Smooth and striated muscles both exhibit a hyperbolic dependence of shortening velocity on load. However, contraction velocities are far slower in smooth muscle than in striated muscle. One factor that underlies these slow velocities is that the myosin isoform in smooth muscle cells has low ATPase activity.
Skeletal muscle cells have a force-velocity curve in which shortening velocities are determined only by load and the myosin isoform (see ). In contrast, both force and shortening velocity, which reflect the number of cycling cross-bridges and their cycling rates, vary in smooth muscle. When activation of smooth muscle is altered, for example, by different frequencies of nerve stimulation or changing hormone concentrations, a “family” of velocity-stress curves can be derived (
Fig. 14.16 ). This implies that both cross-bridge cycling rates and the number of active cross-bridges in smooth muscle are regulated in some way,
Vo(% of max)
Velocity (% of max)
Stress (% of max) •Fig. 14.16 A,Force-velocitycurvesforfastandslowhumanskeletalmusclecellsandsmoothmuscle.B,Smoothmuscleshavevariableforce-velocityrelationshipsthataredeterminedbythelevelofCa++-stimulatedcross-bridgephosphorylation.C,Maximalshorteningvelocitieswithnoload(Vo,whichrepresentstheinterceptsontheordinateinB)aredirectlydependentoncross-bridgephosphorylationbyMLCK.D,Activeforce/stress(abscissainterceptsinB)risesrapidlywithphosphorylation;nearmaximalstress,itmaybegeneratedwithonly20%to30%ofthecross-bridgesinthephosphorylatedstate.
which is in marked contrast to striated muscle. This difference is conferred by a regulatory system that depends on phosphorylation of cross-bridges, which in turn depends on myoplasmic [Ca++]. Because myosin light-chain phosphorylation is required for actin-myosin interaction in smooth muscle, a dependence of maximal force on the degree of myosin phosphorylation is expected (i.e., phosphorylation of more myosin molecules results in more actin-myosin interactions and hence more force generated). The variation in maximal shortening velocity as a function of the degree of myosin phosphorylation may reflect dephosphorylation of the myosin light chain while the myosin is still attached to the actin, thus slowing the rate of detachment (i.e., latch state) at low levels of phosphorylation. At higher levels of phosphorylation, the likelihood of latch states would be reduced and the myosin cross-bridges would be released more quickly from actin, thereby yielding a higher shortening velocity at all loads (see
Fig. 14.16B 1.
Smooth muscle cells are linked by a variety of junctions that serve both mechanical and communication roles. These linkages are essential in cells that must contract uniformly.
2.