Vessel Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 368

Cerebral blood flow is strongly regulated to provide 20% of body energy con-
sumption, even though the brain accounts for only 5% of total weight, and to match
the changing metabolic needs of specific brain regions. Cells that control the blood
flow include endothelial and smooth muscle cells, pericytes, neurons, and astrocytes.
Contraction and relaxation of the vascular smooth myocytes decreases and increases
the artery bore, respectively. Increase in intracellular calcium concentration in
astrocytes regulates the arteriole caliber. In the absence of surrounding smooth
myocytes, blood flow through capillaries is, indeed, regulated by pericytes that
contain contractile machinery [ 683 ]. 42 Groups of pericytes along capillaries and at
capillary junctions of the central nervous system can constrict and relax according
to local neuronal activity. Pericyte constriction can hence redirect the blood flow at
the capillary level.

7.8 Nervous Inputs

Activated q that signals via phospholipase-

C leading to: (1) diacylglycerol that activates protein kinase-C and (2) inositol
trisphosphate that primes calcium influx mainly from the endoplasmic reticulum
(Fig. 7.7 ). In vascular smooth myocytes, G-protein-coupled receptor kinase GRK2
that phosphorylates and desensitizes ligand-bound G-protein-coupled receptors
impedes vasoconstriction caused by

α 1-adrenergic receptors interact with G α

2-Adrenergic re-
ceptors induce relaxation of vascular smooth myocytes (Table 7.7 ).
Smooth muscle myosin-2 consists of 2 heavy chains and 2 types of light
chains. One part of the heavy chains folds into the globular head that forms the
motor domain, the other constitutes the myosin tail that connects to others to
produce the myosin filament. Contraction initiation and termination are switched
by myosin phosphorylation by smooth muscle myosin light chain kinase, and
dephosphorylation by myosin light chain phosphatase. The myosin light chain can
also be phosphorylated by PKC and CamK2 kinases.
Phosphorylation magnitude determines shortening velocity and tension devel-
opment. Isoforms of MLCK in smooth and non-myocytes differ from MLCK in
striated myocytes. Myosin light chain kinase can be phosphorylated by PKA, PKC,
CamK2, and PAK kinases.
Myosin light chain phosphatase is also phosphorylated (inhibited) by RoCK2
kinase, ensuring a sustained vasoconstriction. Integrin-linked protein Ser/Thr kinase
phosphorylates myosin light chain phosphatase as well as myosin light chain.
Regulator caldesmon complexes with actin, tropomyosin, and a Ca 2 + -binding
protein. In the absence of calcium, caldesmon restricts the interaction of the myosin

α 1d-adrenoceptors [ 684 ]. β

42 About 65% of noradrenergic innervation of brain blood vessels ends near capillaries rather than
near arterioles. Pericyte constriction is associated with a rise in intracellular calcium concentration.