Purinergic control of vascular tone in the retina

Abstract

Key pointsr The blood supply to the CNS is controlled by the tone of arteries and arterioles. However, little is known about purinergic regulation of vascular tone in the CNS.r We have investigated purinergic control of vascular tone in the in vivo rat retina. r Reducing endogenous ATP levels by enzyme degradation and inhibiting purinergic signalling with P2X receptor antagonists decreases the tone of retinal arterioles. Raising ATP levels increases vessel tone.r Experiments using fluorocitrate to poison the metabolism of glial cells suggest glia are a source of the ATP that tonically constricts retinal vessels.r The results suggest a novel mechanism of local control of vascular tone through activation of vascular smooth muscle cell P2X receptors.Abstract Purinergic control of vascular tone in the CNS has been largely unexplored. This study examines the contribution of endogenous extracellular ATP, acting on vascular smooth muscle cells, in controlling vascular tone in the in vivo rat retina. Retinal vessels were labelled by I.V. injection of a fluorescent dye and imaged with scanning laser confocal microscopy. The diameters of primary arterioles were monitored under control conditions and following intravitreal injection of pharmacological agents. Apyrase (500 units ml −1 ), an ATP hydrolysing enzyme, dilated retinal arterioles by 40.4 ± 2.8%, while AOPCP (12.5 mM), an ecto-5 -nucleotidase inhibitor that increases extracellular ATP levels, constricted arterioles by 58.0 ± 3.8% (P < 0.001 for both), demonstrating the importance of ATP in the control of basal vascular tone. Suramin (500 μM), a broad-spectrum P2 receptor antagonist, dilated retinal arterioles by 50.9 ± 3.7% (P < 0.001). IsoPPADS (300 μM) and TNP-ATP (50 μM), more selective P2X antagonists, dilated arterioles by 41.0 ± 5.3% and 55.2 ± 6.1% respectively (P < 0.001 for both). NF023 (50 μM), a potent antagonist of P2X1 receptors, dilated retinal arterioles by 32.1 ± 2.6% (P < 0.001). A438079 (500 μM) and AZ10606120 (50 μM), P2X7 antagonists, had no effect on basal vascular tone (P = 0.99 and P = 1.00 respectively). In the ex vivo retina, the P2X1 receptor agonist α,β-methylene ATP (300 nM) evoked sustained vasoconstrictions of 18.7 ± 3.2% (P < 0.05). In vivo vitreal injection of the gliotoxin fluorocitrate (150 μM) dilated retinal vessels by 52.3 ± 1.1% (P < 0.001) and inhibited the vasodilatory response to NF023 (50 μM, 7.9 ± 2.0%; P < 0.01). These findings suggest that vascular tone in rat retinal arterioles is maintained by tonic release of ATP from the retina. ATP acts on P2X1 receptors, although contributions from other P2X and P2Y receptors cannot be ruled out. Retinal glial cells are a possible source of the vasoconstricting ATP.

Glial cell regulation of neuronal activity and blood flow in the retina by release of gliotransmitters

“…Fluorocitrate could also be acting on neurons or on vascular cells to reduce vessel tone. This is unlikely, however, as the toxin is believed to act selectively on glial cells and reduces vascular tone in the retina by decreasing stimulation of P2 receptors [13]. The tonic release of ATP from Mü ller cells could also modulate the excitability of retinal neurons, for instance, inhibiting the firing of RGCs.…”

“…We have found that tonic release of ATP from glial cells could be responsible for generating tone in retinal arterioles [13]. Manipulation of endogenous ATP levels results in large changes in vascular tone.…”

“…Endogenous ATP acts on P2X purinergic receptors to generate vascular tone [13]. Addition of non-selective P2X antagonists, including suramin and isoPPADS, reduces vascular tone, leading to a dilation of retinal arterioles (figure 3c).…”

“…We explored whether glial cells are the source of vasoconstricting ATP in the retina using the selective gliotoxin fluorocitrate [13]. Fluorocitrate is selectively taken up by glial cells in the CNS and blocks the tricarboxylic acid cycle by inhibiting aconitase.…”

Interpreting BOLD: towards a dialogue between cognitive and cellular neuroscience

“…Rather than triggering a rapid vascular response, synaptic activity-evoked astrocyte calcium increases have been proposed to mediate sustained vascular responses to prolonged stimulation [40,43,44]. Furthermore, astrocytes also appear to play a role in maintaining the resting tone of the vasculature [45][46][47], which could affect the size of the blood flow response following neuronal activity [48].…”