Purinergic and Pyriminergic Activation of the Endothelium in Regulation of Tissue Perfusion - Extracellular ATP and Adenosine as Regulators of Endothelial Cell Function - page 3

This is exemplified by the human forearm circulation where adenosine have been
shown also to cause NO dependent increases in blood flow [61].
If the endothelium is damaged or if extracellular nucleotides are administered on
the adventitial side, the vessel constricts as a result of activation of P2Y and P2X
receptors on the VSMC.

1.3 Nucleotide Release in the Vasculature

1.3.1 Endothelial Cells

Shear stress and hypoxia are an important stimuli of both ATP and UTP release from
endothelial cells [17]. The nucleotides can be released from intracellular sources, but
there is also evidence for cell surface ATP synthase involved in shear stress-induced
ATP release [78]. Immunofluorescence staining of human pulmonary arterial ECs
showed that cell surface ATP synthase is distributed in lipid rafts/caveole and crit-
ical for shear stress-induced ATP release. Interestingly, Planck et al., applied a
mathematical model demonstrating that the area directly after a stenotic lesion with
reduced shear stress will have lower levels of extracellular ATP and thereby reduced
endothelium stimulation [50], indicating that lack of ATP mediated endothelial
stimulation might contribute to atherosclerosis development.

1.3.2 Red Blood Cells

The matching of oxygen supply with demand requires a mechanism that increases
blood flow in response to decreased tissue oxygen levels. There is a growing lit-
erature suggesting that the red blood cell (RBC) acts as a sensor for hypoxia and
different mechanisms have been suggested by which the deoxygenated RBC stim-
ulates vasodilatation [22, 26, 46, 82]. The intracellular levels of ATP in RBCs
are at millimolar levels and on the inside of the membrane are abundant gly-
colytic enzymes generating new ATP from adenosine [2, 62, 64]. ATP is released
in response to reductions in oxygen tension and pH [26, 82]. It has been shown
in vitro that vessels dilate in response to low O 2 levels only when blood vessels
are perfused with RBCs [68]. ATP is released in working human skeletal muscle
circulation depending on the number of unoccupied hemoglobin O 2 binding sites
[26, 82]. The released ATP then binds to P2Y receptors on the endothelium and
stimulates vasodilatation. Thus, the RBC may function as an O 2 sensor, contribut-
ing to the regulation of blood flow and O 2 delivery, by releasing ATP depending on
the oxygenation state of hemoglobin (Fig. 1.2).
When ATP is degraded to ADP it can activate P2Y 13 receptors as a nega-
tive feedback pathway for ATP release from human RBCs [73]. Blood consists
of approximately 40% RBCs, containing a 1000-fold higher ATP concentration
than plasma (mmol/L vs. mol/L). With this gradient, even a minor release of

ATP from the high intracellular concentrations raises local extracellular ATP levels

μ