Smooth Myocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 401

Smooth myocytes adapt to a wide range of cell lengths and maintain optimal
contractility. Assembly and disassembly of cytoskeletal filaments in smooth my-
ocytes enable adaptation to large changes in cell geometry. The ability of acto-
myosin filaments to form and dissolve within the myofilament lattice at adequate
sites and suitable instants yields the basic mechanism of cell adaptation [ 742 ].
Formation of myosin filaments is promoted by various filament-stabilizing proteins
such as caldesmon. Many proteins may be assigned, removed, or reassigned in the
context of reversible reorganization of the cytoskeleton. Myosin filament length is
determined by the size of the actin filament network [ 742 ].
In airway smooth myocytes, the amount of contractile filaments can change
rapidly to accommodate the context. Actin filament content actually increases
during contractile activation. Myosin filament content also varies. In relaxed smooth
myocytes, the density of myosin thick filaments is low, although of significant num-
ber. Myosin filaments indeed undergo rapid cycles of assembly and disassembly.
Upon contractile stimulation, the content of myosin filaments in airway smooth
myocytes rises strongly [ 742 ]. In solution, assembly of non-phosphorylated myosin
monomers into filaments depends on monomer concentration. Once a critical
concentration is reached, self-assembly occurs. The critical concentration increases
when ATP is available and decreases when the myosin regulatory light chain is
phosphorylated. In addition, C-terminal isoforms SM1 and SM2 of smooth muscle
myosin heavy chain influence stability of filament assembly. Isoform SM1 confers
a greater stability [ 742 ]. Yet, myosin filament lability is required for adaptive cell
remodeling. Synthesis of SM1 and SM2 isoforms differs among different types of
smooth myocytes as well as during development. Isoform SM2 may enhance SMC
adaptation to large changes in length. In the presence of ATP Mg , phosphorylation
of the myosin regulatory light chain as well as other facilitating factors in smooth
myocytes may regulate thick filament assembly.
Upon large-amplitude length oscillation applied to relaxed trachealis muscle
strips, myosin filaments can partially disassemble [ 742 ]. These filaments reassemble
when smooth muscle is repeatedly activated isometrically, following phosphor-
ylation of regulatory myosin light chains. Moreover, myosin filament formation
increases in the presence of actin filaments. Caldesmon that is detected interspersed
in the actin filament network where myosin filaments are located can also provoke
reassembly of long myosin filaments.

8.4.6.2 Cell Mechanoresponsiveness

Every cell exerts traction on its matrix and experiences more or less large time-
dependent stretch exerted by its environment. The cytoskeleton can stiffen,
hence increasing traction ( reinforcement response ), or can soften ( fluidization
response )[ 744 ]. Therefore, cells respond to imposed stretch using 2 procedures.
(1) Reinforcement refers to rapid actin polymerization and increased focal adhesion