Smooth muscle contraction is regulated by a complex signaling pathway involving various molecules and interactions. Upon activation of G-coupled receptors by multiple agonists, inositol trisphosphate (InsP3) is produced through the action of phospholipase C (PLC) on phosphatidylinositol bisphosphate (PIP2). InsP3 then opens InsP3-gated calcium channels in the sarcoplasmic reticulum (SR), leading to an increase in intracellular calcium concentration ([Ca++]). This increase in [Ca++] activates calmodulin, which in turn activates myosin light-chain kinase (MLCK). MLCK phosphorylates the myosin regulatory light chain (MLC), promoting actin-myosin interaction and subsequent contraction of smooth muscle.

Rho-kinase (ROK) plays a crucial role in smooth muscle contraction. Activated RhoA stimulates ROK, which inhibits myosin phosphatase (MP) by phosphorylating the myosin-binding subunit (MBS) of MP. ROK also indirectly inhibits MP by phosphorylating and activating CPI-17, an inhibitor of MP. These phosphorylation events collectively decrease MP activity, leading to increased myosin light-chain phosphorylation (MLCP) and heightened force of contraction at a given intracellular [Ca++].

The phenomenon of calcium sensitization refers to an increase in the force of contraction at a given intracellular [Ca++]. This is represented by a leftward shift in the calcium dependence curve of smooth muscle contraction. Calcium sensitization can result from a decrease in MP activity and/or an increase in MLCK activity at a given intracellular [Ca++]. Conversely, calcium desensitization refers to a decrease in the force of contraction at a given intracellular [Ca++] and is depicted by a rightward shift in the calcium dependence curve.

Overall, smooth muscle contraction involves a sequence of events starting with Ca++ mobilization and followed by cross-bridge phosphorylation and cycling. Brief stimulation leads to a transient, twitch-like contraction, while prolonged stimulation results in sustained tonic contraction. During tonic contractions, force is maintained at a reduced [Ca++] and low level of myosin light-chain phosphorylation, leading to lower cross-bridge cycling rates, slower shortening velocities, and reduced ATP consumption.
P P Ins(1,4,5)P3 Ca++ SR Ca++ CaM MLCK MLC MLC P GEFs Rho Rho-kinase CPI-17 CPI-17 MBS M20 cat MBS Inactive Active Contraction M20 cat MP Y27632 GTP ? AA PLC GqG12/13 •Fig. 14.8 Rho-kinase(ROK)signalinginsmoothmuscle.A,AvarietyofagonistsofG-coupledreceptorssimultaneouslystimulateInsP3productionandactivateRhoA-ROKsignaling.InsP3(Ins[1,4,5]P3)isproducedbyphospholipaseC(PLC)-mediatedhydrolysisofPIP2.InsP3increasesintracellular[Ca++]byopeningInsP3-gatedCa++ channelsinthesarcoplasmicreticulum(SR),therebyresultinginCa++-calmodulin–dependentactivationofmyosinlight-chainkinase(MLCK)andsubsequentphosphorylationofthemyosinregulatorylightchain(MLC)andpromotionofactin-myosininteraction(contraction).ActivatedRhoA(depictedasRho-GTP)stimulatesROK,whichinhibitsmyosinphosphatase(MP)byphosphorylatingthemyosin-bindingsubunit(MBS)ofMP.ROKalsoinhibitsMPindirectlybyphosphorylating/activatingCPI-17,a17-kDainhibitorofMP.TheneteffectofROKphosphorylationsisadecreaseinMPactivity,whichresultsinanincreasedlevelofmyosinlight-chainphosphorylation(MLCP)andhencegreaterforceofcontractionatagivenintracellular[Ca++](i.e.,increasedsensitivityofcontractiontoCa++).B,Ca++ sensitization referstoanincreaseintheforceofcontractionatagivenintracellular[Ca++]andisdepictedasaleftwardshiftintheCa++ dependenceofcontractionofsmoothmuscle.Ca++ sensitizationcanresultfromadecreaseinMPactivityand/oranincreaseinMLCKactivityatagivenintracellular[Ca++](asshownintheROKsignalingcascadeinpanelA).Conversely,Ca++ desensitization referstoadecreaseintheforceofcontractionatagivenintracellular[Ca++]andisdepictedasarightwardshiftintheCa++ dependenceofcontraction.AA,arachidonicacid;CaM,Ca++-calmodulincomplex;cat,catalyticsubunitofmyosinphosphatase;G12/13andGq,heterotrimericG-proteins;GEF,guanine-nucleotideexchangefactor;M20,subunitofmyosinphosphatase;MP,myosinphosphatasecomplex;Y27632,commercialinhibitorofROK.(FromFukataYetal.Trends Pharmacol Sci 2001;22:32-39.) •Fig. 14.9 Timecourseofeventsincross-bridgeactivationandcontractioninsmoothmuscle.A,AbriefperiodofstimulationisassociatedwithCa++ mobilization,followedbycross-bridgephosphorylationandcyclingtoproduceabriefphasic,twitch-likecontraction.B,Inasustainedtoniccontractionproducedbyprolongedstimulation,theCa++ andphosphorylationlevelstypicallyfallfromaninitialpeak.Forceismaintainedduringtoniccontractionsatareduced[Ca++](andhencealowlevelofmyosinlight-chainphosphorylation),withlowercross-bridgecyclingratesmanifestedbylowershorteningvelocitiesandATPconsumption.