INTRODUCTION

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POSTTRANSLATIONAL modification of cell proteins by phosphorylation on tyrosine residues can alter a variety of physiological functions (9, 10, 12, 13, 30). Protein tyrosine phosphorylation is regulated by the balanced action of protein tyrosine kinases (PTKs), which catalyze phosphorylation of specific tyrosine residues, and protein tyrosine phosphatases (PTPs), which facilitate dephosphorylation of tyrosine residues. PTPs are much more active than PTKs, thus providing a rapid elimination of cellular effects of protein tyrosine phosphorylation (6, 15, 30).

Tyrosine phosphorylation plays an important role in the regulation of smooth muscle tone via multiple endothelium-dependent, as well as endothelium-independent, mechanisms (5, 13, 16, 29, 31, 32). Vascular smooth muscle and vascular endothelium are important cellular targets for protein tyrosine phosphorylation (7, 16, 22). The PTK inhibitors genistein and tyrphostin cause vasorelaxation in endothelium-intact (26, 31) and endothelium-free vascular segments (5, 16, 29, 32) from different vascular beds, including coronary, pulmonary, renal, and cerebral circulations. Mechanisms of endothelium-independent vasorelaxation in response to the PTK inhibitors may include the inhibition of Ca²⁺ mobilization in smooth muscle (11, 13, 33).

In contrast to the PTK inhibitors, vasoactive effects of PTP inhibitors are endothelium dependent. In endothelium-denuded vascular smooth muscle segments from rabbit and rat aortas, the PTP inhibitors sodium orthovanadate and phenylarsine oxide caused strong vasoconstriction (3, 5, 7, 16, 27, 28). However, in vascular segments with intact endothelium, the effect of the PTP inhibitors was reversed to vasorelaxation (7, 21, 22). Vasorelaxant effects of PTP inhibitors could be associated with the activation of endothelium-dependent mechanisms of smooth muscle tone regulation, such as production of endothelium-derived vasorelaxant factor(s). Endothelial nitric oxide (NO) synthase is acutely regulated by tyrosine phosphorylation in vitro (8). N-nitro-L-arginine methyl ester (L-NAME) and N-nitro-L-arginine (L-NNA) abrogate vasorelaxation to PTP inhibitors in several vascular beds (7, 22), indicating that endothelial NO synthase is a physiological target for tyrosine phosphorylation. Activation of tyrosine kinase is essential in NO-mediated vasodilation in physiological response to increasing intraluminal flow in isolated coronary arteries (21).

In the cerebral microcirculation of newborn pigs, endothelium-dependent vascular responses to a variety of physiological and pharmacological stimuli are mediated by vasodilator products of the cyclooxygenase pathway (17), whereas NO-mediated reactions may develop later in ontogenesis (34). We recently demonstrated that tyrosine phosphorylation rapidly stimulates cyclooxygenase activity in cultured endothelial and smooth muscle cells from newborn pig cerebral microvessels. The constitutive isoform of cyclooxygenase, COX-2, appears to be the substrate for the tyrosine phosphorylation (23).

In the present study we investigated the functional contribution of protein tyrosine phosphorylation to cerebral vascular tone in vivo. We tested the hypothesis that cyclooxygenase and NO synthase are physiological targets for tyrosine phosphorylation.

	MATERIALS AND METHODS

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Protocols using animals were approved by the Animal Care and Use Committee at the University of Tennessee, Memphis.

Materials. Indomethacin trihydrate (gift from Merck Sharp & Dohme Research Laboratories, Rahway, NJ), sodium orthovanadate, L-NAME, L-NNA, and isoproterenol (all from Sigma Chemical) were dissolved in PBS. Arachidonic acid (Cayman, Ann Arbor, MI), genistein (Sigma Chemical), and tyrphostin 47 (Biomol) stock solutions were prepared in 95% ethanol. Phenylarsine oxide (Biomol) stock solution was prepared in DMSO. Preliminary studies showed that the vehicles at the concentrations used in our experiments had no effect on pial arteriolar diameter.

Cranial window. Newborn pigs (1-5 days old, 1.5-2.5 kg) were anesthetized with ketamine hydrochloride (33 mg/kg im) and acepromazine (3.3 mg/kg im) and maintained on -chloralose, as described previously (24, 25). The animals were ventilated with room air. During the experiments, arterial blood gases and pH were maintained as follows: 86-94 mmHg PO₂, 32-36 mmHg PCO₂, and pH 7.45-7.52. Arterial blood pressure was between 60 and 80 mmHg. Body temperature was maintained at 37-38°C with a servo-controlled heating pad.

Experimental procedures. Before the experiment the space under the cranial window was flushed several times with aCSF. Two pial arterioles in each animal were selected for observation: a small arteriole (40-50 µm diameter) and a medium-sized arteriole (70-80 µm diameter). To determine the control diameter values, arterioles were measured over a 10-min period under basal conditions. At the end of the control period, control CSF was sampled from under the window for prostanoid determination.

Prostanoid assays. Concentrations of 6-ketoprostaglandin F₁ (6-keto-PGF₁, the hydrolysis product of prostacyclin) and prostaglandin E₂ (PGE₂) in the cortical periarachnoid CSF were determined by RIA, as described previously (24). All samples were assayed at two dilutions. The assay allowed analysis of prostanoid concentrations between 5 and 500 pg/sample.

cAMP and cGMP assays. cAMP and cGMP were measured in CSF samples with use of RIA procedures, as described previously (24). All unknowns were assayed at two dilutions. CSF samples were acetylated with 2:1 triethylamine-acetic anhydride immediately before assay to increase the sensitivity of the method (analysis range 2-128 fmol cAMP or cGMP).

Statistical analysis. Values are means ± SE of absolute values or percentage of control. ANOVA with repeated measures and Fisher's protected least significant difference test were used to isolate differences between groups. P < 0.05 was considered significant in all statistical tests.

	RESULTS

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Effects of phenylarsine oxide on basal pial arteriolar diameter and cortical prostanoid level. Phenylarsine oxide topically applied to the cerebral surface (5-100 µM) caused dose-dependent dilation of pial arterioles (Fig. 1A). No differences in dilator responses were noticed between the groups of small (40-70 µm) and large (80-120 µm) pial arterioles (data not shown). Dilation of pial arterioles in response to 5-20 µM phenylarsine oxide (48 ± 5% above basal diameter) was readily reversible, whereas high concentrations of phenylarsine oxide (>100 µM) caused irreversible maximal vasodilation (90 ± 9% above basal diameter). Dilation of pial arterioles in response to phenylarsine oxide (5-50 µM) was accompanied by a dose-dependent increase in the concentration of 6-keto-PGF₁ in cortical periarachnoid CSF, with a maximal increase of 1.8- to 2.0-fold (Fig. 1B). We observed a similar elevation in the level of another dilator prostanoid, PGE₂ (maximal increase 2.2- to 2.5-fold; data not shown). Increasing the phenylarsine oxide concentration from 100 to 300 µM did not result in further increase in prostanoid levels (Fig. 1B).

View larger version (26K): [in this window] [in a new window]   Fig. 1.   Dose-dependent effects of phenylarsine oxide on pial arteriolar diameter (A) and cortical prostanoid level (B) in control and indomethacin (Indo)-treated animals. 6-Keto-PGF1, 6-ketoprostaglandin F1. Values are means ± SE; n = 5 animals. * P < 0.05 compared with basal values (no phenylarsine oxide);  P < 0.05 compared with control.

Effects of phenylarsine oxide on pial arteriolar responses to arachidonic acid. Cyclooxygenase-mediated cerebral vascular responses were studied using topical application of arachidonic acid (20). Arachidonic acid (2-20 µM) caused dose-dependent dilation of pial arterioles (Fig. 2A) and increased cortical prostanoids (Fig. 2B). Phenylarsine oxide (10 µM) greatly potentiated pial arteriolar dilation and the induction of cortical prostanoid synthesis in response to arachidonic acid (Fig. 2). Indomethacin effectively inhibited the cyclooxygenase activity, as reflected by prostanoid production in response to arachidonic acid (Fig. 2B), and abrogated vascular responses to 10 µM phenylarsine oxide in combination with arachidonic acid (Fig. 2A). These data indicate that phenylarsine oxide stimulated cyclooxygenase-mediated vascular responses to arachidonic acid.

View larger version (29K): [in this window] [in a new window]   Fig. 2.   Effects of phenylarsine oxide (PAO, 10 µM) on cerebral vascular responses to arachidonic acid in control and indomethacin-treated animals. Values are means ± SE; n = 5. * P < 0.05 compared with basal values (no arachidonic acid);  P < 0.05 compared with control.

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Effects of sodium orthovanadate on pial arteriolar responses to arachidonic acid. Sodium orthovanadate (1 mM), topically applied to the cerebral surface, increased the cortical prostanoid level two- to threefold (Fig. 3B) but did not significantly alter the basal pial arteriolar diameter (70 ± 2 and 67 ± 7 µm in absence and presence of sodium orthovanadate, respecively, P > 0.05, n = 12). However, the dilator responses of pial arterioles to 2-20 µM arachidonic acid were greatly potentiated by 1 mM sodium orthovanadate (Fig. 3A). At all concentrations of arachidonic acid used (2-20 µM), vanadate increased cortical prostanoid synthesis in an indomethacin-sensitive manner (Fig. 3B), indicating cyclooxygenase activation. Indomethacin completely prevented vascular responses to sodium orthovanadate applied in combination with arachidonic acid (Fig. 3A) and effectively inhibited cortical prostanoid synthesis (Fig. 3B). These data demonstrate a remarkable similarity in the cerebral vascular effects of both PTP inhibitors, sodium orthovanadate and phenylarsine oxide, on vascular responses to arachidonic acid.

View larger version (27K): [in this window] [in a new window]   Fig. 3.   Effects of sodium orthovanadate (Van, 1 mM) on cerebral vascular responses to arachidonic acid in control and indomethacin-treated animals. Values are means ± SE; n = 5. * P < 0.05 compared with basal values (no sodium orthovanadate);  P < 0.05 compared with control.

Effects of PTK inhibitors on prostanoid production and arteriolar responses to arachidonic acid. Genistein (10-500 µM) caused dose-dependent vasodilation of pial arterioles; the maximal dilation was 75 ± 9% above the basal diameter; the EC₅₀ for genistein was ~150 µM (Fig. 4). However, genistein inhibited the basal production of cortical prostanoids by 30-40% (Fig. 4). Tyrphostin 47 (100 µM), another PTK inhibitor, also had a vasodilator effect on pial arterioles during basal conditions (dilation 25 ± 4%) and did not alter vasodilator responses to 2-20 µM arachidonic acid (Fig. 5A). Tyrphostin 47 decreased the basal cortical prostanoid level and effectively inhibited the prostanoid production from topical arachidonic acid (Fig. 5B). Therefore, vasodilator effects of the PTK inhibitors on cerebral circulation appear to involve mechanisms that are not related to the cyclooxygenase inhibition.

View larger version (20K): [in this window] [in a new window]   Fig. 4.   Dose-dependent effects of genistein on pial arteriolar diameter and cortical prostanoid level (as 6-keto-PGF1). Values are means ± SE; n = 4. * P < 0.05 compared with basal values (no genistein).

View larger version (18K): [in this window] [in a new window]   Fig. 5.   Effects of tyrphostin 47 (Tyr47, 100 µM) on cerebral vascular responses to arachidonic acid. Values are means ± SE; n = 4. * P < 0.05 compared with basal values (no arachidonic acid);  P < 0.05 compared with control.

Effects of PTP inhibitors on cerebral vascular responses to hypercapnia. In newborn pigs, cerebral vascular dilation in response to hypercapnia involves activation of the phospholipase A₂-cyclooxygenase pathway (17, 24). We investigated the effects of PTP inhibitors on hypercapnia-induced cerebral vascular responses. In agreement with our previous reports (17, 24), hypercapnia (70-80 mmHg arterial PCO₂, 80-90 mmHg arterial PO₂, pH 7.0-7.1) caused dilation of pial arterioles and increased cortical levels of 6-keto-PGF₁ and PGE₂ 1.5- to 2-fold. Phenylarsine oxide (20 µM) effectively increased cortical prostanoids but had no effect on the dilation of pial arterioles in response to hypercapnia (54 ± 4 and 42 ± 6% above normocapnia diameter in the absence and presence of inhibitor, respectively, P > 0.05). Similarly, sodium orthovanadate (1 mM) did not alter responses to hypercapnia (pial arteriolar dilation response to hypercapnia was 35 ± 5 and 30 ± 4% above the normocapnic values in the absence and presence of inhibitor, respectively). The effects of PTP inhibitors and hypercapnia on cortical prostanoids were simply additive (data not shown). Together these data indicate that PTP inhibitors and hypercapnia increase cortical prostanoids via different mechanisms.

Effects of L-NAME and L-NNA on vascular responses to PTP inhibitors. To investigate the possible contribution of NO to vasodilator responses to PTP inhibitors, we used L-NAME (30 mg/kg iv) in combination with topical L-NNA (10³ M) (24). Confirming our previous data (24), intravenous administration of L-NAME resulted in an immediate increase in mean arterial blood pressure (66 ± 6 and 90 ± 7 mmHg before and after L-NAME, respectively) and constriction of pial arterioles (10-15% of initial diameter). In animals treated with L-NAME + L-NNA, the vasodilator responses to 5-10 µM phenylarsine oxide were significantly decreased, whereas the response to 20 µM phenylarsine oxide was intact (Fig. 6). In animals treated with L-NAME + L-NNA supplemented with indomethacin, responses to 5-20 µM phenylarsine oxide were completely abolished (Fig. 6). After treatment with NO synthase inhibitors, 1 mM sodium orthovanadate had a significant constrictor effect on pial arterioles (15-20% of control diameter) that substantially added to the vasoconstriction caused by L-NAME (total constriction 25-30% of basal diameter; Fig. 7A).

View larger version (24K): [in this window] [in a new window]   Fig. 6.   Effects of indomethacin (10 mg/kg iv) and nitric oxide synthase inhibitors [N-nitro-L-arginine methyl ester (L-NAME), 30 mg/kg iv, coadministered with topical N-nitro-L-arginine (L-NNA), 1 mM] on cerebral vascular responses to phenylarsine oxide. Arteriolar diameter is expressed as percentage of control (no phenylarsine oxide). Values are means ± SE; n = 5. * P < 0.05 compared with basal values (no phenylarsine oxide);  P < 0.05 compared with control.

View larger version (26K): [in this window] [in a new window]   Fig. 7.   Effects of nitric oxide synthase inhibitors (L-NAME, 30 mg/kg iv, coadministered with topical L-NNA, 1 mM) on cerebral vascular responses to sodium orthovanadate and arachidonic acid. Values are means ± SE; n = 7. * P < 0.05 compared with basal values (C-1 and C-2, no sodium orthovanadate);  P < 0.05 compared with control (no arachidonic acid).

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View larger version (16K): [in this window] [in a new window]   Fig. 8.   Effects of nitric oxide synthase inhibitors (L-NAME, 30 mg/kg iv, coadministered with topical L-NNA, 1 mM) on induction of cortical cAMP level by topical arachidonic acid. Values are means ± SE; n = 9. * P < 0.05 compared with basal values (no arachidonic acid).

	DISCUSSION

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In the present study we investigated the role of protein tyrosine phosphorylation in the regulation of cerebral vascular tone in newborn pigs in vivo. Our data indicate that PTK/PTP inhibitors cause dilation of pial arterioles by acting via multiple mechanisms. The main novel finding in this study is that cyclooxygenase and NO synthase are among the important physiological targets for tyrosine phosphorylation in the cerebral circulation of newborn pigs.

We have shown recently that COX-2 constitutively expressed in endothelial and smooth muscle cells from cerebral microvessels of newborn pigs is posttranslationally regulated by tyrosine phosphorylation (23). The PTP inhibitors phenylarsine oxide and sodium orthovanadate rapidly stimulate the cyclooxygenase activity, whereas the PTK inhibitors genistein and tyrphostin 47 inhibit the enzyme in cerebral microvascular cells (23). COX-2 is a major constitutive isoform involved in prostanoid synthesis in the newborn pig cerebral microcirculation (23). Because endothelial cyclooxygenase greatly contributes to cerebral vascular tone in newborn pigs (17), we hypothesized that modulation of the cyclooxygenase activity by tyrosine phosphorylation has physiological implications in cerebral microcirculation. In the present study we detected the effects of PTK/PTP inhibitors on pial arteriolar diameter and cortical production of dilator prostanoids, prostacyclin (as 6-keto-PGF₁) and PGE₂, under basal conditions and on activation of the cyclooxygenase pathway by topically applied arachidonic acid.

Genistein and tyrphostin 47 are potent dilators of pial arterioles in newborn pigs in vivo. Although the PTK inhibitors rapidly inhibit the cyclooxygenase activity in the cerebral microcirculation in vitro (23) and in vivo (present data), the cerebral vascular effects of PTK inhibitors appear to be prostanoid independent: 1) pial arteriolar dilation in response to genistein is not affected by indomethacin (26), and 2) tyrphostin 47 does not alter the cyclooxygenase-mediated response to arachidonic acid. PTK inhibitors cause relaxation in vascular smooth muscle preparations (5, 16, 32) as well as in endothelium-intact vessels (29, 31). Therefore, PTK activity is necessary for maintaining basal cerebral vascular tone in a variety of vascular beds, including cerebral microcirculation of newborn pigs. The strong vasodilator response to PTK inhibitors, regardless of the presence of vascular endothelium, indicates that tyrosine phosphorylation is intimately involved in fundamental processes that regulate smooth muscle contractility. Indeed, several key proteins in vascular smooth muscle (including proteins responsible for Ca²⁺ mobilization and myosin phosphorylation) are regulated by tyrosine phosphorylation (2, 4, 11, 16, 33). Inhibition of PTK in vascular smooth muscle results in vasorelaxation (2, 4, 13). Therefore, we postulate that, in the newborn pig cerebral circulation in vivo, PTK-mediated pathways contribute mainly to the regulation of endothelium-independent smooth muscle responses.

Data from isolated vascular segments suggest that the vasoactive effects of PTP inhibitors are endothelium dependent. In vascular smooth muscle preparations, sodium orthovanadate and phenylarsine oxide are potent vasoconstrictors (3, 5, 16, 28), consistent with dilator effects of PTK inhibitors. However, in endothelium-intact vessels, PTP inhibitors cause vasodilation (7, 21, 22), indicating that tyrosine phosphorylation may affect vascular smooth muscle via an additional potent mechanism that is endothelium dependent. Therefore, it appears that PTP inhibitors could be used to differentiate between endothelium-mediated and endothelium-independent vascular responses regulated by protein tyrosine phosphorylation in vivo. One would expect that PTP inhibitors placed directly on the cerebral surface, thus first contacting the smooth muscle side of pial arterioles, would cause vasoconstriction. However, topical phenylarsine oxide is a potent vasodilator in the cerebral microcirculation of newborn pigs, indicating possible functional recruitment of vascular endothelium. Dilation in response to phenylarsine oxide was inhibited by indomethacin. Sodium orthovanadate is a strong vasoconstrictor in isolated vascular smooth muscle (5, 16, 28). In contrast, in newborn pigs in vivo, sodium orthovanadate did not alter basal pial arteriolar diameter. This "zero effect" may indirectly indicate that the effect of PTP inhibitor is more pronounced on endothelium in the cerebral microvasculature. Phenylarsine oxide and sodium orthovanadate stimulated the basal production of cortical prostanoids in vivo. This stimulation correlates with effects of the PTP inhibitors on cyclooxygenase activity in cultured cerebral endothelial microvascular cells from newborn pigs (23).

To directly assess whether modification of cyclooxygenase by tyrosine phosphorylation contributes to cerebral vascular tone in newborn pigs, we determined the effects of the PTP inhibitors on responses to arachidonic acid. Topical arachidonic acid, the cyclooxygenase substrate, increases cortical prostanoids and causes indomethacin-dependent dilation of pial arterioles. These results indicate a functional predominance of dilator prostanoid-mediated responses in the cerebral circulation of newborns. Signaling mechanisms for prostacyclin- and PGE₂-mediated responses in cerebral circulation involve cAMP as a second messenger (25). In agreement with these data, we found that arachidonic acid increases the cAMP, but not the cGMP, level in cortical periarachnoid CSF. Phenylarsine oxide and sodium orthovanadate greatly potentiated cerebral vascular responses to arachidonic acid (dilation of cerebral arterioles and increase in dilator prostanoids). In contrast, the response to isoproterenol (endothelium- and prostanoid-independent vasodilator of pial arterioles) was not altered by phenylarsine oxide. Therefore, it appears that the PTP inhibitors selectively stimulate cyclooxygenase-mediated cerebral vascular responses in the newborn pig cerebral circulation.

Hypercapnia is an important physiological stimulus that increases cerebral blood flow by regulatory mechanisms that may be species and age specific (17, 34). In newborn pigs, endothelium-dependent vasodilation of pial arterioles in response to hypercapnia is accompanied by increased production of cortical dilator prostanoids and is abrogated by indomethacin (18, 24). In cultured cerebral vascular endothelial cells, hypercapnia also stimulates the production of dilator prostanoids (14). However, the key enzyme of prostanoid synthesis (phospholipase A₂ or cyclooxygenase) that is activated by hypercapnia remains unknown. We used PTP inhibitors to stimulate cyclooxygenase-mediated pathways in newborn pig cerebral circulation and to evaluate the contribution of cyclooxygenase to vascular responses to hypercapnia. Phenylarsine oxide and sodium orthovanadate did not alter responses of pial arterioles to hypercapnia. The effects of the PTP inhibitors and hypercapnia on cortical prostanoids were simply additive, which may reflect independent mechanisms for the stimulation of prostanoid synthesis by these agents. These data indicate that activation of cyclooxygenase is not a critical event in cerebral vascular responsiveness to hypercapnia. This is consistent with the concept of the permissive role of dilator prostanoids in cerebral vasodilation responses to hypercapnia (18).

Several reports in isolated vessels provide evidence that endothelium-dependent vasodilator effects of PTP inhibitors may be mediated by NO (7, 22). This concept is supported by a recent finding that NO synthase activity is posttranslationally regulated by tyrosine phosphorylation (8). Although direct involvement of NO in the regulation of physiologically significant cerebral vascular responses in piglets remains elusive (17, 24, 34), we addressed the possibility that NO may contribute to the vascular effects of PTP inhibitors. During basal conditions, the NO synthase inhibitors L-NAME and L-NNA partially inhibited the dilation of pial arterioles to 5-10 µM phenylarsine oxide, with less or no effect on responses to higher doses of the PTP inhibitor. When L-arginine analogs were supplemented with indomethacin, complete inhibition of dilator responses of pial arterioles to 5-20 µM phenylarsine oxide was observed. After treatment with NO synthase inhibitors, sodium orthovanadate had a vasoconstrictor effect on basal pial arteriolar diameter. We explored the possibility that L-arginine analogs alter vascular responses to PTP inhibitors indirectly by interacting with prostanoid synthesis. However, we found no interactions between NO- and dilator prostanoid-producing systems in newborn pig cerebral circulation in vivo. This conclusion is based on our observations that NO synthase inhibitors 1) did not alter the prostanoid level during basal conditions or on stimulation of cyclooxygenase activity by arachidonic acid, 2) did not affect pial arteriolar dilation to arachidonic acid, 3) did not affect the ability of sodium orthovanadate and phenylarsine oxide to stimulate prostanoid synthesis, and 4) did not alter potentiation of cerebral vascular responses to arachidonic acid by PTP inhibitors. These data indicate that NO synthase inhibitors do not alter prostanoid synthesis and vascular dilation in response to arachidonic acid and that they do not interfere with the stimulation of cyclooxygenase-mediated cerebral vascular responses by PTP inhibitors. In newborn pigs, possible integration between dilator prostanoids and NO may occur at the level of the signaling mechanisms, which involve cAMP and cGMP, respectively (1). We found that dilator responses of cerebral arterioles to topical arachidonic acid are accompanied by increased cortical levels of cAMP, but not cGMP, thus indicating a selective activation of cAMP-mediated pathways. NO synthase inhibitors did not alter the arachidonic acid-induced increase in cAMP. Therefore, the possibility that NO alters signaling transduction mechanisms for dilator prostanoids under our experimental conditions is unlikely. Together these data indicate that L-arginine analogs alter cerebral vascular responses to PTP inhibitors by a mechanism that is independent of cyclooxygenase-mediated pathways. Dilator prostanoids and NO contribute to the endothelium-dependent vascular effects of PTP inhibitors in newborn pig cerebral circulation. Endothelial NO synthase also might be a physiological target for tyrosine phosphorylation in the cerebral circulation of newborns. It appears that, during basal conditions, phenylarsine oxide more effectively stimulates cyclooxygenase-mediated pathways, whereas sodium orthovanadate affects NO-mediated pathways. However, when cyclooxygenase pathways are activated by arachidonic acid, both PTP inhibitors effectively potentiate dilator prostanoid-mediated cerebral vascular responses. The mechanism of selective effects of sodium orthovanadate and phenylarsine oxide on vascular tone after NO synthase inhibition may be associated with variations in a spectrum of PTPs affected.

In conclusion, protein tyrosine phosphorylation contributes to the regulation of cerebral vascular tone in vivo. Cyclooxygenase and NO synthase are important physiological targets for tyrosine phosphorylation in the cerebral circulation of newborn pigs.