Smooth muscle contraction requires intracellular Ca++ but the sensitivity to Ca++ varies. Certain hormones/agonists increase the force of contraction at a given submaximal intracellular [Ca++], resulting in Ca++ sensitization. Ca++ sensitization is represented as a leftward shift in the Ca++ dependence of smooth muscle contraction. On the other hand, an increase in MP activity at a given intracellular [Ca++] leads to Ca++ desensitization, causing a rightward shift in the Ca++ dependence of smooth muscle contraction. MP and MLCK activities can undergo reciprocal changes, as shown in Fig. 14.8A, where an agonist can stimulate the ROK signaling cascade and inhibit MP while stimulating MLCK, thereby increasing the Ca++ sensitivity of contraction. RhoA activates ROK, which inhibits MP through direct and indirect mechanisms. Direct inhibition involves ROK phosphorylation of the myosin-binding subunit (MBS) of MP. Indirect inhibition involves phosphorylation of CPI-17, a 17-kDa endogenous protein, which then inhibits MP. Hormones/agonists such as catecholamines (acting on α1-adrenergic receptors), vasopressin, endothelin, angiotensin, and muscarinic agonists increase the sensitivity of smooth muscle contraction to Ca++ through activation of RhoA/ROK signaling. ROK can also be activated by arachidonic acid and inhibited by Y-27632, a highly specific inhibitor. Inactive RhoA, typically bound to GDP and an inhibitory protein (Rho-GDP dissociation inhibitor [GDI]), is located in the cytosol. Binding of agonist to various G-coupled receptors can activate RhoA by stimulating guanine nucleotide exchange factor (GEF) to yield RhoA-GTP, which localizes to the sarcolemma and activates ROK. Conversely, stimulation of MP activity reduces the Ca++ sensitivity of smooth muscle contraction, promoting relaxation and vasodilation.
Although intracellular Ca++ is required for smooth muscle contraction, the sensitivity of contraction to Ca++ is variable. Several hormones/agonists, for example, increase the force of contraction at a given submaximal intracellular [Ca++], thereby resulting in Ca++ sensitization (
Fig.
14.8 ). Ca++ sensitization is depicted as a leftward shift in the Ca++ dependence of smooth muscle contraction (see
Fig. 14.8B ) and can occur in response to a decrease in the activity of MP activity at a given intracellular [Ca++]. Likewise an increase in MP activity at a given intracellular [Ca++] promotes a rightward shift in the Ca++ dependence of smooth muscle contraction, resulting in Ca++ desensitization (see
Fig. 14.8B ). Reciprocal changes in the activities of MP and MLCK can occur, as shown in
Fig. 14.8A , where stimulation of the ROK signaling cascade by an agonist can simultaneously inhibit MP and stimulate MLCK at a given intracellular [Ca++], thereby increasing the Ca++ sensitivity of contraction (i.e., Ca++ sensitization).
Fig.
14.8A ).RhoAactivatesRhokinase(ROK),whichinturninhibitsMPbybothdirectandindirectmechanisms.DirectinhibitionofMPbyactivatedROKinvolvesROKphosphorylationofthemyosin-bindingsubunit(MBS)ofMP.IndirectinhibitionofMPbyactivatedROKinvolvesphosphorylationofCPI-17, anendogenous17-kDaprotein,whichtheninhibitsMP.Hormones/agonistssuchascatecholamines(actingonα1-adrenergicreceptors),vasopressin,endothelin,angiotensin,andmuscarinicagonistsincreasethesensitivityofsmoothmusclecontractiontoCa++ throughactivationofRhoA/ROKsignaling.ROKcanalsobeactivatedbyarachidonicacidandinhibitedbyY-27632,ahighlyspecificinhibitor(see
Fig.14.8A ).Thoughnotshownin
Fig.14.8 ,inactiveRhoAistypicallylocatedinthecytosol,boundtoGDPandaninhibitoryprotein(Rho-GDP dissociation inhibitor [GDI]). BindingofagonisttovariousG-coupledreceptorscanactivateRhoAbystimulatingguanine nucleotide exchange factor (GEF) toyieldRhoA-GTP,whichlocalizestothesarcolemmaandactivatesROK.Conversely,stimulationofMPactivityreducestheCa++ sensitivityofcontractionofsmoothmuscle,therebypromotingrelaxation(andhencevasodilation).