Vasodilator and vasoconstrictor substances produced by the endothelium

The Role of Escin as a Topical Agent for Lymphedema Treatment in a Rat Model

Escin, a naturally derived material isolated from horse chestnut, is used as an anti-inflammatory and anti-edema agent. This study aimed to evaluate its effects on lymphedema in a rat tail model. We divided the rats into five groups. The treatment groups received topical application of escin gel at concentrations of 20%, 10%, 2%, and 0.5% for 4 weeks. The fifth group served as a control. We performed volumetric (water displacement) tests, H&E staining, and LYVE-1 immunohistochemical staining, followed by statistical evaluation. All treatment groups showed significant volumetric reductions compared with the control group, but no significant differences were observed between the treatment groups. H&E staining showed a significant reduction in dermal thickness in the 20%, 10%, and 2% escin treatment groups compared to the control group. Within the treatment groups, the 2% escin group showed a significant difference compared with the 20% and 10% escin groups (p = 0.021 for both). LYVE-1 immunohistochemical staining revealed a significantly higher mean lymphatic vessel count in the 2% escin group compared with the 20%, 10%, and 0.5% escin-treated groups and the control group (p = 0.019, p = 0.025, p = 0.019, and p = 0.032 respectively). Topical escin applied to a rat tail model of acute lymphedema resulted in a significant reduction in tail volume, reduced dermal thickness, and increased lymphatic structures. The 2% escin concentration may be the optimal dose for improving lymphedema in this model. Further research is warranted to explore the clinical application of escin in patients with lymphedema.

Sex differences in the participation of endothelial mediators and signaling pathways involved in the vasodilator effect of a selective GPER agonist in resistance arteries of gonadectomized Wistar rats

AIM

Endothelial mechanisms underlying the vascular effects of estrogen modulated by the G protein-coupled estrogen receptor (GPER) are not well understood, especially in gonadal sex hormone deprivation. Thus, we investigated vascular function and endothelial signaling pathways involved in the selective activation of GPER in resistance arteries of gonadectomized rats.

METHODS

Gonadectomy was performed in Wistar rats of both sexes. After 21 days, the animals were euthanized. Concentration-response curves were obtained by cumulative additions of G-1 in third-order mesenteric arteries. The vasodilatory effects of G-1 were evaluated before and after endothelium removal or incubation with pharmacological inhibitors. Tissue protein expression was measured by western blotting. Assays with 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF-FM) and 2',7' dichlorodihydrofluorescein-diacetate (DCF-DA) were performed in the arteries investigated. Immunolocalization was assessed by immunofluorescence.

RESULTS

G-1 induced partially endothelium-dependent relaxation in both sexes. The three isoforms of the enzyme nitric oxide synthase contributed to the production and release of nitric oxide in both gonadectomized groups, but the role of inducible nitric oxide synthase is more expressive in males. The mechanistic pathway by which endothelial nitric oxide synthase is phosphorylated appears to differ between sexes, with the rapid signaling pathway phosphatidylinositol-3-kinase/protein kinase B/endothelial nitric oxide synthase (PI3k-Akt-eNOS) being identified for males and mitogen-activated protein kinase/extracellular signal-regulated kinase/endothelial nitric oxide synthase (MEK-ERK-eNOS) for females. The contribution of hydrogen peroxide as an endothelial relaxation mediator seems to be greater in females.

CONCLUSION

These results provide new insights into the effects of estrogen-induced responses via GPER on vascular function in gonadal sex hormone deprivation.

Horse Chestnut Extract. Update-2022

Horse chestnut is known as a venotonizing agent of plant origin. The main active ingredient of chestnut common extract is aescin. It has anti-edema, anti-inflammatory and venotonizing properties. The aescin medicinal agent should be used for chronic vein disease, hemorrhoidal disease and post-traumatic edema. The pharmacological properties of chestnut horse extract allow the inclusion of medications based on it in the rehabilitation program of patients who have suffered a new coronavirus infection (COVID-19).

The microscopic anatomy of endothelial cells in human atherosclerosis: Focus on ER and mitochondria

Once regarded merely as a bland lipid storage disease consequence of aging, atherosclerosis is currently considered a slow and continuous inflammatory process (partially controllable by treatment) with complex etiology involving a multitude of genetic and environmental risk factors which ultimately result in the formation of the plaque. The vascular endothelium, a monolayer of endothelial cells (ECs), is an important regulatory "organ" critical for cardiovascular homeostasis in health which also contributes significantly to the pathomechanisms of several disease states, including atherosclerosis. Over the years, there has been evidence highlighting the central role of endoplasmic reticulum (ER) in the maintenance of endothelial function and perturbations in ER biology have been proposed to adversely affect a diverse range of endothelial functions. Of particular interest is the evidence that under certain pathophysiological circumstances, abnormal ER ultrastructure correlates with altered ER function and signaling and can contribute to cell injury and apoptosis. Therefore, the ultrastructural traits of ER membranes can have important implications not only for their functional bearings but also for the etiology and pathophysiology of diverse human disorders. With regard to atherosclerosis, the focus of ER research has been centered on the molecular signals originated from the ER to manage conditions of stress, leaving the fine structure of this organelle an almost unexplored (but promising) area of studies. There is, also, increasing evidence that mitochondrial dysfunction plays a critical role in promoting cell apoptosis, inflammation, and oxidative stress, thereby contributing to atheroma growth. It is within this context that the present study has been undertaken to investigate the microscopic architecture of ECs in human atherosclerosis and to determine whether the potential structural abnormalities of ER and mitochondria may play a central pathogenic role in atherogenesis or may merely reflect the condition of a tissue whose integrity has already been disturbed or destroyed. For this purpose, transmission electron microscopy (TEM) remains a powerful technique that can not only provide information about the ultrastructural state of cell organelles but also allow the correlation between different subcellular alterations indicative of a certain pathophysiological condition and cellular response. The present study expands the spectrum of ultrastructural defects known to exist in human atherosclerosis and suggests that ER alterations may be of great importance in the pathogenesis of the disease. The architectural changes of ER may be considered early pathological events that precede any overt histologic abnormalities in the vascular endothelium and its subcellular organelles, primarily the mitochondrial pool.

Endothelial dysfunction and vascular disease - a 30th anniversary update

The endothelium can evoke relaxations of the underlying vascular smooth muscle, by releasing vasodilator substances. The best-characterized endothelium-derived relaxing factor (EDRF) is nitric oxide (NO) which activates soluble guanylyl cyclase in the vascular smooth muscle cells, with the production of cyclic guanosine monophosphate (cGMP) initiating relaxation. The endothelial cells also evoke hyperpolarization of the cell membrane of vascular smooth muscle (endothelium-dependent hyperpolarizations, EDH-mediated responses). As regards the latter, hydrogen peroxide (H₂ O₂ ) now appears to play a dominant role. Endothelium-dependent relaxations involve both pertussis toxin-sensitive G_i (e.g. responses to α₂ -adrenergic agonists, serotonin, and thrombin) and pertussis toxin-insensitive G_q (e.g. adenosine diphosphate and bradykinin) coupling proteins. New stimulators (e.g. insulin, adiponectin) of the release of EDRFs have emerged. In recent years, evidence has also accumulated, confirming that the release of NO by the endothelial cell can chronically be upregulated (e.g. by oestrogens, exercise and dietary factors) and downregulated (e.g. oxidative stress, smoking, pollution and oxidized low-density lipoproteins) and that it is reduced with ageing and in the course of vascular disease (e.g. diabetes and hypertension). Arteries covered with regenerated endothelium (e.g. following angioplasty) selectively lose the pertussis toxin-sensitive pathway for NO release which favours vasospasm, thrombosis, penetration of macrophages, cellular growth and the inflammatory reaction leading to atherosclerosis. In addition to the release of NO (and EDH, in particular those due to H₂ O₂ ), endothelial cells also can evoke contraction of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factors. Recent evidence confirms that most endothelium-dependent acute increases in contractile force are due to the formation of vasoconstrictor prostanoids (endoperoxides and prostacyclin) which activate TP receptors of the vascular smooth muscle cells and that prostacyclin plays a key role in such responses. Endothelium-dependent contractions are exacerbated when the production of nitric oxide is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes). They contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive and diabetic patients. In addition, recent data confirm that the release of endothelin-1 can contribute to endothelial dysfunction and that the peptide appears to be an important contributor to vascular dysfunction. Finally, it has become clear that nitric oxide itself, under certain conditions (e.g. hypoxia), can cause biased activation of soluble guanylyl cyclase leading to the production of cyclic inosine monophosphate (cIMP) rather than cGMP and hence causes contraction rather than relaxation of the underlying vascular smooth muscle.

The fetal cerebral circulation: three decades of exploration by the LLU Center for Perinatal Biology

For more than three decades, research programs in the Center of Perinatal Biology have focused on the vascular biology of the fetal cerebral circulation. In the 1980s, research in the Center demonstrated that cerebral autoregulation operated over a narrower pressure range, and was more vulnerable to insults, in fetuses than in adults. Other studies were among the first to establish that compared to adult cerebral arteries, fetal cerebral arteries were more hydrated, contained smaller smooth muscle cells and less connective tissue, and had endothelium less capable of producing NO. Work in the 1990s revealed that pregnancy depressed reactivity to NO in extra-cerebral arteries, but elevated it in cerebral arteries through effects involving changes in cGMP metabolism. Comparative studies verified that fetal lamb cerebral arteries were an excellent model for cerebral arteries from human infants. Biochemical studies demonstrated that cGMP metabolism was dramatically upregulated, but that contraction was far more dependent on calcium influx, in fetal compared to adult cerebral arteries. Further studies established that chronic hypoxia accelerates functional maturation of fetal cerebral arteries, as indicated by increased contractile responses to adrenergic agonists and perivascular adrenergic nerves. In the 2000s, studies of signal transduction established age-dependent roles for PKG, PKC, PKA, ERK, ODC, IP3, myofilament calcium sensitivity, and many other mechanisms. These diverse studies clearly demonstrated that fetal cerebral arteries were functionally quite distinct compared to adult cerebral arteries. In the current decade, research in the Center has expanded to a more molecular focus on epigenetic mechanisms and their role in fetal vascular adaptation to chronic hypoxia, maternal drug abuse, and nutrient deprivation. Overall, the past three decades have transformed thinking about, and understanding of, the fetal cerebral circulation due in no small part to the sustained research efforts by faculty and staff in the Center for Perinatal Biology.

Endothelial dysfunction and vascular disease

The endothelium can evoke relaxations (dilatations) of the underlying vascular smooth muscle, by releasing vasodilator substances. The best characterized endothelium-derived relaxing factor (EDRF) is nitric oxide (NO). The endothelial cells also evoke hyperpolarization of the cell membrane of vascular smooth muscle (endothelium-dependent hyperpolarizations, EDHF-mediated responses). Endothelium-dependent relaxations involve both pertussis toxin-sensitive G(i) (e.g. responses to serotonin and thrombin) and pertussis toxin-insensitive G(q) (e.g. adenosine diphosphate and bradykinin) coupling proteins. The release of NO by the endothelial cell can be up-regulated (e.g. by oestrogens, exercise and dietary factors) and down-regulated (e.g. oxidative stress, smoking and oxidized low-density lipoproteins). It is reduced in the course of vascular disease (e.g. diabetes and hypertension). Arteries covered with regenerated endothelium (e.g. following angioplasty) selectively loose the pertussis toxin-sensitive pathway for NO release which favours vasospasm, thrombosis, penetration of macrophages, cellular growth and the inflammatory reaction leading to atherosclerosis. In addition to the release of NO (and causing endothelium-dependent hyperpolarizations), endothelial cells also can evoke contraction (constriction) of the underlying vascular smooth muscle cells by releasing endothelium-derived contracting factor (EDCF). Most endothelium-dependent acute increases in contractile force are due to the formation of vasoconstrictor prostanoids (endoperoxides and prostacyclin) which activate TP receptors of the vascular smooth muscle cells. EDCF-mediated responses are exacerbated when the production of NO is impaired (e.g. by oxidative stress, ageing, spontaneous hypertension and diabetes). They contribute to the blunting of endothelium-dependent vasodilatations in aged subjects and essential hypertensive patients.

Nitric oxide and cardiac function

Nitric oxide (NO) participates in the control of contractility and heart rate, limits cardiac remodeling after an infarction and contributes to the protective effect of ischemic pre- and postconditioning. Low concentrations of NO, with production of small amounts of cGMP, inhibit phosphodiesterase III, thus preventing the hydrolysis of cAMP. The subsequent activation of a protein-kinase A causes the opening of sarcolemmal voltage-operated and sarcoplasmic ryanodin receptor Ca(2+) channels, thus increasing myocardial contractility. High concentrations of NO induce the production of larger amounts of cGMP which are responsible for a cardiodepression in response to an activation of protein kinase G (PKG) with blockade of sarcolemmal Ca(2+) channels. NO is also involved in reduced contractile response to adrenergic stimulation in heart failure. A reduction of heart rate is an evident effect of NO-synthase (NOS) inhibition. It is noteworthy that the direct effect of NOS inhibition can be altered if baroreceptors are stimulated by increases in blood pressure. Finally, NO can limit the deleterious effects of cardiac remodeling after myocardial infarction possibly via the cGMP pathway. The protective effect of NO is mainly mediated by the guanylyl cyclase-cGMP pathway resulting in activation of PKG with opening of mitochondrial ATP-sensitive potassium channels and inhibition of the mitochondrial permeability transition pores. NO acting on heart is produced by vascular and endocardial endothelial NOS, as well as neuronal and inducible synthases. In particular, while in the basal control of contractility, endothelial synthase has a predominant role, the inducible isoform is mainly responsible for the cardiodepression in septic shock.

Beta-escin inhibits colonic aberrant crypt foci formation in rats and regulates the cell cycle growth by inducing p21(waf1/cip1) in colon cancer cells

Extracts of Aesculus hippocastanum (horse chestnut) seed have been used in the treatment of chronic venous insufficiency, edema, and hemorrhoids. Most of the beneficial effects of horse chestnut are attributed to its principal component beta-escin or aescin. Recent studies suggest that beta-escin may possess anti-inflammatory, anti-hyaluronidase, and anti-histamine properties. We have evaluated the chemopreventive efficacy of dietary beta-escin on azoxymethane-induced colonic aberrant crypt foci (ACF). In addition, we analyzed the cell growth inhibitory effects and the induction of apoptosis in HT-29 human colon cancer cell line. To evaluate the inhibitory properties of beta-escin on colonic ACF, 7-week-old male F344 rats were fed experimental diets containing 0%, 0.025%, or 0.05% beta-escin. After 1 week, the rats received s.c. injections of azoxymethane (15 mg/kg body weight, once weekly for 2 weeks) or an equal volume of normal saline (vehicle). Rats were continued on respective experimental diets and sacrificed 8 weeks after the azoxymethane treatment. Colons were evaluated histopathologically for ACF. Administration of dietary 0.025% and 0.05% beta-escin significantly suppressed total colonic ACF formation up to approximately 40% (P < 0.001) and approximately 50% (P < 0.0001), respectively, when compared with control diet group. Importantly, rats fed beta-escin showed dose-dependent inhibition (approximately 49% to 65%, P < 0.0001) of foci containing four or more aberrant crypts. To understand the growth inhibitory effects, HT-29 human colon carcinoma cell lines were treated with various concentrations of beta-escin and analyzed by flow cytometry for apoptosis and cell cycle progression. Beta-escin treatment in HT-29 cells induced growth arrest at the G1-S phase, which was associated with the induction of the cyclin-dependent kinase inhibitor p21(WAF1/CIP1), and this correlated with reduced phosphorylation of retinoblastoma protein. Results also indicate that beta-escin inhibited growth of colon cancer cells with either wild-type or mutant p53. This novel feature of beta-escin, a triterpene saponin, may be a useful candidate agent for colon cancer chemoprevention and treatment.

Differential expression of α1-adrenergic receptor subtypes in coronary microvascular endothelial cells in culture

It has been postulated that in blood vessels under alpha(1)-related stimulation, the endothelial intracellular calcium concentration ([Ca(2+)](i)) increases, which is necessary to induce nitric oxide synthesis, is the result of an increase in vascular smooth muscle, which subsequently, flows into the endothelial cells through gap junctions and it is not the result of a direct adrenergic stimulation of endothelial receptors. Others, however, postulate that endothelial alpha(1D)-adrenoceptors, have a direct effect on nitric oxide synthesis. In order to clarify this phenomena, in this work we analyzed the presence of alpha(1)receptor subtypes and their functional association with nitric oxide synthesis in rat coronary microvascular endothelial cells in culture, with pharmacological, immunological and reverse transcriptase polymerase chain reaction approaches. Our results show the presence and functional coupling with nitric oxide synthesis of alpha(1A) and alpha(1D)-adrenoceptor subtypes. alpha(1B)-adrenoceptor subtype is not coupled with nitric oxide production.

Characteristics of the in vitro hypoxic pulmonary vasoconstrictor response in isolated equine and bovine pulmonary arterial rings

OBJECTIVE

Hypoxaemia accompanies dorsal recumbency in the horse and frequently complicates general anaesthesia. The physiology associated with this phenomenon is poorly understood. One possible cause of poor tolerance to dorsal recumbency is an absent or reduced response to hypoxic pulmonary vasoconstriction (HPV). This study compared the HPV response in isolated pulmonary artery vessels from equivalent regions of equine and bovine lung.

ANIMALS

Equine and bovine, in vitro study.

MATERIALS AND METHODS

Equine and bovine pulmonary arteries were removed from the lungs of euthanased horses and cattle. Measurements of isometric tension were made on isolated rings of pulmonary vessels at 37 degrees C in a Krebs' saline solution. Hypoxia was induced by bubbling with a nominally 0% O(2) gas mixture.

RESULTS

A significant HPV response was observed above a baseline tension induced by phenylephrine (PE; 0.3 microm) or 5-hydroxytryptamine (5-HT; 0.1 microm). The HPV response in equine pulmonary vessels was approximately 33% less than the response observed in equivalent bovine vessels (equine 196 +/- 20%versus bovine 290 +/- 32%; p < 0.05). Removal of the endothelium (by rubbing the luminal surface) significantly reduced but did not abolish the HPV response. Incubation with the nitric oxide (NO) synthase inhibitor N-nitro-l-arginine methyl ester (L-NAME; 100 microm), or COX-1/COX-2 inhibitor indomethacin (10 microm) markedly attenuated the HPV response in equine vessels.

CONCLUSIONS

These results suggest that a significant HPV response exists in isolated equine pulmonary vessels; a component of this response requires a functional endothelium. Inhibition of cyclooxygenase and NO synthase attenuated the response, suggesting the involvement of a COX product and/or NO in mediating this effect either directly or indirectly. Alternatively, a non-COX related action of the nonsteroidal anti-inflammatory drug, indomethacin, may be involved.

[Endothelium-derived factors in hypertensive blood vessels, especially nitric oxide and hypertension]

Endothelium-dependent relaxation (EDR) in the blood vessels of spontaneously hypertensive rats (SHR) and the role of nitric oxide (NO) in the initiation of hypertension are reviewed. EDR was impaired in blood vessels of SHR depending on age and degree of hypertension when compared with those of normotensive rats. The cause of the impairment varied among the type of blood vessels: a decrease in the production of NO and endothelium-derived relaxing factor (EDRF) and an increase in the production of endothelium-derived contracting factor (EDCF) are the main causes of the impairment in large arteries, while a decrease in endothelium-dependent hyperpolarization and increased release of EDCF are the main causes of the impairment in small arteries. Interactions among these endothelium-derived factors and changes in the interactions are also causes of impairment. Superoxide may be involved in the impairment of EDR by destroying NO. The endothelium depresses smooth muscle contraction, including spontaneous tone developed in vascular smooth muscle, and the depressing effect of the endothelium is impaired in the preparations from SHR. The endothelium of blood vessels of SHR are structurally injured as demonstrated by scanning electron microscopy. Antihypertensive treatment prevented these functional and structural changes. Chronic treatment with inhibitors of NO production in normotensive rats impaired EDR and elevated blood pressure. The impairment of EDR is a secondary change due to continued hypertension, and early initiation of antihypertensive therapy is recommended.

Aescin: pharmacology, pharmacokinetics and therapeutic profile

Aescin, the major active principle from Aesculus hippocastanum (Hippocastanaceae) the horse chestnut tree, has shown satisfactory evidence for a clinically significant activity in chronic venous insufficiency (CVI), haemorrhoids and post-operative oedema. In one controlled trial aescin was shown to be as effective as compression therapy as an alternative to medical treatment for CVI. The therapeutic benefit is well supported by a number of experimental investigations in different animal models, indicative of clearcut anti-oedematous, anti-inflammatory and venotonic properties, mainly related to the molecular mechanism of the agent, allowing improved entry of ions into channels, thus raising venous tension in both in vitro and in vivo conditions. Other mechanisms, i.e. release of PGF(2) from veins, antagonism to 5-HT and histamine, reduced catabolism of tissue mucopolysaccharides, further underline the wide ranging mechanisms of the therapeutic activity of aescin. The excellent tolerability of aescin in the clinic indicates this treatment is of definite clinical benefit in patients with clinical conditions resulting in CVI, haemorrhoids or peripheral oedema formation.

Factors involved in the time course of response to acetylcholine in mesenteric arteries from spontaneously hypertensive rats

The time course of the response to prolonged application of acetylcholine in mesenteric arteries from stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY) was compared. Only a relaxing response, which was blocked by N(omega)-nitro-L-arginine (L-NOARG), was observed after the prolonged application of a low concentration of acetylcholine (10(-8) M) in both preparations; the response was impaired in SHRSP preparations. Prolonged application of a high concentration of acetylcholine (10(-5) M) induced a second contractile response after a first relaxing response in SHRSP preparations under basal conditions and in WKY preparations in the presence of L-NOARG. This contractile response was attenuated by indomethacin. In the presence of a combination of apamin and charybdotoxin, the relaxing response to the high concentration of acetylcholine was reduced and a contractile response, which was abolished by indomethacin, appeared. In the presence of all of these blockers, a contractile response, which was blocked by cyclo(D-alpha-aspartyl-L-propyl-D-valyl-L-leucyl-D-tryptophyl) (BQ-123), was observed in preparations from WKY but not in preparations from SHRSP. Results indicate that prolonged application of acetylcholine in rat mesenteric arteries induces the release of endothelium-derived relaxing, contracting, hyperpolarizing factors and endothelin-1, and that the mode of action differs between preparations from WKY and SHRSP.

Hypertension and impairment of endothelium-dependent relaxation of arteries from spontaneously hypertensive and L-NAME-treated Wistar rats

Effects of chronic treatment of normotensive Wistar rats with N(omega)-nitro-L-arginine methyl ester (L-NAME) on blood pressure and on endothelium-dependent relaxation of the aorta, carotid and iliac arteries were studied. The endothelium-dependent relaxation was compared in arteries from normotensive Wistar Kyoto rats (WKY) and genetically hypertensive rats (stroke-prone spontaneously hypertensive rats, SHRSP). Chronic treatment of normotensive Wistar rats with L-NAME caused an elevation of blood pressure. The elevated blood pressure at 15 weeks of age was significantly higher in these animals than that of untreated Wistar rats, but lower than that of SHRSP. Endothelium-dependent relaxation of the arteries induced by acetylcholine (ACh) was almost abolished by chronic treatment with L-NAME. The remaining small relaxation in arteries from L-NAME-treated rats was completely inhibited by application of L-NAME (10(-4) M). In such preparations, higher concentrations of ACh induced a contraction, which was abolished by removal of the endothelium or by an application of indomethacin (10(-5) M). Endothelium-independent relaxation induced by sodium nitroprusside was similar between preparations from untreated and L-NAME-treated Wistar rats. Endothelium-dependent relaxation was significantly impaired in preparations from SHRSP, when compared with that in those from WKY. However, the impairment was less prominent in preparations from SHRSP than in those from L-NAME-treated rats. These results suggest that the impairment of endothelium-dependent relaxation in the arteries from L-NAME-treated rats is not due to the elevated blood pressure resulting from the chronic treatment, and that impairment of NO synthesis by the endothelium does not play a major role in the initiation of hypertension in SHRSP.

Difference in effects of stretch on depressive effect of endothelium-derived nitric oxide on noradrenaline- and high-K+-induced contractions between the aortae from normotensive and spontaneously hypertensive rats

Difference in effects of stretch tension on endothelium-derived nitric oxide (EDNO)-dependent depression of noradrenaline (NA)- and high-K+-induced contraction between the aortae from normotensive Wistar Kyoto rats (WKY) a nd stroke-pronespontaneously hypertensive rats (SHRSP) was studied. NA-induced contraction in preparations both from WKY and SHRSP was augmented in the presence of N(omega)-nitro-L-arginine (L-NNA). This augmentation was minimized when the spontaneous tone, which was more prominent in preparations from SHRSP, was subtracted and the effects of L-NNA became less prominent in preparations from SHRSP. The effects of L-NNA were maximal at the stretch tension of 15 mN and, then, decreased as stretch tension increased in both preparations when the spontaneous tone was subtracted. The effects of L-NNA were less prominent when the contraction was initiated by high-K+, although the effects of stretch on high-K+-induced contraction were similar to that of NA-induced contraction. These results suggested 1) that both NA- and high-K+-induced contractions are depressed by EDNO, 2) that the release of EDNO induced by high-K+ is less than that by NA, 3) that increase in stretch tension decreases the release of EDNO, and 4) that the depressive effect of EDNO on contraction is impaired in the aorta of SHRSP.

Effect of vitamin C on forearm blood flow and glucose metabolism in essential hypertension

In 9 patients with essential hypertension, we tested whether a high-dose (12 mg. min(-1)) vitamin C infusion into the brachial artery, by improving endothelium-dependent vasodilatation, would also attenuate the insulin resistance of deep forearm tissues. We measured the effect of vitamin C on acetylcholine (Ach)-induced vasodilatation and on forearm glucose uptake during systemic hyperinsulinemia; in all studies, the contralateral forearm served as the control. Intrabrachial Ach infusion produced a stable increase in forearm blood flow, from 2.6+/-0.3 to 10.6+/-2.1 mL. min(-1). dL(-1); when vitamin C was added, a further rise in forearm blood flow (to 13.4 mL. min(-1). dL(-1); P<0.03 vs Ach alone) was observed. In response to insulin, blood flow in both the infused and control forearms did not significantly change from baseline values (+10+/-16% and +2+/-11%, respectively). In contrast, when vitamin C was added, blood flow in the infused forearm increased significantly (to 3.7+/-0.7 mL. min(-1). dL(-1); P<0.02 vs 2.8+/-0.6 mL. min(-1). dL(-1) in the control forearm). Insulin stimulated whole-body glucose disposal to 20+/-2 micromol. min(-1). kg(-1), compatible with the presence of marked insulin resistance. Forearm glucose uptake was similarly stimulated after 80 minutes of insulin infusion (to 2.11+/-0.42 and 2.06+/-0.43 micromol. min(-1). dL(-1), infused and control, respectively). When intrabrachial vitamin C was added, no difference in glucose uptake was observed between the 2 forearms (infused, 2.37+/-0.44 micromol. min(-1). dL(-1)and control, 2.36+/-0. 53 micromol. min(-1). dL(-1)). Forearm O(2) uptake at baseline was also similar in the 2 forearms (infused, 9.7+/-0.7 micromol. min(-1). dL(-1) and control, 9.6+/-1.1 micromol. min(-1). dL(-1)) and was not changed by either insulin or vitamin C. We conclude that in the deep forearm tissues of patients with essential hypertension and insulin resistance, an acute improvement in endothelial function, obtained with pharmacological doses of vitamin C, restores insulin-mediated vasodilatation but does not improve insulin-mediated glucose uptake. Thus, the endothelial dysfunction of essential hypertension is unlikely to be responsible for their metabolic insulin resistance.

Dexfenfluramine-induced contraction of human and rat isolated pulmonary arteries

Mechanisms of dexfenfluramine-induced vasoconstriction were studied in isolated pulmonary arteries suspended in organ baths for isometric tension recording. Dexfenfluramine (10(-7)-10(-4) M) caused concentration-dependent contractions in rat and human pulmonary arteries with and without endothelium. In pulmonary arteries of the rat, the response to dexfenfluramine was nearly abolished by treatment with the alpha-adrenoceptor antagonists, phentolamine (10(-6) M) or prazosin (10(-7) M). In human pulmonary arteries, the concentration-response curve to dexfenfluramine was unaltered by the presence of phentolamine (10(-6) M), prazosin (10(-7) M), ketanserin (10(-6) M), or indomethacin (3x10(-6) M). The results suggest that dexfenfluramine causes contraction of pulmonary vascular smooth muscle by multiple mechanisms, one of which involves activation of alpha-adrenoceptors within the blood vessel wall. The mechanisms by which dexfenfluramine causes pulmonary vasoconstriction may differ between rat and human pulmonary arteries.

Relaxation induced by histamine is not endothelium dependent in tail arteries of L-NAME-treated rats

The present study was carried out to evaluate the relaxation induced by histamine in tail arteries of rats after chronic inhibition of nitric oxide (NO) synthesis with the inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) compared to tail arteries of control rats. The maximum relaxation induced by histamine was greater in control (88.09% +/-5.50, n=6) than in L-NAME arteries (47.33% +/-6.40, n=6), although pD(2) values were not different between the two groups (control: 4.89+/-0.08; L-NAME: 4.81+/-0.10). After incubation with 100 microM L-NAME in vitro, the maximum relaxation induced by histamine was only reduced in the control arteries (44.93% +/-2.35, n=6), whereas it had no effect on aortas of rats pretreated with this inhibitor. The incubation with 100 microM L-NAME had the same effect as endothelium removal in both arterial groups. Furthermore, the relaxation induced by histamine was unaffected by indomethacin. The combination of L-NAME and the histamine antagonist cimetidine completely abolished the relaxation induced by histamine in both arterial groups. These results show that when NO synthesis is impaired, the relaxation induced by histamine is endothelium independent, and when NO-synthase is active, the relaxation involves both NO released from endothelial cells and an endothelium-independent mechanism that is sensitive to cimetidine.

Effect of long-term ouabain treatment on contractile responses of rat aortae

Male Sprague-Dawley rats were infused with 50 microg/kg/day of ouabain for 4 weeks to address the question whether prolonged exposure to the drug affects blood pressure, the in vitro contractile responses to agonists and high K+ of their aortae, and the influence of endothelium on these responses. Systolic blood pressure was not affected by ouabain treatment. The responsiveness of endothelium-intact aortae from ouabain-treated rats to endothelin-1 increased, that to phenylephrine decreased, and that to high K+ was unchanged, as compared with control. The responses of endothelium-free aortae to endothelin-1, phenylephrine, and high K+ were lower in ouabain-treated than in control rats. The removal of endothelium increased the response to phenylephrine and decreased that to high K+ in either control or ouabain-treated rat aortae, whereas it did not affect the response to endothelin-1 in control rat aortae and decreased it in ouabain-treated rat aortae. The response to caffeine was unaffected by either ouabain treatment or endothelium removal. Thus rat ouabain long-term treatment induces opposing effects on the responsiveness of their intact aortae to an alpha-adrenergic agonist and endothelin-1. If these effects observed in the ex vivo experiments occur also in vivo on rat microvasculature, they could balance out and contribute to the lack of effect on systolic blood pressure of prolonged ouabain treatment.

How to assess endothelial function in human blood vessels

This brief review discusses the ways, if and when available, to examine endothelium-dependent changes diameter in human blood vessels. It stresses the problems in ensuring proper matching between arteries (and veins) from different human sources. It briefly considers the evidence in vitro supporting the role of endothelium-derived nitric oxide, hyperpolarizing factor and contracting factors (including metabolites of arachidonic acid and endothelin). It emphasizes the difficulty in extrapolating observations obtained in isolated arteries (and veins) to the intact human circulation. The overall conclusion is that the interpretations derived from animal work apply to the human vasculature.

Tension oscillation in arteries and its abnormality in hypertensive animals

1. The mechanisms of oscillatory contraction of arterial smooth muscle in vitro are discussed. 2. The membrane potential and cytoplasmic free Ca2+ concentration in smooth muscle cells oscillate in the presence of agonists. 3. The oscillatory change in the membrane potential of smooth muscle cells is related to Ca2+ release from intracellular stores. 4. Gap junctions between smooth muscle cells play important roles in the synchronized oscillation of the cytoplasmic free Ca2+ concentration in this population of cells. 5. Endothelial cells may increase or decrease the tension oscillation of smooth muscle cells. 6. In arteries from hypertensive rats, an increase in membrane excitability and the number of gap junctions between smooth muscle cells and impaired endothelial function are the main factors responsible for the modulation of tension oscillation.

Endothelium-derived relaxing, contracting and hyperpolarizing factors of mesenteric arteries of hypertensive and normotensive rats

Differences in the acetylcholine (ACh)-induced endothelium-dependent relaxation and hyperpolarization of the mesenteric arteries of Wistar Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) were studied. Relaxation was impaired in preparations from SHRSP and tendency to reverse the relaxation was observed at high concentrations of ACh in these preparations. Relaxation was partly blocked by NG-nitro-L-arginine (L-NOARG, 100 microM) and, in the presence of L-NOARG, tendency to reverse the relaxation was observed in response to higher concentrations of ACh, even in preparations from WKY. The relaxation remaining in the presence of L-NOARG was also smaller in preparations from SHRSP. The tendency to reverse the relaxation observed at higher concentrations of ACh in preparations from SHRSP or WKY in the presence of L-NOARG were abolished by indomethacin (10 microM). Elevating the K+ concentration of the incubation medium decreased relaxation in the presence of both indomethacin and L-NOARG. Relaxation in the presence of L-NOARG and indomethacin was reduced by the application of both apamin (5 microM) and charybdotoxin (0.1 microM). This suggests that the relaxation induced by ACh is brought about by both endothelium-derived relaxing factor (EDRF, nitric oxide (NO)) and hyperpolarizing factor (EDHF), which activates Ca2+-sensitive K+ channels. Electrophysiological measurement revealed that ACh induced endothelium-dependent hyperpolarization of the smooth muscle of both preparations in the presence of L-NOARG and indomethacin; the hyperpolarization being smaller in the preparation from SHRSP than that from WKY. These results suggest that the release of both NO and EDHF is reduced in preparations from SHRSP. In addition, indomethacin-sensitive endothelium-derived contracting factor (EDCF) is released from both preparations; the release being increased in preparations from SHRSP.

Hypoxia increases vasodilator release from internal mammary artery and saphenous vein grafts

BACKGROUND

Greater release of endothelium-derived nitric oxide is implicated in the superior patency of the internal mammary artery (IMA) used in coronary artery bypass grafting. This study compared the release of endothelium-derived nitric oxide into the lumen of the IMA and the saphenous vein under normoxic versus hypoxic conditions.

METHODS

Segments of canine IMA and saphenous vein were perfused in vitro. Vasorelaxant activity was measured as vasodilatation of coronary artery smooth muscle induced by the effluent.

RESULTS

Effluents from the IMA and saphenous vein caused comparable vasodilation of coronary artery smooth muscle. The vasodilatation reversed when perfusion was switched to a prosthetic conduit. Vasodilator activity from the IMA and saphenous vein was attenuated by removing the intima of the grafts or by adding N(G)-monomethyl-L-arginine (10(-4) mol/L) or N(G)-nitro-L-arginine (10(-4) mol/L), two inhibitors of nitric oxide synthesis. Indomethacin attenuated vasorelaxant activity from saphenous vein grafts but not IMA grafts (n = 10). Vasodilator release from the IMA and saphenous vein was augmented by hypoxia. This augmentation was inhibited by indomethacin (n = 10, p < 0.05). Hypoxic augmentation reversed with return to normoxia.

CONCLUSIONS

The release of endothelium-derived nitric oxide and prostacyclin from bypass grafts into the lumen, particularly during hypoxemia, could promote the vasodilation of distal coronary arterial beds, enhancing myocardial perfusion.

Hypomagnesemia inhibits nitric oxide release from coronary endothelium: protective role of magnesium infusion after cardiac operations

BACKGROUND

Postoperative hypomagnesemia is common in patients who have undergone cardiac operations and is associated with clinically significant morbidity resulting from atrial and ventricular dysrhythmias. Magnesium supplementation may increase the cardiac index in the early postoperative period.

METHODS

The action of the magnesium cation on coronary vascular reactivity was studied. Segments of canine epicardial coronary artery were suspended in organ chambers to measure isometric force (95% O2/5% CO2, 37 degrees C).

RESULTS

In coronary segments constricted with prostaglandin F2alpha (2 x 10[-6] mol/L), acetylcholine and adenosine diphosphate (10[-9] to 10[-4] mol/L) induced vasodilation in arteries with endothelium (n=10, each group; p < 0.05). Acetylcholine-mediated vasodilation was blocked by NG-monomethyl-L-arginine (10[-4] mol/L) and NG-nitro-L-arginine (10[-4] mol/L), two inhibitors of nitric oxide synthesis from L-arginine (n=10, p < 0.05). The removal of magnesium from the organ chamber solution impaired vasodilation in response to acetylcholine and adenosine diphosphate. However, normal endothelium-dependent vasodilation could be restored by return of magnesium to the bathing solution. Vascular relaxation in response to bradykinin (10[-9] to 10[-6] mol/L), which was found to induce endothelium-dependent vasodilation independent of nitric oxide production, was unaffected by magnesium removal (n=10).

CONCLUSIONS

Hypomagnesemia selectively impaired the release of nitric oxide from the coronary endothelium. Because nitric oxide is a potent endogenous nitro-vasodilator and inhibitor of platelet aggregation and adhesion, hypomagnesemia could promote vasoconstriction and coronary thrombosis in the early postoperative period.

A prostaglandin, not NO, mediates endothelium-dependent dilation in ventral aorta of shark (Squalus acanthias)

In mammals, the vascular endothelium releases a variety of paracrine factors, including the vasodilatory prostaglandin (PG)I2 and nitric oxide (NO), which is generally accepted as the major endothelium-derived relaxing factor (EDRF) in mammals. Current evidence for the vascular NO-EDRF system in fishes is contradictory. In addition, the role of PGs in the control of fish vascular tension is also unclear. We have utilized isolated rings of the ventral aorta of the spiny dogfish shark to examine the ability of various components of the NO system to dilate this vessel. Neither the NO precursor L-arginine, the NO donor sodium nitroprusside, nor NO itself dilated the rings. The Ca2+ ionophore A-23187 did produce an endothelium-dependent dilation that was not inhibited by the NO synthase inhibitor NG-nitro-L-arginine methyl ester but was inhibited by the cyclooxygenase inhibitor indomethacin, suggesting that PGs are involved. PGE1 and carbaprostacyclin, but not PGI2, produced concentration-dependent dilation, and intact aortic rings secreted five times as much PGI2 as PGE in both the unstimulated state and after stimulation with A-23187. Our data suggest strongly that a PG, most probably PGI2, is the EDRF in the ventral aorta of this shark species.

Heterogeneous cholinergic reactions of ringed seal coronary arteries

Coronary blood flow of some seal species is unusual in that it is highly variable in both non-diving and diving conditions and shows intermittent fluctuations, especially during dives when it frequently ceases for brief periods. We sought regulatory mechanisms governing these reactions by studying isometric tension recordings of isolated left circumflex (LC) and left anterior descending (LAD) coronary arteries of ringed seals, Phoca hispida, during reactions to a variety of agents for stimulating or blocking autonomic responses of the vascular smooth muscle. Micromolar acetylcholine (ACh) produced constriction of the small diameter segments of the LAD, but relaxation of the LC and larger segments of LAD. Both constrictions and dilations were prevented by atropine. Small vessel constriction by ACh was prevented by micromolar indomethacin and by a thromboxane receptor antagonist. Large vessel ACh dilations were prevented or reduced by rubbing off the endothelium and by the L-arginine analog, L-NG-nitro-arginine, an inhibitor of nitric oxide synthesis. We conclude that cholinergic, muscarinic, dilation of ringed seal large coronary arteries is mediated by endothelial-derived relaxing factor (EDRF), whereas ACh constriction of small arteries is mediated by a prostaglandin.

Selective large coronary endothelial dysfunction in conscious dogs with chronic coronary pressure overload

Coronary vascular responses to acetylcholine (ACh, 3 micrograms/kg i.v.), nitroglycerin (NTG, 25 micrograms/kg i.v.), and a 20-s coronary artery occlusion (reactive hyperemia, RH) were investigated in seven conscious dogs with severe left ventricular (LV) hypertrophy and chronic coronary pressure overload (CCPO) due to supravalvular aortic banding and in seven control dogs. All dogs were instrumented for measurement of ultrasonic coronary diameter (CD) and Doppler coronary blood flow (CBF). LV-to-body weight ratio was increased by 82% in CCPO dogs. In control dogs, ACh increased CD (+ 5.9 +/- 1.7%). This response was reduced (P < 0.05) in CCPO dogs (+ 1.9 +/- 0.9%). Similarly, flow-mediated increases in CD after RH were blunted (P < 0.01) in CCPO (+ 2.1 +/- 0.8) vs. control dogs (+ 6.8 +/- 1.8%). In contrast, ACh and RH increased CBF similarly in both groups. Increases in both CD and CBF to NTG were not different between control dogs and CCPO. Peak systolic CBF velocity was greater, P < 0.01, in CCPO (94 +/- 17 cm/s) compared with control (35 +/- 7 cm/s) dogs, most likely secondary to the increased systolic coronary perfusion pressure (215 vs. 130 mmHg). Histological analyses of large coronary arteries in CCPO revealed medial thickening, intimal thickening, and disruption of the internal elastic lamina and endothelium. In contrast, small intramyocardial arterioles failed to show the intimal and endothelial lesions. Thus, in CCPO selective to the coronary arteries, i.e., a model independent from systemic hypertension and enhanced levels of plasma renin activity, endothelial control was impaired for both flow-mediated and receptor-mediated large coronary artery function, which could be accounted for by the major morphological changes in the large coronary arteries sparing the resistance vessels. The mechanism may involve chronically elevated systolic coronary perfusion pressure, CBF velocity, and potential disruption of laminar flow patterns.

Inhibition of nitric oxide synthesis increases adenosine production via an extracellular pathway through activation of protein kinase C

BACKGROUND

NO is known to deactivate protein kinase C (PKC). Because we have reported that the activation of PKC activates ecto-5'-nucleotidase, we examined whether the inhibition of NO synthesis increases ecto-5'-nucleotidase activity through the activation of PKC.

METHODS AND RESULTS

The left anterior descending coronary artery (LAD) was cannulated and perfused with blood through a bypass tube from the left carotid artery in 65 open-chest dogs. The intracoronary administration of NG-nitro-L-arginine methyl ester (L-NAME, 10 microg x kg[-1] x min[-1]), an NO synthase inhibitor, for 30 minutes increased (P<.05) adenosine levels in coronary venous blood (123+/-10 versus 21+/-3 pmol/mL) and ecto-5'-nucleotidase activity (64+/-6 versus 41+/-4 nmol x mg[-1] x min[-1]) in the LAD-perfused myocardium. The intracoronary administration of alpha,beta-methyleneadenosine 5'-diphosphate, an inhibitor of ecto-5'-nucleotidase, or GF109203X or calphostin C, both of which are PKC inhibitors, attenuated the L-NAME-induced increases in adenosine levels and ecto-5'-nucleotidase activity. Treatment of cultured human coronary arterial endothelial cells (HCAECs) with L-NAME for 30 minutes increased ecto-5'-nucleotidase activity, which was inhibited by either GF109203X or calphostin C. NO releasers decreased both ecto-5'-nucleotidase and PKC activities in HCAECs. Treatment of HCAECs with zaprinast, a selective inhibitor of cGMP-specific phosphodiesterase, with or without atrial natriuretic peptide, increased intracellular cGMP concentrations but did not change ecto-5'-nucleotidase activity.

CONCLUSIONS

These results indicate that the inhibition of NO synthesis increases both adenosine production and ecto5'-nucleotidase activity through the activation of PKC and that NO modulates ecto-5'-nucleotidase via cGMP-independent mechanisms.

Responses of rat postganglionic sympathetic vasoconstrictor neurons following blockade of nitric oxide synthesis in vivo

The hypothesis was tested that the activation of postganglionic sympathetic neurons contributes to the peripheral vasoconstriction and the blood pressure increase which are observed in rats after systemic blockade of nitric oxide synthase by substituted L-arginine analogues. Single and multifiber postganglionic sympathetic activity supplying hindlimb hairy skin and the activity in the caudal lumbar sympathetic trunk supplying mainly hindlimb skeletal muscle were recorded in anaesthetized, paralysed and artificially ventilated Wistar rats before, during and up to 1 h after intravenous injection of a supramaximal dose (10 or 35 mg/kg) of N(G)-nitro-L-arginine methyl ester. This elicited a sustained rise of arterial blood pressure, a long-lasting decrease in heart rate and vasoconstriction in hindlimb skin and skeletal muscle as measured by laser Doppler flowmetry. With intact buffer nerves all sympathetic neurons analysed responded with a decrease in their ongoing activity in parallel with the vasoconstriction and the increased blood pressure, except for one neuron which was unresponsive. These responses were probably mediated by the arterial baroreceptors. since it was shown that N(G)-nitro-L-arginine methyl ester did not impair the function of both the afferent and the efferent limb of the reflex. Furthermore, baroreceptor denervation almost abolished the inhibitory responses in sympathetic neurons. In baroreceptor denervated animals, with a latency of about 15 min after N(G)-nitro-L-arginine methyl ester there was an increase in sympathetic activity without a further increase in blood pressure. It was concluded that sympathetic vasoconstrictor neurons which supply the rat hindlimb do not contribute by neurogenic means to the vasoconstriction and the blood pressure increase occurring after blockade of the nitric oxide synthase. Instead, the results suggest that sympathetic vasoconstrictor neurons, via the baroreceptor loop, counteract the vasoconstriction caused by the blockade of endothelium-derived nitric oxide. Therefore nitric oxide does not seem to play a role in the central regulation of activity in the sympathetic vasoconstrictor pathways studied here. The long latency increase in sympathetic activity observed after N(G)-nitro-L-arginine methyl ester in baroreceptor-denervated animals may be due to an impairment of blood flow in the brainstem.

Effects of potassium channel blockers on resting tone in isolated coronary arteries

The effects of several potassium channel blockers on resting vasomotor tone were studied in porcine isolated coronary arteries. Coronary artery rings were suspended in organ baths for isometric tension recording. The nonselective potassium channel blockers tetraethylammonium (TEA 10(-5)-3 x 10(-2) M) and 4-aminopyridine (4-AP 10(-5)-10(-2) M) caused concentration-dependent contractions that were similar in rings with and without endothelium. The concentration-response curves to TEA and 4-AP were unaffected by treatment with phentolamine (3 x 10(-6) M),propranolol (10(-6) M), or atropine (10(-6) M). Diltiazem (10(-6) M) almost abolished the contractions evoked by TEA and 4-AP. Charybdotoxin (10(-9)-10(-7) M) and apamin (10(-8)-10(-6) M), selective blockers of large and small calcium-activated potassium channels, respectively, and glyburide (10(-8)-10(-6) M), a selective blocker of ATP-sensitive potassium channels, caused little or no contraction in rings with or without endothelium. Therefore, in isolated coronary arteries, TEA and 4-AP caused contractions that were independent of the release of vasoactive mediators from the endothelium or perivascular nerves. These effects are not mediated by ATP-sensitive potassium channels or by large and small conductance calcium-activated potassium channels. The data are consistent with an effect of TEA and 4-AP on resting membrane potassium conductance in coronary arteries, resulting in contractions that are sensitive to inhibition by diltiazem. This pattern of responsiveness of isolated coronary arteries to potassium channel blockers differs from that observed in vessels from other vascular beds.

Endothelium-dependent relaxation by alpha 2-adrenoceptor agonists in spontaneously hypertensive rat aorta

Differences in alpha(2)-adrenoceptor-induced relaxation of the aorta between stroke-prone spontaneously hypertensive rats (SHRSP) and control normotensive Wistar Kyoto rats (WKY) were studied. Changes in the tension of ring preparations of the aortas were measured isometrically. Relaxation was observed in the preparations precontracted in the presence of ONO-11113, a thromboxane A(2) analogue. The alpha(2)-agonist clonidine and UK-14304 induced dose-dependent relaxation in both the WKY and SHRSP preparations. The relaxation was impaired in the SHRSP preparation. A modified sandwich experiment showed that the relaxing substance from the SHRSP endothelium was decreased. Acetylcholine (ACh) also induced dose-dependent relaxation, and the relaxation was impaired in the SHRSP preparations. alpha(2)-Agonists induced a greater degree of impairment in the relaxation than did ACh. The relaxation induced by alpha(2)-agonists and by ACh was blocked by N G-nitro-L-arginine (L-NNA). Indomethacin improved the relaxation induced by ACh but not that induced by alpha(2)-agonists in the SHRSP aortas. These results suggest that the impairment of relaxation by alpha(2)-agonists in SHRSP is not caused by the increase in the release of endothelium-derived contracting factor (EDCF) but by the reduction in the release of nitric oxide (NO). Alteration of the alpha(2)-adrenoceptors and/or the intracellular mechanism through which NO is synthesized by stimulation of the alpha(2)-adrenoceptors may be the cause of the reduction in relaxation.

Alteration in the release of endothelium-derived relaxing factors by alpha-adrenoceptor stimulation in the aorta of stroke-prone spontaneously hypertensive rats

1. Endothelium-dependent relaxation by alpha-adrenoceptor agonists was examined in the thoracic aorta from normotensive Wistar-Kyoto (WKY) rats and stroke-prone spontaneously hypertensive rats (SHRSP). 2. In ring preparations from both strains, noradrenaline-induced contraction was increased by L-nitro arginine (L-NNA), a NO synthesis inhibitor. 3. L-NNA increased the contraction induced by phenylephrine, an alpha1-adrenoceptor agonist. UK-14304 and clonidine, alpha2-adrenoceptor agonists, did not contract the preparations with intact endothelium. However, these agents contracted preparations when NO synthesis was inhibited. 4. In a precontracted preparation, clonidine and UK-14304 induced relaxations. The relaxations in SHRSP aorta were smaller than those in WKY aorta. 5. These results indicate that alpha-agonists release NO from endothelium in WKY and SHRSP aorta. The mechanism related to NO release by alpha2-adrenoceptor agonist is impaired in SHRSP aorta.

Vasoactivity of the coronary artery of rainbow trout, steelhead trout, and dogfish: lack of support for non-prostanoid endothelium-derived relaxation factors

This study surveyed and compared vasoactive responses of isolated coronary vessels from steelhead trout (Oncorhynchus mykiss), rainbow trout (also O. mykiss), and spiny dogfish (Squalus acanthias). The purpose of the investigation was twofold: to identify vasoactive controls that were possibly mediated by the vascular endothelium and to highlight the possible consequences on vasoactivity of the coronary lesions known to be present in the main coronary of salmonids but not dogfish. The test substances included acetylcholine, adenosine, adenosine triphosphate, adenosine diphosphate, serotonin, thrombin, bradykinin, prostaglandin F2α, prostaglandin I2, prostaglandin E2, and the fatty acids arachidonic acid, docosahexaenoic acid, and eicosapentaenoic acid. Acetylcholine, adenosine, adenosine triphosphate, adenosine diphosphate and prostaglandin F2αtypically produced contractions. Use of endothelial removal techniques and antagonists failed to reveal any relaxations that might involve the endothelium. Thrombin and bradykinin had no vasoactivity. Serotonin, prostaglandin I2, and prostaglandin E2produced relaxations that were not mediated by the endothelium. The powerful relaxations observed with prostaglandin I2and prostaglandin E2and the powerful contractions observed with prostaglandin F2αsuggest a major role of prostanoids in coronary vasoactivity in fish. These prostanoid-mediated mechanisms, in addition to the previously demonstrated powerful contractions with endothelin-1, point to an important role for the endothelium. No major qualitative or quantitative differences in vasoactivity could be related to differences in coronary lesion severity.

The role of nitric oxide in the initiation and in the duration of some vasodilator responses in the coronary circulation

In the coronary bed vasodilation can be mediated by several mechanisms including endothelium-produced nitric oxide. To examine the contribution of nitric oxide, three different techniques to cause vasodilation in the coronary vessels were used in the anaesthetized dog: intracoronary injection of 1 microgram acetylcholine, sudden reduction of the aortic blood pressure inducing a myogenic response and transient occlusion followed by release of the left circumflex coronary artery causing reactive hyperaemia. Each manoeuvre was performed before and after intracoronary administration of 100 mg N-nitro-L-arginine, an inhibitor of the synthesis of nitric oxide. In contrast to previous investigations, the inhibition of nitric oxide synthesis was prevented from causing an increase in blood pressure by the use of a blood-pressure-compensating device. The results observed during each of the three techniques, suggest that the initial cause of the vasodilatation is not the result of the increase of the production of nitric oxide. However, subsequent to the initiation of vasodilation, an increase in the shear stress can result in an increase in the release of nitric oxide from the vascular endothelium, thus prolonging the vasodilatation obtained using each technique.

Control of coronary blood flow by endothelial release of nitric oxide

1. Nitric oxide (NO) is released from vascular endothelium following conversion of L-arginine to L-citrulline by calcium-calmodulin-dependent 'constitutive' NO-synthase. 2. Nitric oxide release occurs under basal conditions, in response to chemical stimuli (acetylcholine, bradykinin, thrombin, prostacyclin, serotonin, etc.) and in response to changes in shear stress (effects of blood velocity on vascular endothelium). 3. Analogues of L-arginine inhibit NO and are widely used to study the effects of NO on the cardiovascular system: in intact animals, these inhibitors cause vasoconstriction, leading to an increase in arterial blood pressure (ABP) and bradycardia. 4. Bradycardia induced by NO inhibitors is due, in part, to baroreceptor activity following the increase in ABP and in part to a direct effect on the sino-atrial node. 5. In the intact animals and isolated perfused heart, NO inhibitors cause coronary vasoconstriction and hence a reduction in basal coronary flow. This effect, however, is not seen in isolated coronary vessels. 6. From experiments in which ABP did not change, NO does not appear to have an important role in regulating coronary vasomotor tone under basal conditions. 7. Nitric oxide appears to be involved in the duration of reactive hyperaemia following coronary vascular occlusion but is not involved to any significant extent in the peak amplitude of hyperaemia. 8. Responses to vasodilator stimuli which do not involve NO in the initiation of the vasodilation may be prolonged by the effect of increased blood flow (shear stress) which releases NO and potentiates hyperaemia.

Augmented agonist-induced Ca(2+)-sensitization of coronary artery contraction in genetically hypertensive rats. Evidence for altered signal transduction in the coronary smooth muscle cells

The Ca2+ responsiveness of vascular smooth muscle myofilaments is not unique: it is increased during neuro-humoral activation and decreased during beta-adrenergic stimulation. In this study we tested whether an augmented Ca2+ responsiveness of smooth muscle myofilaments may contribute to the increased coronary tone observed in hypertension using beta-escin-permeabilized coronary arteries from 3-mo-old stroke-prone spontaneously hypertensive rats (SHRSP) and their age matched normotensive reference strain (WKY rats). In intact coronary arteries, the response to 5-hydroxytryptamine (5-HT) but not to KCl was larger in SHRSP than in WKY rats. In beta-escin permeabilized coronary arteries in which the receptor effector coupling is still intact, 5-HT enhanced force at constant submaximal (Ca2+) (pCa 6.38) to a greater extent in SHRSP. The Ca2+ sensitizing effect of 5-HT was mimicked by GTP gamma S (0.01-10 microM); again this effect was larger in SHRSP. In the absence of 5-HT or GTP gamma S the Ca2+ force relation was similar in both groups. Forskolin induced relaxation at constant submaximal (Ca2+). This desensitizing effect was smaller in SHRSP than in WKY rats. In conclusion, this study shows that intracellular signalling pathways involved in modulating the Ca2+ responsiveness of coronary smooth muscle myofilaments are altered in the genetically hypertensive animals favoring a hypercontractile state in the coronary circulation.