Sự gia tăng canxi trong tế bào miễn phí có thể do tăng nhập hoặc canxi vào trong tế bào thông qua các kênh canxi loại L-hoặc phát hành của canxi từ các cửa hàng nội bộ (ví dụ, sarcoplasmic lưới | CELLULAR STRUCTURE AND FUNCTION 53 stimulates vascular smooth muscle contraction. An increase in free intracellular calcium can result from either increased entry of calcium into the cell through L-type calcium channels or release of calcium from internal stores . sarcoplasmic reticulum . The free calcium binds to a special calcium-binding protein called calmodulin. The calciumcalmodulin complex activates myosin light chain kinase an enzyme that phosphorylates myosin light chains in the presence of ATP. Myosin light chains are regulatory subunits found on the myosin heads. Myosin light chain phosphorylation leads to cross-bridge formation between the myosin heads and the actin filaments thus leading to smooth muscle contraction. Intracellular calcium concentrations therefore are very important in regulating smooth muscle contraction. The concentration of intracellular calcium depends on the balance between the calcium that enters the cells the calcium that is released by intracellular storage sites and the movement of calcium either back into intracellular storage sites or out of the cell. Calcium is resequestered by the sarcoplasmic reticulum by an ATP-dependent calcium pump similar to the SERCA pump found in cardiac myocytes. Calcium is removed from the cell to the external environment by either an ATP-dependent calcium pump or the sodium-calcium exchanger as in cardiac muscle see Chapter 2 . Several signal transduction mechanisms modulate intracellular calcium concentration and therefore the state of vascular tone. This section describes three different pathways 1 IP3 via Gq-protein activation of phospholipase C 2 cAMP via Gs-protein activation of adenylyl cyclase and 3 cyclic guanosine monophosphate cGMP via nitric oxide NO activation of guanylyl cyclase Fig. 3-10 . Epi Ado PGI2 FIGURE 3-10 Receptors and signal transduction pathways that regulate vascular smooth muscle contraction. R receptor Gs stimulatory G-protein Gq phospholipase C-coupled G-protein AC .
