Neurokinin Receptor Mechanisms in Smooth Muscle

In smooth muscle, a rise in free intracellular calcium is considered to be of primary importance in determining the degree of actin-myosin interaction, and resultant contraction, although other processes may also operate. Contraction due to hormone-receptor interaction utilises the two main sources of calcium, namely extracellular calcium and stored intracellular calcium, which can be made available by some combination of three mechanisms: (1) namely (a) entry of extracellular calcium via voltage-operated channels (VOCs). (b) Entry of extracellular calcium via receptor-operated channels (ROCs). (c) Release of intracellular calcium from storage sites such as the sarcoplasmic reticulum (SR) within the cell. The extent and efficiency of different agonists in using these three calcium translocation mechanisms varies depending both on the particular type of smooth muscle concerned, and also the agonist-receptor system mediating the response.