Calcium-Handling Studies Using Isolated Smooth Muscle Membranes

Intracellular calcium ion concentration is the major determinant in the contractile state of smooth muscles (see Chapters 2–4). A decrease in this concentration to ~0.1 µM is associated with relaxation and an increase in it correlates with contraction of the smooth muscle. How is the intracellular Ca2+ concentration regulated? Studies using tissue strips to answer this question have been presented in Chapters 3 and 4. It is generally agreed that the cellular membranes, namely, plasma membranes (PM), endoplasmic reticulum (ER), and mitochondria are responsible for regulating the intracellular Ca2+ concentration. Four types of Ca2+ interactions with the membranes have been identified: (a) Ca2+ extrusion from cytoplasm by PM or into ER by an ATP-dependent Ca2+ transport system, (b) transmembrane Ca2+ movements by Na2+—Ca2+ exchange, (c) passive Ca2+ entry from ER or through PM into cytoplasm, and (d) Ca2+ binding to and release from various membranes. Even though these processes have been identified, there is considerable dispute concerning the role of the three different types of membranes (PM, ER, and mitochondria) and the relative importance of each of the processes in the excitation—contraction coupling cycle of the smooth muscle.