Smooth Myocytes - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 379

that the entire cell set contracts in bulk, as a syncytium. Single-unit smooth muscles
line blood vessels, except large elastic arteries. (2) In multi-unit smooth muscles,
such as in the trachealis muscle that connects the ends of U-shaped cartilaginous
rings of the trachea and media of large arteries, the autonomic nervous system
innervates many cells to finely tune nervous control and cause gradual responses.
Smooth myocytes can contribute to the formation of the extracellular matrix, as
they secrete collagens, mostly type-1 and -3, elastin, glycoproteins, and proteogly-
cans. They possess on their surface elastin and collagen receptors that interact with
these extracellular proteins.
The plasma membrane, or sarcolemma, possesses specialized nanodomains
devoted to cell signaling with ion carriers and receptors for hormones, growth
factors, neurotransmitters (e.g., acetylcholine and cathecholamines), and regulators
of the SMC tone (e.g., nitric oxide and endothelin). However, smooth myocytes
in various regions of the vascular bed, airways, and kidneys differ in their production
of ion carriers and receptors, as well as other proteins that determine their function.

8.1 Markers of Smooth Myocytes

Smooth myocytes of walls of the vasculature and ventilatory tract possess various
contractile proteins. Contractility of smooth myocytes relies on smooth muscle α -

actin, myosin heavy chain, calponin, 2 caldesmon hMW , 3 . β -tropomyosin, and myosin

light chain kinase (MLCK).
Hypercontractility markers include myosin light chain kinase, smooth muscle-
specific myosin heavy chain (smMHC) isoforms, and 22-kDa actin-binding, smooth
muscle protein (SM22), or transgelin (TaGln). In addition to cytoskeleton constitu-
tents, calmodulin is synthesized in smooth myocytes to participate in the control of
cytoskeleton functioning.
Myosin-2 and actin generate cell contraction via stress fibers. In contracting
smooth myocytes, -actin and myosin-2

filaments remodel. Myosin-2 is activated by Ca 2 + -calmodulin myosin light chain
kinase and inhibited by myosin light chain phosphatase.
Contraction of smooth myocytes is regulated not only by phosphorylation of the
myosin light chain by Ca 2 + -calmodulin-dependent myosin light chain kinase, but
also by the caldesmon-calmodulin complex, protein kinase-C, and calponin.

β -actin network is conserved, whereas α

2 Calponin is a calcium-binding inhibitor of the ATPase activity of myosin in smooth muscle.
Phosphorylation of calponin by a calcium-calmodulin-dependent protein kinase relieves calponin-
induced inhibition.
3 High-molecular-weight caldesmon hMW is predominantly synthesized in smooth myocytes.
The low-molecular-weight isoform, caldesmon lMW is widespread in non-myocytes. Caldesmon
alternatively binds (flip-flop binding) either to calmodulin or

F actin according to the Ca 2 +

concentration.