In vivo microscopy of the cerebral microcirculation using neonatal allografts in hamsters

Salt, Angiotensin II, Superoxide, and Endothelial Function

Proper function of the vascular endothelium is essential for cardiovascular health, in large part due to its antiproliferative, antihypertrophic, and anti-inflammatory properties. Crucial to the protective role of the endothelium is the production and liberation of nitric oxide (NO), which not only acts as a potent vasodilator, but also reduces levels of reactive oxygen species, including superoxide anion (O2•-). Superoxide anion is highly injurious to the vasculature because it not only scavenges NO molecules, but has other damaging effects, including direct oxidative disruption of normal signaling mechanisms in the endothelium and vascular smooth muscle cells. The renin-angiotensin system plays a crucial role in the maintenance of normal blood pressure. This function is mediated via the peptide hormone angiotensin II (ANG II), which maintains normal blood volume by regulating Na+ excretion. However, elevation of ANG II above normal levels increases O2•- production, promotes oxidative stress and endothelial dysfunction, and plays a major role in multiple disease conditions. Elevated dietary salt intake also leads to oxidant stress and endothelial dysfunction, but these occur in the face of salt-induced ANG II suppression and reduced levels of circulating ANG II. While the effects of abnormally high levels of ANG II have been extensively studied, far less is known regarding the mechanisms of oxidant stress and endothelial dysfunction occurring in response to chronic exposure to abnormally low levels of ANG II. The current article focuses on the mechanisms and consequences of this less well understood relationship among salt, superoxide, and endothelial function.

Reduced angiotensin II levels cause generalized vascular dysfunction via oxidant stress in hamster cheek pouch arterioles

OBJECTIVES

We investigated the effect of suppressing plasma angiotensin II (ANG II) levels on arteriolar relaxation in the hamster cheek pouch.

METHODS

Arteriolar diameters were measured via television microscopy during short-term (3-6days) high salt (HS; 4% NaCl) diet and angiotensin converting enzyme (ACE) inhibition with captopril (100mg/kg/day).

RESULTS

ACE inhibition and/or HS diet eliminated endothelium-dependent arteriolar dilation to acetylcholine, endothelium-independent dilation to the NO donor sodium nitroprusside, the prostacyclin analogs carbacyclin and iloprost, and the KATP channel opener cromakalim; and eliminated arteriolar constriction during KATP channel blockade with glibenclamide. Scavenging of superoxide radicals and low dose ANG II infusion (25ng/kg/min, subcutaneous) reduced oxidant stress and restored arteriolar dilation in arterioles of HS-fed hamsters. Vasoconstriction to topically-applied ANG II was unaffected by HS diet while arteriolar responses to elevation of superfusion solution PO2 were unaffected (5% O2, 10% O2) or reduced (21% O2) by HS diet.

CONCLUSIONS

These findings indicate that sustained exposure to low levels of circulating ANG II leads to widespread dysfunction in endothelium-dependent and independent vascular relaxation mechanisms in cheek pouch arterioles by increasing vascular oxidant stress, but does not potentiate O2- or ANG II-induced constriction of arterioles in the distal microcirculation of normotensive hamsters.

Nitric oxide contributes to AT2 but not AT1 angiotensin II receptor-mediated vasodilatation of porcine pial arteries and arterioles

Angiotensin II elicits pial artery dilation by activating angiotensin AT1 and angiotensin AT2 receptors. This study determined if vasodilatation in response to angiotensin AT2 receptor activation is due to stimulated release of nitric oxide (NO) in newborn pigs equipped with a closed cranial window. Angiotensin II (10(-8), 10(-6) M) elicited pial artery dilatation that was unchanged by the NO synthase inhibitor N omega-Nitro-L-Arginine (L-NNA) (10(-6) M) (12+/-3 and 18+/-2 versus 12+/-3 and 21+/-4%). Angiotensin II was not associated with changes in artificial cerebrospinal fluid (CSF) cGMP concentration, an indicator of NO release. Similar data were obtained for the angiotensin AT1 receptor agonist L 162,313. In contrast, the angiotensin AT2 receptor agonist CGP 42112A (10(-8), 10(-6) M) induced vasodilatation that was blocked by L-NNA (9+/-2 and 18+/-3 versus 1+/-1 and 1+/-1%). CGP 42112A dilatation was associated with elevated artificial CSF cGMP concentration (757+/-18, 1590+/-89, and 2101+/-116 fmol/ml) and such stimulated release was blocked by L-NNA. These data indicate that stimulated NO release contributes to angiotensin AT2 but not angiotensin AT1 induced vasodilatation. These data suggest that angiotensin II primarily elicits dilatation via angiotensin AT1 receptor activation.

Prostaglandins contribute to impaired angiotensin II-induced cerebral vasodilation after brain injury

This study characterized the effects of fluid percussion brain injury (FPI) on angiotensin II (AII)-induced cerebral vasodilation, determined the role of prostaglandins in such changes and evaluated the contribution of two subtypes of AII receptors (AT(1) and AT(2)) to the effects of AII on cerebrovascular regulation. Topical AII (10(-8), 10(-6), 10(-4) M) elicited vasodilation, which was attenuated by FPI (10 +/- 1; 18 +/- 2; 27 +/- 1% vs. 2 +/- 1; 4 +/- 1; 7 +/- 1%). Such changes in diameter were associated with increases in CSF 6-keto-PGF(1alpha), the stable breakdown product of PGI(2) (1.5 +/- 0.1; 2.1 +/- 0.1; 4.0 +/- 0.3 fold) and TXB(2), the stable breakdown product of TXA(2) (1.2 +/- 0.1; 1.4 +/- 0.1; 1.6 +/- 0.1 fold). However, after FPI, increases in 6-keto PGF(1alpha) were blocked (1.0 +/- 0.1; 1.0 +/- 0.1; 1.1 +/- 0.1 fold) whereas TXB(2) release was enhanced (1.5 +/- 0.1; 1.8 +/- 0.1; 1.9 +/- 0.1 fold). Pretreatment with the cyclooxygenase inhibitor indomethacin (5 mg/kg i.v.) in FPI animals partially protected AII vasodilation (8 +/- 1; 14 +/- 2; 19 +/- 3%). CGP 42112A, a putative AT(2) agonist, elicited vasodilation, which was also blunted by FPI. Such dilation was not associated with CSF prostaglandin changes, and indomethacin did not protect responses altered by FPI. Vasodilatation caused by low concentrations of AII was blunted by an AT(1) antagonist ZD 7155 but unchanged by an AT(2) antagonist PD 123,319. The high AII concentration produced dilation that was blunted by both antagonists. These data show that FPI impairs AII-mediated vasodilation. These data suggest that FPI causes these changes via alteration in an AT(1)-mediated production of prostaglandins. These data additionally suggest that FPI induced impairment of AT(2) mediated vasodilation is independent of an altered production of prostaglandins.

Effect of renin on brain arterioles and cerebral blood flow in rabbits

There is evidence of an intrinsic renin-angiotensin system in the brain. The goal of the study was to determine whether stimulation of endogenous angiotensin production by applying renin to the brain surface has an effect on pial arteriolar caliber and CBF. Pial vessel diameters were measured through a closed cranial window in anesthetized rabbits. Percent changes of blood flow in the cortical area under the cranial window were simultaneously measured by laser-Doppler flowmetry. Topical application of 0.01-0.1 U/ml renin induced maximum dilation of 18.9 +/- 4% (mean +/- SD) of pial arterioles within 2 min. Arteriolar calibers thereafter decreased slowly. Flow gradually increased to peak at 38 +/- 15% 50 min after renin application. Angiotensin I levels in jugular blood, as measured by radioimmunoassay, increased to a peak 40 min after topical renin application. Angiotensin II levels in jugular blood and both angiotensin I and II levels in blood samples from the femoral artery did not change. Diameter and flow changes were inhibited by intravenous pretreatment with the converting enzyme blocker captopril (10 mg/kg body wt i.v.). Captopril did not affect the vasodilation and flow increase in response to hypercapnia. Topically applied captopril (10(-5) M) blocked renin-induced arteriolar dilation. We conclude that renin increases pial arteriolar diameters and cortical blood flow in the rabbit brain. Stimulation of angiotensin production is likely to be a mediator of this response.

Angiotensin II AT2 receptor stimulation increases cerebrovascular resistance during hemorrhagic hypotension in rats

The effects of the angiotensin II (ANG II) AT2 ligand PD 123319 and the AT1 antagonist losartan on cerebral blood flow (CBF) were studied during hemorrhagic hypotension in anesthetized rats using laser-Doppler flowmetry. In the control group CBF remained stable when mean arterial blood pressure (MABP) was lowered from 84 mmHg (baseline) to 45 mmHg, whereafter there was a pressure dependent decrease in CBF indicating inadequacy of autoregulation. Cerebrovascular resistance (CVR) was reduced until MABP 40 mmHg, where a maximum dilation was reached. PD 123319 dose-dependently (3-30 mg/kg i.v.) increased CVR through all blood pressures. Losartan 3 mg/kg i.v. had an effect similar to PD 123319. Selective stimulation of AT2 receptors with intravenous ANG II infusion, in the presence of AT1 receptor blockade by losartan, also increased CVR. As a result, reduced CBF was seen in the treatment groups. The effects of ANG II and PD 123319 30 mg/kg were antagonized by the nonselective ANG II antagonist Sar1,Ile8-ANG II (4 micrograms/kg/min i.v.). None of the treatments affected baseline CBF. The results confirm that ANG II contributes to cerebrovascular resistance and participates in the regulation of CBF apparently through AT2 receptors.

Cerebral blood flow and cerebral blood flow velocity during angiotensin-induced arterial hypertension in dogs

Pressure-passive perfusion beyond the upper limit of cerebral blood flow (CBF) autoregulation may be deleterious in patients with intracranial pathology. Therefore, monitoring of changes in CBF would be of clinical relevance in situations where clinical evaluation of adequate cerebral perfusion is impossible. Noninvasive monitoring of cerebral blood flow velocity using transcranial Doppler sonography (TCD) may reflect relative changes in CBF. This study correlates the effects of angiotensin-induced arterial hypertension on CBF and cerebral blood flow velocity in dogs. Heart rate (HR) was recorded using standard ECG. Catheters were placed in both femoral arteries and veins for measurements of mean arterial blood pressure (MAP), blood sampling and drug administration. A left ventricular catheter was placed for injection of microspheres. Cerebral blood flow velocity was measured in the basilar artery through a cranial window using a pulsed 8 MHz transcranial Doppler ultrasound system. CBF was measured using colour-labelled microspheres. Intracranial pressure (ICP) was measured using an epidural probe. Arterial blood gases, arterial pH and body temperature were maintained constant over time. Two baseline measures of HR, MAP, CBF, cerebral blood flow velocity and ICP were made in all dogs (n = 10) using etomidate infusion (1.5 mg.kg-1 x hr-1) and 70% N2O in O2 as background anaesthesia. Following baseline measurements, a bolus of 1.25 mg angiotensin was injected i.v. and all variables were recorded five minutes after the injection. Mean arterial blood pressure was increased by 76%. Heart rate and ICP did not change. Changes in MAP were associated with increases in cortical CBF (78%), brainstem CBF (87%) and cerebellum CBF (64%).(ABSTRACT TRUNCATED AT 250 WORDS)

Nonpeptide angiotensin AT1 and AT2 receptor ligands modulate the upper limit of cerebral blood flow autoregulation in rats

We investigated the effect of angiotensin AT1 and AT2 receptor blockade on the upper limit of CBF autoregulation in pentobarbital-anesthetized rats. CBF was measured by laser-Doppler flowmetry from the parietal cortex and MABP was increased by intravenous phenylephrine infusion. Neither the AT1 antagonist losartan nor the AT2 ligand PD 123319 nor angiotensin II (ANG II) in the presence of losartan affected baseline CBF. When the blood pressure was increased in the control group, CBF remained fairly constant up to 145 mm Hg and increased steeply after 150 mm Hg. Both PD 123319 (7-10 mg/kg) and losartan (1-10 mg/kg) shifted the upper limit of CBF autoregulation toward higher pressures. Intravenous infusion of PD 123319 was more effective than bolus injection. The losartan effect was dose dependent. Selective stimulation of AT2 receptors with an intravenous ANG II infusion (0.54 micrograms/min) in the presence of losartan did not reverse the effect of losartan on CBF autoregulation, but, on the contrary, appeared to further shift the upper limit of autoregulation toward higher pressures. The results implicate a role for both AT1 and AT2 angiotensin receptors in the regulation of CBF.

Morphological study of bovine lung grafted into the hamster cheek pouch

The hamster cheek pouch has been used extensively to study the modulation of microvascular responsiveness of native and transplanted tissues, because it is immunologically privileged. The purpose of this study was to determine the structural changes that occur over time in bovine lung tissue (donor) that was grafted into the hamster cheek pouch (recipient). Lungs from adult cows were cut into 1-mm-thick slices and grafted into the cheek pouch of adult Syrian golden hamsters (n = 60). After induction of anesthesia, bovine lung tissue was placed under the avascular tissue covering the cheek pouch, and the overlying skin was sutured. Intravital microscopy (IM) and transmission electron microscopy (TEM) of the cheek pouch and grafted tissue were performed 1, 7, 14, and 28 days after grafting. By using IM, we found blood flow throughout the grafted bovine lung tissue between days 7 and 14 post-transplantation. Both IM and TEM showed that the grafted tissue contained patent microvessels anastomosing with cheek pouch microvessels, alveolar structures, and interstitial tissue. Mast cell infiltration around microvessels of the grafted tissue was evident in all animals between days 14 and 28 post-grafting. No other inflammatory cells were identified throughout the observation period. By day 28 post-grafting, the entire lung tissue became fibrotic. We conclude that bovine lung tissue can be successfully transplanted into the hamster cheek pouch, that blood flow is established throughout the graft, and that prominent mast cell infiltration is associated with fibrosis of the graft 28 days after transplantation. We suggest that this model can be useful in studying pulmonary microvascular responses in situ.

Oxygen transport in intraspinal fetal grafts: graft-host relations

In the present experiments, we determined tissue oxygen tension (PtO2) levels within the injured spinal cords of adult rats following transplantation of fetal spinal cord tissue. Partial resection cavities were made at L1-L2 levels after which whole pieces of 14-day donor tissue were placed into the cavities. Analysis of recordings obtained from graft tissues at 1 and 2 months after transplantation revealed low PtO2 values in many cases. Even the more extensively developed transplants at 3 months continued to show mean PtO2 levels lower than those taken from normal, mature spinal cord tissue. Measurements from host tissue, adjacent to a lesion in which no graft was introduced, showed normal or elevated PtO2 levels. In contrast, where extensive host and graft integration had occurred, the PtO2 levels of adjacent host resembled those obtained within the transplants. On the other hand, in cases of poor host-graft integration, characterized by either cellular or fibrotic graft-host interfaces or large cysts, the PtO2 tensions exceeded normal levels. Therefore, the present results show that when fetal grafts are placed acutely into an aspiration cavity within the adult spinal cord, the transplants quickly establish an oxygen microenvironment resembling that found during normal fetal development. Oxygen transport is therefore a regulated variable in the graft neuropil as it is in the normally developing spinal cord. Furthermore, in the presence of closely approximated fetal transplants, adjacent host tissue assumes tissue oxygen levels that mimic those in the graft. This "inductive" effect gradually diminishes as development proceeds and may be the hallmark of successful graft-host integration.

Separation of bovine bronchial epithelial cell subpopulations by density centrifugation: a method to isolate ciliated and nonciliated cell fractions

Bronchial epithelial cells isolated by protease digestion can be cultured in vitro for the study of proliferation and differentiation. However, these cells represent a heterogenous population, the components of which likely interact with one another. We attempted to utilize density gradient centrifugation as a method to prepare subpopulations of these bronchial epithelial cells. The suspension of the cells obtained by protease digestion of the bovine bronchi was mixed with an equal volume of colloidal silica reagent, Sepracell-MN, and centrifuged to form a continuous density gradient. Two distinct cell layers were identified in addition to a cell pellet at the bottom. Cells from fraction A (top layer) were more than 95% ciliated cells by morphologic examination. These ciliated cells were recovered intact as assessed by trypan blue dye exclusion and by watching beating of their cilia. The cells from fraction C (bottom layer) were 89.9 +/- 3.88% nonciliated, small round cells with a densely staining nucleus and scant cytoplasm. Comparison of cell morphology of these cells with basal cells in vivo and electron microscopic examinations suggested that these cells were basal cells. These basal cells showed an exponential cell proliferation until confluence in Ham's F12 with supplements, LHC9, and a 1:1 mixture of Medium-199 and modified Eagle's medium with 2% fetal calf serum. In contrast, the cells from fraction A grew minimally in all conditions tested. This difference was also shown in the study of DNA synthesis by [3H]thymidine uptake. Enzyme-linked immunosorbent assay for release of bovine fibronectin into cultured media indicated that fraction C cells secreted much more fibronectin (532 +/- 5.28 ng/10(6) cells/h) than fraction A cells (73.4 +/- 1.00). We also used Percoll as a density-gradient reagent and showed potential usefulness in the preparation of cell fractions of bronchial epithelial cells. In conclusion, it was possible to separate ciliated and nonciliated, presumably basal, cells of bovine bronchial epithelial cells. These differed in growth and fibronectin secretion. Studies of airway cell biology may be aided by the availability of more homogenous cell populations.

Spinal cord repair: is tissue oxygenation an important variable?

We have demonstrated the reliability and feasibility of making PtO2 recordings from graft and host tissue in the injured spinal cord. The data suggest that the oxygen microenvironment of developing graft and host spinal tissue is clearly different from that found in normal spinal tissue or in transplants that have not survived or integrated well. These same constraints seem to apply to cavitation in developing grafts and poorly developed graft/host interfaces. The similarity between these findings and those from previous studies in other fetal vertebrates suggests that oxygen tensions in the spinal cord probably reflect the developmental status of the regenerating tissue. Our future studies will seek to define the relationship between anatomical development of transplant tissue and these functional (PtO2, microvascular development and tissue metabolism) indicators of graft development. These investigations should also provide a background for those later studies which seek to establish the mechanisms by which these relationships come about i.e. oxygen consumption of host/transplant tissue, blood flow to transplants, studies of glycolytic metabolism (2-DG autoradiography), etc. In this way, we can begin to understand the role of tissue metabolism in graft-mediated repair.