Citations

The endothelium can secrete both relaxing and contracting substances. One of the most powerful stimuli to the release of the former are thrombin and aggregating platelets. This contributes to the protective role of the endothelium against inappropriate intraluminal platelet aggregation and coagulation in blood vessels with an intact intima. Thrombin-induced, endothelium-dependent relaxations have been obtained in isolated arteries of different species, including humans. Endothelium-dependent relaxations can be evoked by autologous platelets in isolated blood vessels of the dog, pig and rat; they can be obtained in canine coronary arteries with human platelets. The major platelet-products involved in these endothelium-dependent relaxations are 5-hydroxytryptamine (serotonin) and the adenine nucleotides. Although platelet-activating factor (PAF) can evoke endothelium-dependent relaxation it only does so at concentrations much higher than those occurring under physiological conditions; since the relaxations are not prevented by PAF-antagonists, they are non-specific in nature.The receptor mediating the endothelium-dependent relaxations to serotonin released from the aggregating platelets can be subtyped as a S1~(5HT1) serotonergic receptor;those mediating the response to the adenine nucleotides as P2y-purinergic receptors. In the absence of the endothelium aggregating platelets cause contractions of vascular smooth muscle; these are mediated by a mixture of S1-like and S2~serotoner-gic receptors in coronary arteriesof the dog, and by S2-serotonergic receptors in those of the pig. Thus, in the porcine coronary artery, the S2-serotonergic antagonist ketanserin markedly enhances the platelet-induced endothelium-dependent relaxation. After previous (four weeks) injury, the regenerated endothelium of the porcine coronary artery loses the ability to respond to serotonin,and is unable to prevent the constrictionsevoked by aggregating platelets. The endothelium-dependent relaxations of porcine coronary arteries evoked by aggregating platelets are potentiated by chronic treatmentof the donor animals with cod liver oil. These studies emphasize the protective roleof the endothelial cells against the vasoconstriction (vasospasm) induced by aggregating platelets. This role is depressed after previous injury, and can be facilitatedby dietary adj ustments.

Endothelium‐dependent relaxation of blood vessels is produced by a large number of agents (e.g., acetylcholine, ATP and ADP, substance P, bradykinin, histamine, thrombin, serotonin). With some agents, relaxation may be limited to certain species and/or blood vessels. Relaxation results from release of a very labile non‐prostanoid endothelium‐derived relaxing factor (EDRF) or factors. EDRF stimulates guanylate cyclase of the vascular smooth muscle, with the resulting increase in cyclic GMP activating relaxation. EDRF is rapidly inactivated by hemoglobin and superoxide. There is strong evidence that EDRF from many blood vessels and from cultured endothelial cells is nitric oxide (NO) and that its precursor is L‐arginine. There is evidence for other relaxing factors, including an endothelium‐derived hyperpolarizing factor in some vessels. Flow‐induced shear stress also stimulates EDRF release. Endothelium‐dependent relaxation occurs in resistance vessels as well as in larger arteries, and is generally more pronounced in arteries than veins. EDRF also inhibits platelet aggregation and adhesion to the blood vessel wall. Endothelium‐derived contracting factors appear to be responsible for endothelium‐dependent contractions produced by arachidonic acid and hypoxia in isolated systemic vessels and by certain agents and by rapid stretch in isolated cerebral vessels. In all such experiments, the endothelium‐derived contracting factor appears to be some product or by‐product of cyclooxygenase activity. Recently, endothelial cells in culture have been found to synthesize a peptide, endothelin, which is an extremely potent vasoconstrictor. The possible physiological roles and pathophysiological significance of endothelium‐derived relaxing and contracting factors are briefly discussed.— F urchgott , R. F.; V anhoutte , P. M. Endothelium‐derived relaxing and contracting factors. FASEB J. 3: 2007‐2018; 1989.

Electrical stimulation induces tetrodotoxin-insensitive relaxation of the canine coronary artery. The present study was designed to verify whether this relaxation involves the production of oxygen-derived free radicals. Isolated rings of canine coronary arteries were suspended for isometric tension recording in organ chambers filled with Krebs-Ringer bicarbonate solution. They were stimulated electrically (9 V, 3 Hz, 2 ms for 2 min) by means of two platinum electrodes during contractions evoked by various vasoactive agonists. Under control conditions, electrical stimulation caused rapid, reversible relaxations. Superoxide dismutase in association with catalase or mannitol, sodium ascorbate, dimethyl sulfoxide, and glutathione did not inhibit the relaxation caused by stimulation applied for only 2 min; neither did the removal of chloride ions from the salt solution nor the association of Cl-free solution in the presence of mannitol, superoxide dismutase, and catalase. Prolonging the electrical stimulation (9 V, 3 Hz, 2 ms) for up to 20 min produced a secondary relaxation. This second phase was inhibited by sodium ascorbate. These experiments indicate that the rapid relaxation induced by short-lasting electrical stimulation is probably not due to the generation of oxygen-derived free radicals. However, prolonged stimulation causes the production of such radicals, which then evoke irreversible inhibition of the vascular smooth muscle of the canine coronary artery.

The effects of nitric oxide and acetylcholine (ACh) were investigated on the electrical and mechanical properties of vascular smooth muscle cells of the canine mesenteric artery. Isolated tissues with or without the endothelium were contracted with prostaglandin F2 alpha. Nitric oxide caused comparable concentration-dependent relaxations in rings with and without endothelium. ACh induced concentration-dependent relaxations only in arteries with endothelium. The relaxations to both nitric oxide and ACh were inhibited by methylene blue or oxyhemoglobin. Either in the presence or absence of prostaglandin F2 alpha, ACh caused transient hyperpolarization of the cell membrane of the vascular smooth muscle. The ACh-induced transient hyperpolarization was not observed after mechanical removal of the endothelial cells or after treatment with atropine. Nitric oxide (less than or equal to 8 X 10(-6) M) did not alter membrane potential, in either the presence or absence of the endothelium. The excitatory junction potentials generated by perivascular nerve stimulation were inhibited by ACh but not by nitric oxide. These results suggest that in the canine mesenteric artery 1) the endothelium-derived hyperpolarizing factor generated by ACh is not nitric oxide; 2) nitric oxide relaxes vascular smooth muscle by a direct effect; and 3) nitric oxide does not modify adrenergic neurotransmission.

Chronic treatment with estrogens enhances some endothelium-dependent relaxations. Whether or not progesterone would exert a similar effect is unknown. Experiments were designed to determine the effect of chronic treatment with progesterone on endothelium-dependent responses. Adult female dogs were ovariectomized and pellets containing carrier substance, estrogen, progesterone, or estrogen plus progesterone were implanted subcutaneously. After 14-21 days coronary arteries were removed, cut into rings, and suspended for the measurement of isometric force in organ chambers in the presence of indomethacin. Endothelium-dependent relaxations to ADP, bradykinin, or the calcium ionophore did not differ among groups. However, relaxations to acetylcholine and to the alpha 2-adrenergic agonist BHT-920 were greater in the estrogen-treated group than in the estrogen plus progesterone-treated group. In rings without endothelium, relaxations to nitric oxide and isoproterenol did not differ among groups. However, relaxations of the smooth muscle to ADP were greater in the progesterone-treated group than in the progesterone plus estrogen group. These results suggest that progesterone alone minimally affects endothelium-dependent responses. However, progesterone seems to antagonize the stimulatory effects of estrogen on two endothelium-dependent responses that are associated with pertussis toxin-sensitive guanine nucleotide regulatory proteins and the production of nitric oxide. These studies suggest that a specific receptor/second messenger system can be modulated by female reproductive steroid hormones.

Experiments were designed to determine the effects of chronic estrogen treatment on endothelium-dependent responses to arachidonic acid in the aorta of the rabbit. Ovariectomized rabbits were treated with either placebo or 17 beta-estradiol for 14 days. The aortas were removed, and rings cut from each blood vessel were suspended for measurement of isometric force. In rings contracted with a submaximal concentration of norepinephrine, endothelium-dependent contractions to arachidonic acid were augmented by estrogen treatment. Prostaglandin F2 alpha, E2, the thromboxane analogue U 46619, and prostacyclin contracted aortic smooth muscle. Only the contractions to prostacyclin were augmented by estrogen treatment. Estrogen treatment enhanced the contractions to norepinephrine in aortas with, but not in those without, endothelium; these contractions were not affected by inhibition of uptake of the neurotransmitter. Indomethacin inhibited contractions to norepinephrine in rings with endothelium from estrogen-treated rabbits. These results indicate that chronic treatment with estrogens can affect arterial reactivity to norepinephrine through an endothelium-dependent mechanism that may involve the metabolism of arachidonic acid by cyclooxygenase. The altered sensitivity of the smooth muscle to prostacyclin may contribute in part to the augmented contractions to arachidonic acid with estrogen treatment.

This study was designed to investigate whether the respiratory epithelium can modulate the tone of the underlying smooth muscle in response to decreases in partial pressure of O2 (PO2). Canine bronchial segments with or without epithelium (diameter, 4-6 mm; length, 50-60 mm) were mounted in organ chambers and perfused intraluminally with modified Krebs-Ringer bicarbonate solution [temperature, 37 degrees C; PO2 varying from 600 (control) to 40 mmHg; PCO2, 36 mmHg]. Isometric tension was recorded by means of stirrups passed through the wall of the central part of the bronchial segment. During contractions to carbachol, the tissues with epithelium showed epithelium-dependent relaxations when the PO2 was decreased. The level of relaxation was dependent on the PO2. The epithelium-dependent relaxation could not be blocked by the following agents: indomethacin, methylene blue, propranolol, or tetrodotoxin (antagonists or blockers of cyclooxygenase, guanylate cyclase, beta-adrenoceptors, and sodium channels, respectively). The epithelium-dependent relaxation was not accompanied by the release of an assayable relaxing factor in the bronchial lumen. The experiments suggest that 1) lowering the PO2 induces the epithelium to release a relaxing factor(s), which is neither a product of cyclooxygenase nor endothelium-derived relaxing factor; 2) a local reflex mechanism is not involved in the phenomenon; and 3) the relaxing factor(s) either is not released into the bronchial lumen or, if it is, is catalyzed rapidly in the lumen on release.

The present study was designed to determine the role of the respiratory epithelium as a diffusion barrier and a modulator of the responsiveness of airway smooth muscle to bronchoactive agents. Segments of canine bronchi, with or without epithelium, were suspended in organ chambers and perfused intraluminally. The isometric tension was recorded. Acetylcholine, given intraluminally, induced significantly smaller contractions in bronchi with than in bronchi without epithelium. When this agonist was given extraluminally, no difference in contractions was noted between the tissues. In the presence of acetylcholine and phentolamine, norepinephrine, given either intra- or extraluminally, induced significantly larger relaxations in bronchi with than in bronchi without epithelium. High potassium given intraluminally induced contractions only in bronchi without epithelium; however, in the presence of ouabain, both tissues contracted similarly. When high potassium was given extraluminally, no difference in contraction between tissues with and without epithelium was noted. When [3H]acetylcholine and [3H]norepinephrine were perfused intraluminally, the accumulation of 3H radioactivity in the extraluminal solutions was significantly less in bronchi with than in bronchi without epithelium. These observations suggest that the epithelium acts as both a diffusion barrier and a modulator of the responses of canine airways to bronchoactive agents.