Hypercapnia-induced Vasodilation Research Articles

Dynamic resistance exercise-induced pressor response does not alter hypercapnia-induced cerebral vasodilation in young adults.

Excessive arterial pressure elevation induced by resistance exercise (RE) attenuates peripheral vasodilatory function, but its effect on cerebrovascular function is unknown. We aimed to evaluate the effect of different pressor responses to RE on hypercapnia-induced vasodilation of the internal carotid artery (ICA), an index of cerebrovascular function. To manipulate pressor responses to RE, 15 healthy young adults (11M/4F) performed two RE: high intensity with low repetitions (HL) and low intensity with high repetitions (LH) dynamic knee extension. ICA dilation, induced by 3min of hypercapnia, was measured before and 10min after RE using Doppler ultrasound. HL exercise elicited a greater pressor response than LH exercise. In relaxation phases of RE, ICA blood velocity increased in both HL and LH trials. However, ICA shear rate did not significantly increase in either trial (P = 0.06). Consequently, neither exercise altered post-exercise hypercapnia-induced ICA dilation (HL, 3.9 ± 1.9% to 5.1 ± 1.7%; LH, 4.6 ± 1.4% to 4.8 ± 1.8%; P > 0.05 for all). When viewed individually, the changes in ICA shear rate were positively correlated with changes in end-tidal partial pressure of carbon dioxide (PETCO2) (r = 0.46, P < 0.01) than with mean arterial pressure (r = 0.32, P = 0.02). These findings suggest that the effects of RE-induced pressor response on cerebrovascular function may be different from peripheral arteries. An increase in PETCO2 during the relaxation phase may play a more crucial role than elevated pressure in increasing cerebral shear during dynamic RE.

Microglia modulate blood flow, neurovascular coupling, and hypoperfusion via purinergic actions.

Microglia, the main immunocompetent cells of the brain, regulate neuronal function, but their contribution to cerebral blood flow (CBF) regulation has remained elusive. Here, we identify microglia as important modulators of CBF both under physiological conditions and during hypoperfusion. Microglia establish direct, dynamic purinergic contacts with cells in the neurovascular unit that shape CBF in both mice and humans. Surprisingly, the absence of microglia or blockade of microglial P2Y12 receptor (P2Y12R) substantially impairs neurovascular coupling in mice, which is reiterated by chemogenetically induced microglial dysfunction associated with impaired ATP sensitivity. Hypercapnia induces rapid microglial calcium changes, P2Y12R-mediated formation of perivascular phylopodia, and microglial adenosine production, while depletion of microglia reduces brain pH and impairs hypercapnia-induced vasodilation. Microglial actions modulate vascular cyclic GMP levels but are partially independent of nitric oxide. Finally, microglial dysfunction markedly impairs P2Y12R-mediated cerebrovascular adaptation to common carotid artery occlusion resulting in hypoperfusion. Thus, our data reveal a previously unrecognized role for microglia in CBF regulation, with broad implications for common neurological diseases.

Hypercapnia-induced vasodilation in the cerebral circulation is reduced in older adults with sleep-disordered breathing.

The prevalence of sleep-disordered breathing (SDB) is higher in older adults compared with younger individuals. The increased propensity for ventilatory control instability in older adults may contribute to the increased prevalence of central apneas. Reductions in the cerebral vascular response to CO2 may exacerbate ventilatory overshoots and undershoots during sleep. Thus, we hypothesized that hypercapnia-induced cerebral vasodilation (HCVD) will be reduced in older compared with younger adults. In 11 older and 10 younger adults with SDB, blood flow velocity in the middle cerebral artery (MCAV) was measured using Doppler transcranial ultrasonography during multiple steady-state hyperoxic hypercapnic breathing trials while awake, interspersed with room air breathing. Changes in ventilation, MCAV, and mean arterial pressure (MAP) via finger plethysmography during the trials were compared with baseline eupneic values. For each hyperoxic hypercapnic trial, the change (Δ) in MCAV for a corresponding change in end-tidal CO2 and the HCVD or the change in cerebral vascular conductance (MCAV divided by MAP) for a corresponding change in end-tidal CO2 was determined. The hypercapnic ventilatory response was similar between the age groups, as was ΔMCAV/Δ[Formula: see text]. However, compared with young, older adults had a significantly smaller HCVD (1.3 ± 0.7 vs. 2.1 ± 0.6 units/mmHg, P = 0.004). Multivariable analyses demonstrated that age and nadir oxygen saturation during nocturnal polysomnography were significant predictors of HCVD. Thus, our data indicate that older age and SDB-related hypoxia are associated with diminished HCVD. We hypothesize that this impairment in vascular function may contribute to breathing instability during sleep in these individuals.NEW & NOTEWORTHY This study demonstrates, for the first time, in individuals with sleep-disordered breathing (SDB) that aging is associated with decreased hypercapnia-induced cerebral vasodilation (HCVD). In addition to advanced age, the magnitude of nocturnal oxygen desaturation due to SDB is an equal independent predictor of HCVD.

High-but not moderate-intensity exercise acutely attenuates hypercapnia-induced vasodilation of the internal carotid artery in young men.

Exercise-induced increases in shear rate (SR) across different exercise intensities may differentially affect hypercapnia-induced vasodilation of the internal carotid artery (ICA), a potential index of cerebrovascular function. We aimed to elucidate the effects of exercise intensity on ICA SR during exercise and post-exercise hypercapnia-induced vasodilation of the ICA in young men. Twelve healthy men completed 30min of cycling at moderate [MIE; 65 ± 5% of age-predicted maximal heart rate (HRmax)] and high (HIE; 85 ± 5% HRmax) intensities. Hypercapnia-induced vasodilation was induced by 3min of hypercapnia (target end-tidal partial pressure of CO2 + 10mmHg) and was assessed at pre-exercise, 5min and 60min after exercise. Doppler ultrasound was used to measure ICA diameter and blood velocity during exercise and hypercapnia tests. SR was not altered during either exercise (interaction and main effects of time; both P > 0.05). ICA conductance decreased during HIE from resting values (5.1 ± 1.3 to 3.2 ± 1.0mL·min-1·mmHg-1; P < 0.01) but not during MIE (5.0 ± 1.3 to 4.0 ± 0.8mL·min-1·mmHg-1; P = 0.11). Consequently, hypercapnia-induced vasodilation declined immediately after HIE (6.9 ± 1.7% to 4.0 ± 1.4%; P < 0.01), but not after MIE (7.2 ± 2.1% to 7.3 ± 1.8%; P > 0.05). Sixty minutes after exercise, hypercapnia-induced vasodilation returned to baseline values in both trials (MIE 8.0 ± 3.1%; HIE 6.4 ± 2.9%; both P > 0.05). The present study showed blunted hypercapnia-induced vasodilation of the ICA immediately after high-intensity exercise, but not a moderate-intensity exercise in young men. Given that the acute response is partly linked to the adaptive response in the peripheral endothelial function, the effects of aerobic training on cerebrovascular health may vary depending on exercise intensity.

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.

Mechanisms Associated with the Adverse Vascular Consequences of Rapid Posthypothermic Rewarming and Their Therapeutic Modulation in Rats.

We previously demonstrated that rapid posthypothermic rewarming in noninjured animals was capable of damaging cerebral arterioles both at endothelial and smooth muscle levels. Such adverse consequences could be prevented with antioxidants, suggesting the involvement of free radicals. In this study, we further investigate the mechanisms associated with free radicals production by using two radical scavengers, superoxide dismutase (SOD) and catalase. Employing rats, the cerebral vascular response was evaluated at 2, 3, and 4 hours after onset of hypothermia. Before rapid rewarming, SOD treatment, but not catalase, preserved the NO-mediated dilation induced by acetylcholine (ACh). On the contrary, catalase preserved the hypercapnia-induced relaxation of the smooth muscle cells, whereas SOD offered only partial protection. Adding SOD to catalase treatment offered no additional benefit. These results suggest that rapid posthypothermic rewarming impairs ACh- and hypercapnia-induced vasodilation through different subcellular mechanisms. In the case of diminished vascular response to ACh, it appears to act on the endothelial front primarily by superoxide anions, as evidenced by its full preservation after SOD treatment. In terms of impaired dilation to hypercapnia, hydrogen peroxide and/or its derivatives are the likely candidates in targeting the smooth muscle cells. The partial protection of SOD to hypercapnia-induced dilation is believed to be the reduced amount of superoxide that would otherwise spontaneously dismutate to produce hydrogen peroxide. Although SOD exerts some indirect influence on the hydrogen peroxide production downstream, catalase apparently has no influence on upstream superoxide production.

Blunted shear-mediated dilation of the internal but not common carotid artery in response to lower body negative pressure

Shear-mediated dilation in peripheral conduit arteries is blunted with sympathetic nervous system (SNS) activation; however, the effect of SNS activation on shear-mediated dilation in carotid arteries is unknown. We hypothesized that SNS activation reduces shear-mediated dilation in common and internal carotid arteries (CCA and ICA, respectively), and this attenuation is greater in the ICA compared with the CCA. Shear-mediated dilation in the CCA and ICA were measured in nine healthy men (24 ± 1 yr) with and without SNS activation. Shear-mediated dilation was induced by 3 min of hypercapnia (end-tidal partial pressure of carbon dioxide +10 mmHg from individual baseline); SNS activity was increased with lower body negative pressure (LBNP; -20 mmHg). CCA and ICA measurements were made using Doppler ultrasound during hypercapnia with (LBNP) or without (Control) SNS activation. LBNP trials began with 5 min of LBNP with subjects breathing hypercapnic gas during the final 3 min. Shear-mediated dilation was calculated as the percent rise in peak diameter from baseline diameter. Sympathetic activation attenuated shear-mediated dilation in the ICA (Control vs. LBNP, 5.5 ± 0.7 vs. 1.8 ± 0.4%, P < 0.01), but not in the CCA (5.1 ± 1.2 vs. 4.2 ± 1.0%, P = 0.31). Moreover, absolute reduction in shear-mediated dilation via SNS activation was greater in the ICA than the CCA (-3.6 ± 0.7 vs. -0.9 ± 0.8%, P = 0.02). Our data indicate that shear-mediated dilation is attenuated during LBNP to a greater extent in the ICA compared with the CCA. These results potentially provide insight into the role of SNS activation on cerebral perfusion, as the ICA is a key supplier of blood to the brain. NEW & NOTEWORTHY We explored the effect of acute sympathetic nervous system (SNS) activation on shear-mediated dilation in the common and internal carotid arteries (CCA and ICA, respectively) in young healthy men. Our data demonstrate that hypercapnia-induced vasodilation of the ICA is attenuated during lower body negative pressure to a greater extent than the CCA. These data may provide novel information related to the role of SNS activation on cerebral perfusion in humans.

The Mechanism of Increased Blood Flow in the Brain and Spinal Cord During Hemodilution

Hemodilution is accompanied by an increase in cerebral blood flow, but whether this is due to vasodilation in response to reduced arterial oxygen content, reduced blood viscosity, or a combination of these mechanisms is a matter of debate. We performed the current study to gain insight into this question by evaluating the effect of hemodilution on (1) vasodilator reserve and (2) the level of blood flow during hypercapnia-induced vasodilation in regions of the brain and spinal cord. Sixteen mongrel dogs were anesthetized with halothane 0.9% (1 minimum alveolar concentration) while their lungs were mechanically ventilated. Radioactive microspheres (15 μm) were used to measure regional blood flow (RBF) in the cerebral cortex, cerebellum, pons, medulla, and spinal cord (cervical, thoracic, and lumbar segments). Arterial blood pressure was measured via an aortic catheter. Vasodilator reserve was assessed from the ratio of RBF during hypercapnia (PaCO2 approximately 65 mm Hg) to RBF before hypercapnia. PaCO2 was increased by the addition of dead-space tubing without changing the ventilator settings. The dilating effects of hypercapnia within the central nervous system (CNS) were assessed with hematocrit normal (group 1; n = 8) and after induction of isovolemic hemodilution to a hematocrit of 19 ± 4 (SD) with 5% dextran (group 2; n = 8). Hemodilution increased RBF (P < 0.0001) and decreased the vasodilator reserve ratio (P < 0.05) in all regions of the brain and spinal cord; the ratios during hemodilution (group 2) were only 48% to 68% of those without hemodilution (group 1). The level of RBF during hypercapnia was not significantly different in the absence and presence of hemodilution (cerebral cortex: mean, 122 mL/min/100 g vs mean, 108 mL/min/100 g; 95% confidence interval of the difference (95% CID), -53 to 26; P = 0.46; cerebellum: mean, 117 mL/min/100 g vs mean, 100 mL/min/100 g; 95% CID, -52 to 18; P = 0.32; pons: mean, 83 mL/min/100 g vs mean, 73 mL/min/100 g; 95% CID, -12 to 31; P = 0.35; medulla: mean, 96 mL/min/100 g vs mean, 82 mL/min/100 g; 95% CID, -11 to 40; P = 0.25; cervical spinal cord: mean, 61 mL/min/100 g vs mean, 52 mL/min/100 g; 95% CID, -18 to 34; P = 0.51; thoracic spinal cord: mean, 35 mL/min/100 g vs mean, 46 mL/min/100 g; 95% CID, -30 to 8; P = 0.24; lumbar spinal cord: mean, 54 mL/min/100 g vs mean, 58 mL/min/100 g; 95% CID, -25 to 15; P = 0.61). Neither hypercapnia alone nor combined with hemodilution affected mean arterial blood pressure (P = 0.78 and P = 0.81, respectively). Hemodilution caused recruitment of the vasodilator reserve, suggesting that vasodilation played a role in the increase in RBF throughout the CNS. Although the mean values for RBF during hypercapnia were similar with and without hemodilution, a large variation in the responses precluded a conclusive determination of whether or not reduced blood viscosity also contributed to the hemodilution-induced increases in RBF. A dependence on vasodilation would limit autoregulatory capability throughout the CNS during hemodilution, which would enhance the risk for ischemia if hypotension was superimposed.

Selective Inhibitors Differentially Affect Cyclooxygenase-Dependent Pial Arteriolar Responses in Newborn Pigs

Cyclooxygenase (COX)-derived prostanoids play an important role in the cerebrovascular control of newborns. In humans and in the widely accepted model of piglets, both the COX-1 and the COX-2 isoforms are expressed in cerebral arteries. However, the involvement of these isoforms in cerebrovascular control is unknown. Therefore we tested if specific inhibitors of COX-1 and/or COX-2 would differentially affect pial arteriolar responses to COX-dependent stimuli in piglets. Anesthetized, ventilated piglets (n = 35) were equipped with a closed cranial window, and changes in pial arteriolar diameters (baseline approximately 100 microm) to hypercapnia (ventilation with 5-10% CO(2), 21% O(2), balance N(2)), arterial hypotension (40 mm Hg MABP achieved by blood withdrawal), and Ach (Ach, 10-100 microM) were determined via intravital microscopy. Arteriolar responses were repeatedly tested 15 min after IV administration of selective COX-1 and COX-2 inhibitors SC-560 and NS-398 (1-1 mg/kg), and nonselective inhibitors indomethacin (0.3-1 mg/kg), acetaminophen (30 mg/kg), and ibuprofen (30 mg/kg). Hypercapnia resulted in concentration-dependent, reversible, (approximately 20-40%) increases in pial arteriolar diameters that were unaffected by NS-398, SC-560, acetaminophen and ibuprofen. In contrast, 0.3 mg/kg indomethacin significantly reduced, 1 mg/kg virtually abolished the vasodilation. Arterial hypotension elicited (approximately 15-20%) vasodilation that was similarly reduced by NS-398 and indomethacin but was unaltered by SC-560. Ach dose-dependently constricted pial arterioles. This response was similarly attenuated by NS-398, indomethacin, and ibuprofen, but left intact by SC-560. We conclude that the assessed COX-dependent vascular reactions appear to depend largely on COX-2 activity. However, hypercapnia-induced vasodilation was found indomethacin-sensitive instead of a COX-dependent response in the piglet.

Cerebrovascular vasodilation to extraluminal acidosis occurs via combined activation of ATP-sensitive and Ca2+-activated potassium channels.

Albeit controversial, it has been suggested by several authors that nitric oxide (NO) serves as a permissive factor in the cerebral blood flow response to systemic hypercapnia. Potassium channels are important regulators of cerebrovascular tone and may be modulated by a basal perivascular NO level. To elucidate the functional targets of the proposed NO modulation during hypercapnia-induced vasodilation, the authors performed experiments in isolated, cannulated, and pressurized rat middle cerebral arteries (MCA). Extracellular pH was reduced from 7.4 to 7.0 in the extraluminal bath to induce NO dependent vasodilation. Acidosis increased vessel diameter by 35 +/- 10%. In separate experiments, ATP-sensitive potassium channels (KATP) were blocked by extraluminal application of glibenclamide (Glib), Ca2+-activated potassium channels (KCa) by tetraethylammonium (TEA), voltage-gated potassium channels (Kv) by 4-aminopyridine, and inward rectifier potassium channels (KIR) by BaCl2. Na+-K+-ATP-ase was inhibited by ouabain. Application of TEA slightly constricted the arteries at pH 7.4 and slightly but significantly attenuated the vasodilation to acidosis. Inhibition of the other potassium channels or Na+-K+-ATP-ase had no effect. Combined blockade of KATP and KCa channels further reduced resting diameter, and abolished acidosis induced vasodilation. The authors conclude that mainly KCa channels are active under resting conditions. KATP and KCa channels are responsible for vasodilation to acidosis. Activity of one of these potassium channel families is sufficient for vasodilation to acidosis, and only combined inhibition completely abolishes vasodilation. During NO synthase inhibition, dilation to the KATP channel opener pinacidil or the KCa channel opener NS1619 was attenuated or abolished, respectively. The authors suggest that a basal perivascular NO level is necessary for physiologic KATP and KCa channel function in rat MCA. Future studies have to elucidate whether this NO dependent effect on KATP and KCa channel function is a principle mechanism of NO induced modulation of cerebrovascular reactivity and whether the variability of findings in the literature concerning a modulatory role of NO can be explained by different levels of vascular NO/cGMP concentrations within the cerebrovascular tree.

Role of nitric oxide in the control of ocular blood flow.

In the recent years it has been recognized that nitric oxide is an important regulator of ocular blood flow. Nitric oxide is involved in the control of basal blood flow in the choroid, optic nerve and the retina. In addition, nitric oxide mediates a number of vasodilator responses in ocular vessels to agonists such as acetylcholine, bradykinin, histamine, substance P and insulin. Nitric oxide also plays a role in hypercapnia-induced vasodilation in the choroid and is a modulator of pressure autoregulation in this vascular bed. Abnormalities of the L-arginine/nitric oxide system have been observed in a variety of ocular diseases including glaucoma, diabetic retinopathy and retinopathy of prematurity. This makes the L-arginine/nitric oxide pathway an attractive target for therapeutic interventions. Additional research is required, particularly in characterizing the role of the three nitric oxide synthase isoforms in the control of ocular perfusion, to implement this concept into the clinical management of ocular diseases.

Hypercapnia stimulates prostaglandin E 2 but not prostaglandin I 2 release in endothelial cells cultured from microvessels of human fetal brain

Hypercapnia-induced cerebral vasodilation involves prostanoids, in newborns. The source of these prostanoids, however, is not yet determined. In the present study we address the hypothesis that microvascular endothelial cells of human fetal cerebrum increase the synthesis of dilator prostanoids in response to high pCO 2. Cells were isolated from a 22-week-old human fetus. Indication of induced abortion was 46 XY-t(3,10) 3q-25 chromosome abnormality. Normocapnia or hypercapnia was performed during normoxic and normothermic conditions in the medium of the cell culture. After normocapnic or hypercapnic stimuli, the amounts of released prostaglandin E 2 and 6-keto-prostaglandin F 1α (the stable metabolite of prostaglandin I 2) were measured by radioimmunoassay. Endothelial cells cultured from human fetal brain microvessels express PGE 2 and 6-keto-PGF 1α in different degrees. Hypercapnic stimulus induced a significant increase of PGE 2, while expression of 6-keto-PGF 1α was not augmented by the same stimulus. PGE 2 of endothelial origin, therefore, could be a factor in the mediation of the hypercapnia-induced vasodilation in human fetuses.

Carbon dioxide and the cerebral circulation.

Carbon dioxide and the cerebral circulation.

Role of NO in the O2 and CO2 responsiveness of cerebral and ocular circulation in humans.

It is well known that changes in PCO2 or PO2 strongly influence cerebral and ocular blood flow. However, the mediators of these changes have not yet been completely identified. There is evidence from animal studies that NO may play a role in hypercapnia-induced vasodilation and that NO synthase inhibition modulates the response to hyperoxia in the choroid. Hence we have studied the effect of NO synthase inhibition by NG-monomethyl-L-arginine (L-NMMA, 3 mg/kg over 5 min as a bolus followed by a continuous infusion of 30 micrograms.kg-1.min-1) on the changes of cerebral and ocular hemodynamic parameters elicited by hypercapnia and hyperoxia in healthy young subjects. Mean flow velocities in the middle cerebral artery and the ophthalmic artery were measured with Doppler ultrasound, and ocular fundus pulsation amplitude, which estimates pulsatile choroidal blood flow, was measured with laser interferometry Administration of L-NMMA reduced ocular fundus pulsation. (-19%, P < 0.005) but only slightly reduced mean flow velocities in the larger arteries. Hypercapnia (PCO2 = 48 mmHg) significantly increased mean flow velocities in the middle cerebral artery (+26%, P < 0.01) and fundus pulsation amplitude (+16%, P < 0.005) but did not change mean flow velocity in the ophthalmic artery. The response to hypercapnia in the middle cerebral artery (P < 0.05) and in the choroid (P < 0.05) was significantly blunted by L-NMMA. On the contrary, L-NMMA did not affect hyperoxia-induced (PO2 = 530 mmHg) hemodynamic changes. The hemodynamic effects of L-NMMA (at baseline and during hypercapnia) were reversed by coadministration of L-arginine. The present study supports the concept that NO has a role in hypercapnia induced vasodilation in humans.

Protein kinase Cs and tyrosine kinases in permissive action of prostacyclin on cerebrovascular regulation in newborn pigs.

The involvement of protein kinase C (PKC) and protein tyrosine kinase (PTK) in hypercapnia-induced cerebral vasodilation in newborn pigs was investigated with closed cranial windows using the PKC stimulator phorbol 12-myristate 13-acetate (PMA), and the PTK inhibitors, genistein and herbimycin A. The influence of prostaglandin I2 was eliminated using the prostaglandin cyclooxygenase inhibitor, indomethacin. Changes in pial arteriolar diameters in response to hypercapnia [partial pressure of arterial CO2 approximately 9.3 kPa (70 torr)] were analyzed. Genistein (40 micrograms/mL), herbimycin A (10 microM), or PMA (1 microM) did not affect cerebral vasodilation to hypercapnia when applied topically. Indomethacin (5 mg/kg i.v.) treatment blocked the dilation to hypercapnia and attenuated hypercapnia-induced increase in cortical cAMP. Genistein and herbimycin A restored the response to hypercapnia to indomethacin-treated piglets. PMA also restored the pial arteriolar dilation and the cAMP response to hypercapnia to indomethacin-treated piglets. One-hour exposure to 10 microM PMA, to down-regulate PKC, blocked vasodilation to hypercapnia but did not inhibit vasodilation to sodium nitroprusside. After prolonged (2 h) topical exposure of indomethacin-treated piglets to 10 microM PMA, neither genistein nor iloprost could restore dilation to hypercapnia. These results indicate that PKC stimulation and/or PTK inhibition may permit hypercapnia-induced vasodilation. These data further suggest that PKC is downstream from PTK in the regulatory pathway. Because previous data showed prostaglandin I2 at subdilator concentrations can also return dilation to hypercapnia to piglets treated with indomethacin, prostaglandin I2 could provide its permissive input by activating PKC and/or inhibiting PTK.

Prostanoid synthesis in response to high CO2 in newborn pig brain microvascular endothelial cells

Hypercapnia-induced cerebral vasodilation involves prostanoids in newborn pigs. However, the source of prostanoids has not been determined. The current study was designed to address the hypothesis that piglet cerebral microvascular endothelial cells increase their synthesis of prostanoids in response to high CO2. Microvascular endothelial cells, smooth muscle cells, and glia were isolated and grown in primary culture. They were identified morphologically and by indirect immunofluorescence staining. Cerebral microvascular endothelial cell cultures from newborn pigs produced equal amounts of 6-ketoprostaglandin (PG) F1 alpha (stable hydrolysis product of PGI2), PGE2 and a small amount of PGF2 alpha under basal conditions. Administration of calcium ionophore A23187 to the endothelial cells increased release of all three prostanoids in a dose- and time-dependent manner. Exposure of piglet cerebral microvascular endothelial cells to higher than normal CO2 increased the production of 6-keto-PGF1 alpha and PGE2 but not of PGF2 alpha. The enhanced prostanoid biosynthesis was concentration dependent, peaking at 14% CO2, and was detected during the first 10 min exposure to 14% CO2. Hypercapnia-induced increased synthesis of prostanoids was blocked dose dependently by the simultaneous addition of PGH synthase inhibitor indomethacin. High CO2 did not increase prostanoid production by cerebral microvascular smooth muscle cells or glia, although A23187 enhanced prostanoid formation by both cell types. These data show that high CO2 stimulates prostanoid synthesis by newborn pig cerebral microvascular endothelial cells, which is consistent with an involvement of cerebral vascular endothelium in hypercapnia-induced vasodilation.