Interaction between ATP-sensitive K+ channels and nitric oxide on pial arterioles in piglets

KATP channels in cerebral hemodynamics: a systematic review of preclinical and clinical studies

Cumulative evidence suggests that ATP-sensitive potassium (KATP) channels act as a key regulator of cerebral blood flow (CBF). This implication seems to be complicated, since KATP channels are expressed in several vascular-related structures such as smooth muscle cells, endothelial cells and pericytes. In this systematic review, we searched PubMed and EMBASE for preclinical and clinical studies addressing the involvement of KATP channels in CBF regulation. A total of 216 studies were screened by title and abstract. Of these, 45 preclinical and 6 clinical studies were included. Preclinical data showed that KATP channel openers (KCOs) caused dilation of several cerebral arteries including pial arteries, the middle cerebral artery and basilar artery, and KATP channel inhibitor (KCI) glibenclamide, reversed the dilation. Glibenclamide affected neither the baseline CBF nor the baseline vascular tone. Endothelium removal from cerebral arterioles resulted in an impaired response to KCO/KCI. Clinical studies showed that KCOs dilated cerebral arteries and increased CBF, however, glibenclamide failed to attenuate these vascular changes. Endothelial KATP channels played a major role in CBF regulation. More studies investigating the role of KATP channels in CBF-related structures are needed to further elucidate their actual role in cerebral hemodynamics in humans. Systematic review registration: Prospero: CRD42023339278 (preclinical data) and CRD42022339152 (clinical data).

KATP channels modulate cerebral blood flow and oxygen delivery during isocapnic hypoxia in humans

KEY POINTS

ATP-sensitive K⁺ (K_ATP ) channels mediate hypoxia-induced cerebral vasodilatation and hyperperfusion in animals. We tested whether K_ATP channels blockade affects the increase in human cerebral blood flow (CBF) and the maintenance of oxygen delivery (CDO₂ ) during hypoxia. Hypoxia-induced increases in the anterior circulation and total cerebral perfusion were attenuated under K_ATP channels blockade affecting the relative changes of brain oxygen delivery. Therefore, in humans, K_ATP channels activation modulates the vascular tone in the anterior circulation of the brain, contributing to CBF and CDO₂ responses to hypoxia.

ABSTRACT

ATP-sensitive K⁺ (K_ATP ) channels mediate hypoxia-induced cerebral vasodilatation and hyperperfusion in animals. We tested whether K_ATP channels blockade affects the increase in cerebral blood flow (CBF) and the maintenance of oxygen delivery (CDO₂ ) during hypoxia in humans. Nine healthy men were exposed to 5-min trials of normoxia and isocapnic hypoxia (IHX, 10% O₂ ) before (BGB) and 3 h after glibenclamide ingestion (AGB). Mean arterial pressure (MAP), arterial saturation ( S a O 2 ), partial pressure of oxygen ( P a O 2 ) and carbon dioxide ( P aC O 2 ), internal carotid artery blood flow (ICABF), vertebral artery blood flow (VABF), total (t)CBF (Doppler ultrasound) and CDO₂ were quantified during the trials. IHX provoked similar reductions in S a O 2 and P a O 2 , while MAP was not affected by oxygen desaturation or K_ATP blockade. A smaller increase in ICABF (ΔBGB: 36 ± 23 vs. ΔAGB 11 ± 18%, p = 0.019) but not in VABF (∆BGB 26 ± 21 vs. ∆AGB 27 ± 27%, p = 0.893) was observed during the hypoxic trial under K_ATP channels blockade. Thus, IHX-induced increases in tCBF (∆BGB 32 ± 19 vs. ∆AGB 14 ± 13%, p = 0.012) and CDO₂ relative changes (∆BGB 7 ± 13 vs. ∆AGB -6 ± 14%, p = 0.048) were attenuated during the AGB hypoxic trial. In a separate protocol, 6 healthy men (5 from protocol 1) underwent a 5-min exposure to normoxia and IHX before and 3 h after placebo (5 mg of cornstarch) ingestion. IHX reduced S a O 2 and P a O 2 , but placebo did not affect the ICABF, VABF, tCBF, or CDO₂ responses. Therefore, in humans, K_ATP channels activation modulates vascular tone in the anterior rather than the posterior circulation of the brain, contributing to tCBF and CDO₂ responses to hypoxia.

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.

Endothelium-dependent responses in the microcirculation observed in vivo

Endothelium-dependent responses were first demonstrated 40 years ago in the aorta. Since then, extensive research has been conducted in vitro using conductance vessels and materials derived from them. However, the microcirculation controls blood flow to vital organs and has been the focus of in vivo studies of endothelium-dependent dilation beginning immediately after the first in vitro report. Initial in vivo studies employed a light/dye technique for selectively damaging the endothelium to unequivocally prove, in vivo, the existence of endothelium-dependent dilation and in the microvasculature. Endothelium-dependent constriction was similarly proven. Endothelium-dependent agonists include acetylcholine (ACh), bradykinin, arachidonic acid, calcium ionophore A-23187, calcitonin gene-related peptide (CGRP), serotonin, histamine and endothelin-1. Normal and disease states have been studied. Endothelial nitric oxide synthase, cyclooxygenase and cytochrome P450 have been shown to generate the mediators of the responses. Some of the key enzyme systems generate reactive oxygen species (ROS) like superoxide which may prevent EDR. However, one ROS, namely H₂ O₂ , is one of a number of hyperpolarizing factors that cause dilation initiated by endothelium. Depending upon microvascular bed, a single agonist may use different pathways to elicit an endothelium-dependent response. Interpretation of studies using inhibitors of eNOS is complicated by the fact that these inhibitors may also inhibit ATP-sensitive potassium channels. Other in vivo observations of brain arterioles failed to establish nitric oxide as the mediator of responses elicited by CGRP or by ACh and suggest that a nitrosothiol may be a better fit for the latter.

Role of BKCa Channels in Cephalic Vasodilation Induced by CGRP, NO and Transcranial Electrical Stimulation In The Rat

Both calcitonin gene-related peptide (CGRP) and nitric oxide (NO) are potent vasodilators that have been shown to induce headache in migraine patients. Their antagonists are effective in the treatment of migraine attacks. In the present study, we hypothesize that vasodilation induced by the NO donor glyceryltrinitrate (GTN) or by CGRP is partially mediated via large conductance calcium-activated potassium (BKCa) channels. The effects of the BKCa channel selective inhibitor iberiotoxin on dural and pial vasodilation induced by CGRP, GTN and endogenously released CGRP by transcranial electrical stimulation (TES) were examined. Iberiotoxin significantly attenuated GTN-induced dural and pial artery dilation in vivo and in vitro, but had no effect on vasodilation induced by CGRP and TES. Our results show that GTN- but not CGRP-induced dural and pial vasodilation involves opening of BKCa channels in rat.

Astrocyte-induced cortical vasodilation is mediated by D-serine and endothelial nitric oxide synthase

Astrocytes play a critical role in neurovascular coupling by providing a physical linkage from synapses to arterioles and releasing vaso-active gliotransmitters. We identified a gliotransmitter pathway by which astrocytes influence arteriole lumen diameter. Astrocytes synthesize and release NMDA receptor coagonist, D-serine, in response to neurotransmitter input. Mouse cortical slice astrocyte activation by metabotropic glutamate receptors or photolysis of caged Ca(2+) produced dilation of penetrating arterioles in a manner attenuated by scavenging D-serine with D-amino acid oxidase, deleting the enzyme responsible for D-serine synthesis (serine racemase) or blocking NMDA receptor glycine coagonist sites with 5,7-dichlorokynurenic acid. We also found that dilatory responses were dramatically reduced by inhibition or elimination of endothelial nitric oxide synthase and that the vasodilatory effect of endothelial nitric oxide synthase is likely mediated by suppressing levels of the vasoconstrictor arachidonic acid metabolite, 20-hydroxy arachidonic acid. Our results provide evidence that D-serine coactivation of NMDA receptors and endothelial nitric oxide synthase is involved in astrocyte-mediated neurovascular coupling.

Coactivation of NMDA receptors by glutamate and D-serine induces dilation of isolated middle cerebral arteries

N-methyl-D-aspartate (NMDA) receptors are glutamate-gated cation channels that mediate excitatory neurotransmission in the central nervous system. In addition to glutamate, NMDA receptors are also activated by coagonist binding of the gliotransmitter, D-serine. Neuronal NMDA receptors mediate activity-dependent blood flow regulation in the brain. Our objective was to determine whether NMDA receptors expressed by brain endothelial cells can induce vasodilation of isolated brain arteries. Adult mouse middle cerebral arteries (MCAs) were isolated, pressurized, and preconstricted with norepinephrine. N-methyl-D-aspartate receptor agonists, glutamate and NMDA, significantly dilated MCAs in a concentration-dependent manner in the presence of D-serine but not alone. Dilation was significantly inhibited by NMDA receptor antagonists, D-2-amino-5-phosphonopentanoate and 5,7-dichlorokynurenic acid, indicating a response dependent on NMDA receptor glutamate and D-serine binding sites, respectively. Vasodilation was inhibited by denuding the endothelium and by selective inhibition or genetic knockout of endothelial nitric oxide synthase (eNOS). We also found evidence for expression of the pan-NMDA receptor subunit, NR1, in mouse primary brain endothelial cells, and for the NMDA receptor subunit NR2C in cortical arteries in situ. Overall, we conclude that NMDA receptor coactivation by glutamate and D-serine increases lumen diameter in pressurized MCA in an endothelial and eNOS-dependent mechanism.

N-methyl-D-aspartate induces cortical hyperemia through cortical spreading depression-dependent and -independent mechanisms in rats

OBJECTIVE

N-methyl-d-aspartate (NMDA) is a powerful cerebrovascular dilator in vivo. Cortical spreading depression (CSD) has recently been shown to contribute to the pial arteriolar dilation in mice. Our main aim was to examine the participation of CSD in the overall cerebrovascular response to NMDA in the rat.

METHODS

Anesthetized Wistar rats (eight weeks old) were equipped with a closed cranial window to allow topical application of NMDA (10(-5)-10(-3) M) to the parietal cortex. Cortical blood flow (CoBF) under and outside the cranial window was simultaneously monitored by using a two-channel laser-Doppler flowmeter. CSDs were detected by recording the changes in the cortical DC potential.

RESULTS

Concentrations of 10(-4) and 10(-3) M of NMDA evoked single CSDs associated with rapid, transient hyperemia, followed by a sustained, but reduced, increase in CoBF. The latency and magnitude of the CoBF responses were dose dependent. The higher dose resulted in shorter latency (100+/-5* vs. 146+/-11 seconds, *P<0.05; mean+/-standard error of the mean) and larger overall flow response (77+/-12* vs. 28+/-3% from baseline) under, but not outside, the cranial window.

CONCLUSIONS

NMDA elicits dose-dependent increases in CoBF that are composed of CSD-dependent and -independent components in rats.

Hypertension and cerebral vasoreactivity: a continuous arterial spin labeling magnetic resonance imaging study

Hypertension is associated with microvascular and macrovascular brain injury but its direct influence on the cerebral circulation is not fully clear. Our objective was to investigate the association of hypertension with global and regional cerebral vasoreactivity to CO(2) using continuous arterial spin labeling MRI, independent of stroke and white matter hyperintensities. Participants (n=62; mean age: 66.7±1.0 years, 55% women, 84% white, 65% hypertension, 47% stroke) underwent arterial spin labeling perfusion MRI during normal breathing, 5% CO(2) rebreathing, and hyperventilation, as well as 24-hour ambulatory blood pressure monitoring. Vasoreactivity was the slope of the regression between cerebral perfusion and end-tidal CO(2). White matter hyperintensity volumes were quantified. Nighttime dipping was calculated as the percentage decline in nighttime/daytime blood pressure. After accounting for stroke and white matter hyperintensity volume, hypertensive participants had lower global vasoreactivity (1.11±0.13 versus 0.43±0.1 mL/100 g per minute per millimeter of mercury; P=0.0012). Regionally, this was significant in the frontal, temporal, and parietal lobes. Higher mean systolic blood pressure was associated with lower vasoreactivity (decreased by 0.11 U/10-mm Hg increase in systolic blood pressure; P=0.04), but nighttime dipping was not (P=0.2). The magnitude of decrease in vasoreactivity in hypertension without stroke was comparable to the magnitude of decrease in vasoreactivity in stroke without hypertension. Hypertension has a direct negative effect on the cerebrovascular circulation independent of white matter hyperintensities and stroke that is comparable to that seen with stroke. Because lower vasoreactivity is associated with poor outcomes, studies of the impact of antihypertensive on vasoreactivity are important.

Renin angiotensin system gene polymorphisms and cerebral blood flow regulation: the MOBILIZE Boston study

BACKGROUND AND PURPOSE

Our objective was to investigate the associations between polymorphisms in representative genes of the renin angiotensin system with measures of cerebral blood flow regulation in older adults.

METHODS

Participants in this analysis were white subjects (n=335) in the MOBILIZE Boston study (Maintenance of Balance, Independent Living, Intellect, and Zest in the Elderly of Boston), an observational study of community-dwelling elders who underwent transcranial Doppler while sitting and standing and during hypercapnea and hypocapnea. Autoregulation phenotype was the change in cerebrovascular resistance from sit to stand. Vasoreactivity phenotype was the slope of the change in cerebrovascular conductance versus change in end-tidal CO2. A total of 33 tagged single nucleotide polymorphisms were selected in the angiotensinogen gene, the angiotensin converting enzyme gene, and the angiotensin receptor gene. Regression analyses adjusted for age, gender, body mass index, mean arterial blood pressure, stroke, and use of antihypertensives were conducted for each single nucleotide polymorphism and outcome. Bonferroni corrections were used to adjust P values for multiple testing.

RESULTS

In the angiotensinogen gene, only the rs699 single nucleotide polymorphism was associated with vasoreactivity after Bonferroni correction (P=0.00028). Homozygous carriers of the CC genotype of this single nucleotide polymorphism had lower vasoreactivity compared with the CT or TT genotypes. There were no significant associations with autoregulation measures. None of the single nucleotide polymorphisms in the other genes were associated with our phenotypes.

CONCLUSION

This analysis suggests that the angiotensinogen gene may be involved in vasoreactivity independent of blood pressure. Larger studies are needed to confirm the role of this gene in cerebrovascular health and aging.

PACAP and VIP differentially preserve neurovascular reactivity after global cerebral ischemia in newborn pigs

Pituitary adenylate cyclase activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP) are neuroprotective in numerous models. Impairment of cerebrovascular reactivity (CR) contributes to ischemia/reperfusion (I/R)-induced neuronal damage. We tested whether PACAP and/or VIP preserve CR to I/R-sensitive dilator responses dependent on endothelial and/or neuronal function. Accordingly, changes in pial arteriolar diameters in response to hypercapnia (5-10% CO(2) ventilation) or topical N-methyl-d-aspartate (NMDA, 10(-4) M) were determined before and after I/R via intravital microscopy in anesthetized/ventilated piglets. Local pretreatment with non-vasoactive doses of PACAP (10(-8) M) and VIP (10(-9) M) prevented the attenuation of postischemic CR to hypercapnia; to 10% CO(2), the CR values were 27+/-8% vs 92+/-5% vs 88+/-13% (vehicle vs PACAP38 vs VIP, CR expressed as a percentage of the response before I/R, mean+/-SEM, n=8-8, p<0.05). PACAP, but not VIP, preserved CR to NMDA after I/R, with CR values of 31+/-10% vs 87+/-8% vs 35+/-12% (vehicle vs PACAP38 vs VIP, n=6-6). Unlike PACAP, VIP-induced vasodilation has not yet been investigated in the piglet. We tested whether VIP-induced arteriolar dilation was sensitive to inhibitors of cyclooxygenase (COX)-1 (SC-560, 1 mg/kg), COX-2 (NS-398, 1 mg/kg), indomethacin (5 mg/kg), and nitric oxide synthase (L-NAME, 15 mg/kg). VIP (10(-8)-10(-7)-10(-6) M, n=8) induced reproducible, dose-dependent vasodilation of 16+/-3%, 33+/-6%, and 70+/-8%. The response was unaffected by all drugs, except that the vasodilation to 10(-8) M VIP was abolished by SC-560 and indomethacin. In conclusion, PACAP and VIP differentially preserve postischemic CR; independent of their vasodilatory effect.

Mechanisms involved in the cerebrovascular dilator effects of N-methyl-d-aspartate in cerebral cortex

Glutamate and its synthetic analogues N-methyl-d-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) are potent dilator agents in the cerebral circulation. The close linkage between neural activity-based release and actions of glutamate on neurons and the related decrease in cerebral vascular resistance is a classic example in support of the concept of tight coupling between increased neural activity and cerebral blood flow. However, mechanisms involved in promoting cerebral vasodilator responses to glutamatergic agents are controversial. Here we review the development and current status of this important field of research especially in respect to cerebrovascular responses to NMDA receptor activation.

Impaired cerebral CO2 vasoreactivity: association with endothelial dysfunction

Conflicting data exist on the role of nitric oxide (NO) in cerebral blood flow (CBF) autoregulation. Previous studies involving human and animal subjects seem to indicate that NO involvement is limited to the CO(2)-dependent mechanism (chemoregulation) and not to the pressure-dependent autoregulation (mechanoregulation). We tested this hypothesis in patients with impaired endothelial function compared with healthy controls. Blood pressure, heart rate, end-tidal Pco(2), CBF velocities (CBFV), forearm blood flow, and reactive hyperemia were assessed in 16 patients with diabetes mellitus and/or hypertension and compared with 12 age- and sex-matched healthy controls. Pressure-dependent autoregulation was determined by escalating doses of phenylephrine. CO(2) vasoreactivity index was extrapolated from individual slopes of mean CBFV during normocapnia, hyperventilation, and CO(2) inhalation. Measurements were repeated after sodium nitroprusside infusion. Indexes of endothelial function, maximal and area under the curve (AUC) of forearm blood flow (FBF) changes, were significantly impaired in patients (maximal flow: 488 +/- 75 vs. 297 +/- 31%; P = 0.01, AUC DeltaFBF: 173 +/- 17 vs. 127 +/- 11; P = 0.03). Patients and controls showed similar changes in cerebrovascular resistance during blood pressure challenges (identical slopes). CO(2) vasoreactivity was impaired in patients compared with controls: 1.19 +/- 0.1 vs. 1.54 +/- 0.1 cm.s(-1).mmHg(-1); P = 0.04. NO donor (sodium nitroprusside) offsets this disparity. These results suggest that patients with endothelial dysfunction have impaired CO(2) vasoreactivity and preserved pressure-dependent autoregulation. This supports our hypothesis that NO is involved in CO(2)-dependent CBF regulation alone. CBFV chemoregulation could therefore be a surrogate of local cerebral endothelial function.

Effects of the Long-Term Administration of Nicorandil on Vascular Endothelial Function and the Progression of Arteriosclerosis

This study compared the effects of long-term administration of nicorandil and isosorbide dinitrate (ISDN) on vascular endothelial function and the progression of arteriosclerosis. Forty-two patients with ischemic heart disease were randomly allocated to receive nicorandil (N group; 15 mg/d) or ISDN (I group, 40 mg/d). Twelve normal subjects served as controls. Vascular endothelial function and the progression of arteriosclerosis (intima-media thickness, IMT), as determined by carotid vascular ultrasound, were assessed 1 week before and 3 months after drug administration. Reactive hyperemia was induced in the forearm for 5 minutes, and the percentage change in the diameter of the brachial artery (% change in flow-mediated dilation, %FMD) was calculated. FMD was significantly lower in CAD groups than in controls. The %FMD significantly decreased (7.2 +/- 1.9 to 4.2 +/- 2.8) in the I group, while rising from 6.8 +/- 1.6 to 8.0 +/- 2.0 in the N group. IMT increased by 0.036 +/- 0.015 mm in the I group but showed no significant change in the N group (-0.01 +/- 0.012 mm). Thus, ISDN deteriorates IMT and FMD, whereas a beneficial effect of nicorandil is seen on FMD with no effect on IMT. Long-term treatment with nicorandil may be desirable for prevention of cardiovascular events.

Effect of systemic administration of L-kynurenine on corticocerebral blood flow under normal and ischemic conditions of the brain in conscious rabbits

Kynurenic acid, the only known endogenous antagonist of the excitatory amino acid receptors, exerts neuroprotective effect in focal cerebral ischemia. Kynurenic acid poorly while its bioprecursor, l-kynurenine (L-KYN) completely crosses the blood-brain barrier. The aim of our study was to investigate the effect of intravenous l-KYN (0.3, 1, and 3 mg/kg) on the normal and the unilateral carotid artery occlusion induced ischemic corticocerebral blood flow (cCBF) measured by hydrogen polarography in conscious rabbits. Administration of l-KYN produced a significant increase in the normal cCBF; the peak values were recorded at the dose of 1 mg/kg (187% at 120 and 150 mins. respectively). The cCBF-improving effect of l-KYN was immediate and highly pronounced also in rabbits with carotid occlusion (peak value was 192% at 120 mins. at the dose of 1 mg/kg). Pretreatment with either atropine or Nomega-nitro-L-arginine-methyl-ester (L-NAME) prevented the l-KYN induced enhancement of the normal and the ischemic cCBF alike. It is suggested that the cCBF-increasing effect of l-KYN might be mediated by activation of cholinergic and nitric oxide pathways.

The pharmacology of nitric oxide in the peripheral nervous system of blood vessels

Unanticipated, novel hypothesis on nitric oxide (NO) radical, an inorganic, labile, gaseous molecule, as a neurotransmitter first appeared in late 1989 and into the early 1990s, and solid evidences supporting this idea have been accumulated during the last decade of the 20th century. The discovery of nitrergic innervation of vascular smooth muscle has led to a new understanding of the neurogenic control of vascular function. Physiological roles of the nitrergic nerve in vascular smooth muscle include the dominant vasodilator control of cerebral and ocular arteries, the reciprocal regulation with the adrenergic vasoconstrictor nerve in other arteries and veins, and in the initiation and maintenance of penile erection in association with smooth muscle relaxation of the corpus cavernosum. The discovery of autonomic efferent nerves in which NO plays key roles as a neurotransmitter in blood vessels, the physiological roles of this nerve in the control of smooth muscle tone of the artery, vein, and corpus cavernosum, and pharmacological and pathological implications of neurogenic NO have been reviewed. This nerve is a postganglionic parasympathetic nerve. Mechanical responses to stimulation of the nerve, mainly mediated by NO, clearly differ from those to cholinergic nerve stimulation. The naming "nitrergic or nitroxidergic" is therefore proposed to avoid confusion of the term "cholinergic nerve", from which acetylcholine is released as a major neurotransmitter. By establishing functional roles of nitrergic, cholinergic, adrenergic, and other autonomic efferent nerves in the regulation of vascular tone and the interactions of these nerves in vivo, especially in humans, progress in the understanding of cardiovascular dysfunctions and the development of pharmacotherapeutic strategies would be expected in the future.

ATP-sensitive potassium channels in the cerebral circulation

BACKGROUND

In brain blood vessels, electrophysiological studies proving the existence of ATP-sensitive potassium channels (KATP) are scarce. However, numerous pharmacological studies establish the importance of KATP channels in these blood vessels. This review emphasizes the data supporting the importance of vascular KATP in the responses of brain blood vessels.

SUMMARY OF REVIEW

Electrophysiological data show the existence of KATP in smooth muscle and endothelium of brain vessels. A much larger number of studies in virtually all experimental species have shown that classic openers of KATP dilate brain arteries and arterioles. This response can by blocked by glibenclamide, a selective inhibitor of KATP opening. Several physiological or pathophysiological responses are also blocked by glibenclamide. KATP contains a multiplicity of potential sites of interaction with drugs of diverse, sometimes unrelated, structures. Drugs with imidazole or guanidinium groups are particularly likely to have effects on KATP. This complicates interpretation of the actions of such drugs when used as supposedly selective pharmacological probes for other putative targets. A pH-sensitive site on the internal surface of cloned channels may explain the glibenclamide-inhibitable dilation produced by intracellular acidosis and perhaps by CO2. In some situations KATP appears to be involved in either the synthesis/release or action of endothelium-derived mediators of cerebrovascular tone. The importance of KATP may be dependent on the portion of the cerebrovascular tree being studied and on diverse experimental conditions, age, species, and the presence of disease.

CONCLUSIONS

KATP have been shown to mediate a wide range of cerebrovascular response in physiologic or pathologic circumstances in a variety of experimental conditions. Their relevance to cerebrovascular responses in humans remains to be explored.

Role of nitric oxide in the regulation of cerebral blood flow in humans: chemoregulation versus mechanoregulation

BACKGROUND

From animal studies it emerged that nitric oxide is important for the modulation of CO2-mediated cerebral blood flow (CBF chemoregulation) but not for the pressor-dependent mechanism (mechanoregulation). This hypothesis was tested in 18 healthy subjects.

METHODS AND RESULTS

Peak velocity (PV), diastolic velocity (DV), and mean velocity (MV) were measured by transcranial Doppler of the middle cerebral artery. Chemoregulation was assessed during normocapnia, hypocapnia, and after inhaled mixture of 95% O2+5% CO2. Mechanoregulation was evaluated by incremental doses of phenylephrine. Measurements were repeated during infusion of sodium nitroprusside (SNP). Regional cerebrovascular resistance (CVR) was calculated as mean blood pressure (BP)/MV. SNP infusion decreased mean BP by 7 mm Hg and CVR decreased from 1.38+/-0.08 to 1.29+/-0.09 mm Hg/cm x s(-1); P=0.01, resulting in unaffected CBF. Phenylephrine (25 to 250 microg) caused a similar increase in BP in a dose-response fashion before and during SNP infusion. Despite the increments in BP and CVR, CBF remained unaffected. During hyperventilation (end-tidal CO2 approximately 24 mm Hg), CVR increased by 75+/-3% and PV and DV decreased by 27+/-2% and 43+/-2%, respectively (P<0.001 for all). SNP infusion blunted the vasoconstrictive effect of hypocapnia; CVR increased only by 57+/-5%, and PV and DV decreased by 23+/-2% and 35+/-3%, respectively, (P<0.05 for all). Similarly, SNP augmented the vasodilatory effect of hypercapnia.

CONCLUSIONS

Exogenous nitric oxide donor affects the basal cerebral vascular tone without affecting the CBF mechanoregulation. However, it selectively affects only the chemoregulatory mechanism (CO2-dependent). Thus, the CO2-NO axis is a cardinal pathway for CBF regulation in humans.

Role of Nociceptin/Orphanin FQ in the physiologic and pathologic control of the cerebral circulation

Nociceptin/orphanin FQ is a newly described member of the opioid family. Previous minireviews in this series have described the contribution of important factors, including opioids, in the regulation of the cerebral circulation during physiologic and pathologic conditions. The present review extends these initial comments to an opioid whose vascular actions have only very recently been appreciated. In particular, this review discusses the contribution of nociceptin/orphanin FQ to impaired cerebral hemodynamics after cerebral hypoxia/ischemia and traumatic brain injury.

N-methyl-D-aspartate-induced vasodilation is mediated by endothelium-independent nitric oxide release in piglets

N-methyl-D-aspartate (NMDA) elicits pial arteriolar dilation that has been associated with neuronal nitric oxide (NO) production. However, endothelial factors or glial P-450 epoxygenase products may play a role. We tested whether NMDA-induced pial vasodilation 1) primarily involves NO diffusion from the parenchyma to the surface arterioles, 2) involves intact endothelial function, and 3) involves a miconazole-sensitive component. Arteriolar diameters were determined using closed cranial window-intravital microscopy in anesthetized piglets. NMDA (10-100 microM) elicited virtually identical dose-dependent dilations in paired arterioles (r = 0.94, n = 15). However, NMDA- but not bradykinin (BK)-induced dilations of arteriolar sections over large veins were reduced by 31 +/- 1% (means +/- SE, P < 0.05, n = 4) compared with adjacent sections on the cortical surface. Also, 100 microM NMDA increased cerebrospinal fluid levels of NO metabolites from 3.7 +/- 1.0 to 5.3 +/- 1.2 microM (P < 0.05, n = 6). Endothelial stunning by intracarotid injection of phorbol 12,13-dibutyrate did not affect NMDA-induced vasodilation but attenuated vascular responses to hypercapnia and BK by approximately 70% (n = 7). Finally, miconazole (n = 6, 20 microM) pretreatment and coapplication with NMDA did not alter vascular responses to NMDA. In conclusion, NMDA appears to dilate pial arterioles exclusively through release and diffusion of NO from neurons to the pial surface in piglets.

Interactions between the endothelium-derived relaxing factor/nitric oxide system and the endogenous opiate system in the modulation of cerebral and spinal vascular CO2 responsiveness

The role of the L-arginine-nitric oxide (NO) system, the role of the endogenous morphine-like substances (endorphins), and the possible interaction between these two systems in the modulation of regional cerebral and spinal CO2 responsiveness was investigated in anesthetized, ventilated, normotensive, normoxic cats. Regional cerebral blood flow was measured with radiolabeled microspheres in hypocapnic, normocapnic, and hypercapnic conditions in nine individual cerebral and spinal cord regions. General opiate receptor blockade by 1 mg/kg naloxone intravenously alone or NO synthase blockade by 3 mg/kg N(omega)-nitro-L-arginine-methyl ester (L-NAME) intravenously alone caused no changes in regional CO2 responsiveness. Combined administration of these two blocking agents in the very same doses, however, resulted in a strong potentiation, with a statistically significant reduction of the CO2 responsiveness observed. Separation of the blood flow response to hypercapnia and hypocapnia indicates that this reduction occurs only during hypercapnia. Specific mu and delta opiate receptors were blocked by 0.5 mg kg(-1) IV beta-funaltrexamine and 0.4 mg kg(-1) IV naltrindole, respectively. The role of specific mu and delta opiate receptors in the NO-opiate interaction was found to be negligible because neither mu nor delta receptor blockade along with simultaneous NO blockade were able to decrease CO2 responsiveness. The current findings suggest a previously unknown interaction between the endothelium-derived relaxing factor/nitric oxide (EDRF/NO) system and the endogenous opiate system in the cerebrovascular bed during hypercapnic stimulation, with the phenomenon not mediated by mu or delta opiate receptors.

Evidence for a K(ATP) ion channel link in the inhibition of hypercapnic dilation of pial arterioles by 7-nitroindazole and tetrodotoxin

7-Nitroindazole, an inhibitor of neuronal nitric oxide synthase, reportedly inhibits hypercapnic dilation, but tetrodotoxin, an inhibitor of neuronal transmission, reportedly does not. Thus, evidence does not uniformly support the hypothesis of a neurogenic link to the hypercapnic response. Others suggest the hypercapnic response is mediated by a K(ATP) ion channel. In the following studies, we observed that topically administered tetrodotoxin inhibited dilations produced by hypercapnia. In addition, topical tetrodotoxin and either topical or intraperitoneal 7-nitroindazole, inhibited dilations produced by the K(ATP) channel openers, cromakalim and pinacidil. Inhibition of hypercapnic dilation and inhibition of dilation by the openers of the K(ATP) channel was immediately reversed by either L-lysine or L-arginine, amino acids previously shown to facilitate opening of the channel. The data strongly supports the previous conclusion that there is a K(ATP) ion channel link in the response of pial arterioles to hypercapnia. The location of the channel is not established by these data, nor is it known whether the action of tetrodotoxin on the channel was direct or indirect.

Nociceptin/orphanin FQ dilates pial arteries by K(ATP) and K(ca) channel activation

Nociceptin/orphanin FQ (NOC/oFQ) is a recently discovered endogenous ligand for the opioid like receptor, ORL-1. In the piglet, cGMP activates the ATP sensitive (K(ATP)) while cAMP activates both the K(ATP) and the calcium sensitive (K(ca)) K(+) channel to elicit vasodilation. The present study was designed to characterize the role of cGMP, cAMP, K(ATP), and K(ca) channel activation in NOC/oFQ-induced pial artery dilation in newborn pigs equipped with a closed cranial window. NOC/oFQ (10(-8), 10(-6) M) induced pial arteriole dilation was decreased by the protein kinase A inhibitor Rp 8-Br cAMPs (16+/-1 and 30+/-1 vs. 5+/-1 and 10+/-1%). NOC/oFQ dilation was associated with elevated CSF cAMP (1037+/-58 vs. 1919+/-209 fmol/ml for control and 10(-6) M NOC/oFQ). Glibenclamide and iberiotoxin, K(ATP) and K(ca) channel antagonists, attenuated NOC/oFQ induced dilation (15+/-1 and 28+/-1 vs. 10+/-1 and 19+/-1% before and after iberiotoxin). In contrast, the nitric oxide synthase inhibitor, L-NNA, and the protein kinase G inhibitor, Rp 8-Br cGMPs had no effect on NOC/oFQ dilation while such dilation was not associated with a change in CSF cGMP. The putative ORL-1 receptor antagonist [F/G] NOC/oFQ (1-13)-NH(2) blocked NOC/oFQ dilation while responses were unchanged after naloxone (17+/-1 and 30+/-2 vs. 3+/-1 and 5+/-1%, before and after [F/G] NOC/oFQ (1-13)-NH(2)). Dilation to other opioids (e.g., methionine enkephalin) was unchanged by [F/G] NOC/oFQ (1-13)-NH(2). These data show that NOC/oFQ elicits pial artery dilation, at least in part, via cAMP, K(ATP), and K(ca) channel dependent mechanisms. These data suggest that such a mechanism involves the sequential release of cAMP and subsequent K(ATP) and K(ca) channel activation.

Age-dependent impairment of K(ATP) channel function following brain injury

Previous studies observed that endothelin-1 (ET-1) contributed to ATP-sensitive K+ (K(ATP)) channel impairment 1 h following fluid percussion brain injury (FPI) in the newborn pig. The present study was designed to determine the effect of FPI on K(ATP) channel activity as a function of time in newborn (1-5 days old) and juvenile (3-4 weeks old) pigs equipped with a closed cranial window. FPI of moderate severity (1.9-2.1 atm) was produced by using a pendulum to strike a piston on a saline-filled cylinder that was fluid coupled to the brain via a hollow screw inserted through the cranium. Cromakalim, a K(ATP) agonist, produced dilation that was blunted for at least 72 h post FPI, but dilator responsiveness was restored within 168 h post FPI in the newborn pig (15+/-1% and 27+/-2% vs. 5+/-1% and 11+/-1% vs. 13+/-1% and 26+/-2% for responses to 10(-8), 10(-6) M cromakalim before, and 72 and 168 h after FPI). Similar inhibited responses were observed for calcitonin gene-related peptide, 8-Bromo cGMP, and the nitric oxide (NO) releasers SNP and SNAP. In contrast, cromakalim-induced dilation was blunted for at least 4 h, but dilator responsiveness was restored within 8 h post FPI in the juvenile pig (15+/-1% and 27+/-1% vs. 9+/-1% and 15+/-2% vs. 18+/-1% and 28+/-1% for 10(-8), 10(-6) M cromakalim before, and 4 and 8 h post FPI). Similar inhibition of dilations of other agonists also occurred in the juvenile. CSF ET-1 increased to a greater level and remained elevated for a longer period of time in the newborn compared to the juvenile pig. BQ123, an ET-1 antagonist, pretreatment partially restored decremented agonist induced dilation following FPI in the newborn and juvenile pig (5+/-1% and 11+/-1% vs. 11+/-1% and 21+/-1% for responses to 10(-8), 10(-6) M cromakalim 72 h post FPI in the newborn in the absence and presence of BQ123). These data indicate that K(ATP) channel function is impaired to a greater extent and for a longer time period in the newborn versus the juvenile pig. These data also show that ET-1 contributes to such impaired vascular responsiveness to a greater extent in the newborn versus the juvenile pig. These data furthermore suggest that the newborn is more sensitive to traumatic vascular injury than the juvenile.

Hypotension dilates pial arteries by KATP and kca channel activation

Hypotension induced pial artery dilation is prostaglandin-dependent in the newborn pig. Prostaglandins, in turn, elicit vasodilation through cGMP and cAMP dependent mechanisms and K+ channel activation contributes to cyclic nucleotide induced vasodilation. The present study was designed to characterize the role of ATP sensitive (KATP) and calcium sensitive (Kca) channel activation in hypotension induced pial artery dilation in newborn pigs equipped with a closed cranial window. Glibenclamide and iberiotoxin, KATP and Kca channel antagonists, attenuated hypotension induced dilation (36+/-1 vs. 14+/-2% before and after iberiotoxin). Combined administration of these K+ channel antagonists eliminated the vascular response. Hypotension induced dilation was associated with elevated cerebrospinal fluid (CSF) cAMP but not cGMP concentration (1023+/-29 vs. 1566+/-39 fmol/ml for cAMP). L-NNA, a nitric oxide (NO) synthase inhibitor, and Rp 8-Br cGMPs, a protein kinase G inhibitor, had no effect but Rp 8-Br cAMPs, a protein kinase A inhibitor, attenuated hypotensive dilation (35+/-1 vs. 16+/-2% before and after Rp 8-Br cAMPs). Dilation by the cAMP analogue 8-Bromo cAMP (10(-8), 10(-6) M) was attenuated by glibenclamide and iberiotoxin (8+/-1 and 17+/-1 vs. 4+/-1 and 9+/-1% before and after glibenclamide). These data show that both KATP and Kca channel activation contribute to hypotension induced dilation. These data suggest that dilation during hypotension results from the sequential release of prostaglandins and cAMP, which, in turn, activates both the KATP and Kca channel.

Superoxide generation links protein kinase C activation to impaired ATP-sensitive K+ channel function after brain injury

BACKGROUND AND PURPOSE--Endothelin-1, in concentrations similar to that present in cerebrospinal fluid after fluid percussion brain injury (FPI), increases superoxide anion (O2-) production. Endothelin-1 also contributes to altered cerebral hemodynamics after FPI through impairment of ATP-sensitive K+ (KATP) channel function through protein kinase C (PKC) activation. Generation of O2- additionally occurs after FPI. Nitric oxide and cGMP elicit pial artery dilation through KATP channel activation. The present study was designed to determine whether PKC activation generates O2-, which, in turn, could link such activation to impaired KATP channel function after FPI. METHODS--Injury of moderate severity (1.9 to 2.1 atm) was produced by the lateral FPI technique in anesthetized newborn pigs equipped with a closed cranial window. Superoxide dismutase-inhibitable nitroblue tetrazolium (NBT) reduction was determined as an index of O2- generation. RESULTS--Phorbol 12, 13-dibutyrate (10(-6) mol/L), a PKC activator, increased superoxide dismutase-inhibitable NBT reduction from 1+/-1 to 37+/-5 pmol/mm2. Staurosporine (10(-7) mol/L), a PKC antagonist, blocked the NBT reduction after phorbol 12,13-dibutyrate and blunted the NBT reduction observed after FPI (1+/-1 to 15+/-2 versus 1+/-1 to 5+/-1 pmol/mm2 after FPI in the absence versus presence of staurosporine). Exposure of the cerebral cortex to a xanthine oxidase O2--generating system increased NBT reduction in a manner similar to FPI and blunted pial artery dilation to the KATP channel agonists cromakalim and calcitonin gene-related peptide, the nitric oxide releasers sodium nitroprusside and S-nitroso-N-acetylpenicillamine, and the cGMP analogue 8-bromo-cGMP (10+/-1% and 21+/-1% versus 4+/-1% and 9+/-1% for 10(-8) and 10(-6) mol/L cromakalim before and after activated oxygen-generating system exposure). CONCLUSIONS--These data show that PKC activation increases O2- production and contributes to such production observed after FPI. These data also show that an activated system that generates an amount of O2- similar to that observed with FPI blunted pial artery dilation to KATP channel agonists and nitric oxide/cGMP. These data suggest, therefore, that O2- generation links PKC activation to impaired KATP channel function after FPI.

Neuronal nitric oxide synthase inhibitor, 7-nitroindazole, delays motor dysfunction and spinal motoneuron degeneration in the wobbler mouse

Gene mutations of superoxide dismutase (SOD) have been discovered in familial amyotrophic lateral sclerosis (ALS). Neuronal nitric oxide synthase (NOS), endothelial NOS and 3-nitrotyrosine immunoreactivities are selectively increased in the spinal motoneurons of sporadic ALS. Other study suggests that 3-nitrotyrosine immunoreactivity is enhanced in the spinal motoneurons of sporadic and familial ALS patients. The hypothesis is postulated that increased production of radical species, such as superoxide and peroxynitrite, may cause motoneuron degeneration in ALS. There are increased amounts of nitric oxide and SOD hypoactivities in the brain and spinal cord of wobbler mice. NOS is also induced in the vacuolated spinal motoneurons or axons in this animal. Free radicals might contribute to the pathogenesis of wobbler mouse motoneuron disease. Lecithinized SOD treatment has retarded the progression of this disease. This evidence allowed us to determine whether NOS inhibitors delay progression of wobbler mouse motoneuron disease. After clinical diagnosis at age 3-4 weeks, wobbler mice were injected with intraperitoneal non-selective NOS inhibitor, N(G)-nitro-L-arginine methyl ester (L-NAME, 50 mg/kg), two doses of neuronal NOS inhibitor, 7-nitroindazole (5 or 50 mg/kg) or a vehicle solution, daily for 4 weeks in a blind fashion. In comparison with vehicle, 7-nitroindazole-treated mice potentiated grip strength and attenuated deformities in the forelimbs. 7-Nitroindazole treatment increased the biceps muscle weight, reduced denervation muscle atrophy, and suppressed degeneration of spinal motoneurons. To a lesser degree, L-NAME-treated mice displayed slowed progression of disease. The present studies indicate that neuronal NOS inhibitor may be a candidate for promising therapy in lower motoneuron disease or motor neuropathy.

Relationship among NO, the KATP channel, and opioids in hypoxic pial artery dilation

Nitric oxide (NO), opioids, and ATP-sensitive K+ (KATP) channel activation contribute to hypoxia-induced pial artery dilation. NO releasers and cGMP analogs increase opioid concentration in cerebrospinal fluid (CSF) and elicit dilation via KATP channel activation. Opioids themselves also elicit dilation via KATP channel activation. This study was designed to investigate the relationships among the above mechanisms in hypoxic pial artery dilation using newborn pigs equipped with a closed cranial window. Cromakalim (10(-8) and 10(-6) M), a KATP agonist, produced dilation that was unchanged by the NO synthase inhibitor N-nitro-L-arginine (L-NNA, 10(-6) and 10(-3) M): 13 +/- 1 and 31 +/- 1 vs. 14 +/- 1 and 31 +/- 1% before and after 10(-3) M L-NNA. Cromakalim dilation also was not associated with increased CSF cGMP and was unchanged by the Rp diastereomer of 8-bromoguanosine 3',5'-cyclic monophosphothioate, a cGMP antagonist. Glibenclamide (10(-6) M), a KATP antagonist, attenuated hypoxic dilation but hypoxia-associated CSF cGMP release was unchanged: 457 +/- 12 and 935 +/- 30 vs. 458 +/- 11 and 921 +/- 22 fmol/ml. Coadministration of L-NNA with glibenclamide had no further effect on the already diminished hypoxic dilation but blocked the hypoxia-associated rise in CSF cGMP. Cromakalim had no effect on CSF methionine enkephalin: 1,012 +/- 28 and 1,062 +/- 32 pg/ml. These data show that KATP channel agonists do not elicit dilation via NO/cGMP and do not release opioids. NO release during hypoxia also is independent of KATP channel activation. These data suggest that hypoxic dilation results from the sequential release of NO, cGMP, and opioids, which in turn activate the KATP channel.

Brain injury impairs prostaglandin cerebrovasodilation

Previous studies have observed that ATP- and calcium-sensitive K+ (KATP and Kca) channel function is impaired after fluid percussion brain injury (FPI). The present study was designed to characterize the effect of FPI on prostaglandin (PG)E2 and 12 pial artery dilation and the role of activation of these K+ channels in that dilation in newborn pigs equipped with a closed cranial window. FPI of moderate severity (1.9-2.1 atm) was produced by using a pendulum to strike a piston on a saline-filled cylinder that was fluid coupled to the brain via a hollow screw inserted through the cranium. PGE2 vasodilation was blunted by FPI (9+/-1%, 13+/-1%, and 19+/-1% vs. 2+/-1%, 5+/-1%, and 9+/-1%, for 1, 10, and 100 ng/ml PGE2 before and after FPI, respectively). PGE2 dilation was associated with increased CSF cGMP and cAMP concentration and such changes in cyclic nucleotides were blunted by FPI (448+/-10 and 793+/-38 vs. 316+/-11 and 403+/-27 fmol/ml for control and PGE2 induced change in cGMP before and after FPI, respectively). PGI2-induced dilation and associated changes in CSF cyclic nucleotide concentration were similarly blunted by FPI. PGE2 dilation was attenuated by either glibenclamide or iberiotoxin, KATP and K,ca channel antagonists, and coadministration of both K+ channel antagonists further decremented the dilator response (9+/-1%, 14+/-1%, and 21+/-1%; vs. 4+/-1%, 7+/-1%, and 12+/-1%; vs. 2+/-1%, 4+/-1%, and 7+/-1%, for 1, 10, and 100 ng/ml PGE2 during control, after glibenclamide, and after combined glibenclamide and iberiotoxin, respectively). Glibenclamide and iberiotoxin had similar effects on PGI2 dilation. These data show that prostaglandin dilation is attenuated after FPI. These data also show that prostaglandin dilation is dependent on activation of both KATP and Kca channels. Further, these data suggest that attenuated prostaglandin dilation following FPI results from diminished prostaglandin-associated elevation in cyclic nucleotide concentration and impaired KATP and Kca channel function.

Contribution of kca channel activation to hypoxic cerebrovasodilation does not involve NO

Although nitric oxide (NO) and calcium sensitive K+ channel (Kca) activation contribute to hypoxic pial artery dilation in the piglet, responses to the NO releasers SNP and SNAP are unchanged by the Kca channel antagonist iberiotoxin. These data suggest that NO does not elicit dilation via Kca channel activation. The present study was designed to determine if dilation by Kca channel activation is mediated by NO in newborn pigs equipped with a closed cranial window. NS1619 (10(-8), 10(-6) M), a Kca agonist, produced dilation that was unchanged by the NO synthase inhibitor, L-NNA (10(-6) or 10(-3) M) (11+/-1 and 20+/-1 vs. 11+/-1 and 18+/-1% before and after L-NNA 10(-3) M). NS1619 dilation also was not associated with increased CSF cGMP and was unchanged by Rp 8-Bromo cGMPs, a cGMP antagonist (9+/-1 and 17+/-1 vs. 9+/-1 and 16+/-2% before and after Rp 8-Bromo cGMPs 10(-5) M). Iberiotoxin (10(-7) M) attenuated hypoxic dilation but hypoxia associated CSF cGMP release was unchanged (418+/-11 and 897+/-31 vs. 419+/-10 and 896+/-25 fmol/ml for control and moderate hypoxia before and after iberiotoxin). Coadministration of L-NNA with iberiotoxin further decremented hypoxic pial dilation and blocked the hypoxia-associated rise in CSF cGMP. These data show that pial artery dilation by Kca channel activation is not mediated by NO/cGMP. Further, these data suggest that NO and the Kca channel act at different sites in their contributions to hypoxic pial artery dilation.

Maintained vasodilatory response to cromakalim after inhibition of nitric oxide synthesis

Activation of vascular smooth-muscle adenosine triphosphate-sensitive potassium channels (KATP channels) causes membrane hyperpolarization, reduced entry of Ca2+ through L-type voltage-gated Ca2+ channels, and subsequent smooth-muscle relaxation. Conversely, opening of endothelial KATP channels elicits hyperpolarization but may induce Ca2+ influx and stimulation of endothelium-derived nitric oxide (EDNO) because these cells appear not to possess L-type Ca2+ channels. We therefore hypothesized that EDNO contributes to KATP channel-mediated vasodilation. To test this hypothesis, we examined vasodilatory responses to the KATP channel opener cromakalim in conscious rats, perfused rat tail artery segments, and isolated perfused rat lungs in the presence or absence of the EDNO synthesis inhibitor Nomega-nitro-L-arginine (L-NNA). Additionally, we compared the effect of cromakalim with the EDNO-dependent dilator bradykinin on NO production and intracellular Ca2+ in cultured rat pulmonary artery endothelial cells. Vasodilatory profiles to cromakalim were unaffected by L-NNA in conscious rats, tail arteries, and isolated lungs. Consistent with these results, cromakalim had no apparent effect on either NO synthesis or Ca2+ levels in cultured endothelial cells. These data suggest a lack of a role for EDNO in contributing to KATP-channel-mediated vasodilation in the rat.

Calcium-dependent and ATP-sensitive potassium channels and the 'permissive' function of cyclic GMP in hypercapnia-induced pial arteriolar relaxation

The conclusion that cyclic 3'-5 guanosine monophosphate (cGMP) functions in a 'permissive' manner in promoting cerebrovasodilation during hypercapnia was based on findings showing that the nitric oxide synthase (NOS) inhibitor-induced repression of the CO2 response could be reversed upon addition of exogenous cGMP. We hypothesized that the action of cGMP revealed in those studies does not define its normal role in hypercapnic cerebral vasodilation, but rather is a unique function of the artificial situation of NOS inhibition coupled with cGMP repletion. Thus, although CO2 reactivity may be the same in normal versus cGMP-repleted animals, the factors contributing to that response may differ. To test that possibility, the effects of calcium-dependent (KCa) or ATP-sensitive (KATP) potassium channel blockers on pial arteriolar CO2 reactivity, in vivo, were evaluated in the presence and absence of NOS inhibition plus administration of a cGMP analogue. Pial arteriolar diameter changes in hypercapnia were measured in three principal groups of anesthetized rats: (I) KCa channel-inhibited (via iberiotoxin); (II) KATP channel-inhibited (via glibenclamide); and (III) controls. Group I and II rats were further divided into: (a) those treated with the neuronal NOS (nNOS) inhibitor, 7-nitroindazole (7-NI), followed by successive suffusions of the cGMP analogue, 8-bromo-cGMP (8Br-cGMP) and 8Br-cGMP+K-channel blocker; and (b) rats where 7-NI and 8Br-cGMP applications were omitted. Group III rats were divided into time and 8Br-cGMP controls. Hypercapnia (PCO2 congruent with60 mmHg, 3 min)-induced dilations were reduced by 70-80% following 7-NI and restored by 8Br-cGMP. That restoration was reversed by both K-channel blockers. In the absence of 7-NI and exogenous cGMP, CO2 reactivity was unaffected by K-channel inhibition. These findings confirmed that nNOS-derived NO is critically important to the hypercapnic reactivity of cerebral arterioles, and that cGMP repletion, following NOS inhibition, could restore CO2 reactivity. The observation that KCa and KATP channel blockade did not alter CO2 reactivity under baseline conditions, but attenuated CO2 reactivity only in the presence nNOS inhibition (and cGMP repletion), suggests that multiple, redundant, and interactive mechanisms participate in CO2-induced vasodilation. These results also imply that current strategies for revealing permissive actions of cGMP (or NO) may need to be re-evaluated.

Effects of anoxic stress on prostaglandin H synthase isoforms in piglet brain

We examined effects of ischemia and asphyxia on levels of prostaglandin H synthase-1 (PGHS-1) and prostaglandin H synthase-2 (PGHS-2) in piglet brain. Ischemia was induced by increasing intracranial pressure and asphyxia was induced by turning off the respirator. Duration of anoxic stress was 10 min. In some animals, indomethacin (5 mg/kg, i.v.) or 7-nitroindazole (7-NI) was administered prior to ischemia to block PGHS or brain nitric oxide synthase (bNOS), respectively. Tissues from cerebral cortex and hippocampus were removed and fixed and/or frozen after 1, 2, 4 and 8 h of recovery from anoxic stress. In addition, tissues were obtained from untreated animals or from time control animals. Levels of mRNA and proteins were determined using RNase protection assay and immunohistochemical approaches, respectively. In the tissues studied, only a few neurons were immunopositive for PGHS-1, and neither ischemia or asphyxia affected PGHS-1 immunostaining at 8 h after recovery. Likewise, PGHS-1 mRNA did not increase following anoxic stress. In contrast, substantial PGHS-2 immunoreactivity was present in neurons and glial cells in the cerebral cortex and hippocampus and there was no difference between time control and non treated animals. PGHS-2 mRNA increased by 2-4 h after ischemia, and heightened immunoreactivity for PGHS-2 was present at 8 h after ischemia in cerebral cortex and hippocampus. However, asphyxia did not increase PGHS-2 mRNA or immunostaining. Indomethacin pretreatment inhibited increases in mRNA and protein for PGHS-2 after ischemia, while 7-NI had little effect on increases in PGHS-2 immunoreactivity. We conclude that: (1) PGHS-2 is the predominant isoform present in piglet cerebral cortex and hippocampus; (2) Ischemia but not asphyxia increases levels of PGHS-2; (3) Ischemia does not increase levels of PGHS-1; and (4) Indomethacin but not 7-NI attenuates ischemia-induced increases in PGHS-2.

Role of endothelial nitric oxide synthase in hypoxia-induced pial artery dilation

Nitric oxide (NO) contributes to hypoxia-induced pial artery dilation, at least in part, through the formation of cGMP and the subsequent release of methionine enkephalin and leucine enkephalin in the newborn pig. In separate studies, these opioids also were observed to elicit NO-dependent pial artery dilation, whereas light/dye endothelial injury reduced hypoxic pial dilation. The current study was designed to investigate the role of the endothelial isoform of NO synthase in hypoxic pial dilation, associated opioid release, and opioid dilation in piglets equipped with a closed cranial window. N-iminoethyl-L-ornithine (L-NIO) (10(-6) mol/L), an antagonist that may have greater endothelial NO synthase inhibitory selectivity, had no effect on dilation elicited by hypoxia (PO2 approximately 35 mm Hg) (24 +/- 2 versus 24 +/- 2% in the absence and presence of L-NIO, respectively, n = 8). Hypoxic dilation was accompanied by increased CSF cGMP, which also was unchanged in the presence of L-NIO (394 +/- 19 and 776 +/- 63 versus 323 +/- 13 and 739 +/- 25 fmol/mL for control and hypoxia in the absence and presence of L-NIO, respectively, n = 6). Additionally, hypoxic pial dilation was associated with increased CSF methionine enkephalin, which also was unchanged in the presence of L-NIO (992 +/- 73 and 2469 +/- 197 versus 984 +/- 18 and 2275 +/- 185 pg/mL, respectively, n = 6). In contrast, methionine enkephalin-induced dilation was blocked by L-NIO (6 +/- 1, 10 +/- 1, and 16 +/- 1 versus 1 +/- 1, 1 +/- 1, and 2 +/- 1% for 10(-10), 10(-8), 10(-6) mol/L methionine enkephalin, respectively, before and after L-NIO, n = 8). Substance P-induced pial dilation was blunted by L-NIO, whereas responses to sodium nitroprusside and N-methyl-D-aspartate were unchanged. These data indicate that endothelial NO synthase contributes to opioid-induced pial artery dilation but not hypoxia-induced dilation. Additionally, these data suggest that neuronally derived NO contributes to hypoxic pial dilation.

ATP-dependent K+ channel activation reduces loss of opioid dilation after brain injury

ATP-dependent K+ (KATP) channel function is impaired after fluid percussion brain injury (FPI). Additionally, the nitric oxide (NO) releaser sodium nitroprusside and a cGMP analog elicit pial dilation via KATP channel activation, whereas opioids such as methionine enkephalin (Met) elicit pial dilation via NO and KATP channel activation. Decremented Met dilation contributes to reductions in pial artery diameter and altered cerebral hemodynamics after FPI. This study was designed to investigate the role of KATP channel activation before FPI in the loss of opioid dilation subsequent to FPI in newborn pigs equipped with a closed cranial window. FPI was produced by allowing a pendulum to strike a piston on a saline-filled cylinder that was fluid coupled to the brain via a hollow screw in the cranium. FPI blunted dilation to Met (7 +/- 1, 11 +/- 1, and 17 +/- 1% before FPI vs. 1 +/- 1, 4 +/- 1, and 6 +/- 1% after FPI for 10(-10), 10(-8), and 10(-6) M Met, respectively). Met-associated elevation in cerebrospinal fluid (CSF) cGMP was similarly blunted (350 +/- 12 and 636 +/- 12 fmol/ml before FPI vs. 265 +/- 5 and 312 +/- 17 fmol/ml after FPI for control and 10(-6) M Met, respectively). In piglets pretreated with cromakalim (10(-10) M) 20 min before FPI, Met dilation was partially restored (7 +/- 1, 10 +/- 1, and 15 +/- 1% before FPI vs. 4 +/- 1, 7 +/- 1, and 11 +/- 1% after FPI for 10(-10), 10(-8), and 10(-6) M Met, respectively). Met cGMP release was similarly partially restored (400 +/- 9 and 665 +/- 25 fmol/ml before FPI vs. 327 +/- 11 and 564 +/- 23 fmol/ml after FPI for control and 10(-6) Met, respectively). Cromakalim (10(-10) M) had no effect on pial diameter itself but prevented pial artery constriction by FPI (148 +/- 5 to 124 +/- 5 microns vs. 139 +/- 4 to 141 +/- 4 microns in the absence vs. presence of cromakalim pretreatment, respectively). In contrast, pretreatment with a subthreshold concentration of NS-1619, a calcium-dependent K+ channel agonist, did not restore vascular and biochemical parameters after FPI. These data indicate that prior KATP channel activation reduces the loss of opioid dilation after FPI.

Inhibitory effects of hypoxia and adenosine on N-methyl-D-aspartate-induced pial arteriolar dilation in piglets

Our previous studies have indicated that oxygen radicals, produced during reoxygenation following short-term arterial hypoxia, lead to sustained suppression of cerebral arteriolar responses to N-methyl-D-aspartate (NMDA). However, whether arteriolar dilator responses to NMDA are reduced during arterial hypoxia has never been examined. In this study, we determined whether hypoxia or hypoxia-related metabolites such as adenosine or nitric oxide (NO) will reduce NMDA-induced arteriolar dilation. We have also determined the location of NMDA receptor- and brain nitric oxide synthase (bNOS)-positive neurons in the cerebral cortex. In anesthetized piglets, pial arteriolar diameters were determined using intravital microscopy. Baseline arteriolar diameters were approximately 100 microns. Topical application of NMDA at concentrations of 10(-5), 5 x 10(-5) and 10(-4) M resulted in dose-dependent vasodilation (9 +/- 2, 18 +/- 2 and 29 +/- 2% above baseline, respectively, n = 21). Administration of theophylline (20 mg/kg, i.v.) had no effect on NMDA-dependent vasodilation, but it did block dilation to hypoxia (inhalation of 8.5% O2). In theophylline-treated animals, NMDA responses were completely abolished during hypoxia (28 +/- 2 vs. 2 +/- 1%, respectively to 10(-4) M, n = 7) while sodium nitroprusside (SNP, 10(-4) M) still dilated pial arterioles normally. NMDA-induced vasodilation was not modified after application and removal of adenosine (10(-4) M; n = 5) or SNP (10(-5) M; n = 4), or when SNP (10(-7) M) was coapplied with NMDA (n = 6). Conversely, coapplication of adenosine (10(-6) M) attenuated NMDA responses (31 +/- 5 vs. 20 +/- 3%, n = 7). We also found that NMDA receptor- and bNOS-containing neurons were located predominantly in layers II/III of the cortex. Proximity of these neurons to the cortical surface is consistent with diffusion of NO to pial arterioles as the mechanism of dilation to NMDA. We conclude that NMDA-induced cerebral arteriolar dilation is inhibited by hypoxia alone and by exogenous adenosine, but not by NO.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Effects of ischemia on cerebral arteriolar dilation to arterial hypoxia in piglets

BACKGROUND AND PURPOSE

Arterial hypoxia mediates cerebral arteriolar dilation primarily via mechanisms involving activation of ATP-sensitive K+ channels (K[ATP]), which we have shown to be sensitive to ischemic stress. In this study, we determined whether ischemia/reperfusion alters cerebral arteriolar responses to arterial hypoxia in anesthetized piglets. Since adenosine plays an important role in cerebrovascular responses to hypoxia, we also determined whether adenosine-induced arteriolar dilation is affected by ischemic stress. We tested the hypothesis that reductions in cerebral arteriolar dilator responses after ischemia would be proportional to the contribution of K(ATP) to hypoxia and adenosine.

METHODS

Pial arteriolar diameters were measured using a cranial window and intravital microscopy. We examined arteriolar responses to arterial hypoxia (inhalation of 8.5% and 7.5% O2), to topical adenosine (10[-5] and 10[-4] mol/L) and to arterial hypercapnia (inhalation of 5% and 10% CO2 in air) before and after 10 minutes of global ischemia. Ischemia was achieved by increasing intracranial pressure. Arterial hypercapnia was used as a positive control for the effectiveness of the ischemic insult. In addition, we evaluated cerebral arteriolar responses to 10(-5) and 10(-4) mol/L adenosine applied topically with or without glibenclamide, a selective inhibitor of K(ATP) (10[-5] and 10[-6] mol/L). Finally, we administered theophylline (20 mg/kg, i.v.) to assess the contribution of adenosine to cerebral arteriolar dilation to arterial hypoxia.

RESULTS

Before ischemia, cerebral arterioles dilated by 19+/-3% to moderate and 29+/-4% to severe hypoxia (n=7; P<.05); 13+/-2% to 10(-5) and 20+/-1% to 10(-4) mol/L adenosine (n=9; P<.05); and by 17+/-2% to moderate and 28+/-3% to severe hypercapnia (n=6; P<.05). After ischemia, cerebral arteriolar responses to hypoxia and adenosine were unchanged. In contrast, cerebral arteriolar dilation to hypercapnia was impaired by ischemia (1+/-1% and 2+/-1% at 1 hour; n=6). Glibenclamide reduced cerebral arteriolar dilation to adenosine by approximately one half (n= 7). In addition, blockade of adenosine receptors by theophylline (20 mg/kg, i.v.) almost totally suppressed cerebral arteriolar dilation to arterial hypoxia (n = 6).

CONCLUSIONS

Cerebrovascular responsiveness is selectively affected by anoxic stress. In addition, cerebral arteriolar dilation to hypoxia and adenosine is maintained after ischemia despite the expected impairment in K(ATP) function.

Endothelin impairs ATP-sensitive K+ channel function after brain injury

In piglets, pial arteries constrict, ATP-sensitive K+ (KATP) channel function is impaired, and cerebrospinal fluid endothelin-1 (ET-1) increases to 10(-10) M after brain injury [fluid percussion injury (FPI)]. Nitric oxide (NO) elicits dilation via guanosine 3',5'-cyclic monophosphate (cGMP) and KATP channel activation. This study was designed to characterize the relationship between ET-1 and impaired function of KATP channels after FPI. Injury was produced via the lateral FPI technique in piglets equipped with a closed cranial window. Cromakalim, a KATP agonist, produced dilation that was attenuated by FPI and partially restored by BQ-123, an ET-1 antagonist (11 +/- 1 and 23 +/- 2 vs. 2 +/- 1 and 4 +/- 1 vs. 8 +/- 1 and 17 +/- 2% for responses to 10(-8) and 10(-6) M cromakalim before FPI, after FPI, and after FPI with BQ-123, respectively). Because ET-1 constriction may antagonize dilation, separate experiments were conducted under conditions of equivalent baseline diameter in the absence and presence of ET-1 (10(-10) M). Cromakalim dilation was attenuated by ET-1 and partially restored by the protein kinase C (PKC) inhibitor staurosporine (12 +/- 1 and 28 +/- 1 vs. 2 +/- 1 and 21 +/- 3 vs. 9 +/- 1 and 29 +/- 2% for 10(-8) and 10(-6) M cromakalim, cromakalim with ET-1, and cromakalim with ET-1 + staurosporine, respectively). Similar interactions were observed with calcitonin gene-related peptide, 8-bromoguanosine 3',5'-cyclic monophosphate, and the NO releasers sodium nitroprusside and S-nitroso-N-acetylpenicillamine. These data show that ET-1 blunts KATP channel-, NO-, and cGMP-mediated dilation. These data suggest that ET-1 contributes to altered cerebral hemodynamics after FPI through impairment of KATP channel function via PKC activation.

Brain injury impairs ATP-sensitive K+ channel function in piglet cerebral arteries

BACKGROUND AND PURPOSE

Traumatic injury is the leading cause of death for infants and children, and mortality is increased with head injury. Previous studies have shown that pial arteries constricted and that responses to several nitric oxide (NO)-dependent dilator stimuli were blunted after fluid percussion injury (FPI) in newborn pigs. Membrane potential of vascular muscle is a major determinant of vascular tone, and activity of K+ channels is a major regulator of membrane potential. Recent data show that the NO releasers sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) and 8-bromo-cGMP elicit dilation via ATP-sensitive K+ channel (KATP) activation. The present study was designed to investigate the effect of FPI on KATP channel function.

METHODS

Chloralose-anesthetized newborn pigs equipped with a closed cranial window were connected to a percussion device that consisted of a saline-filled cylindrical reservoir and a metal pendulum. Brain injury of moderate severity (1.9 to 2.1 atm) was produced by allowing the pendulum to strike a piston on the cylinder. Pial artery diameter was measured with a video microscaler. Data were analyzed by repeated measures ANOVA. An alpha level of P < .05 was considered significant.

RESULTS

FPI blunted dilation to cromakalim (10(-8), 10(-6) mol/L), a KATP agonist (10 +/- 1% and 27 +/- 2% versus 3 +/- 1% and 7 +/- 2% before and after FPI, respectively, n = 8). Similarly, FPI blunted dilation to calcitonin gene-related peptide, an endogenous KATP activator. FPI also blunted dilator responses to SNP, S-nitroso-N-acetylpenicillamine, and 8-bromo-cGMP (10(-6) to 10(-8) mol/L) (10 +/- 1% and 20 +/- 1% versus 2 +/- 1% and 8 +/- 2% for SNP before and after FPI; 9 +/- 1% and 16 +/- 1% versus 2 +/- 1% and 4 +/- 1% for 8-bromo-cGMP before and after FPI, respectively, n = 8). In contrast, responses to papaverine and brain natriuretic peptide were unchanged after FPI.

CONCLUSIONS

These data show that KATP channel function is impaired after FPI. Furthermore, these data suggest that impaired function of mechanisms distal to NO synthase contribute to altered cerebral hemodynamics after FPI.

Calcium-activated K+ channels in cerebral arterioles in piglets are resistant to ischemia

Our previous studies indicate that function of ATP-dependent K+ channels (K(ATP)) in cerebral arterioles is suppressed after ischemia. In the current study, we examined pial arteriolar responses to forskolin, dibutyryl-cAMP, NS-1619, and methionine (met)-enkephalin, activators of calcium-dependent K+ channels (K(Ca)) before and 1 hour after 10 minutes of total, global ischemia in anesthetized piglets. Arteriolar diameters were measured using a closed cranial window and intravital microscopy. All pharmacologic agents were given topically. Baseline diameters were approximately 100 microm, and diameters had returned to normal by 1 hour after ischemia. Forskolin dilated arterioles by 9 +/- 3%, 18 +/- 4%, and 31 +/- 12% at 5 x 10(-8), 5 x 10(-7), and 10(-6) mol/L, respectively (P < 0.05, n = 10). In addition, dibutyryl-cAMP dilated arterioles by 8 +/- 2% at 10(-4) mol/L and 14 +/- 2% at 3 x 10(-4) mol/L (P < 0.05, n = 6). Also, NS-1619 increased diameter of arterioles by 9 +/- 2% at 10(-7) mol/L and 17 +/- 9% at 10(-5) mol/L (P < 0.05, n = 5). Finally, met-enkephalin dilated arterioles by 9 +/- 2% at 10(-8) mol/L and 16 +/- 3% at 10(-6) mol/L (P < 0.05, n = 5). At 1 hour after ischemia, arteriolar dilator effects to forskolin, dibutyryl-cAMP and NS-1619, and met-enkephalin were intact. Thus, in contrast to K(ATP), K(Ca) in cerebral arterioles are resistant to ischemic stress.

Role of neuronal NO synthase in relationship between NO and opioids in hypoxia-induced pial artery dilation

Nitric oxide (NO) contributes to hypoxia-induced pial artery dilation, at least in part, via the formation of guanosine 3',5'-cyclic monophosphate (cGMP) and subsequent release of Met-enkephalin and Leu-enkephalin in the newborn pig. In separate studies, these opioids were also observed to elicit NO-dependent pial dilation. The present study was designed to investigate the role of the neuronal isoform of NO synthase (NOS) in hypoxic pial dilation, associated opioid release, and opioid dilation in piglets equipped with a closed cranial window. Tetrodotoxin (10(-6) M) attenuated the dilation resulting from hypoxia (PO2 approximately 35 mmHg; 25 +/- 1 vs. 14 +/- 1%). Similarly, 7-nitroindazole, sodium salt (7-NINA, 10(-6) M), a purported neuronal NOS inhibitor, attenuated hypoxic pial dilation (26 +/- 1 vs. 14 +/- 2%). Hypoxic dilation was accompanied by elevated cerebrospinal (CSF) cGMP, which was blocked by 7-NINA (433 +/- 19 and 983 +/- 36 vs. 432 +/- 19 and 441 +/- 19 fmol/ml for control and hypoxia in absence and presence of 7-NINA, respectively). Additionally, hypoxic dilation was also accompanied by elevated CSF Met-enkephalin, which was attenuated by 7-NINA (1,027 +/- 47 and 2,871 +/- 134 vs. 779 +/- 78 and 1,551 +/- 42 pg/ml for control and hypoxia in absence and presence of 7-NINA, respectively). In contrast, Met-enkephalin (10(-10), 10(-8), and 10(-6) M) induced dilation that was unchanged by 7-NINA (7 +/- 1, 12 +/- 1, and 18 +/- 1 vs. 6 +/- 1, 10 +/- 1, and 17 +/- 1%, respectively). N-methyl-D-aspartate (NMDA, 10(-8) and 10(-6) M), an activator of neuronal NOS, induced pial dilation that was blocked by 7-NINA (10 +/- 1 and 20 +/- 2 vs. 1 +/- 1 and 2 +/- 1%, respectively). However, sodium nitroprusside-induced dilation was unchanged by 7-NINA. These data indicate that neuronal NOS contributes to hypoxic pial artery dilation but not to opioid-induced dilation. Furthermore, these data suggest that neuronally derived NO contributes to hypoxic dilation, at least in part, via formation of cGMP and the subsequent release of opioids.

Kainate-induced cerebrovascular dilation is resistant to ischemia in piglets

BACKGROUND AND PURPOSE

Cerebral arteriolar dilation to N-methyl-D-aspartate (NMDA) is drastically reduced by anoxic stress. The effects of anoxic stress on cerebrovascular dilation to activation of other types of glutamate receptors are unknown. The purpose of this study was to examine the effects of ischemia on cerebral arteriolar responses to kainate in anesthetized piglets.

METHODS

Arteriolar responses to 5 x 10(-5) mol/L and 10(-4) mol/L kainate were evaluated before and 10 minutes after total, global ischemia. Ischemia was induced by increasing intracranial pressure. We measured pial arteriolar diameters (approximately 100 microns) using a cranial window and intravital microscopy.

RESULTS

Before ischemia, kainate dilated arterioles by 16 +/- 2% at 5 x 10% mol/L and 30 +/- 2% at 10(-4) mol/L (mean +/- SEM; n = 6). After ischemia, the diameter of arterioles increased by 17 +/- 3% and 26 +/- 3% to 5 x 10% and 10(-4) mol/L kainate, respectively (P > .05). We also investigated the mechanisms involved in mediating arteriolar dilation to kainate. Intravenous administration of N omega-nitro-L-arginine methyl ester (L-NAME) (15 mg/kg) (n = 7) or indomethacin (10 mg/kg) (n = 6) individually reduced arteriolar dilation to kainate by approximately one half. Coadministration of L-NAME and indomethacin almost completely eliminated arteriolar dilation to kainate (n = 10). Administration of theophylline (20 mg/kg IV) did not affect dilator responses to kainate (n = 7). Blockade of NMDA receptors by MK801 had minimal effects on arteriolar dilation to kainate (n = 6).

CONCLUSIONS

There are three main findings from this study: (1) kainate is a potent dilator agent in the neonatal cerebral circulation; (2) nitric oxide and prostaglandins both participate in the vasodilator response to kainate; and (3) in contrast to NMDA, cerebral arteriolar dilator responses to kainate are resistant to ischemic stress.