Abstract
This chapter discusses the calcium sensitivity of contraction. It describes the current understanding of mechanisms by which the Ca2+ sensitivity of smooth-muscle contraction may be modulated. Selected experimental examples are presented to illustrate important concepts. This relation between the concentration of Ca2+ surrounding the myofilaments and contractile force in the steady state has since been documented in both intact and permeabilized smooth-muscle preparations. The general features of this relation suggest that muscle is relaxed at an intracellular Ca2+ concentration ([Ca2+]i) of about 100 nM. Smooth-muscle contraction arises from the MgATP-dependent cyclic interaction of myosin in thick filaments with actin in thin filaments. The ability of actin to activate myosin MgATPase is inhibited by the nonphosphorylated 20-kDa regulatory light chain (RLC) subunit of myosin.
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