Vascular Endothelium - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 593

effects of hypoxia involves the endothelium. In cerebral arteries, the net contribution
of endothelial factors to hypoxic vasodilatation is moderate. In addition, hypoxia
activates ATP-sensitive K + channels of vascular smooth myocytes of cerebral
arteries, thereby causing membrane hyperpolarization and reducing calcium influx.
Hypoxia also lower the calcium sensitivity of contractile proteins.
The cerebral circulation is similarly sensitive to hypoxia than the coronary
circulation, but more sensitive to hypercapnia [ 1159 ]. Hypercapnia also activates
K AT P channel and supports NO synthesis [ 1160 ]. Two CO 2 regulatory mechanisms
enable pH or CO 2 homeostasis to maintain activity of numerous enzymes and ion
channels operating in neural activities [ 1161 ]: the central respiratory chemoreflex
aimed at maintaining CO 2 level and repressing associated respiratory acidosis
(hyperventilation) and cerebrovascular CO 2 reactivity that controls cerebral blood
flow. The onset of the latter is faster than that of the former. At rest, cerebral
vasodilation attenuates activation of the central chemoreflex and subsequently
ventilatory response onset. The sensitivity of cerebral vasoreactivity to hypercapnia
is restrained by augmented sympathetic activity [ 1162 ].

9.10.10 Vasomotor Tone during Pregnancy

Pregnancy is characterized by the development of the uteroplacental vasculature.
Pregnancy potentiates endothelium-derived hyperpolarizing factor that enables va-
sodilation [ 1163 ]. A cumulative exposure of acetylcholine causes a rapid dilatation
in non-pregnant and late pregnant rats. This vasodilation results from reduced
cytosolic Ca 2 + concentration and membrane hyperpolarization in smooth myocytes.
On the other hand, endothelial cytosolic Ca 2 + concentration increases. Small- (SK)
and intermediate-conductance (IK) Ca 2 + -activated K + channels are involved in the
vasodilation of the maternal uterine resistive vasculature [ 1163 ].

9.10.11 Modeling of Mechanotransduction in Arteries

Vascular endothelial and smooth muscle cells at the interface between flowing blood
and underlying layer of the vessel wall such as the media in arteries interact to
locally regulate the vasomotor tone. This process pertains to the set of regulations
carried out by endothelial cells (Fig. 9.11 ). The endothelial interface serves thus as
a local controller of tissue homeostasis for both blood and constituent tissues of the
vessel wall.
Vascular endothelial and smooth muscle cells can be supposed to be mainly shear
and stretch sensor, respectively. Different types of mechanotransduction responses
exist in the arterial bed. The autoregulation , also termed myogenic or Bayliss effect,
is observed in small resistive arteries, i.e., relatively far downstream the heart exit.
The autoregulation aims at ensuring tissue perfusion. In other words, the flow rate