Modulation by endothelium of the vascular effects of angiotensin II.

Modulation by vascular endothelium of the effects of AII was studied in the isolated rabbit aortic and superior mesenteric artery strips. The contractile effect of AII was enhanced in rubbed aortic strips. Similar enhancement was obtained in hydroquinone pretreated unrubbed strips. The relaxing effect of acetylcholine in AII--induced precontracted aortic strips was abolished after rubbing and hydroquinone pretreatment. However, no significant changes were observed in the contractile response to AII on aspirin and nicotine pretreated strips. In the isolated mesenteric artery strips AII produced a biphasic responses. The contractile effect of AII was enhanced in rubbed strips. Similar potentiation was also obtained in hydroquinone, aspirin and nicotine pretreated unrubbed strips. The relaxation phase of AII response was completely abolished in rubbed strips but partially inhibited in hydroquinone, aspirin and nicotine pretreated unrubbed strips. From these results it was concluded that EDRF is the main endothelial humoral factor which modulates the effect of AII in the rabbit aorta while both EDRF and PGI2 are involved for the modulation of the effects of octapeptide in the mesenteric artery.

Endothelium-derived relaxing factor in cultured cells.

Many vasoactive agents stimulate release of an endothelium-derived relaxing factor (EDRF). EDRF stimulates cyclic guanosine 3',5'-monophosphate (cGMP) accumulation and relaxation of vascular smooth muscle in a manner similar to that produced by sodium nitroprusside. Endothelium and vascular smooth muscle were isolated from porcine, bovine, and rat thoracic aorta. The capacity of sodium nitroprusside to stimulate cGMP accumulation in cultured bovine, porcine, and rat vascular smooth muscle was found to increase with time in culture to a maximum of 12 to 14 days after plating. In addition, bovine and porcine vascular smooth muscle, but not rat vascular smooth muscle, lost the sodium nitroprusside-stimulated cGMP response after the fifth passage. Cultured endothelial cells did not respond to endothelium-dependent vasodilators or sodium nitroprusside with increased cGMP levels. Vascular smooth muscle cells responded only to sodium nitroprusside. Mixed cultures of porcine and bovine endothelium and vascular smooth muscle and bovine endothelium and rat vascular smooth muscle responded to endothelium-dependent vasodilators with increased cGMP levels. Short-term (4 hours) coculture experiments using bovine endothelium grown on microcarriers to assess the need for long-term contact between the two cell types produced similar results. Release of EDRF from bovine endothelium was studied by loading endothelium-covered microcarrier beads into a column superfused with physiological buffer. Treatment of the column with bradykinin, the calcium ionophore A23187, melittin, and arachidonate released EDRF from the column as measured by cGMP changes in denuded aortic rings and vascular smooth muscle cells and by relaxation of rings when bathed in column effluent. The time course of cGMP changes and relaxation were similar and could be reversed by hydroquinone.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-dependent transfer of ethanol tolerance in the aorta.

Recent studies have suggested that the endothelium-dependent tolerance to the direct vasoconstrictor effect of ethanol in the rat aorta is mediated by endothelium-derived relaxing factor (EDRF). This hypothesis was tested directly by employing a sandwich technique which has been used to demonstrate the release and action of EDRF. These experiments measured the ethanol-induced contraction of a spirally-cut aortic strip without endothelium obtained from an ethanol naive control rat. The response of the spiral strip was measured before and after it was sandwiched with a longitudinally-cut aortic strip with or without endothelium obtained from either control or ethanol tolerant rats. There was no significant difference in the ethanol-induced contraction of the spiral strip after beginning sandwiched with a longitudinal strip with or without endothelium obtained from a control rat or with a longitudinal strip without endothelium from a tolerant rat. In contrast, when a longitudinal strip with endothelium from a tolerant rat was sandwiched with the spiral strip the ethanol-induced contraction was significantly reduced. This effect was inhibited by methylene blue but not by indomethacin. Further, the magnitude of the carbachol-induced relaxation of the sandwiched preparation was significantly greater when the longitudinal strip with endothelium was obtained from a tolerant rat than from control rat. These data demonstrate the involvement of EDRF in the endothelium-dependent tolerance to ethanol in the rat aorta.

The role of endothelium in tolerance to ethanol-induced contraction of the rat aorta.

After 2 days of intoxication an endothelium-dependent tolerance to ethanol-induced contraction of the rat aorta develops. This study investigates the possibility that the observed tolerance may be due to the recently described endothelium-dependent vasodilator system which mediates the action of several potent vasodilators. The maximum response induced by ethanol in control and intoxicated aortic rings with endothelium was significantly increased by treatment with eicosatetraynoic acid and methylene blue. In addition, these agents significantly decreased the EC50 of ethanol in intoxicated but not control rings with endothelium. The response of control and intoxicated aortic rings without endothelium was unaltered by eicosatetraynoic acid or methylene blue. Indomethacin, nordihydroguaiaretic acid, and FPL 55712 did not alter either the maximum response or the EC50 of ethanol in control or intoxicated rings with endothelium. These observations support the hypothesis that the endothelium-dependent tolerance observed in the aorta may be due to the release of an endothelium-derived relaxing factor.

Venous and arterial endothelia: different dilator abilities in dog vessels.

It has been demonstrated by other investigators that endothelium-dependent vasodilators are more effective on arterial tissue than on venous tissue. The purpose of this study was to determine if this was due to a difference in the sensitivity of arterial and venous smooth muscle to the endothelial dilator (EDRF) or to a difference in the ability of arterial and venous endothelia to release EDRF. To differentiate between these two possibilities, an in vitro "sandwich" preparation was used in which the mechanical response to endothelium-dependent dilators of a de-endothelialized vessel was determined when "sandwiched" with an endothelialized vessel. Using dog femoral artery and saphenous vein, it was determined that acetylcholine (ACh), the ionophore A23187, and thrombin were endothelium-dependent dilators of the femoral artery, but their dilatory ability was significantly less in the saphenous vein. However, if the de-endothelialized saphenous vein was "sandwiched" with an endothelialized femoral artery, both ACh and A23187 significantly relaxed the vein. No relaxation of the de-endothelialized femoral artery occurred when it was "sandwiched" with an intact saphenous vein. Sodium nitroprusside, thought to act by a mechanism similar to EDRF, relaxed equally the saphenous vein and femoral artery. These observations suggest that the difference in responsiveness between femoral arteries and saphenous veins to endothelium-dependent dilators is due more to differences in the ability of their endothelia to release EDRF than to an inability of their smooth muscle to respond to EDRF.

Vasodilatation and the discovery of endothelium-derived relaxing factor.

Whether endothelium-derived relaxing factor (EDRF) is important in the physiology and pathology of vascular reactivity must await the discovery of the nature of the factor and an appropriate antagonist substance. Nevertheless the demonstration that EDRF is powerful, short-lived and can be released by a wide variety of stimuli adds a new dimension to our knowledge of the control of the circulation. The macroenvironment of the vessel wall is influenced by nerves, blood-borne factors and the architecture of the wall. The discovery of a role of the endothelial cell has for the first time forced pharmacologists to consider cell-cell interactions in the micro-environment of the blood-vessel wall. This is a potentially important target for new therapy.

Development of a large fibromuscular intimal thickening does not impair endothelium-dependent relaxation in the rabbit carotid artery.

The release of endothelium-derived relaxing factor (EDRF) was examined in the rabbit carotid artery 6 weeks after denudation with an inflated balloon catheter in vivo. A concentric, fibromuscular intimal thickening of variable width developed in all areas lined with either regenerated endothelium or modified luminal smooth muscle cells. In vitro studies showed that in vessels precontracted with serotonin, only the re-endothelialized areas could relax to the endothelium-dependent dilators methacholine, substance P and the Ca2+ ionophore A23187. Re-endothelialized areas with large concentric, fibromuscular intimal thickening (between 10 and 20 cells thick) relaxed with a similar sensitivity and maximum to methacholine compared with control areas. It is concluded that newly generated endothelial cells release EDRF whilst the specialized lining smooth muscle cells present 6 weeks after injury do not, and that the presence of a large fibromuscular intima does not prevent EDRF from reaching the media to cause relaxation.

Actions of parathyroid hormone in the cardiovascular system.

Recent studies from several laboratories have suggested that the cardiovascular system may be considered a new target organ system for parathyroid hormone (PTH). PTH caused a sharp, dose-dependent reduction in blood pressure of several mammalian models. This hypotensive response was mediated by direct interaction of PTH with the vascular smooth muscle of arteries and resistance vascular beds. Increases in intracellular cyclic AMP concentrations were temporally and quantitatively correlated with smooth muscle relaxation by PTH. Direct (nonreflex) positive inotropic and chronotropic effects have been observed in cardiac tissue after treatment with PTH. Considering these and other observations, a physiological role for PTH as a homeostatic regulator of the cardiovascular system may be speculated.

Evidence for endothelium-dependent vasodilation of resistance vessels by acetylcholine.

In the perfused mesenteric arterial vasculature of the rabbit, vasodilation by acetylcholine (ACh) was almost completely blocked after a 15-min perfusion of the vasculature with 0.2% collagenase, an enzyme capable of removing endothelial cells. In the perfused mesenteric arterial vasculature of the rat, vasodilation by ACh was markedly, though not completely, inhibited by hemoglobin (10 microM), an agent which can inactivate endothelium-derived relaxing factor (EDRF). These results suggest that a major component of vasodilation of mesenteric resistance vessels in rabbit and rat by ACh is mediated by EDRF.

The Malpighi lecture. From 'Porositates carnis' to cellular microcirculation.

After Malpighi's discovery of the capillaries almost 200 years elapsed before clear concepts about their main function, solute transport between blood and tissue, were formulated. The German physiologist Carl Ludwig played a significant role in this development because of his will to seek physico-chemical explanations of physiological phenomena. Ludwig's pupil Christian Bohr formulated the first (and almost definitive) mathematical model of transcapillary diffusion, later extended by Bohr's pupil August Krogh, to describe blood-tissue exchange in peripheral tissues. In the 1950s the two-pore model of transcapillary transport became the main paradigm (Pappenheimer, Grotte). The morphological substrate for this concept is found in tortuous pathways in the interendothelial junctions together with a small number of 'leaks' or focal openings in the capillary wall. The concept makes transcapillary transport of hydrophilic molecules a paracellular phenomenon. Recent studies in the author's laboratory based on electrophysiological methods on single capillaries have disclosed the rapidity of changes in microvascular permeability in response to many different substances--indicating reactions elicited in the endothelial cells. It is almost certain that in many cases, these changes are mediated via a transitory increase in cytosolic free calcium concentration. The overlap between permeability-increasing substances and agents that induce formation of endothelial-derived relaxing factor, EDRF, is so striking that a possible connection between vasodilatation and subtle increase in permeability should be considered. It is likely that the endothelial system of plasmalemmal invaginations ('vesicles') plays a role in the control of cytosolic calcium concentration.

Endothelium-dependent relaxation and experimental atherosclerosis in the rabbit aorta.

This study was undertaken to determine whether the production or release of the endothelium-dependent relaxatory factor is impaired in atherosclerotic New Zealand White rabbits. Atherosclerosis was induced by feeding a diet containing 2% cholesterol for 6 weeks. The production or release of endothelium-dependent relaxatory factor was assayed as follows. A 5-cm length of aorta donor was perfused with Krebs-bicarbonate buffer and the perfusate drained over a deendothelialized ring of recipient aorta set up for recording isometric tension. The recipient was precontracted with norepinephrine (0.2 mumol/L) in the perfusate. When acetylcholine was added to the perfusate, the recipient relaxed in a dose-dependent manner. This assay was used to compare the relaxatory responses produced in recipient rings by adding acetylcholine to donors from atherosclerotic and control rabbits. The relaxation produced by atherosclerotic donors were smaller than those generated by control donors (16.5 +/- 4.9 vs. 32.7 +/- 5.3%; n = 10, p less than 0.05). It is suggested that in atherosclerotic rabbits the ability of aortic endothelium to produce or release endothelium-dependent relaxatory factor is impaired.

Tracheal epithelium releases a vascular smooth muscle relaxant factor: demonstration by bioassay.

Acetylcholine caused no relaxation of a rubbed strip from rabbit aorta (RASR) which was precontracted with phenylephrine. When the same strip was taken into epithelium-intact guinea-pig trachea, acetylcholine produced a slow-developing relaxation which was antagonized by atropine but not by propranolol, indomethacin, theophylline or hydroquinone. RASR was not relaxed by acetylcholine when it was taken from epithelium-rubbed trachea. The results indicate that acetylcholine releases vascular smooth muscle relaxant factor from tracheal epithelium.