NO and prostanoids: age dependence of hypercapniaand histamine-induced dilations of pig pial arterioles

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.

NMDA attenuates the neurovascular response to hypercapnia in the neonatal cerebral cortex

Cortical spreading depolarization (SD) involves activation of NMDA receptors and elicit neurovascular unit dysfunction. NMDA cannot trigger SD in newborns, thus its effect on neurovascular function is not confounded by other aspects of SD. The present study investigated if NMDA affected hypercapnia-induced microvascular and electrophysiological responses in the cerebral cortex of newborn pigs. Anesthetized piglets were fitted with cranial windows over the parietal cortex to study hemodynamic and electrophysiological responses to graded hypercapnia before/after topically applied NMDA assessed with laser-speckle contrast imaging and recording of local field potentials (LFP)/neuronal firing, respectively. NMDA increased cortical blood flow (CoBF), suppressed LFP power in most frequency bands but evoked a 2.5 Hz δ oscillation. The CoBF response to hypercapnia was abolished after NMDA and the hypercapnia-induced biphasic changes in δ and θ LFP power were also altered. MK-801 prevented NMDA-induced increases in CoBF and the attenuation of microvascular reactivity to hypercapnia. The neuronal nitric oxide synthase (nNOS) inhibitor (N-(4 S)-4-amino-5-[aminoethyl]aminopentyl-N′-nitroguanidin) also significantly preserved the CoBF response to hypercapnia after NMDA, although it didn’t reduce NMDA-induced increases in CoBF. In conclusion, excess activation of NMDA receptors alone can elicit SD-like neurovascular unit dysfunction involving nNOS activity.

Changes in Endothelial Nitric Oxide Production in Systemic Vessels during Early Ontogenesis-A Key Mechanism for the Perinatal Adaptation of the Circulatory System

Nitric oxide (NO) produced in the wall of blood vessels is necessary for the regulation of vascular tone to ensure an adequate blood supply of organs and tissues. In this review, we present evidence that the functioning of endothelial NO-synthase (eNOS) changes considerably during postnatal maturation. Alterations in NO-ergic vasoregulation in early ontogeny vary between vascular beds and correlate with the functional reorganization of a particular organ. Importantly, the anticontractile effect of NO can be an important mechanism responsible for the protectively low blood pressure in the immature circulatory system. The activity of eNOS is regulated by a number of hormones, including thyroid hormones which are key regulators of the perinatal developmental processes. Maternal thyroid hormone deficiency suppresses the anticontractile effect of NO at perinatal age. Such alterations disturb perinatal cardiovascular homeostasis and lead to delayed occurring cardiovascular pathologies in adulthood. The newly discovered role of thyroid hormones may have broad implications in cardiovascular medicine, considering the extremely high prevalence of maternal hypothyroidism in human society.

Juvenile growth reduces the influence of epithelial sodium channels on myogenic tone in skeletal muscle arterioles

Previous studies have documented that rapid juvenile growth is accompanied by functional changes in the arteriolar endothelium, but much less is known about functional changes in arteriolar smooth muscle over this period. In this study, we investigate the possible contribution of epithelial sodium channels (ENaC) to the myogenic behaviour of arterioles at two stages of juvenile growth. The effects of the ENaC inhibitor benzamil on different levels of myogenic tone were studied in isolated gracilis muscle arterioles from rats aged 21-28 days ("weanlings") and 42-49 days ("juveniles"). ENaC subunit expression in the arteriolar wall was also determined, and the interaction between ENaC and nitric oxide (NO) in regulating vascular tone was explored by combined use of benzamil and N^G -monomethyl-l-arginine (l-NMMA). At physiological pressures, both steady-state myogenic tone and the dynamic adjustments in this tone triggered by acute pressure changes were less in juvenile arterioles than in weanling arterioles. α, β and γ ENaC protein was present in arterioles at both ages, but benzamil only had an effect on myogenic tone in weanling arterioles. In these vessels, benzamil increased, rather than decreased, myogenic tone, and this effect was prevented by l-NMMA or endothelial removal. These findings suggest that although ENaC is present in gracilis muscle arterioles of both weanling and juvenile rats, it is not obligatory for the genesis of myogenic activity in these vessels at either age. However, ENaC activity can significantly modulate the level of myogenic tone through stimulation of endothelial NO release at an early stage of growth.

Sildenafil, a cyclic GMP phosphodiesterase inhibitor, induces microglial modulation after focal ischemia in the neonatal mouse brain

BACKGROUND

Perinatal ischemic stroke is the most frequent form of cerebral infarction in neonates; however, evidence-based treatments are currently lacking. We have previously demonstrated a beneficial effect of sildenafil citrate, a PDE-5 inhibitor, on stroke lesion size in neonatal rat pups. The present study investigated the effects of sildenafil in a neonatal mouse stroke model on (1) hemodynamic changes and (2) regulation of astrocyte/microglia-mediated neuroinflammation.

METHODS

Ischemia was induced in C57Bl/6 mice on postnatal (P) day 9 by permanent middle cerebral artery occlusion (pMCAo), and followed by either PBS or sildenafil intraperitoneal (i.p.) injections. Blood flow (BF) velocities were measured by ultrasound imaging with sequential Doppler recordings and laser speckle contrast imaging. Animals were euthanized, and brain tissues were obtained at 72 h or 8 days after pMCAo. Expression of M1- and M2-like microglia/macrophage markers were analyzed.

RESULTS

Although sildenafil (10 mg/kg) treatment potently increased cGMP concentrations, it did not influence early collateral recruitment nor did it reduce mean infarct volumes 72 h after pMCAo. Nevertheless, it provided a significant dose-dependent reduction of mean lesion extent 8 days after pMCAo. Suggesting a mechanism involving modulation of the inflammatory response, sildenafil significantly decreased microglial density at 72 h and 8 days after pMCAo. Gene expression profiles indicated that sildenafil treatment also modulates M1- (ptgs2, CD32 and CD86) and M2-like (CD206, Arg-1 and Lgals3) microglia/macrophages in the late phase after pMCAo. Accordingly, the number of COX-2(+) microglia/macrophages significantly increased in the penumbra at 72 h after pMCAo but was significantly decreased 8 days after ischemia in sildenafil-treated animals.

CONCLUSIONS

Our findings argue that anti-inflammatory effects of sildenafil may provide protection against lesion extension in the late phase after pMCAo in neonatal mice. We propose that sildenafil treatment could represent a potential strategy for neonatal ischemic stroke treatment/recovery.

Ischemic postconditioning in cerebral ischemia: Differences between the immature and mature brain?

Ischemic postconditioning (postC), defined as serial mechanical interruptions of blood flow at reperfusion, effectively reduces myocardial infarct size in all species tested so far, including humans. In the brain, ischemic postC leads to controversial results regardless of variations in factors such as onset time of beginning, the duration of ischemia and/or reperfusion, and the number of cycles of occlusion/reperfusion. Thus, many major issues remain to be resolved regarding its protective effects. Future studies should aim to identify the parameters that yield the strongest protection, as well as to understand why the efficacy of ischemic postC differs between models. This review will focus on initial hemodynamic changes and their consequences, and on specific features such as NO-dependent vascular tone and/or prolonged acidosis in cerebral ischemia-reperfusion in order to better understand the dynamics of ischemic postC in the developing brain.

Contributions of KATP and KCa channels to cerebral arteriolar dilation to hypercapnia in neonatal brain

Mechanisms by which Pco₂ controls cerebral vascular tone remain uncertain. We hypothesize that potassium channel activation contributes to the neonatal cerebrovascular dilation in response to increases in Paco₂. To test this hypothesis, experiments were performed on newborn pigs with surgically implanted, closed cranial windows. Hypercapnia was induced by ventilation with elevated Pco₂ gas in the absence and presence of the K_ATP channel inhibitor, glibenclamide and/or the K_Ca channel inhibitor, paxillin. Dilations to pinacidil, a selective K_ATP channel activator, without and with glibenclamide, were used to evaluate the efficacy of K_ATP channel inhibition. Dilations to NS1619, a selective K_Ca channel activator, without and with paxillin, were used to evaluate the efficacy of K_Ca channel inhibition. Cerebrovascular responses to the K_ATP and K_Ca channel activators, pinacidil and NS1619, respectively, cAMP‐dependent dilator, isoproterenol, and cGMP‐dependent dilator, sodium nitroprusside (SNP), were used to evaluate the selectivity of glibenclamide and paxillin. Glibenclamide blocked dilation to pinacidil, but did not inhibit dilations to NS1619, isoproterenol, or SNP. Glibenclamide prior to hypercapnia decreased mean pial arteriole dilation ~60%. Glibenclamide treatment during hypercapnia constricted arterioles ~35%. The level of hypercapnia, Paco₂ between 50 and 75 mmHg, did not appear to be involved in efficacy of glibenclamide in blocking dilation to Paco₂. Similarly to glibenclamide and K_ATP channel inhibition, paxillin blocked dilation to the K_Ca channel agonist, NS1619, and attenuated, but did not block, arteriolar dilation to hypercapnia. Treatment with both glibenclamide and paxillin abolished dilation to hypercapnia. Therefore, either glibenclamide or paxillin that block dilation to their channel agonists, pinacidil or NS1619, respectively, only partially inhibit dilation to hypercapnia. Block of both K_ATP and K_Ca channels completely prevent dilation hypercapnia. These data suggest hypercapnia activates both K_ATP and K_Ca channels leading to cerebral arteriolar dilation in newborn pigs.

Mechanisms by which Pco₂ controls vascular tone remain uncertain. We hypothesize K_ATP and K_Ca channel activation contributes to the neonatal cerebrovascular dilation in response to increases in Paco₂. Presented data support this hypothesis.

Cerebral vascular regulation and brain injury in preterm infants

Cerebrovascular lesions, mainly germinal matrix hemorrhage and ischemic injury to the periventricular white matter, are major causes of adverse neurodevelopmental outcome in preterm infants. Cerebrovascular lesions and neuromorbidity increase with decreasing gestational age, with the white matter predominantly affected. Developmental immaturity in the cerebral circulation, including ongoing angiogenesis and vasoregulatory immaturity, plays a major role in the severity and pattern of preterm brain injury. Prevention of this injury requires insight into pathogenesis. Cerebral blood flow (CBF) is low in the preterm white matter, which also has blunted vasoreactivity compared with other brain regions. Vasoreactivity in the preterm brain to cerebral perfusion pressure, oxygen, carbon dioxide, and neuronal metabolism is also immature. This could be related to immaturity of both the vasculature and vasoactive signaling. Other pathologies arising from preterm birth and the neonatal intensive care environment itself may contribute to impaired vasoreactivity and ineffective CBF regulation, resulting in the marked variations in cerebral hemodynamics reported both within and between infants depending on their clinical condition. Many gaps exist in our understanding of how neonatal treatment procedures and medications have an impact on cerebral hemodynamics and preterm brain injury. Future research directions for neuroprotective strategies include establishing cotside, real-time clinical reference values for cerebral hemodynamics and vasoregulatory capacity and to demonstrate that these thresholds improve long-term outcomes for the preterm infant. In addition, stimulation of vascular development and repair with growth factor and cell-based therapies also hold promise.

Changes in eNOS phosphorylation contribute to increased arteriolar NO release during juvenile growth

Nitric oxide (NO) mediates a major portion of arteriolar endothelium-dependent dilation in adults, but indirect evidence has suggested that NO contributes minimally to these responses in the young. Isolated segments of arterioles were studied in vitro to verify this age-related increase in NO release and investigate the mechanism by which it occurs. Directly measured NO release induced by ACh or the Ca(2+) ionophore A-23187 was five- to sixfold higher in gracilis muscle arterioles from 42- to 46-day-old (juvenile) rats than in those from 25- to 28-day-old (weanling) rats. There were no differences between groups in arteriolar endothelial NO synthase (eNOS) expression or tetrahydrobiopterin levels, and arteriolar l-arginine levels were lower in juvenile vessels than in weanling vessels (104 ± 6 vs.126 ± 3 pmol/mg). In contrast, agonist-induced eNOS Thr(495) dephosphorylation and eNOS Ser(1177) phosphorylation (events required for maximal activity) were up to 30% and 65% greater, respectively, in juvenile vessels. Juvenile vessels did not show increased expression of enzymes that mediate these events [protein phosphatases 1 and 2A and PKA and PKB (Akt)] or heat shock protein 90, which facilitates Ser(1177) phosphorylation. However, agonist-induced colocalization of heat shock protein 90 with eNOS was 34-66% greater in juvenile vessels than in weanling vessels, and abolition of this difference with geldanamycin also abolished the difference in Ser(1177) phosphorylation between groups. These findings suggest that growth-related increases in arteriolar NO bioavailability may be due at least partially to changes in the regulation of eNOS phosphorylation and increased signaling activity, with no change in the abundance of eNOS signaling proteins.

Prostanoids suppress the coronary vasoconstrictor influence of endothelin after myocardial infarction

Myocardial infarction (MI) is associated with endothelial dysfunction resulting in an imbalance in endothelium-derived vasodilators and vasoconstrictors. We have previously shown that despite increased endothelin (ET) plasma levels, the coronary vasoconstrictor effect of endogenous ET is abolished after MI. In normal swine, nitric oxide (NO) and prostanoids modulate the vasoconstrictor effect of ET. In light of the interaction among NO, prostanoids, and ET combined with endothelial dysfunction present after MI, we investigated this interaction in control of coronary vasomotor tone in the remote noninfarcted myocardium after MI. Studies were performed in chronically instrumented swine (18 normal swine; 13 swine with MI) at rest and during treadmill exercise. Furthermore, endothelial nitric oxide synthase (eNOS) and cyclooxygenase protein levels were measured in the anterior (noninfarcted) wall of six normal and six swine with MI. eNOS inhibition with N(ω)-nitro-L-arginine (L-NNA) and cyclooxygenase inhibition with indomethacin each resulted in coronary vasoconstriction at rest and during exercise, as evidenced by a decrease in coronary venous oxygen levels. The effect of l-NNA was slightly decreased in swine with MI, although eNOS expression was not altered. Conversely, in accordance with the unaltered expression of cyclooxygenase-1 after MI, the effect of indomethacin was similar in normal and MI swine. L-NNA enhanced the vasodilator effect of the ET(A/B) receptor blocker tezosentan but exclusively during exercise in both normal and MI swine. Interestingly, this effect of L-NNA was blunted in MI compared with normal swine. In contrast, whereas indomethacin increased the vasodilator effect of tezosentan only during exercise in normal swine, indomethacin unmasked a coronary vasodilator effect of tezosentan in MI swine both at rest and during exercise. In conclusion, the present study shows that endothelial control of the coronary vasculature is altered in post-MI remodeled myocardium. Thus the overall vasodilator influences of NO as well as its inhibition of the vasoconstrictor influence of ET on the coronary resistance vessels were reduced after MI. In contrast, while the overall prostanoid vasodilator influence was maintained, its inhibition of ET vasoconstrictor influences was enhanced in post-MI remote myocardium.

Carbon monoxide as an endogenous vascular modulator

Carbon monoxide (CO) is produced by heme oxygenase (HO)-catalyzed heme degradation to CO, iron, and biliverdin. HO has two active isoforms, HO-1 (inducible) and HO-2 (constitutive). HO-2, but not HO-1, is highly expressed in endothelial and smooth muscle cells and in adjacent astrocytes in the brain. HO-1 is expressed basally only in the spleen and liver but can be induced to a varying extent in most tissues. Elevating heme, protein phosphorylation, Ca(2+) influx, and Ca(2+)/calmodulin-dependent processes increase HO-2 activity. CO dilates cerebral arterioles and may constrict or dilate skeletal muscle and renal arterioles. Selected vasodilatory stimuli, including seizures, glutamatergic stimulation, hypoxia, hypotension, and ADP, increase CO, and the inhibition of HO attenuates the dilation to these stimuli. Astrocytic HO-2-derived CO causes glutamatergic dilation of pial arterioles. CO dilates by activating smooth muscle cell large-conductance Ca(2+)-activated K(+) (BK(Ca)) channels. CO binds to BK(Ca) channel-bound heme, leading to an increase in Ca(2+) sparks-to-BK(Ca) channel coupling. Also, CO may bind directly to the BK(Ca) channel at several locations. Endothelial nitric oxide and prostacyclin interact with HO/CO in circulatory regulation. In cerebral arterioles in vivo, in contrast to dilation to acute CO, a prolonged exposure of cerebral arterioles to elevated CO produces progressive constriction by inhibiting nitric oxide synthase. The HO/CO system is highly protective to the vasculature. CO suppresses apoptosis and inhibits components of endogenous oxidant-generating pathways. Bilirubin is a potent reactive oxygen species scavenger. Still many questions remain about the physiology and biochemistry of HO/CO in the circulatory system and about the function and dysfunction of this gaseous mediator system.

Hydrogen sulfide and cerebral microvascular tone in newborn pigs

Hydrogen sulfide (H2S) is a gaseous signaling molecule that appears to be involved in numerous biological processes, including regulation of blood pressure and vascular tone. The present study is designed to address the hypothesis that H2S is a functionally significant, endogenous dilator in the newborn cerebrovascular circulation. In vivo experiments were conducted using newborn pigs with surgically implanted, closed, cranial windows. Topical application of H2S concentration-dependently (10(-6) to 2×10(-4) M) dilated pial arterioles. This dilation was blocked by glibenclamide (10(-6) M). L-cysteine, the substrate of the H2S-producing enzymes cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS), also dilated pial arterioles. The dilation to L-cysteine was blocked by the CSE inhibitor d,l-propargylglycine (PPG, 10 mM) but was unaffected by the CBS inhibitor amino-oxyacetate (AOA, 1 mM). Western blots detected CSE, but not CBS, in cerebral microvessels, whereas CBS is detected in brain parenchyma. Immunohistological CSE expression is predominantly vascular while CBS is expressed mainly in neurons and astrocytes. L-cysteine (5 mM) increased H2S concentration in cerebrospinal fluid (CSF), measured by GC-MS, from 561±205 to 2,783±818 nM before but not during treatment with PPG (1,030±70 to 622±78 nM). Dilation to hypercapnia was inhibited by PPG but not AOA. Hypercapnia increased CSF H2S concentration from 763±243 to 4,337±1789 nM before but not during PPG treatment (357±178 vs. 425±217 nM). These data show that H2S is a dilator of the newborn cerebral circulation and that endogenous CSE can produce sufficient H2S to decrease vascular tone. H2S appears to be a physiologically significant dilator in the cerebral circulation.

Endothelium-dependent control of vascular tone during early postnatal and juvenile growth

Endothelial dysfunction can develop at an early age in children with risk factors for cardiovascular disease. A clear understanding of the nature of this dysfunction and how it can worsen over time requires detailed information on the normal growth-related changes in endothelial function on which the pathological changes are superimposed. This review summarizes our current understanding of these normal changes, as derived from studies in four different mammalian species. Although the endothelium plays an important role in controlling vascular tone from birth onward, the vasoactive molecules that mediate this control often change during postnatal or juvenile growth. The specifics of this transition to an adult endothelial cell phenotype can vary depending on the vascular bed. During growth, the contribution of nitric oxide to endothelium-dependent dilation generally increases in the lung, cerebral cortex, and skeletal muscle, but decreases in the intestine. Endothelial capacity for release of other vasoactive factors (e.g., cyclooxygenase products, hydrogen peroxide, carbon monoxide) can also increase or decrease during growth. Although these changes have been well documented, there is less information on their underlying cellular or molecular events. Further research is required to clarify these mechanisms, and to evaluate the functional significance of such shifts in endothelial phenotype.

Urokinase plasminogen activator impairs SNP and PGE2 cerebrovasodilation after brain injury through activation of LRP and ERK MAPK

Pial artery dilation in response to prostaglandin (PG)E(2) and the nitric oxide (NO) releaser sodium nitroprusside (SNP) are blunted after fluid percussion brain injury (FPI), whereas responses to papaverine are unchanged. Urokinase plasminogen activator (uPA) and ERK mitogen-activated protein kinase (MAPK) are upregulated and contribute to the impairment of cerebrohemodynamics seen after FPI. PA vascular activity is mediated through the low-density lipoprotein receptor (LRP). Therefore, we investigated the role of uPA, LRP, and ERK MAPK in the impaired cerebrovasodilation response to PGE(2) and SNP after FPI. Lateral FPI (2 atm) was induced in anesthetized piglets equipped with a closed cranial window. Cerebrospinal fluid (CSF) ERK MAPK was quantified by enzyme-linked immunosorbent assay (ELISA). Pretreatment with soluble uPA receptor (suPAR), which antagonizes the vascular action of uPA, blunted the impairment of SNP and PGE(2)-mediated dilation seen after FPI. Pretreatment with the LRP antagonist RAP, a monoclonal antibody against LRP (Mab ag LRP) and the ERK MAPK antagonist, U 0126, all provided similar protection, whereas control immunoglobulin G (IgG) had no effect. Responses to papaverine were unchanged after FPI. Upregulation of ERK MAPK phosphorylation in CSF after FPI was blunted in animals pretreated with suPAR, RAP, MAb ag LRP, or U 0126, whereas control IgG had no effect. These data indicate that uPA contributes to the impairment of SNP and PGE(2)-mediated cerebrovasodilation seen after brain injury through activation of LRP and ERK MAPK.

Human cerebral arteriovenous vasoactive exchange during alterations in arterial blood gases

Cerebral blood flow (CBF) is highly regulated by changes in arterial Pco2and arterial Po2. Evidence from animal studies indicates that various vasoactive factors, including release of norepinephrine, endothelin, adrenomedullin, C-natriuretic peptide (CNP), and nitric oxide (NO), may play a role in arterial blood gas-induced alterations in CBF. For the first time, we directly quantified exchange of these vasoactive factors across the human brain. Using the Fick principle and transcranial Doppler ultrasonography, we measured CBF in 12 healthy humans at rest and during hypercapnia (4 and 8% CO2), hypocapnia (voluntary hyperventilation), and hypoxia (12 and 10% O2). At each level, blood was sampled simultaneously from the internal jugular vein and radial artery. With the exception of CNP and NO, the simultaneous quantification of norepinephrine, endothelin, or adrenomedullin showed no cerebral uptake or release during changes in arterial blood gases. Hypercapnia, but not hypocapnia, increased CBF and caused a net cerebral release of nitrite (a marker of NO), which was reflected by an increase in the venous-arterial difference for nitrite: 57 ± 18 and 150 ± 36 μmol/l at 4% and 8% CO2, respectively (both P < 0.05). Release of cerebral CNP was also observed during changes in CO2(hypercapnia vs. hypocapnia, P < 0.05). During hypoxia, there was a net cerebral uptake of nitrite, which was reflected by a decreased venous-arterial difference for nitrite: −96 ± 14 μmol/l at 10% O2( P < 0.05). These data indicate that there is a differential exchange of NO across the brain during hypercapnia and hypoxia and that CNP may play a complementary role in CO2-induced CBF changes.

Prostacyclin receptor deletion aggravates hippocampal neuronal loss after bilateral common carotid artery occlusion in mouse

Transient global cerebral ischemia causes delayed neuronal death in the hippocampal CA1 region. It also induces an increase in cyclooxygenase 2 (COX-2), which generates several metabolites of arachidonic acid, known as prostanoids, including prostacyclin (PGI(2)). To determine the role of the PGI(2) receptor (IP) in post-ischemic delayed cell death, wild-type and IP knockout (IP(-/-)) C57Bl/6 mice were subjected to 12-min bilateral common carotid artery occlusion or sham surgery, followed by 7 days of reperfusion. In the sham-operated mice, no statistical difference in CA1 hippocampal neuronal density was observed between the wild-type (2836+/-18/mm(2)) and IP(-/-) (2793+/-43/mm(2)) mice. Interestingly, in animals subjected to ischemia, surviving neuronal density in wild-type mice decreased to 50.5+/-7.9% and that of IP(-/-) mice decreased to 23.0+/-4.5% of their respective sham-operated controls (P<0.05). The results establish a role for the IP receptor in protecting pyramidal hippocampal neurons after this global ischemic model and suggest that IP receptor agonists could be developed to prevent delayed pyramidal neuronal cell death.

Increased myogenic responsiveness of skeletal muscle arterioles with juvenile growth

Previous studies from this laboratory suggest that during juvenile growth, structural changes in the arteriolar network are accompanied by changes in some of the mechanisms responsible for regulation of tissue blood flow. To test the hypothesis that arteriolar myogenic behavior is altered with growth, we studied gracilis muscle arterioles isolated from Sprague-Dawley rats at two ages: 21-28 and 42-49 days. When studied at their respective in vivo pressures, the myogenic index (instantaneous slope of the active pressure-diameter curve) of arterioles from 42-49-day-old rats was more negative than that of arterioles from 21-28-day-old rats, indicating greater myogenic responsiveness. Endothelial denudation, or prostaglandin H(2) (PGH(2))/thromboxane A(2) (TxA(2)) receptor antagonism without denudation, significantly reduced the myogenic responsiveness of arterioles from the older rats over a wide range of pressures but had no consistent effects on the myogenic responsiveness of arterioles from the younger rats. The heme oxygenase inhibitor chromium (III) mesoporphyrin IX chloride had no effect on the myogenic activity of arterioles from either age group. These findings indicate that microvascular growth in young animals is accompanied by an increase in the myogenic behavior of arterioles, possibly because PGH(2) or TxA(2) assumes a role in reinforcing myogenic activity over this period. As a result, the relative contribution of myogenic activity to blood flow regulation in skeletal muscle may increase during rapid juvenile growth.

Age and species dependence of pial arteriolar responses to topical carbon monoxide in vivo

In newborn pigs, carbon monoxide (CO) contributes to regulation of cerebrovascular circulation. Results from isolated adult cerebral arteries suggest CO may have less dilatory potential in mature animals. However, few data are available on the direct effects of CO on cerebrovascular circulation in vivo except for those from newborn pigs. Therefore, we tested the hypothesis that i) rat cerebral arterioles dilate to CO in vivo and ii) CO-induced cerebrovascular dilatory responses are age dependent in pigs. Also, we examined whether the permissive role of nitric oxide in CO-induced dilation observed in piglets is present in older pigs and rats. Experiments used anesthetized newborn, 7-week-old, and juvenile (3- to 4-month-old) pigs and 3- to 4-month-old rats with closed cranial windows and topical applications of CO and sodium nitroprusside (SNP). Dilations to SNP were not different at different ages in pigs or between pigs and rats. CO produced pial arteriolar dilations in all groups. Dilation to 10(-5) M CO was reduced in juvenile pigs as compared to newborn and 7-week-old pigs, and tended to less at 10(-6) M CO. Dilations of rat pial arterioles to all concentrations were less than those of newborn and 7-week-old pigs, but not different from those of juvenile pig pial arterioles. In newborn and 7-week-old pigs, l-nitro-arginine (LNA) inhibited the dilation to CO, an effect reversed by a constant background of SNP. In contrast, LNA did not reduce dilation to CO in juvenile pigs or rats. In conclusion, rat pial arterioles like those in piglets dilate to CO in vivo, but there are age and species differences with regard to reactivity and interaction with NO.

Role of nitric oxide in cerebrovascular reactivity to NMDA and hypercapnia during prenatal development in sheep

Cerebral vasodilatory responses evoked by activation of NMDA receptors and by hypercapnia are important factors in the integrated vascular response to perinatal cerebral ischemia. Cerebral vasodilation to NMDA is mediated by nitric oxide in adult and newborn animals, whereas vasodilation to hypercapnia is thought to become modulated by nitric oxide, at least in swine, after the newborn period. The developmental role of nitric oxide in the cerebral blood flow response to NMDA and hypercapnia was investigated at mid- and late gestation in fetal sheep. Superfusion of 300microM NMDA over the cerebral cortex through a closed cranial window on the exteriorized head of an anesthetized fetus increased laser-Doppler flow by 41+/-7% (+/-S.E.) at 0.65 gestation. The increase was reduced by superfusion of a nitric oxide synthase inhibitor (18+/-8%). At 0.9 gestation, the response to NMDA was augmented (85+/-24%) compared to that at 0.65 gestation and was reduced by a nitric oxide synthase inhibitor (32+/-6%). In unanesthetized fetal sheep, hypercapnic reactivity of microsphere-determined cerebral blood flow was not significantly attenuated by nitric oxide synthase inhibition at 0.65 gestation (4.6+/-0.7 to 3.7+/-1.0% change/mmHg pCO2) or at 0.9 gestation (4.0+/-0.7 to 3.5+/-0.9% change/mmHg pCO2). Therefore, nitric oxide-dependent cerebrovascular dilation to NMDA-receptor activation is present as early as 0.65 gestation in fetal sheep and increases further during the last trimester, whereas vasodilation to hypercapnia remains unchanged and independent of nitric oxide during the last trimester. Hence, cerebrovascular reactivities to different stimuli do not mature concurrently.

Carbon monoxide contributes to hypotension-induced cerebrovascular vasodilation in piglets

The gaseous compound carbon monoxide (CO) has been identified as an important endogenous biological messenger in brain and is a major component in regulation of cerebrovascular circulation in newborns. CO is produced endogenously by catabolism of heme to CO, free iron, and biliverdin during enzymatic degradation of heme by heme oxygenase (HO). The present study was designed to test the hypothesis that endogenously produced CO contributes to hypotension-induced vasodilation of cerebral arterioles. Experiments used anesthetized piglets with implanted, closed cranial windows. Topical application of the HO substrate heme-l-lysinate caused dilation of pial arterioles that was blocked by a metal porphyrin inhibitor of HO, chromium mesoporphyrin (CrMP). In normotensive piglets (arterial pressure 64 +/- 4 mmHg), CrMP did not cause vasoconstriction of pial arterioles but rather a transient dilation. Hypotension (50% of basal blood pressure) increased cerebral CO production and dilated pial arterioles from 66 +/- 2 to 92 +/- 7 microm. In hypotensive piglets, topical CrMP or intravenous tin protoporphyrin decreased cerebral CO production and produced pial arteriolar constriction to normotensive diameters. In additional experiments, because prostacyclin and nitric oxide (NO) are also key dilators that can contribute to cerebrovascular dilation, we held their levels constant. NO/prostacyclin clamp was accomplished with continuous, simultaneous application of indomethacin, N(omega)-nitro-l-arginine, and minimal dilatory concentrations of iloprost and sodium nitroprusside. With constant NO and prostacyclin, the transient dilator and prolonged constrictor responses to CrMP of normotensive and hypotensive piglets, respectively, were the same as when NO and prostaglandins were not held constant. These data suggest that endogenously produced CO contributes to cerebrovascular dilation in response to reduced perfusion pressure.

Contributions of astrocytes and CO to pial arteriolar dilation to glutamate in newborn pigs

Astrocytes can act as intermediaries between neurons and cerebral arterioles to regulate vascular tone in response to neuronal activity. Release of glutamate from presynaptic neurons increases blood flow to match metabolic demands. CO is a gasotransmitter that can be related to neural function and blood flow regulation in the brain. The present study addresses the hypothesis that glutamatergic stimulation promotes perivascular astrocyte CO production and pial arteriolar dilation in the newborn brain. Experiments used anesthetized newborn pigs with closed cranial windows, piglet astrocytes, and cerebrovascular endothelial cells in primary culture and immunocytochemical visualization of astrocytic markers. Pial arterioles and arteries of newborn pigs are ensheathed by astrocytes visualized by glial fibrillary acidic protein staining. Treatment (2 h) of astrocytes in culture with L-2-alpha-aminoadipic acid (L-AAA), followed by 14 h in toxin free medium, dose-dependently increased cell detachment, suggesting injury. Conversely, 16 h of continuous exposure to L-AAA caused no decrease in endothelial cell attachment. In vivo, topical L-AAA (2 mM, 5 h) disrupted the cortical glia limitans histologically. Such treatment also eliminated pial arteriolar dilation to the astrocyte-dependent dilator ADP and to glutamate but not to isoproterenol or CO. Glutamate stimulated CO production by the brain surface that also was abolished following L-AAA. In contrast, tetrodotoxin blocked dilation to N-methyl-D-aspartate but not to glutamate, isoproterenol, or CO or the glutamate-induced increase in CO. The concurrent loss of CO production and pial arteriolar dilation to glutamate following astrocyte injury suggests astrocytes may employ CO as a gasotransmitter for glutamatergic cerebrovascular dilation.

Nitric oxide increases carbon monoxide production by piglet cerebral microvessels

Carbon monoxide (CO) and nitric oxide (NO) can be involved in the regulation of cerebral circulation. Inhibition of production of either one of these gaseous intercellular messengers inhibits newborn pig cerebral arteriolar dilation to the excitatory amino acid glutamate. Glutamate can increase NO production. Therefore, the present study tests the hypothesis that NO, which is increased by glutamate, stimulates the production of CO by cerebral microvessels. Experiments used freshly isolated cerebral microvessels from piglets that express only heme oxygenase-2 (HO-2). CO production was measured by gas chromatography-mass spectrometry. Although inhibition of nitric oxide synthase (NOS) with N(omega)-nitro-l-arginine (l-NNA) did not alter basal HO-2 catalytic activity or CO production, l-NNA blocked glutamate stimulation of HO-2 activity and CO production. Furthermore, the NO donor sodium nitroprusside mimicked the actions of glutamate on HO-2 and CO production. The action of NO appears to be via cGMP because 8-bromo-cGMP mimics and 1H-[1,2,4]oxadiazole-[4,3-a]quinoxalin-1-one (ODQ) blocks glutamate stimulation of CO production and HO-2 catalytic activity. Inhibitors of neither casein kinase nor phosphotidylinositol 3-kinase altered HO-2 catalytic activity. Conversely, inhibition of calmodulin with calmidazolium chloride blocked glutamate stimulation of CO production and reduced HO-2 catalytic activity. These data suggest that glutamate may activate NOS producing NO that leads to CO synthesis via a cGMP-dependent elevation of HO-2 catalytic activity. These results are consistent with the findings in vivo that either HO or NOS inhibition blocks cerebrovascular dilation to glutamate in piglets.

Interaction between prostanoids and nitric oxide in regulation of systemic, pulmonary, and coronary vascular tone in exercising swine

Prostacyclin and nitric oxide (NO) are produced by the endothelium in response to physical forces such as shear stress. Consequently, both NO and prostacyclin may increase during exercise and contribute to metabolic vasodilation. Conversely, NO has been hypothesized to inhibit prostacyclin production. We therefore investigated the effect of cyclooxygenase (COX) inhibition on exercise-induced vasodilation of the porcine systemic, pulmonary, and coronary beds before and after inhibition of NO production. Swine were studied at rest and during treadmill exercise at 1-5 km/h, before and after COX inhibition with indomethacin (10 mg/kg iv), and in the absence and presence of NO synthase inhibition with N(omega)-nitro-l-arginine (l-NNA; 20 mg/kg iv). COX inhibition produced systemic vasoconstriction at rest, which waned during exercise. The systemic vasoconstriction by COX inhibition was enhanced after l-NNA, particularly at rest. In the coronary circulation, COX inhibition also resulted in vasoconstriction at rest and during exercise. However, vasoconstriction was not modified by pretreatment with l-NNA. In contrast, COX inhibition had no effect on the pulmonary circulation, either at rest or during exercise. Moreover, a prostanoid influence in the pulmonary circulation could not be detected after l-NNA. In conclusion, endogenous prostanoids contribute importantly to systemic and coronary tone in awake swine at rest but are not mandatory for exercise-induced vasodilation in these beds. Endogenous prostanoids are not mandatory for the regulation of pulmonary resistance vessel tone. Finally, NO blunts the contribution of prostanoids to vascular tone regulation in the systemic but not in the coronary and pulmonary beds.

Age-Dependent Changes in the Regulatory Roles of Nitric Oxide and Vasodilator Prostanoids on the Mechanical Activities of Isolated Rabbit Spinal Arterioles

The developmental changes in acetylcholine (ACh)-induced vasodilator response in isolated rabbit spinal arterioles were investigated with special reference to endogenous nitric oxide (NO) and vasodilator prostaglandins. Spinal arterioles from juvenile (1-3 months) and adult (12-14 months) Japanese white rabbits were dissected, cannulated, and perfused. In both age groups, ACh produced a comparable vasodilation of the arterioles preconstricted with KCl (20-40 mM). In juveniles, a major part of the ACh-induced vasodilation was eliminated by the cyclo-oxygenase inhibitor indomethacin (10 microM), whereas the effect of the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 30 microM) was smaller. In contrast, L-NAME completely eliminated the ACh-induced vasodilation in the adult group, which was partially restored with additional treatment with L-arginine (1 mM). Vasodilations induced by isocarbacyclin (a prostacyclin analogue) and cilostazol (a cAMP-phosphodiesterase inhibitor) was attenuated in the adult, whereas that induced by sodium nitroprusside was unaltered. These results suggest that the mechanisms of ACh-induced vasodilation of rabbit spinal arterioles change with the development: prostanoid-associated in juveniles and NO-associated in the adult. Such age-related modulation of the mediator roles of prostanoids and NO might represent an increase in NO production and a decreased sensitivity of the vascular smooth muscle to cAMP-mediated responses with age.

Developmental changes in cerebral autoregulatory capacity in the fetal sheep parietal cortex

We validated laser Doppler flowmetry (LDF) for long-term monitoring and detection of acute changes of local cerebral blood flow (lCBF) in chronically instrumented fetal sheep. Using LDF, we estimated developmental changes of cerebral autoregulation. Single fibre laser probes (0.4 mm in diameter) were implanted in and surface probes were placed on the parietal cerebral cortex at 105 +/- 2 (n = 7) and 120 +/- 2 days gestational age (dGA, n = 7). Basal lCBF was monitored over 5 days followed by a hypercapnic challenge (fetal arterial partial pressure of CO(2), P(a,CO2): 83 +/- 3 mmHg) during which lCBF changes obtained by LDF were compared to those obtained with coloured microspheres (CMSs). Mean arterial blood pressure (MABP) was increased and decreased using phenylephrine and sodium nitroprusside at 110 +/- 2 and 128 +/- 2 dGA. Intracortical and cortical surface laser probes gave stable measurements over 5 days. The lCBF increase during hypercapnia obtained by LDF correlated well with flows obtained using CMS (r = 0.89, P < 0.01). The signals of intracortical and surface laser probes also correlated well (r = 0.91, P < 0.01). Gliosis of 0.35 +/- 0.06 mm around the tip of intracortical probes did not affect the measurements. The range of MABP over which cerebral autoregulation was observed increased from 20-48 mmHg at 110 dGA to 35 to > 95 mmHg at 128 dGA (P < 0.05). Since MABP increased from 33 to 54 mmHg over this period (P < 0.01), the range between the lower limit of cerebral autoregulation and the MABP increased from 13 mmHg at 110 dGA to 19 mmHg at 128 dGA (P < 0.01). LDF is a reliable tool to assess dynamic changes in cerebral perfusion continuously in fetal sheep.

Compensatory role of NO in cerebral circulation of piglets chronically treated with indomethacin

We hypothesize that inhibitory effects exist between prostanoids and nitric oxide (NO) in their contributions to cerebral circulation. Piglets (1-4 days old) were divided into three chronically treated (6-8 days) groups: control piglets, piglets treated with indomethacin (75 mg/day), and piglets treated with N(omega)-nitro-L-arginine methyl ester (L-NAME, 100 mg x kg(-1) x day(-1)). Pial arterioles dilated in response to hypercapnia similarly among the three groups (41 +/- 4, 40 +/- 6, and 45 +/- 11%). Cerebrospinal fluid cAMP increased in control piglets, while cGMP increased in indomethacin-treated piglets. L-NAME, but not 7-nitroindazole, inhibited the response to hypercapnia only in indomethacin-treated piglets (40 +/- 6 vs. 17 +/- 5%). Topical sodium nitroprusside or iloprost restored dilation in response to hypercapnia. Similar results were obtained when the dilator was bradykinin. Pial arterioles of control and L-NAME-treated piglets constricted in response to ACh (-24 +/- 3%). However, those of indomethacin-treated piglets dilated in response to ACh (15 +/- 2%). This dilation was inhibited by L-NAME. NO synthase activity, but not endothelial NO synthase expression, increased after chronic indomethacin treatment. These data suggest that chronic inhibition of cyclooxygenase can increase the contribution of NO to cerebrovascular circulatory control in piglets.

Endothelial NO and prostanoid involvement in newborn and juvenile pig pial arteriolar vasomotor responses

Specific cerebrovascular dilatory responses in newborn piglets are entirely prostanoid dependent, but require both nitric oxide (NO) and prostanoids in juveniles. We examined endothelial dependency and mechanisms of NO- and prostanoid-mediated cerebrovascular responses in anesthetized newborn and juvenile pigs implanted with closed cranial windows. Light/dye endothelial injury inhibited newborn and juvenile hypercapnic and bradykinin (BK) responses and inhibited dilation to acetylcholine in juveniles. Iloprost and NO act permissively in restoring light/dye inhibited newborn and juvenile responses, respectively. Differences in sensitivity to iloprost and sodium nitroprusside were not observed. Juvenile (not newborn) hypercapnic and BK cerebrovascular responses were sensitive to soluble guanylyl cyclase inhibition. Pial arteriolar diameter and cortical production of prostacyclin, cAMP, and cGMP in response to BK were measured under control conditions, after treatment with indomethacin and/or N(omega)-nitro-L-arginine methyl ester (L-NAME). Indomethacin inhibited BK responses in newborns. Juvenile responses were inhibited by L-NAME, and mildly by indomethacin. Cortical 6-keto-PGF(1 alpha), cAMP, and cGMP increased in response to BK in both age groups. Newborn cerebrovascular responses are largely NO independent, but NO becomes more important with maturation.

Prostanoid receptors: ontogeny and implications in vascular physiology

Prostanoids exert significant effects on circulatory beds. They play a role in the response of the vasculature to adjustments in perfusion pressure and oxygen and carbon dioxide tension, and they mediate the actions of numerous factors. The role of prostanoids in governing circulation of the perinate is suggested to surpass that in the adult. Prostanoids are abundantly generated in the perinate. They have been implicated in autoregulation of blood flow as studied in brain and eyes. Prostaglandins are also dominant regulators of ductus arteriosus tone. The effects of these autacoids are mediated through specific G protein-coupled receptors. In addition to the pharmacological characterization of the prostanoid receptors, important advances in understanding the biology of these receptors have been made in the last decade. Their cloning and the development of animals with disrupted genes of these receptors have been very informative. The involvement of prostanoid receptors in the developing subject, especially on brain and ocular vasculature and on ductus arteriosus, has also begun to be investigated; the expression of these receptors changes with development. Some but not all of the ontogenic changes in these receptors are attributed to homologous regulation. Interestingly, in the process of elucidating their effects, functional perinuclear prostaglandin E2 receptors have been uncovered. This article reviews prostanoid receptors and addresses implications on the developing subject with attention to vascular physiology.

Developmental changes in endothelium-derived vasorelaxant factors in cerebral circulation

Endothelium-derived prostanoids are predominant vasorelaxant factors in the cerebral circulation of newborn pigs in vivo, whereas in older pigs nitric oxide (NO)-mediated responses also contribute to the regulation of cerebral vascular tone. We compared the expression and activities of NO synthase and cyclooxygenase in the cerebral microcirculation of newborn and adult pigs. In adult animals, expression and activity of endothelial NO synthase in cerebral microvessels and in cultured cerebral endothelial cells is two- to threefold higher than in newborn pigs; acetylcholine and bradykinin cause a greater increase in NO production in adult pigs. Expression and activity of cyclooxygenase in cerebral microvascular endothelial cells is similar in newborn and adult pigs; acetylcholine and bradykinin stimulated dilator prostanoid production to the same degree in both age groups. Endothelial prostanoid synthesis in cerebral microvessels and cultured endothelial cells was inhibited 30-70% by NS-398, reflecting a large contribution of COX-2 in both newborn and adult animals. These data indicate that in the cerebral circulation of pigs, NO synthase is age-dependently upregulated, whereas endothelial cyclooxygenase is not altered during postnatal development.