ca2+]c) in smooth muscle by permitting Ca2+ entry across the plasma membrane and release of the ion from intracellular Ca2+ stores. The control of [Ca2+]c relies on feedback modulation of the entry and release channels by Ca2+ itself. Local rises in [Ca2+]c may promote or inhibit channel activity directly or indirectly. The latter may arise from Ca2+ regulation of ionic conductances in the plasma membrane to provide control of cell excitability and so [Ca2+]c entry. Organelles such as mitochondria may also contribute significantly to the feedback regulation of ion channel activity by the control of Ca2+ or redox status of the cell. This brief review describes the feedback regulation of Ca2+ release from the internal Ca2+ store and of plasma membrane excitability in smooth muscle.