Vessel Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 374

product (MDR, P-glycoprotein-1, ABC20, or ABCb1), and sulfonylurea receptor
(SUR, or ABCc8), which functions as a modulator of K AT P channels and insulin
release, can not only utilize ATP as an energy source for active transport, but also
export the purine nucleotide out of the cell, enabling auto- and paracrine purinergic
signaling [ 686 ].
However, these ATP-binding cassette transporters expressed in endothelial cells
are not involved in ATP release [ 695 ]. Nonetheless, CFTR contributes to ATP
release from red blood capsules upon activation of PKA that fosters CFTR
activity [ 686 ].

7.9.3.3 Plasmalemmal F 1 -F 0 AT P a s e

ATP synthase is composed of 2 fractions: F 0 membrane-anchoring subunit, which
forms the pore for proton transport, and F 1 catalytic subunit. This enzyme resides
not only on the inner mitochondrial membrane, but also on the plasma membrane
of numerous cell types such as vascular endothelial cells [ 686 ]. 45 Synthesis of ATP
at the endothelial cell surface requires the existence of a proton gradient across the
plasma membrane.

7.9.3.4 Connexin Hemichannels

Connexins form hexameric connexons in the endoplasmic reticulum and Golgi
body, which are then transferred to and inserted into the plasma membrane to
build gap junctions between apposed cells or hemichannels between the cytosol
and the extracellular space. Glial cells in the central nervous system, granulocytes,
monocytes, and vascular endothelial and smooth muscle cells exhibit connexin
hemichannels [ 686 ]. At rest, connexin hemichannels remain impermeable; under
strong membrane depolarizations and mechanical stress, these hemichannels open.
Four connexin isoforms abound in vascular smooth muscle and endothelial cells
(Cx37, Cx40, Cx43, and Cx45). They construct gap junctions between adjacent
vascular smooth myocytes on the one hand and endothelial cells on the other, as
well as between these 2 cell types at the myoendothelial junctions (Sect. 9.1.2.3 ).
Connexin hemichannels at the non-junctional plasma membrane of endothelial cells
may release ATP agent.
Macula densa cells of the juxtaglomerular apparatus coat the wall of the distal
tubule at the glomerular vascular pole. They detect changes in distal tubular flow
rate and sodium chloride concentration. They release ATP through gap junctions
into the interstitium of the juxtaglomerular apparatus to control the glomerular

45 Angiostatin, an anti-angiogenic product of plasminogen cleavage in the fibrinolytic system, has
a high binding affinity for

α β

subunits of F 1 fraction of ATP synthase and prevents its activity,
hence secretion of ATP messenger.

and