Increased 20-HETE Signaling Suppresses Capillary Neurovascular Coupling After Ischemic Stroke in Regions Beyond the Infarct

Neurovascular coupling, the process by which neuronal activity elicits increases in the local blood supply, is impaired in stroke patients in brain regions outside the infarct. Such impairment may contribute to neurological deterioration over time, but its mechanism is unknown. Using the middle cerebral artery occlusion (MCAO) model of stroke, we show that neuronal activity-evoked capillary dilation is reduced by ∼75% in the intact cortical tissue outside the infarct border. This decrease in capillary responsiveness was not explained by a decrease in local neuronal activity or a loss of vascular contractility. Inhibiting synthesis of the vasoconstrictive molecule 20-hydroxyeicosatetraenoic acid (20-HETE), either by inhibiting its synthetic enzyme CYP450 ω-hydroxylases or by increasing nitric oxide (NO), which is a natural inhibitor of ω-hydroxylases, rescued activity-evoked capillary dilation. The capillary dilation unmasked by inhibiting 20-HETE was dependent on PGE2 activation of endoperoxide 4 (EP4) receptors, a vasodilatory pathway previously identified in healthy animals. Cortical 20-HETE levels were increased following MCAO, in agreement with data from stroke patients. Inhibition of ω-hydroxylases normalized 20-HETE levels in vivo and increased cerebral blood flow in the peri-infarct cortex. These data identify 20-HETE-dependent vasoconstriction as a mechanism underlying capillary neurovascular coupling impairment after stroke. Our results suggest that the brain’s energy supply may be significantly reduced after stroke in regions previously believed to be asymptomatic and that ω-hydroxylase inhibition may restore healthy neurovascular coupling post-stroke.

Increased 20-HETE signaling suppresses neurovascular coupling after ischemic stroke in regions beyond the infarct

Neurovascular coupling, the process by which neuronal activity elicits increases in the local blood supply, is impaired in stroke patients in brain regions outside the infarct. Such impairment may contribute to neurological deterioration over time, but its mechanism is unknown. Using the middle cerebral artery occlusion (MCAO) model of stroke, we show that neuronal activity-evoked capillary dilation is reduced by ~75% in the intact cortical tissue outside the infarct border. This decrease in capillary responsiveness was not explained by a decrease in local neuronal activity or a loss of vascular contractility. Inhibiting synthesis of the vasoconstrictive molecule 20-HETE, either by inhibiting its synthetic enzyme CYP450 ω-hydroxylases or by increasing nitric oxide (NO), which is a natural inhibitor of ω-hydroxylases, rescued activity-evoked capillary dilation. The capillary dilation unmasked by inhibiting 20-HETE was dependent on PGE2 activation of EP4 receptors, a vasodilatory pathway previously identified in healthy animals. Cortical 20-HETE levels were increased following MCAO, in agreement with data from stroke patients. Inhibition of ω-hydroxylases normalized 20-HETE levels in vivo and increased cerebral blood flow in the peri-infarct cortex. These data identify 20-HETE-dependent vasoconstriction as a mechanism underlying neurovascular coupling impairment after stroke. Our results suggest that the brain's energy supply may be significantly reduced after stroke in regions previously believed to be asymptomatic and that ω-hydroxylase inhibition may restore healthy neurovascular coupling post-stroke.

The impact of capillary dilation on the distribution of red blood cells in artificial networks

Recent studies suggest that pericytes around capillaries are contractile and able to alter the diameter of capillaries. To investigate the effects of capillary dilation on network dynamics, we performed simulations in artificial capillary networks of different sizes and complexities. The unequal partition of hematocrit at diverging bifurcations was modeled by assuming that each red blood cell (RBC) enters the branch with the faster instantaneous flow. Network simulations with and without RBCs were performed to investigate the effect of local dilations. The results showed that the increase in flow rate due to capillary dilation was less when the effects of RBCs are included. For bifurcations with sufficient RBCs in the parent vessel and nearly equal flows in the branches, the flow rate in the dilated branch did not increase. Instead, a self-regulation of flow was observed due to accumulation of RBCs in the dilated capillary. A parametric study was performed to examine the dependence on initial capillary diameter, dilation factor, and tube hematocrit. Furthermore, the conditions needed for an efficient self-regulation mechanism are discussed. The results support the hypothesis that RBCs play a significant role for the fluid dynamics in capillary networks and that it is crucial to consider the blood flow rate and the distribution of RBCs to understand the supply of oxygen in the vasculature. Furthermore, our results suggest that capillary dilation/constriction offers the potential of being an efficient mechanism to alter the distribution of RBCs locally and hence could be important for the local regulation of oxygen delivery.

Longterm Treatment with Endothelin Receptor Antagonist Bosentan and Iloprost Improves Fingertip Blood Perfusion in Systemic Sclerosis

Objective.To evaluate the longterm effects of endothelin-1 (ET-1) antagonism on peripheral blood perfusion (PBP) in patients with systemic sclerosis (SSc).Methods.Twenty-six patients with SSc already receiving cyclic intravenous iloprost (ILO) for severe Raynaud phenomenon were enrolled. Thirteen patients continued the treatment for a further 3 years (ILO group) and 13 patients, because of the appearance of digital ulcers, received in addition bosentan (BOS; 125 mg twice/day) for 3 years (ILO + BOS group). Both PBP at fingertips and nailfold microangiopathy were evaluated yearly by laser Doppler flowmetry and nailfold videocapillaroscopy, respectively.Results.A progressive significant increase of PBP was observed in the ILO + BOS group during the 3 followup years (p = 0.0007, p = 0.0002, p = 0.01, respectively). In contrast, an insignificant progressive decrease of PBP was observed in the ILO group. Difference of perfusion between the PBP evaluations at basal temperature and at 36°C (to test capillary dilation capacity), was found progressively decreased during the 3-year followup only in the ILO group (p = 0.05, p = 0.26, p = 0.09, respectively). A progressive increase of nailfold capillary number was observed only in the ILO + BOS group after 2 and 3 years of followup (p = 0.05).Conclusion.Longterm treatment of SSc patients with ET-1 antagonism, in combination with ILO, seems to increase fingertip blood perfusion, as well as both capillary dilation capacity and number.

Capillary Dilation and Rarefaction Are Correlated with Intracapillary Inflammation in Antibody-Mediated Rejection

Antibody-mediated rejection (ABMR) remains one of the major causes of graft loss after renal transplantation. It is dominated by endothelial damage in microcirculation. Clarifying the mechanism of microcirculating damage is obviously a key step to understand the pathogenesis of ABMR. Here we characterized capillary variation in ABMR and its possible mechanisms. Compared with T cell-mediated rejection and stable grafts, there was a significant dilation and rarefaction in peritubular capillaries (PTCs) of the ABMR group; Image-Pro Plus revealed a significantly larger intra-PTC area. Interestingly, the dilation of PTCs was strongly correlated with the intra-PTC cell counting. Moreover, peritubular capillary inflammation is correlated within situT-bet expression, and there was a good correlation between the intra-PTC expression of T-bet and the PTC diameter. HIF-1αup-regulation could be observed in ABMR but it was not necessary for capillary dilation. In general, ABMR is characterized with early capillary dilation and rarefaction; our data confirmed that the dilation is strongly correlated with intracapillary inflammation, which in turn is correlated within situT-bet expression. T-bet plays an important role in the development of microcirculating injury, and thus it is a potential target for the treatment of ABMR.

Aging is associated with an increased susceptibility to ischaemic necrosis due to microvascular perfusion failure but not a reduction in ischaemic tolerance

In the present study in a murine model of chronic ischaemia, we analysed: (i) whether aging was associated with an increased susceptibility to ischaemic necrosis, and (ii) whether this was based on microvascular dysfunction or reduced ischaemic tolerance. An ischaemic pedicled skin flap was created in the ear of homozygous hairless mice. The animals were assigned to three age groups, including adolescent (2±1 months), adult (10±2 months) and senescent (19±3 months). Microvascular perfusion of the ischaemic flap was assessed over 5 days by intravital microscopy, evaluating FCD (functional capillary density), capillary dilation response and the area of tissue necrosis. Expression of the stress-protein HO (haem oxygenase)-1 was determined by immunohistochemistry and Western blotting. Induction of chronic ischaemia stimulated a significant expression of HO-1 without a significant difference between the three age groups. This was associated with capillary dilation, which, however, was more pronounced in adolescent (10.5±2.8 μm compared with 3.95±0.79 μm at baseline) and adult (12.1±3.1 μm compared with 3.36±0.45 μm at baseline) animals compared with senescent animals (8.5±1.7 μm compared with 3.28±0.69 μm at baseline; P value not significant). In senescent animals, flap creation further resulted in complete cessation of capillary flow in the distal area of the flap (FCD, 0±0 cm/cm2), whereas adult (11.9±13.5 cm/cm2) and, in particular, adolescent animals (58.4±33.6 cm/cm2; P<0.05) were capable of maintaining residual capillary perfusion. The age-associated microcirculatory dysfunction resulted in a significantly increased flap necrosis of 49±8% (P<0.05) and 42±8% (P<0.05) in senescent and adult animals respectively, compared with 31±6% in adolescent mice. Of interest, functional inhibition of HO-1 by SnPP-IX (tin protoporphyrin-IX) in adolescent mice abrogated capillary dilation, decreased functional capillary density and aggravated tissue necrosis comparably with that observed in senescent mice. Thus aging is associated with an increased susceptibility to tissue necrosis, which is due to a loss of vascular reactivity to endogenous HO-1 expression, rather than a reduction in ischaemic tolerance.