Dependence of acetylcholine and ADP dilation of pial arterioles on heme oxygenase after transfusion of cell-free polymeric hemoglobin

Polynitroxylated PEGylated hemoglobin protects pig brain neocortical gray and white matter after traumatic brain injury and hemorrhagic shock

Polynitroxylated PEGylated hemoglobin (PNPH, aka SanFlow) possesses superoxide dismutase/catalase mimetic activities that may directly protect the brain from oxidative stress. Stabilization of PNPH with bound carbon monoxide prevents methemoglobin formation during storage and permits it to serve as an anti-inflammatory carbon monoxide donor. We determined whether small volume transfusion of hyperoncotic PNPH is neuroprotective in a porcine model of traumatic brain injury (TBI) with and without accompanying hemorrhagic shock (HS). TBI was produced by controlled cortical impact over the frontal lobe of anesthetized juvenile pigs. Hemorrhagic shock was induced starting 5 min after TBI by 30 ml/kg blood withdrawal. At 120 min after TBI, pigs were resuscitated with 60 ml/kg lactated Ringer's (LR) or 10 or 20 ml/kg PNPH. Mean arterial pressure recovered to approximately 100 mmHg in all groups. A significant amount of PNPH was retained in the plasma over the first day of recovery. At 4 days of recovery in the LR-resuscitated group, the volume of frontal lobe subcortical white matter ipsilateral to the injury was 26.2 ± 7.6% smaller than homotypic contralateral volume, whereas this white matter loss was only 8.6 ± 12.0% with 20-ml/kg PNPH resuscitation. Amyloid precursor protein punctate accumulation, a marker of axonopathy, increased in ipsilateral subcortical white matter by 132 ± 71% after LR resuscitation, whereas the changes after 10 ml/kg (36 ± 41%) and 20 ml/kg (26 ± 15%) PNPH resuscitation were not significantly different from controls. The number of cortical neuron long dendrites enriched in microtubules (length >50 microns) decreased in neocortex by 41 ± 24% after LR resuscitation but was not significantly changed after PNPH resuscitation. The perilesion microglia density increased by 45 ± 24% after LR resuscitation but was unchanged after 20 ml/kg PNPH resuscitation (4 ± 18%). Furthermore, the number with an activated morphology was attenuated by 30 ± 10%. In TBI pigs without HS followed 2 h later by infusion of 10 ml/kg LR or PNPH, PNPH remained neuroprotective. These results in a gyrencephalic brain show that resuscitation from TBI + HS with PNPH protects neocortical gray matter, including dendritic microstructure, and white matter axons and myelin. This neuroprotective effect persists with TBI alone, indicating brain-targeting benefits independent of blood pressure restoration.

Regulation of the Cerebral Circulation During Development

The cerebral microcirculation undergoes dynamic changes in parallel with the development of neurons, glia, and their energy metabolism throughout gestation and postnatally. Cerebral blood flow (CBF), oxygen consumption, and glucose consumption are as low as 20% of adult levels in humans born prematurely but eventually exceed adult levels at ages 3 to 11 years, which coincide with the period of continued brain growth, synapse formation, synapse pruning, and myelination. Neurovascular coupling to sensory activation is present but attenuated at birth. By 2 postnatal months, the increase in CBF often is disproportionately smaller than the increase in oxygen consumption, in contrast to the relative hyperemia seen in adults. Vascular smooth muscle myogenic tone increases in parallel with developmental increases in arterial pressure. CBF autoregulatory response to increased arterial pressure is intact at birth but has a more limited range with arterial hypotension. Hypoxia-induced vasodilation in preterm fetal sheep with low oxygen consumption does not sustain cerebral oxygen transport, but the response becomes better developed for sustaining oxygen transport by term. Nitric oxide tonically inhibits vasomotor tone, and glutamate receptor activation can evoke its release in lambs and piglets. In piglets, astrocyte-derived carbon monoxide plays a central role in vasodilation evoked by glutamate, ADP, and seizures, and prostanoids play a large role in endothelial-dependent and hypercapnic vasodilation. Overall, homeostatic mechanisms of CBF regulation in response to arterial pressure, neuronal activity, carbon dioxide, and oxygenation are present at birth but continue to develop postnatally as neurovascular signaling pathways are dynamically altered and integrated. © 2021 American Physiological Society. Compr Physiol 11:1-62, 2021.

Exposure to Blast Overpressure Impairs Cerebral Microvascular Responses and Alters Vascular and Astrocytic Structure

Exposure to blast overpressure may result in cerebrovascular impairment, including cerebral vasospasm. The mechanisms contributing to this vascular response are unclear. The aim of this study was to evaluate the relationship between blast and functional alterations of the cerebral microcirculation and to investigate potential underlying changes in vascular microstructure. Cerebrovascular responses were assessed in sham- and blast-exposed male rats at multiple time points from 2 h through 28 days after a single 130-kPa (18.9-psi) exposure. Pial microcirculation was assessed through a cranial window created in the parietal bone of anesthetized rats. Pial arteriolar reactivity was evaluated in vivo using hypercapnia, barium chloride, and serotonin. We found that exposure to blast leads to impairment of arteriolar reactivity >24 h after blast exposure, suggesting delayed injury mechanisms that are not simply attributed to direct mechanical deformation. Observed vascular impairment included a reduction in hypercapnia-induced vasodilation, increase in barium-induced constriction, and reversal of the serotonin effect from constriction to dilation. A reduction in vascular smooth muscle contractile proteins consistent with vascular wall proliferation was observed, as well as delayed reduction in nitric oxide synthase and increase in endothelin-1 B receptors, mainly in astrocytes. Collectively, the data show that exposure to blast results in delayed and prolonged alterations in cerebrovascular reactivity that are associated with changes in the microarchitecture of the vessel wall and astrocytes. These changes may contribute to long-term pathologies involving dysfunction of the neurovascular unit, including cerebral vasospasm.

Cerebral Vasoactivity and Oxygenation with Oxygen Carrier M101 in Rats

The severity of traumatic brain injury (TBI) may be reduced if oxygen can be rapidly provided to the injured brain. This study evaluated if the oxygen-carrier M101 causes vasoconstricton of pial vasculature in healthy rats (Experiment 1) and if M101 improves brain tissue oxygen (PbtO₂) in rats with controlled cortical impact (CCI)-TBI (Experiment 2). M101 (12.5 mL/kg intravenous [IV] over 2 h) caused a mild (9 mm Hg) increase in the mean arterial blood pressure (MAP) of healthy rats without constriction of cerebral pial arterioles. M101 (12 mL/kg IV over 1 h) caused a modest (27 mm Hg) increase in MAP (peak, 123 ± 5 mm Hg [mean ± standard error of the mean]) of CCI-TBI rats and restored PbtO₂ to near pre-injury levels. In both M101 and untreated control (NON) groups, PbtO₂ was ∼30 ± 2 mm Hg pre-injury and decreased (p ≤ 0.05) to ∼16 ± 2 mm Hg 15 min after CCI. In NON, PbtO₂ remained ∼50% of baseline but M101 administration resulted in a sustained increase in PbtO₂ (peak, 25 ± 5 mm Hg), which was not significantly different from pre-injury until the end of the study, when it decreased again below pre-injury (but was still higher than NON). Histopathology showed no differences between groups. In conclusion, M101 increased systemic blood pressures without concurrent cerebral pial vasoconstriction (in healthy rats) and restored PbtO₂ to 86% of pre-injury for at least 80 min when given soon after CCI-TBI. M101 should be evaluated in a clinically-relevant large animal model for pre-hospital treatment of TBI.

Cerebral Microvascular and Systemic Effects Following Intravenous Administration of the Perfluorocarbon Emulsion Perftoran

Oxygen-carrying perfluorocarbon (PFC) fluids have the potential to increase tissue oxygenation during hypoxic states and to reduce ischemic cell death. Regulatory approval of oxygen therapeutics was halted due to concerns over vasoconstrictive side effects. The goal of this study was to assess the potential vasoactive properties of Perftoran by measuring brain pial arteriolar diameters in a healthy rat model. Perftoran, crystalloid (saline) or colloid (Hextend) solutions were administered as four sequential 30 min intravenous (IV) infusions, thus allowing an evaluation of cumulative dose-dependent effects. There were no overall changes in diameters of small-sized (<50 μm) pial arterioles within the Perftoran group, while both saline and Hextend groups exhibited vasoconstriction. Medium-sized arterioles (50–100 μm) showed minor (~8–9%) vasoconstriction within saline and Hextend groups and only ~5% vasoconstriction within the Perftoran group. For small- and medium-sized pial arterioles, the mean percent change in vessel diameters was not different among the groups. Although there was a tendency for arterial blood pressures to increase with Perftoran, pressures were not different from the other two groups. These data show that Perftoran, when administered to healthy anesthetized rats, does not cause additional vasoconstriction in cerebral pial arterioles or increase systemic blood pressure compared with saline or Hextend.

Sanguinate's effect on pial arterioles in healthy rats and cerebral oxygen tension after controlled cortical impact

Sanguinate, a polyethylene glycol-conjugated carboxyhemoglobin, was investigated for cerebral vasoactivity in healthy male Sprague-Dawley rats (Study 1) and for its ability to increase brain tissue oxygen pressure (PbtO2) after controlled cortical impact (CCI) - traumatic brain injury (TBI) (Study 2). In both studies ketamine-acepromazine anesthetized rats were ventilated with 40% O2. In Study 1, a cranial window was used to measure the diameters of medium - (50-100μm) and small-sized (<50μm) pial arterioles before and after four serial infusions of Sanguinate (8mL/kg/h, cumulative 16mL/kg IV), volume-matched Hextend, or normal saline. In Study 2, PbtO2 was measured using a phosphorescence quenching method before TBI, 15min after TBI (T15) and then every 10min thereafter for 155min. At T15, rats received either 8mL/kg IV Sanguinate (40mL/kg/h) or no treatment (saline, 4mL/kg/h). Results showed: 1) in healthy rats, percentage changes in pial arteriole diameter were the same among the groups, 2) in TBI rats, PbtO2 decreased from 36.5±3.9mmHg to 19.8±3.0mmHg at T15 in both groups after TBI and did not recover in either group for the rest of the study, and 3) MAP increased 16±4mmHg and 36±5mmHg after Sanguinate in healthy and TBI rats, respectively, while MAP was unchanged in control groups. In conclusion, Sanguinate did not cause vasoconstriction in the cerebral pial arterioles of healthy rats but it also did not acutely increase PbtO2 when administered after TBI. Sanguinate was associated with an increase in MAP in both studies.

Effects of the Oxygen-Carrying Solution OxyVita C on the Cerebral Microcirculation and Systemic Blood Pressures in Healthy Rats

The use of hemoglobin-based oxygen carriers (HBOC) as oxygen delivering therapies during hypoxic states has been hindered by vasoconstrictive side effects caused by depletion of nitric oxide (NO). OxyVita C is a promising oxygen-carrying solution that consists of a zero-linked hemoglobin polymer with a high molecular weight (~17 MDa). The large molecular weight is believed to prevent extravasation and limit NO scavenging and vasoconstriction. The aim of this study was to assess vasoactive effects of OxyVita C on systemic blood pressures and cerebral pial arteriole diameters. Anesthetized healthy rats received four intravenous (IV) infusions of an increasing dose of OxyVita C (2, 25, 50, 100 mg/kg) and hemodynamic parameters and pial arteriolar diameters were measured pre- and post-infusion. Normal saline was used as a volume-matched control. Systemic blood pressures increased (P ≤ 0.05) with increasing doses of OxyVita C, but not with saline. There was no vasoconstriction in small (<50 µm) and medium-sized (50–100 µm) pial arterioles in the OxyVita C group. In contrast, small and medium-sized pial arterioles vasoconstricted in the control group. Compared to saline, OxyVita C showed no cerebral vasoconstriction after any of the four doses evaluated in this rat model despite increases in blood pressure.

Effects of estrogen on postischemic pial artery reactivity to ADP

OBJECTIVE

The aims of this work were to determine if 1) ischemia alters pial artery responsiveness to the partially nitric oxide (NO)-dependent dilator, ADP, 2) the alteration depends on 17beta-estradial (E2), and 3) NO contributes to E2 protective effects.

MATERIALS AND METHODS

Response to ADP and the non-NO-dependent dilator, PGE(2), were examined through closed cranial windows. Ovariectomized (OVX) and E2-replaced (E25, 0.025 mg; or E50, 0.05 mg) rats were subjected to 15-minute forebrain ischemia and one-hour reperfusion. Endothelial NO synthase (eNOS) expression was determined in pre- and postischemic isolated cortical microvessels.

RESULTS

In OVX rats, ischemia depressed pial responses to ADP, but not to PGE(2). Both doses of E2 maintained responses to ADP and had no effect on the response to PGE(2). eNOS inhibition decreased the ADP response by 60% in the E25 rats and 50% in the E50 rats, but had no effect in the OVX rats. Compared to the OVX group, microvessel expression of eNOS was increased by E2, but postischemic eNOS was unchanged in both groups.

CONCLUSIONS

The nearly complete loss of postischemic dilation to ADP suggests that normal non-NO-mediated dilatory mechanisms may be acutely impaired after ischemic injury. Estrogen's protective action on ADP dilation may involve both NO- and non-NO-mediated mechanisms.

Endothelin rather than 20-HETE contributes to loss of pial arteriolar dilation during focal cerebral ischemia with and without polymeric hemoglobin transfusion

Partial exchange transfusion with a cell-free hemoglobin (Hb) polymer during transient middle cerebral artery occlusion (MCAO) reduces infarct volume but fails to increase blood flow, as might be expected with the induced decrease in hematocrit. In ischemic brain, endothelin antagonists are known to produce vasodilation. In nonischemic brain, pial arterioles constrict after Hb exchange transfusion, and the constriction is blocked by an inhibitor of 20-HETE synthesis. We tested the hypothesis that a 20-HETE synthesis inhibitor and an endothelin A receptor antagonist increase pial arteriolar dilation after Hb exchange transfusion during MCAO. Pial arteriolar diameter was measured in the ischemic border region of the distal MCA border region through closed cranial windows in anesthetized rats subjected to the filament model of MCAO. During 2 h of MCAO, pial arteriolar dilation gradually subsided from 37 +/- 3 to 7 +/- 5% (+/-SE). Compared with residual dilation at 2 h of MCAO with vehicle superfusion (14 +/- 3%), loss of dilation was not prevented by superfusion of a 20-HETE synthesis inhibitor (21 +/- 5%), partial Hb exchange transfusion (7 +/- 5%) that decreased hematocrit to 23%, or a combination of the two (5 +/- 5%). However, loss of dilation was prevented by superfusion of an endothelin A receptor antagonist with (35 +/- 4%) or without (32 +/- 5%) Hb transfusion. Pial artery constriction during reperfusion was attenuated by HET0016 alone and by BQ610 with or without Hb transfusion. Systemic administration of the endothelin antagonist during prolonged MCAO increased blood flow in the border region. Thus loss of pial arteriolar dilation in the ischemic border region during prolonged MCAO depends on endothelin A receptor activation, and this effect was independent of the presence of cell-free Hb polymers in the plasma. In contrast to previous work in nonischemic brain, inhibition of oxygen-dependent 20-HETE synthesis does not significantly influence the pial arteriolar response to polymeric Hb exchange transfusion during focal ischemia.

Roles of glia limitans astrocytes and carbon monoxide in adenosine diphosphate-induced pial arteriolar dilation in newborn pigs

BACKGROUND AND PURPOSE

Astrocytes, neurons, and microvessels together form a neurovascular unit allowing blood flow to match neuronal activity. Adenosine diphosphate (ADP) is an important signaling molecule in the brain, and dilation in response to ADP is astrocyte-dependent in rats and newborn pigs. Carbon monoxide (CO), produced endogenously by catabolism of heme to CO, iron, and biliverdin via heme oxygenase, is an important cell-signaling molecule in the neonatal cerebral circulation. We hypothesize ADP stimulates CO production by glia limitans astrocytes and that this CO causes pial arteriolar dilation.

METHODS

Experiments were performed using anesthetized piglet with closed cranial windows, and freshly isolated piglet astrocytes and microvessels. Astrocyte injury was caused by topical application of L-2-alpha aminoadipic acid (2 mmol/L, 5 hours). Cerebrospinal fluid was collected from under the cranial windows for measurement of ADP-stimulated CO production. CO was measured by gas chromatography-mass spectroscopy analysis.

RESULTS

Before, but not after, astrocyte injury in vivo, topical ADP stimulated both CO production and dilation of pial arterioles. Astrocyte injury did not block dilation to isoproterenol or bradykinin. Chromium mesoporphyrin, an inhibitor of heme oxygenase, also prevented the ADP-induced increase in cerebrospinal fluid CO and pial arteriolar dilation caused by ADP, but not dilation to sodium nitroprusside. ADP also increased CO production by freshly isolated piglet astrocytes and cerebral microvessels, although the increase was smaller in the microvessels.

CONCLUSIONS

These data suggest that glia limitans astrocytes use CO as a gasotransmitter to cause pial arteriolar dilation in response to ADP.

Role of heme oxygenase-2 in pial arteriolar response to acetylcholine in mice with and without transfusion of cell-free hemoglobin polymers

Carbon monoxide derived from heme oxygenase (HO) may participate in cerebrovascular regulation under specific circumstances. Previous work has shown that HO contributes to feline pial arteriolar dilation to acetylcholine after transfusion of a cell-free polymeric hemoglobin oxygen carrier. The role of constitutive HO2 in the pial arteriolar dilatory response to acetylcholine was determined by using 1) HO2-null mice (HO2-/-), 2) the HO inhibitor tin protoporphyrin IX (SnPPIX), and 3) 4,5,6,7-tetrabromobenzotriazole (TBB), an inhibitor of casein kinase-2 (CK2)-dependent phosphorylation of HO2. In anesthetized mice, superfusion of a cranial window with SnPPIX decreased arteriolar dilation produced by 10 microM acetylcholine by 51%. After partial polymeric hemoglobin exchange transfusion, the acetylcholine response was normal but was reduced 72% by SnPPIX and 95% by TBB. In HO2-/- mice, the acetylcholine response was modestly reduced by 14% compared with control mice and was unaffected by SnPPIX. After hemoglobin transfusion in HO2-/- mice, acetylcholine responses were also unaffected by SnPPIX and TBB. In contrast, nitric oxide synthase inhibition completely blocked the acetylcholine responses in hemoglobin-transfused HO2-/- mice. We conclude 1) that HO2 activity partially contributes to acetylcholine-induced pial arteriolar dilation in mice, 2) that this contribution is augmented in the presence of a plasma-based hemoglobin polymer and appears to depend on a CK2 kinase mechanism, 3) that nitric oxide synthase activity rather than HO1 activity contributes to the acetylcholine reactivity in HO2-/- mice, and 4) that plasma-based polymeric hemoglobin does not scavenge all of the nitric oxide generated by cerebrovascular acetylcholine stimulation.

Insensitivity of cerebral oxygen transport to oxygen affinity of hemoglobin-based oxygen carriers

The cerebrovascular effects of exchange transfusion of various cell-free hemoglobins that possess different oxygen affinities are reviewed. Reducing hematocrit by transfusion of a non-oxygen-carrying solution dilates pial arterioles on the brain surface and increases cerebral blood flow to maintain a constant bulk oxygen transport to the brain. In contrast, transfusion of hemoglobins with P50 of 4-34 Torr causes constriction of pial arterioles that offsets the decrease in blood viscosity to maintain cerebral blood flow and oxygen transport. The autoregulatory constriction is dependent on synthesis of 20-HETE from arachidonic acid. This oxygen-dependent reaction is apparently enhanced by facilitated oxygen diffusion from the red cell to the endothelium arising from increased plasma oxygen solubility in the presence of low or high-affinity hemoglobin. Exchange transfusion of recombinant hemoglobin polymers with P50 of 3 and 18 Torr reduces infarct volume from experimental stroke. Cell-free hemoglobins do not require a P50 as high as red blood cell hemoglobin to facilitate oxygen delivery.