Role of interaction of the tanycytes of the third ventricle of the brain with the vascular bed endothelium in enzymatic activation of the parkinsonism-inducing proneurotoxin MPTP in rat brain

The authors describe the localization of monoaminooxidases oxidizing the protoxin MPTP to the active neurotoxic MPP+ ion, which induces parkinsonism by destroying the dopaminergic neurons of the nigrostriatum. Apart from some types of magnocellular neurons of the hypothalamus, enzymatic activity is localized in the endothelium of segments of the circumventricular vascular bed communicating with the enzymatically equally positive processes of the tanycytes of the third brain ventricle. The findings are briefly discussed from the aspect of the pathogenesis of experimental parkinsonism.     

Role of myocardium and endothelium in coronary vascular smooth muscle responses to hypoxia

Hypoxia triggers a mechanism that induces vasodilation in the whole heart but not necessarily in isolated coronary arteries. We therefore studied the role of cardiomyocytes (CM), smooth muscle cells (SMC), and endothelial cells (EC) in coronary responses to hypoxia (PO(2) of 5-10 mmHg). In an attempt to determine the factor(s) released in response to hypoxia, we inhibited the contribution of adenosine, ATP-sensitive K(+) channels, prostaglandins, and nitric oxide. Isolated rat septal artery segments without (-T) and with a layer of cardiac tissue (+T) were mounted in a double wire myograph, and constriction was induced. Hypoxia induced a decrease in isometric force of 21% and 61% in -T and +T segments, respectively (P < 0.05). EC removal increased the relaxation to hypoxia in -T segments to 33% but had the same effect in +T segments (61%). Only one of the inhibitors, the adenosine antagonist in +T segments, partially affected the relaxation due to hypoxia. The role of adenosine is thus limited and other mechanisms have to contribute. We conclude that hypoxia induces a relaxation of SMC that is augmented by the presence of CM and blunted by the endothelium. A single mediator does not induce those effects.     

Tolerance to ethanol-induced contractions of vascular smooth muscle: Role of endothelium

Ethanol, at high concentrations, produced a dose-dependent contraction of male rat aortic rings, $\text{in vitro}$. Mechanical removal of endothelial cells from aortic rings of control rats resulted in a small, but significant, shift of the ethanol dose-response curve to the right without a change in the maximal contraction. Removing the endothelial cells of aortic rings obtained from rats intoxicated with ethanol for two days significantly shifted the ethanol dose-response curve to the left and significantly increased the maximal contraction induced by ethanol. A comparison of the ethanol dose-response curves in aortic rings with endothelium obtained from control rats with those obtained from intoxicated rats indicated a significant shift to the right with no change in maximal response. No significant changes were observed when the responses of aortic rings without endothelium obtained from control and intoxicated rats were compared. These observations confirm that tolerance to ethanol can be demonstrated in vascular smooth muscle. In addition, they demonstrate that the endothelium is required for the development of tolerance to ethanol in the aorta.     

Nanomechanics of vascular endothelium

The mechanical characteristics of endothelial cells reveal four distinct compartments, namely glycocalyx, cell cortex, cytoplasm and nucleus. There is accumulating evidence that endothelial nanomechanics of these individual compartments control vascular physiology. Depending on protein composition, filament formation and interaction with cross-linker proteins, these four compartments determine endothelial stiffness. Structural organization and mechanical properties directly influence physiological processes such as endothelial barrier function, nitric oxide release and gene expression. This review will focus on endothelial nanomechanics and its impact on vascular function.     

Cytochemical studies of the vascular endothelium

Cytochemical methods have been used to examine the vascular endothelium. With hemeproteins and immunocytochemistry, investigators have demonstrated the pathways that blood-borne molecules can take to gain access to the extravascular space (Ghitescu et al. 1986; Milici et al. 1987; Schneeberger and Karnovsky 1971; Simionescu et al. 1975). These same cytochemical methods have also provided evidence that morphologically similar endothelia may have different permeability properties (Hart and Pino 1985b, 1986; Pino 1985; Pino and Essner 1980, 1981). Differences in the location and chemical composition of cell surface moieties have been ascertained with enzyme digestion methods, lectins, and cationic ferritin (De Bruyn and Michelson 1978; Pino 1984c, 1986a, b; Simionescu et al. 1981a). The author hopes that he has provided the reader with representative examples of how investigators have used these cytochemical methods for their studies. As new methods are developed and applications are found for existing techniques such as ultracryomicrotomy (Milici et al. 1987) and colloidal gold markers (Pino 1987b), cytochemistry will remain a fundamental tool for the study of the structure and function of the vascular endothelium.     

Role of the endothelium in the development of the contractile reactions of vascular smooth muscle during decreased oxygenation

Noradrenaline-preactivated vascular smooth muscles (VSM) of the rat thoracic aorta showed two-phase reactions in response to decreased oxygenation: significant relaxation was preceded by transient constriction. When the endothelium was removed only VSM relaxation phase was retained, with no constriction observed. The data obtained suggest an endothelium-dependent nature of VSM constriction reaction to hypoxia, in contrast to endothelium-independent VSM relaxation. Intracellular calcium is also assumed to play an essential role in the formation of endothelium-dependent constriction VSM reaction to hypoxia.     

Mechanisms underlying mechanosensitivity of mesenteric afferent fibers to vascular flow

Spinal afferent neurons, with endings in the intestinal mesenteries, have been shown to respond to changes in vascular perfusion rates. The mechanisms underlying this sensitivity were investigated in an in vitro preparation of the mesenteric fan devoid of connections with the gut wall. Afferent discharge increased when vascular perfusion was stopped ("flow off"), a response localized to the terminal vessels just prior to where they entered the gut wall. The flow-off response was compared following pharmacological manipulations designed to determine direct mechanical activation from indirect mechanisms via the vascular endothelium or muscle. Under Ca(2+)-free conditions, responses to flow off were significantly augmented. In contrast, the myosin light chain kinase inhibitor wortmannin (1 microM, 20 min) did not affect the flow-off response despite blocking the vasoconstriction evoked by 10 microM l-phenylephrine. This ruled out active tension, generated by vascular smooth muscle, in the response to flow off. Passive changes caused by vessel collapse during flow off were speculated to affect sensory nerve terminals directly. The flow-off response was not affected by the N-, P-, and Q-type Ca(2+) channel blocker omega-conotoxin MVIIC (1 muM intra-arterially) or the P2X receptor/ion channel blocker PPADS (50 microM). However, ruthenium red (50 microM), a blocker of nonselective cation channels, greatly reduced the flow-off response and also abolished the vasodilator response to capsaicin. Our data support the concept that mesenteric afferents sense changes in vascular flow during flow off through direct mechanisms, possibly involving nonselective cation channels. Passive distortion in the fan, caused by changes in blood flow, may represent a natural stimulus for these afferents in vivo.     

Flow-activated ion channels in vascular endothelium

The ability of vascular endothelial, cells (ECs) to respond to fluid mechanical forces associated with blood flow is essential for flow-mediated vasoregulation and arterial wall remodeling. Abnormalities in endothelial responses to flow also play a role in the development of atherosclerosis. Although our understanding of the endothelial signaling pathways stimulated by flow has greatly increased over the past two decades, the mechanisms by which ECs sense flow remain largely unknown. Activation of flow-sensitive ion channels is among the fastest known endothelial responses to flow; therefore, these ion channels have been proposed as candidate flow sensors. This review focuses on: 1) describing the various types of flow-sensitive ion channels that have been reported in ECs, 2) discussing the implications of activation of these ion channels for endothelial function, and 3) proposing candidate mechanisms for activation of flow-sensitive ion channels.     

Secretory functions of the vascular endothelium.

The endothelial cells which line the blood vessels as a monolayer exert a remarkable control over the vascular system. Indeed, the endothelium can be regarded as a highly active metabolic and endocrine organ in its own right. On the hand, vasoactive substances such as serotonin and bradykinin are inactivated and on the other the cells can enzymatically produce the vasoconstrictor, angiotensin II and secrete endothelin-1 ((ET-1). Perhaps more importantly, the cells also produce two unstable vasodilator substances, which potently inhibit platelet clumping: prostacyclin and endothelium-derived relaxing factor (EDRF) which has been identified as nitric oxide (NO; 1). Both substances seem well designated as local hormones, released to influence adjacent cells. The endothelial cell, therefore, exerts control over the cardiovascular system by elaborating dilator substances as well as vasconstrictors.     

Role of endothelin in alpha-adrenoceptor coronary vasoconstriction

It has been proposed that alpha-adrenoceptor vasoconstriction in coronary resistance vessels results not from alpha-adrenoceptors on coronary smooth muscle but from alpha-adrenoceptors on cardiac myocytes that stimulate endothelin (ET) release. The present experiments tested the hypothesis that the alpha-adrenoceptor-mediated coronary vasoconstriction that normally occurs during exercise is due to endothelin. In conscious dogs (n = 10), the endothelin ET(A)/ET(B) receptor antagonist tezosentan (1 mg/kg iv) increased coronary venous oxygen tension at rest but not during treadmill exercise. This result indicates that basal endothelin levels produce a coronary vasoconstriction at rest that is not observed during the coronary vasodilation during exercise. In contrast, the alpha-adrenoceptor antagonist phentolamine increased coronary venous oxygen tension during exercise but not at rest. The difference between the endothelin blockade and alpha-adrenoceptor blockade results indicates that alpha-adrenoceptor coronary vasoconstriction during exercise is not due to endothelin. However, in anesthetized dogs, bolus intracoronary injections of the alpha-adrenoceptor agonist phenylephrine produced reductions in coronary blood flow that were partially antagonized by endothelin receptor blockade with tezosentan. These results are best explained if alpha-adrenoceptor-induced endothelin release requires high pharmacological concentrations of catecholamines that are not reached during exercise.     

Blood-forming potential of vascular endothelium in the human embryo

Hematopoietic cells arise first in the third week of human ontogeny inside yolk sac developing blood vessels, then, one week later and independently, from the wall of the embryonic aorta and vitelline artery. To address the suggested derivation of emerging hematopoietic stem cells from the vessel endothelium, endothelial cells have been sorted by flow cytometry from the yolk sac and aorta and cultured in the presence of stromal cells that support human multilineage hematopoiesis. Embryonic endothelial cells were most accurately selected on CD34 or CD31 surface expression and absence of CD45, which guaranteed the absence of contaminating hematopoietic cells. Yet, rigorously selected endothelial cells yielded a progeny of myelo-lymphoid cells in culture. The frequency of hemogenic endothelial cells in the yolk sac and aorta reflected the actual blood-forming activity of these tissues, as a function of developmental age. Even less expected, a subset of endothelial cells sorted similarly from the embryonic liver and fetal bone marrow also exhibited blood-forming potential. These results suggest that a part at least of emerging hematopoietic cells in the human embryo and fetus originate in vascular walls.     

Augmentation of antioxidant enzymes in vascular endothelium

The endothelium is a key site of injury from reactive oxygen species that can potentially be protected by the antioxidant enzymes superoxide dismutase and catalase. Large proteins, such as superoxide dismutase and catalase, do not readily penetrate cell membranes, which limits their efficacy in protecting cells from cellular reactions involving both intracellularly and extracellularly generated reactive oxygen species. Two methods are described that promote enzyme delivery to cultured endothelial cells and confer increased resistance to oxidative stress. The first method is to entrap the antioxidant enzymes within liposomes, which then become incorporated by endothelial cells and can increase enzyme specific activities by as much as 44-fold within 2 h. The second method involves covalent conjugation of polyethylene glycol (PEG) to superoxide dismutase and catalase, a technique that increases circulatory half-life and reduces protein immunogenicity. Conjugation of PEG to superoxide dismutase and catalase increased cellular-specific activities of these enzymes in cultured endothelial cells (but at a slower rate than for liposome entrapped enzymes) and rendered these cells more resistant to oxidative stress. Both liposome-mediated delivery and PEG conjugation offer an additional benefit over native superoxide dismutase and catalase because they can increase cellular antioxidant activities in a manner that can provide protection from both intracellular and extracellular superoxide and hydrogen peroxide.