Acute modulation of endothelial Akt/PKB activity alters nitric oxide-dependent vasomotor activity in vivo

The serine/threonine protein kinase Akt (protein kinase B) phosphorylates endothelial cell nitric oxide synthase (eNOS) and enhances its ability to generate nitric oxide (NO). Because NO is an important regulator of vasomotor tone, we investigated whether Akt can regulate endothelium-dependent vasomotion in vivo using a rabbit femoral artery model of gene transfer. The endothelium of isolated femoral arteries was infected with replication-defective adenoviral constructs expressing beta-galactosidase, constitutively-active Akt (myr-Akt), or dominant-negative Akt (dn-Akt). Femoral arteries transduced with myr-Akt showed a significant increase in resting diameter and blood flow, as assessed by angiography and Doppler flow measurements, respectively. L-NAME, an eNOS inhibitor, blocked myr-Akt-mediated vasodilatation. In contrast, endothelium-dependent vasodilatation in response to acetylcholine was attenuated in vessels transduced with dn-Akt, although these vessels showed normal responses to nitroglycerin, an endothelium-independent vasodilator. Similarly, relaxation of murine aorta ex vivo in response to acetylcholine, but not nitroglycerin, was inhibited by transduction of dn-Akt to the endothelium. These data provide evidence that Akt functions as key regulator of vasomotor tone in vivo.

Optic nerve sheath fenestration for a reversible optic neuropathy in radiation oncology

To the authors' knowledge, there is a paucity of published accounts of management of radiation-induced optic neuropathy (RION) by optic nerve sheath fenestration (ONSF) in the conventional medical literature. With higher doses of radiation being given by using conformal techniques, more radiation-induced optic neuritis and neuropathy will be identified. We report here the successful use of ONSF to restore vision to three consecutive patients with pending anterior RION, and the importance of early identification and intervention in these potentially reversible cases.

Temporal events underlying arterial remodeling after chronic flow reduction in mice: correlation of structural changes with a deficit in basal nitric oxide synthesis

To define the cellular events of vascular remodeling in mice, we measured blood flow and analyzed the morphology of remodeled vessels at defined points after a flow-reducing remodeling stimulus for 3, 7, 14, and 35 days. Acute ligation of the left external carotid artery reduced blood flow in the left common carotid artery (LC) compared with sham and contralateral right common carotid arteries (RCs). In morphometric analyses, the decrease in diameter in LCs was reversible by vasodilator perfusion 3 days after ligation, whereas ligation for 7 days or greater resulted in a permanent diameter reduction. Coincident with structural remodeling at day 7 was an increase in cell death in remodeled LCs. Functionally, rings from remodeled LCs contracted to prostaglandin F(2alpha) and relaxed to acetylcholine in a manner identical to that of control arteries. However, remodeled LCs were hypersensitive to the nitrovasodilator sodium nitroprusside (at day 7) and exhibited a marked reduction in basal NO synthesis at 7 and 14 days after ligation. The impairment of endothelial NO synthase function was likely due to post-translational mechanisms, given that endothelial NO synthase mRNA and protein levels did not change in remodeled LCs. These data define the ontogeny of flow-triggered luminal remodeling in adult mice and suggest that endothelial dysfunction occurs during reorganization of the vessel wall.

Vascular endothelial growth factor-stimulated actin reorganization and migration of endothelial cells is regulated via the serine/threonine kinase Akt

Vascular endothelial growth factor (VEGF) induces endothelial cell proliferation, migration, and actin reorganization, all necessary components of an angiogenic response. However, the distinct signal transduction mechanisms leading to each angiogenic phenotype are not known. In this study, we examined the ability of VEGF to stimulate cell migration and actin rearrangement in microvascular endothelial cells infected with adenoviruses encoding beta-galactosidase (beta-gal), activation-deficient Akt (AA-Akt), or constitutively active Akt (myr-Akt). VEGF increased cell migration in cells transduced with beta-gal, whereas AA-Akt blocked VEGF-induced cell locomotion. Interestingly, myr-Akt transduction of bovine lung microvascular endothelial cells stimulated cytokinesis in the absence of VEGF, suggesting that constitutively active Akt, per se, can initiate the process of cell migration. Treatment of beta-gal-infected endothelial cells with an inhibitor of NO synthesis blocked VEGF-induced migration but did not influence migration initiated by myr-Akt. In addition, VEGF stimulated remodeling of the actin cytoskeleton into stress fibers, a response abrogated by infection with dominant-negative Akt, whereas transduction with myr-Akt alone caused profound reorganization of F-actin. Collectively, these data demonstrate that Akt is critically involved in endothelial cell signal transduction mechanisms leading to migration and that the Akt/endothelial NO synthase pathway is necessary for VEGF-stimulated cell migration.

Angiopoietin-1 inhibits endothelial cell apoptosis via the Akt/survivin pathway

A productive angiogenic response must couple to the survival machinery of endothelial cells to preserve the integrity of newly formed vessels. Angiopoietin-1 (Ang-1) is an endothelium-specific ligand essential for embryonic vascular stabilization, branching morphogenesis, and post-natal angiogenesis, but its contribution to endothelial cell survival has not been completely elucidated. Here we show that Ang-1 acting via the Tie 2 receptor induces phosphorylation of the survival serine-threonine kinase, Akt (or protein kinase B). This is associated with up-regulation of the apoptosis inhibitor, survivin, in endothelial cells and protection of endothelium from death-inducing stimuli. Moreover, dominant negative survivin negates the ability of Ang-1 to protect cells from undergoing apoptosis. The activation of anti-apoptotic pathways mediated by Akt and survivin in endothelial cells may contribute to Ang-1 stabilization of vascular structures during angiogenesis, in vivo.

Enhanced electron flux and reduced calmodulin dissociation may explain "calcium-independent" eNOS activation by phosphorylation

Bovine endothelial nitric oxide synthase (eNOS) is phosphorylated directly by the protein kinase Akt at serine 1179. Mutation of this residue to the negatively charged aspartate (S1179D eNOS) increases nitric oxide (NO) production constitutively, in the absence of agonist challenge. Here, we examine the potential mechanism of how aspartate at 1179 increases eNOS activity using purified proteins. Examination of NO production and cytochrome c reduction resulted in no substantial changes in the K(m)/EC(50) for L-arginine, calmodulin, and calcium, whereas there was a 2-fold increase in the rate of NO production for S1179D and a 2-4-fold increase in reductase activity (based on cytochrome c reduction). The observed increase in activity for both assays of NOS function indicates that a faster rate of electron flux through the reductase domain is likely the rate-limiting step in NO formation from eNOS. In addition, S1179D eNOS did show an increased resistance to inactivation by EGTA compared with wild type eNOS. These results suggest that a negative charge imposed at serine 1179, either by phosphorylation or by replacement with aspartate, increases eNOS catalytic activity by increasing electron flux at the reductase domain and by reducing calmodulin dissociation from activated eNOS when calcium levels are low.

NCIC-CTG phase II study of gemcitabine in patients with malignant glioma (IND.94)

PURPOSE

We conducted a phase II multicentre study of gemcitabine in patients with anaplastic astrocytoma and glioblastoma multiforme at first relapse.

PATIENTS AND METHODS

Patients with anaplastic astrocytoma or glioblastoma multiforme receiving a stable dose of steroids and ECOG performance status < or = 3 were eligible for this study at the time of first relapse. One adjuvant chemotherapy regimen was permissible. Patients received gemcitabine 1000 mg/m2 i.v. weekly x 3, repeated on a four-weekly cycle.

RESULTS

Of 20 patients enrolled, 15 were evaluable for response, 19 for non-hematological toxicity and 18 for hematological toxicity. Seven patients had anaplastic astrocytoma (AA) and twelve glioblastoma multiforme (GBM). Age ranged from 28-71 years (median 50). Fifteen patients discontinued therapy due to disease progression. The median number of cycles administered was 1 (range 1-11); only two patients received more than three cycles. Hematologic toxicity was acceptable and no grade 4 toxicity was seen. One patient developed Pneumocystis pneumonia and eventual pulmonary embolism; one died of gastric hemorrhage related to steroid therapy. No objective responses were seen. Nine patients had stable disease (median duration 2.7 months, range 0.9-11.2).

CONCLUSIONS

Gemcitabine given in this dose and schedule seems well tolerated but is not active in patients with recurrent high-grade gliomas.

Quantification of eNOS mRNA in the canine cardiac vasculature by competitive PCR

The goal of the present study was to develop a competitive PCR assay to measure changes in the expression of endothelial nitric oxide synthase (eNOS) mRNA levels throughout the canine vascular tree. A partial sequence of canine eNOS cDNA (1.86 kb), inducible NOS (1.95 kb), and neuronal NOS (1.16 kb) was cultured from canine aortic endothelial cells, LPS-treated canine splenic vein endothelial cells, and from canine left ventricle, respectively. Competitor eNOS cDNA (eNOS-C) was constructed via recombinant PCR. Thus, with the use of a standard curve competitive PCR with eNOS-C, the amount of eNOS mRNA in 500 ng of total RNA was greatest in the circumflex > right coronary artery > left anterior descending coronary artery > aorta. The isolation of coronary microvessels from the left ventricle was associated with an enrichment of endothelial cell markers such as eNOS, von Willebrand factor, and caveolin-1, an observation supported by the detection of up to 15-fold higher levels of eNOS mRNA in coronary microvessels relative to the larger arteries. The ability to quantify changes in eNOS mRNA levels throughout the canine vasculature should provide greater insight into the molecular mechanisms of how this gene is regulated in physiological and pathophysiological states.

A phase I trial of 1,3-bis(2-chloroethyl)-1-nitrosourea plus temozolomide: a North American Brain Tumor Consortium study

The North American Brain Tumor Consortium conducted a phase I trial of the combination 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and temozolomide. Eligibility included a patient with a cancer type that was considered refractory to standard therapy. Prior nitrosourea treatments were not permitted. There were parallel dose escalations in two treatment schedules. Forty-five patients were enrolled during an 18-month period. The maximum tolerated doses (MTDs) when temozolomide followed BCNU (Arm A) were temozolomide at 550 mg/m2/p.o. and BCNU at 150 mg/m2/i.v.), whereas the MTD when temozolomide preceded BCNU (Arm B) was temozolomide at 400 mg/m2/p.o. and BCNU at 100 mg/m2/i.v. Toxicity was predominantly hematologic, although there were three instances of pulmonary toxicity, which in one case could have represented potentiation of nitrosourea-induced pulmonary fibrosis. The half-life of temozolomide was 1.86 (+/-0.31) h. There was a moderate relationship between dose and peak concentration and a strong relationship between dose and plasma concentration time curve. Pharmacokinetic parameters of temozolomide were unaffected by the treatment schedule, so the difference in MTD between the schedules is likely due to a biologic rather than a pharmacokinetic sequence interaction. There were 9 partial responses among 43 patients evaluable for response, including 5 of 25 with a histologic diagnosis of glioblastoma. The recommended dose and schedule for phase II trials of this regimen are BCNU 150 mg/m2/i.v. followed in 2 h by temozolomide 550 mg/m2/p.o. repeated every 6 weeks. We are also recommending screening and periodic pulmonary function testing during treatment to assess the possible potentiation of nitrosourea-induced pulmonary fibrosis.

Specificity of starch synthase isoforms from potato

In higher plants several isoforms of starch synthase contribute to the extension of glucan chains in the synthesis of starch. Different isoforms are responsible for the synthesis of essentially linear amylose chains and branched, amylopectin chains. The activity of granule-bound starch synthase I from potato has been compared with that of starch synthase II from potato following expression of both isoforms in Escherichia coli. Significant differences in their activities are apparent which may be important in determining their specificities in vivo. These differences include affinities for ADPglucose and glucan substrates, activation by amylopectin, response to citrate, thermosensitivity and the processivity of glucan chain extension. To define regions of the isoforms determining these characteristic traits, chimeric proteins have been produced by expression in E. coli. These experiments reveal that the C-terminal region of granule-bound starch synthase I confers most of the specific properties of this isoform, except its processive elongation of glucan chains. This region of granule-bound starch synthase I is distinct from the C-terminal region of other starch synthases. The specific properties it confers may be important in defining the specificity of granule-bound starch synthase I in producing amylose in vivo.

Renal cytochrome P450 omega-hydroxylase and epoxygenase activity are differentially modified by nitric oxide and sodium chloride

Renal function is perturbed by inhibition of nitric oxide synthase (NOS). To probe the basis of this effect, we characterized the effects of nitric oxide (NO), a known suppressor of cytochrome P450 (CYP) enzymes, on metabolism of arachidonic acid (AA), the expression of omega-hydroxylase, and the efflux of 20-hydroxyeicosatetraenoic acid (20-HETE) from the isolated kidney. The capacity to convert [(14)C]AA to HETEs and epoxides (EETs) was greater in cortical microsomes than in medullary microsomes. Sodium nitroprusside (10-100 microM), an NO donor, inhibited renal microsomal conversion of [(14)C]AA to HETEs and EETs in a dose-dependent manner. 8-bromo cGMP (100 microM), the cell-permeable analogue of cGMP, did not affect conversion of [(14)C]AA. Inhibition of NOS with N(omega)-nitro-L-arginine-methyl ester (L-NAME) significantly increased conversion of [(14)C]AA to HETE and greatly increased the expression of omega-hydroxylase protein, but this treatment had only a modest effect on epoxygenase activity. L-NAME induced a 4-fold increase in renal efflux of 20-HETE, as did L-nitroarginine. Oral treatment with 2% sodium chloride (NaCl) for 7 days increased renal epoxygenase activity, both in the cortex and the medulla. In contrast, cortical omega-hydroxylase activity was reduced by treatment with 2% NaCl. Coadministration of L-NAME and 2% NaCl decreased conversion of [(14)C]AA to HETEs without affecting epoxygenase activity. Thus, inhibition of NOS increased omega-hydroxylase activity, CYP4A expression, and renal efflux of 20-HETE, whereas 2% NaCl stimulated epoxygenase activity.

Which glioblastoma multiforme patient will become a long-term survivor? A population-based study

In this clinical and histopathological study, the frequency of long-term glioblastoma multiforme (GBM) survivors (LTGBMSs) was determined in a population-based study. The Alberta Cancer Registry was used to identify all patients diagnosed with GBM in Alberta between January 1, 1975, and December 31, 1991. Patient charts were reviewed and histology reexamined. LTGBMSs were defined as GBM patients surviving 3 years after diagnosis. Each LTGBMS was compared with 3 age-, sex-, and year of diagnosis-matched controls, and patient/treatment or tumor characteristics that predicted long-term survival were determined. There were 689 GBMs diagnosed in the study period; 15 (2.2%) of these patients survived 3 years. LTGBMSs (average age, 43.5 +/- 3.3 years) were significantly younger when compared with all GBM patients (average age, 53.0 +/- 0.56 years). LTGBMSs had a higher Karnofsky Performance Status score at diagnosis compared with controls. LTGBMSs were much more likely to have had a gross total resection and adjuvant chemotherapy than control GBM patients. Tumors from LTGBMSs tended to have fewer mitoses and a significantly lower Ki-67 cellular proliferation index compared with controls. Radiation-induced dementia was common and disabling in LTG-BMSs. In conclusion, conventionally treated GBM patients in an unselected population have a very small chance of long-term survival. The use of aggressive surgical resection and adjuvant chemotherapy may make long-term survival more likely in GBM patients if their performance status is high at diagnosis.

Regulation of endothelium-derived nitric oxide production by the protein kinase Akt

Endothelial nitric oxide synthase (eNOS) is the nitric oxide synthase isoform responsible for maintaining systemic blood pressure, vascular remodelling and angiogenesis. eNOS is phosphorylated in response to various forms of cellular stimulation, but the role of phosphorylation in the regulation of nitric oxide (NO) production and the kinase(s) responsible are not known. Here we show that the serine/threonine protein kinase Akt (protein kinase B) can directly phosphorylate eNOS on serine 1179 and activate the enzyme, leading to NO production, whereas mutant eNOS (S1179A) is resistant to phosphorylation and activation by Akt. Moreover, using adenovirus-mediated gene transfer, activated Akt increases basal NO release from endothelial cells, and activation-deficient Akt attenuates NO production stimulated by vascular endothelial growth factor. Thus, eNOS is a newly described Akt substrate linking signal transduction by Akt to the release of the gaseous second messenger NO.

Pharmacological evaluation of an epoxide as the putative hyperpolarizing factor mediating the nitric oxide-independent vasodilator effect of bradykinin in the rat heart

A cytochrome P450-derived metabolite of arachidonic acid, namely an epoxyeicosatrienoic acid (EET), has many of the properties of a hyperpolarizing factor that mediates endothelium-dependent, nitric oxide-independent vasodilation. As there are four EET regioisomers, we used pharmacological criteria, based on previous observations with bradykinin (BK), to evaluate which, if any, of the EETs could be considered a potential mediator of vasodilator responses to BK in the rat isolated heart treated with indomethacin and nitroarginine to eliminate prostaglandin and nitric oxide components of the response. Nifedipine, used as a probe for dilator mechanisms dependent on closure of voltage-dependent Ca++ channels, almost abolished the vasodilator effect of cromakalim and attenuated those of BK and 5,6 EET. The vasodilator effects of the other EETs were not reduced and were excluded from consideration as mediators of BK-induced vasodilation. The vasodilator effect of 5,6 EET, as with that of BK, was markedly reduced by charybdotoxin but not iberiotoxin, suggesting the contribution of a similar type K+ channel to the vascular response to both agents. As expected for a putative endothelium- and cytochrome P450-derived mediator, the coronary vasodilator effect of 5,6 EET was not affected by either removal of the endothelium or inhibition of cytochrome P450 with clotrimazole, interventions that virtually abolished the vasodilator activity of BK. Thus, of the four EET regioisomers, 5,6 EET is the most likely mediator of the vasodilator effect of BK in the isolated heart under these experimental conditions.

Evidence against anandamide as the hyperpolarizing factor mediating the nitric oxide-independent coronary vasodilator effect of bradykinin in the rat

The mediator of nitric oxide-(NO) independent vasodilation attributed to endothelium-derived hyperpolarizing factor remains unidentified although there is evidence for a cytochrome P450-derived eicosanoid. Anandamide, the ethanolamide of arachidonic acid and an endogenous ligand for cannabinoid receptors, was proposed as an endothelium-derived hyperpolarizing factor-mediating mesenteric vasodilation to acetylcholine and the hypotensive effect of bradykinin. Using pharmacological interventions that attenuate responses to bradykinin, we examined the possibility of anandamide as a mediator of the NO-independent vasodilator effect of bradykinin in the rat perfused heart by determining responses to anandamide and arachidonic acid. Hearts were treated with indomethacin to exclude prostaglandins and nitroarginine to inhibit NO synthesis and elevate perfusion pressure. The cannabinoid receptor antagonist, SR 141716A (2 microM), reduced dose-dependent vasodilator responses to anandamide (1-10 microgram) but was without effect on responses to AA (1-10 microgram), bradykinin (10-1000 ng) or cromakalim (1-10 microgram). Inhibition of voltage-dependent Ca++ channels with nifedipine (5 nM) attenuated vasodilation to anandamide and arachidonic acid whereas inhibition of Ca++-activated K+ channels with charybdotoxin (10 nM) reduced responses to arachidonic acid but had no effect on vasodilation induced by anandamide. Inhibition of cytochrome P450 with clotrimazole (1 microM) greatly reduced vasodilator responses to bradykinin with less effect on those to anandamide. Finally, the time course of the coronary vasodilator responses to anandamide and bradykinin were dissimilar. These results argue against a role of anandamide in the vasodilator effect of bradykinin in the rat heart.

A method for the determination of 5,6-EET using the lactone as an intermediate in the formation of the diol

The 5,6 epoxyeicosatrienoic acid (5,6-EET) exhibits a range of biological activities but the functional significance of this labile eicosanoid is unknown due, in part, to difficulties of quantitation in biological samples. We have developed a sensitive and specific method to measure 5,6-EET utilizing its selective capacity to form a lactone. The initial conversion of 5,6-EET and 5,6-dihydroxyeicosatrienoic acid (5,6-DHT) to 5,6-delta-lactone is followed by selective purification using reverse phase high performance liquid chromatography (HPLC), reconversion to 5,6-DHT and quantitation by gas chromatography-mass spectrometry (GCMS). In oxygenated Krebs' buffer, 5,6-EET degrades to 5,6-delta-lactone and 5,6-DHT with a t1/2 approximately 8 min. In the presence of camphorsulfonic acid, 5,6-EET and 5,6-DHT convert to a single HPLC peak (lambda = 205) comigrating with 5,6-delta-lactone. Incubation of 5,6-delta-lactone with triethylamine resulted in a single HPLC peak with the retention time of 5,6-DHT. In the perfusate from the isolated kidney, release of 5,6-EET (20 +/- 5 pg/ml), measured indirectly via conversion to 5,6-DHT, was approx. 6-fold less than that reported for prostaglandin E2 (PGE2) and 20-HETE. The coronary perfusate concentration of 5,6 EET was 9 +/- 2 pg/ml. 5,6-EET recovered from renal and coronary perfusates was increased 2-fold to 45.5 +/- 5.5 pg/ml and 21.6 +/- 6.3 pg/ml, respectively, by arachidonic acid.

Long-term glioblastoma multiforme survivors: a population-based study

BACKGROUND

Long-term glioblastoma multiforme survivors (LTGBMS) are uncommon. The frequency which these occur in an unselected population and factors which produce these unusually long survivors are unknown.

OBJECTIVES

To determine in a population-based study 1) the frequency of LTGBMS in a population and 2) identify which patient, treatment or tumor characteristics would predict which glioblastoma (GBM) patient would become a LTGBMS.

METHODS

The Alberta Cancer Registry was used to identify all patients diagnosed with GBM in southern Alberta between 1/1/75-12/31/91. Patient charts were reviewed and histology re-examined by a blinded neuropathologist. LTGBMS were defined as GBM patients surviving > or = 3 years after diagnosis. Each LTGBMS was compared to three age-, gender-, and year of diagnosis-matched controls to compare patient, treatment, and tumor factors to GBM patients without long-term survival.

RESULTS

There were 279 GBMs diagnosed in the study period. Five (1.8%) survived > or = three years (range, 3.2-15.8 years). Seven additional long-term survivors, who carried a diagnosis of GBM, were excluded after neuropathologic review; the most common revised diagnosis was malignant oligodendroglioma. LTGBMS (avg. age = 45 years) were significantly younger when compared to all GBM patients (avg. age = 59 years, p = 0.0001) diagnosed in the study period. LTGBMS had a higher KPS at diagnosis (p = 0.001) compared to controls. Tumors from LTGBMS tended to have fewer mitoses and a lower Ki-67 cellular proliferative index compared to controls. Radiation-induced dementia was common and disabling in LTGBMS.

CONCLUSIONS

These data highlight the dismal prognosis for GBM patients who have both a short median survival and very small chance (1.8%) of long-term survival. The LTGBMS were younger, had a higher performance status, and their tumors tended to proliferate less rapidly than control GBM patients. When long-term survival does occur it is often accompanied by severe treatment-induced dementia.

Phase I study of paclitaxel in patients with recurrent malignant glioma: a North American Brain Tumor Consortium report

PURPOSE

To determine the maximum-tolerated dose (MTD) of paclitaxel administered as a 3-hour infusion in patients with recurrent malignant glioma.

PATIENTS AND METHODS

Patients were stratified by starting dose of paclitaxel and concurrent anticonvulsant (AC) use and were treated in cohorts of three patients. The starting dose was 240 mg/m2 administered intravenously with escalations of 30 mg/m2 until the MTD was established. Pharmacokinetic data were obtained for each patient for the first infusion. Tumor response was assessed at 6-week intervals and treatment was continued until documented tumor progression, unacceptable toxicity, or a total of 12 paclitaxel infusions.

RESULTS

From April 1995 to December 1996, 34 patients were treated; 27 patients in the AC group and seven patients in the non-AC group. The MTD for patients who received ACs was established at 360 mg/m2 and the dose-limiting toxicity (DLT) was central neurotoxicity, characterized as transient encephalopathy and seizures. In contrast, the MTD for patients who did not receive ACs was 240 mg/m2, and myelosuppression, gastrointestinal toxicity, and fatigue were the DLTs. Pharmacokinetic data confirmed that the plasma drug levels and clearance rates were similar for patients in both groups at the respective dose levels that produced DLTs.

CONCLUSION

The pharmacokinetics of paclitaxel are altered by ACs, and significantly larger doses of the drug can be administered to patients with brain tumors on AC therapy. The toxicity profile is different for patients on AC therapy treated at these higher doses. A phase II study has been initiated that uses a dose of 330 mg/m2 for patients on AC therapy and 210 mg/m2 for patients not on AC therapy.

NO-independent vasodilation to acetylcholine in the rat isolated kidney utilizes a charybdotoxin-sensitive, intermediate-conductance Ca(++)-activated K+ channel

The role of K+ channels in the nitric oxide-independent renal vasodilator effect of acetylcholine (Ach) was examined to address the hypothesis that the mechanism underlying this response was different from that of bradykinin, because an earlier study indicated the possibility of different mediators. We used the rat isolated, perfused kidney that was constricted with phenylephrine and treated with nitroarginine and indomethacin to inhibit nitric oxide synthase and cyclooxygenase, respectively. The nonspecific K+ channel inhibitors, procaine and tetraethylammonium (TEA), reduced vasodilator responses to Ach and cromakalim, but not those to nitroprusside. Glibenclamide, an inhibitor of ATP-sensitive K+ channels, reduced vasodilator responses to cromakalim but did not affect those to Ach or nitroprusside. Charybdotoxin, an inhibitor of Ca(++)-activated K+ channels, reduced vasodilator responses to Ach without affecting those to cromakalim or nitroprusside. Iberiotoxin and apamin, inhibitors of large- and small-conductance Ca(++)-activated K+ channels, respectively, did not reduce vasodilation induced by Ach, cromakalim or nitroprusside. The inhibitor of cytochrome P450, clotrimazole, reduced the renal vasodilator effects of Ach and bradykinin but not those of nitroprusside or SCA 40, an agonist for Ca(++)-activated K+ channels. These results suggest that in the rat kidney, Ach, like bradykinin, utilizes a charybdotoxin-sensitive Ca(++)-activated K+ channel of intermediate conductance to elicit vasodilation and that this effect may be dependent on cytochrome P450 activity.

Hyperpolarizing factors

There is now overwhelming evidence for factors, other than nitric oxide (NO), that mediate endothelium-dependent vasodilation by hyperpolarizing the underlying smooth muscle via activation of Ca2+-activated K+ channels. Although the identity of endothelium-derived hyperpolarizing factor (EDHF) remains to be established, cytochrome P450 (CYP)-dependent metabolites of arachidonic acid (AA), namely, the epoxides, fulfill several of the criteria required for consideration as putative mediators of endothelium-dependent hyperpolarization. They are produced by the endothelium, released in response to vasoactive hormones, and elicit vasorelaxation via stimulation of Ca2+-activated K+ channels. Our studies in the rat indicate that, of the epoxides, 5,6-epoxyeicosatrienoic acid (5,6-EET) is the most likely mediator of NO-independent, but CYP-dependent coronary vasodilation in response to bradykinin. Studies in the rat kidney, however, support the existence of additional EDHFs as acetylcholine also exhibits NO-independent vasodilation that is unaffected by CYP inhibitors in concentrations that attenuate responses to bradykinin. In some blood vessels, NO may tonically suppress the expression of CYP-dependent EDHF. In the event of impaired NO synthesis, therefore, a CYP-dependent vasodilator mechanism may serve as a backup to a primary NO-dependent mechanism, although they may act in concert. In other vessels, particularly microvessels, an EDHF may constitute the major vasodilator mechanism for hormones and other physiological stimuli. EDHFs appear to be important regulators of vascular tone; alterations in this system can be demonstrated in hypertension and diabetes, conditions associated with altered endothelium-dependent vasodilator responsiveness.

Analysis of eicosanoid mediation of the renal functional effects of hyperchloremia

Depression of GFR and antinatriuresis in response to high chloride has been linked to a cyclooxygenase (COX)-dependent mechanism involving thromboxane A2 (TxA2) and prostaglandin endoperoxide (PGH2), because inhibition of COX prevented the fall in GFR and antinatriuresis produced by hyperchloremia. However, hyperchloremia did not increase, but unexpectedly decreased, renal prostaglandin and TxA2 efflux (Yin et al., 1995). To resolve questions regarding the role of eicosanoids in mediating the renal functional effects of high chloride (117 mM), by stimulating either TxA2 synthesis or TxA2/PGH2 receptors, we compared the ability of indomethacin to block high-chloride effects in the rat isolated kidney with that of BMS 180291 and SQ 29548, antagonists of the TxA2/PGH2 receptor. These antagonists differ in terms of their selectivity and their capacity to inhibit isoforms of the TxA2/PGH2 receptor. Indomethacin and SQ 29548 had identical actions, preventing the decrease of GFR and antinatriuresis evoked by hyperchloremia, e.g., sodium excretion rate in the SQ 29548 and indomethacin groups increased to 7.2 +/- 1.3 and 7.1 +/- 1.2 microEq/min, respectively, compared with 2.6 +/- 0.7 microEq/min in the control group. In contrast, neither BMS 180291 nor the TxA2 synthase inhibitors, OKY 046 and CGS 13080, modified the negative effects of high chloride on GFR or sodium excretion. These results argue against either TxA2 or PGH2 acting as mediator of the effects of high chloride on renal function and suggest a product of COX activity such as a 20-HETE analog of prostaglandin endoperoxide. Evidence to support this proposal was obtained: 1) Hyperchloremia increased 20-HETE release from the rat kidney by 2-fold when compared with low-chloride conditions of renal perfusion. 2) The renal vasoconstrictor action of 20-HETE was shown to be dependent on COX activity and to be antagonized by blockade of the TxA2/PGH2 receptor.

Evidence against a cytochrome P450-derived reactive oxygen species as the mediator of the nitric oxide-independent vasodilator effect of bradykinin in the perfused heart of the rat

The coronary vasodilator effect of bradykinin (BK) in the rat is independent of NO but dependent on activation of phospholipases with involvement of cytochrome P450 mono-oxygenase (P450) and stimulation of Ca++-activated K+ channels, implicating an unidentified hyperpolarizing factor generated via P450 metabolism of arachidonic acid (AA). Because P450 activity also generates free radicals, such as superoxide, which can lead to the formation of hydrogen peroxide and hydroxyl radicals, which are vasoactive, we addressed the contribution of superoxide to the vasodilator effect of BK in the rat heart. Using rat renal microsomes as a source of P450, we verified that P450-dependent metabolism of AA generated superoxide, as detected by chemiluminescence with lucigenin. The signal was almost abolished by inhibition of P450 with clotrimazole and the superoxide scavenger 4,5-dihydroxy-1,3-benzene sulfonic acid. However, base-line superoxide formation, detected by chemiluminescence, in cardiac slices and perfused hearts was unchanged in response to BK or AA. Furthermore, in perfused hearts treated with nitroarginine and indomethacin to eliminate NO and prostaglandins and elevate perfusion pressure, dose-dependent vasodilator responses to BK were unaffected by superoxide dismutase plus catalase, a combination that abolished dilator responses to hydrogen peroxide. Similarly, the superoxide scavengers 4,5-dihydroxy-1,3-benzene sulfonic acid and 4-hydroxy-2,2,6,6-tetramethylpiperidine-noxyl were without effect on vasodilator responses to BK. Thus, the coronary vasodilator action of BK is independent of superoxide or its derivatives, which can be excluded as hyperpolarizing factors mediating NO-independent vasodilation in the rat.

Role of phospholipase C and phospholipase A2 in the nitric oxide-independent vasodilator effect of bradykinin in the rat perfused heart

The cytochrome P450-dependent component of the coronary vasodilator action of bradykinin which requires activation of K+ channels was examined in terms of the contribution of phospholipases in the rat Langendorff heart preparation. This component was isolated by inhibition of nitric oxide synthase with nitroarginine and cyclooxygenase with indomethacin, neither of which affects the coronary vasodilator action of bradykinin. However, nitroarginine elevated coronary perfusion pressure from approximately 40 to 130 mm Hg. The phospholipase C inhibitor, U73122 {1-(6-((17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl) amino)hexyl)-1H-pyrrole-2,5-dione}, reduced coronary vasodilator responses to bradykinin by greater than 80%. U73122 also diminished the coronary vasodilator action of cromakalim which activates ATP-sensitive K+ channels. The maleimide moiety of U73122 that has the capacity to affect K+ channels inhibited cromakalim-induced coronary vasodilation, but did not affect that to bradykinin. Inhibition of diacylglycerol lipase with RHC 80267 {1,6-bis-(cyclohexyloximinocarbonylamino)-hexane} was without an overall effect on coronary vasodilator responses to bradykinin. The cytosolic phospholipase A2 inhibitor, AACOCF3 {arachidonyl trifluoromethyl ketone¿} decreased responses to bradykinin by up to 90% whereas inhibitors of the secretory form of phospholipase A2 oleyloxyethyl phosphorylcholine and ONO-RS-082 {2-(p-amylcinnamoyl)amino-4-chlorobenzoic acid} were less effective than either AACOCF3 or U73122. The phospholipase inhibitors demonstrated selectivity as they did not affect the coronary vasodilator responses to nitroprusside. We obtained additional evidence for the antiphospholipase activity of the inhibitors by demonstrating their capacity to suppress bradykinin-stimulated increases in the release of prostacyclin, measured as 6-keto prostaglandin F1 alpha. The phospholipase inhibitors did not affect cyclooxygenase activity as the ability of arachidonic acid to stimulate prostaglandin formation was unimpaired. These results indicate that the coronary vasodilator action of bradykinin is linked to the activities of both phospholipase C and A2.