A novel potent vasoconstrictor peptide produced by vascular endothelial cells

Presence of immunoreactive endothelin in human plasma

A highly specific and sensitive radioimmunoassay has been established for measurement of human endothelin (hET) in human plasma. After extraction of plasma with an octyl-silica column, this assay allowed for detection of immunoreactive (IR) hET as low as 0.2 fmol/ml. In 16 healthy subjects, the mean concentration of plasma IR-hET was 0.6 fmol/ml. Reverse-phase HPLC coupled with radioimmunoassay revealed two major IR-hET components, one corresponding to authentic hET(1-21) and another with more hydrophilicity than hET(1-21). These data indicate that ET is a circulating vasoconstrictor hormone in man.

The role of EDRF in flow distribution: a microangiographic study of the rabbit isolated ear

A microangiographic technique was used to study the influence of endothelium-derived relaxing factor (EDRF) on vasomotor control mechanisms in resistance vessels of intact buffer-perfused rabbit ear. Selective inhibition of EDRF activity by hemoglobin unmasked an intrinsic ("myogenic") constrictor response to sudden increases in flow rate. EDRF activity was greatest in arteries in which calculated shear stress and hydraulic resistance were maximal, namely the central ear artery and its first generation of branch arteries: these are proximal "feed" vessels (150-700 microns internal diameter) in this bed. The findings are consistent with enhancement of EDRF release by the physical stimulus of shear stress in resistance vessels as previously demonstrated in conduit vessels-a phenomenon which is likely to exert a major influence on flow in vascular networks. EDRF activity thus reduced perfusion pressure and power losses, particularly in highly constricted preparations. Shear-induced release of EDRF may provide an integrating link between flow and arterial topography by optimizing perfusion characteristics over a wide range of flow rates.

Endothelium-mediated spontaneous response in aortic rings of deoxycorticosterone acetate-hypertensive rats

Aortic rings isolated from normotensive Sprague-Dawley rats (CONT) exhibited spontaneous tone when the preparations were stretched. After administering deoxycorticosterone acetate (DOCA), the rats became hypertensive, and this spontaneous tone increased remarkably. The spontaneous tone was dependent on the extracellular calcium concentration. Incubation with the calcium entry blocker D-600 attenuated the spontaneous response to a greater degree in rings from DOCA rats than in rings from CONT rats. Nifedipine relaxed the already developed spontaneous tone. Removal of the endothelium greatly depressed spontaneous tone, but did not diminish the contraction caused by norepinephrine. On the basis of our findings, we conclude that 1) spontaneous tone depends on calcium influx, presumably through specific stretch-operated membrane channels, 2) these stretch-dependent channels are blocked by D-600 and nifedipine, 3) spontaneous tone is enhanced in DOCA hypertension, and 4) the endothelium appears to act as a receptor for stretch, mediating--at least in part--the spontaneous contractile response by releasing a constrictor agent.

Culture of human corpus cavernosum endothelium

A method for culturing endothelial cells (HCC-EC) from surgical specimens of human corpus cavernosum has been developed. The approach involves selective endothelial outgrowth from explants and may be generally applicable to tissues whose endothelium is not amenable to isolation by routine mechanical or enzymatic methods. The tissue is minced into pieces which are placed onto gelatin- or fibronectin-coated tissue culture plastic, and grown in medium suitable for microvascular endothelial cell growth (Carson and Haudenschild, In Vitro 22:344-354, 1986). By Days 5 to 7 EC colonies are found. Within a day or two after the appearance of the EC colonies, a non-EC cell type appears and, if undisturbed, quickly overgrows the EC. An exploitable temporal separation between the emergence of EC and non-EC is obtained when both conditioned medium (from bovine aortic endothelium) and retinal extract are present during the outgrowth period. Explants are removed by pipetting at the first sign of the emergence of the non-EC cell type. Once isolated, HCC-EC do not require conditioned medium but do require either retinal extract or acidic fibroblast growth factor for survival and growth. Approximately 60% of the first passage cultures are at least 80% EC as judged by DiI-Ac-LDL labeling. One corpus (0.3 x 0.3 x 0.5 cm) usually produces 120 cm2 of primary culture within 2 wk. These EC form contact-inhibited monolayers and stain positively for Factor VIII. They have a doubling time at 6th passage of 48 h and a plateau density of 5 to 7 x 10(4) cells/cm2. The availability of such cultures should facilitate the study of endothelium-mediated responses which play an important role in the erectile function of human penile corpus cavernosum.

Endothelin is a positive inotropic agent in human and rat heart in vitro

We have investigated the response to endothelin of isolated atrial and ventricular trabeculae from failing human hearts obtained at transplant. Results indicate that endothelin exerts a significant positive inotropic effect on human atrial and ventricular tissue, with increases in developed tension of 74.6 +/- 14.1% (+/- SEM) and 9.9 +/- 4.0%, respectively. Further studies on rat cardiac muscle demonstrate that the greater inotropic effect on atrial than ventricular muscle is also exhibited by the rat heart in vitro, with 39.9 +/- 10.7% and 17.1 +/- 5.9% increases in developed tension for atria and papillary muscle, respectively. Studies in rat atria also provide no evidence for an effect of endothelin on the frequency of spontaneous contractions. These results suggest that the potential exists for regulation of cardiac function in humans and rats by endothelial-derived factors such as endothelin, possibly via augmentation of atrial systole.

Endothelin induces two types of contractions of rat uterus: phasic contractions by way of voltage-dependent calcium channels and developing contractions through a second type of calcium channels

Effects of endothelin on nonvascular smooth muscle have been examined using rat uterine horns and two modes of endothelin action have been revealed. Endothelin (0.3 nM) caused rhythmic contractions of isolated uterus in the presence of extracellular calcium. The rhythmic contractions were completely inhibited by calcium channel antagonists. These characteristics of endothelin-induced contractions were very similar to those induced by oxytocin. Binding assays using 125I-endothelin showed that endothelin and the calcium channel blockers did not compete for the binding sites. However, endothelin was unique in that it caused, in addition to rhythmic contractions, a slowly developing monophasic contraction that was insensitive to calcium channel blockers. This developing contraction became dominant at higher concentrations of endothelin and was also calcium dependent.

Autoradiographic visualization of the binding sites for [125I]endothelin in rat and human brain

Putative receptors for endothelin were localised autoradiographically in frozen sections of rat and human brain using [125I]endothelin as a ligand. In the rat brain the highest densities were in the granular layer of the cerebellum, choroid plexus, hippocampus, and habenular nucleus. Similar brain high densities were found in the human cerebellum and hippocampus. The non-vascular pattern of distribution suggests that endothelin may have a function as a modulator of neuronal function in addition to its possible involvement in the regulation of vascular tone.

Endothelin-mediated stimulation of DNA synthesis in vascular smooth muscle cells

Effects of endothelin on DNA synthesis were investigated in two clones of vascular smooth muscle cells, 1YB4 and A7r5. The peptide stimulated DNA synthesis in both clones with apparent EC50 of less than 1 ng/ml. More than 17 h was required before initiating endothelin-stimulated DNA synthesis. The platelet-derived growth factor at a concentration which had no effects by itself on DNA synthesis enhanced the effect of low concentrations of endothelin. A calcium antagonist, nifedipine, inhibited endothelin-induced DNA synthesis. These data suggest that endothelin stimulates DNA synthesis in vascular smooth muscle cells through nifedipine-sensitive mechanisms that can be modulated by platelet-derived growth factor.

Pharmacological modulation of the bronchopulmonary action of the vasoactive peptide, endothelin, administered by aerosol in the guinea-pig

Administration by aerosol for 1 min of solutions of endothelin (ENDO; 1, 5 or 10 micrograms/ml) to anaesthetized and ventilated guinea-pigs induced a dose-dependent bronchopulmonary response (BR) which was maximal within 4 to 5 min. In contrast, no significant change of the mean arterial blood pressure was observed. Pretreatment of guinea-pigs with propranolol (1 mg/kg, i.v.), mepyramine (1 mg/kg, i.v.), nifedipine (50 mg/kg, i.p.) or verapamil (0.3 mg/kg, i.v.) did not significantly affect the BR induced by an aerosol of a solution of 10 micrograms/ml ENDO. In contrast, BR was significantly reduced when the animals were pretreated with the cyclooxygenase inhibitor, indomethacin (10 mg/kg, i.v.) or the platelet-activating factor (PAF) receptor antagonist, BN 52021 (10 mg/kg, i.v.). These results indicate that aerosolized ENDO induces a BR via the generation of secondary mediators such as cyclooxygenase products and PAF in a process which is unaffected by the blockers of the voltage-dependent calcium channels.

Hemoglobin causes both endothelium-dependent and endothelium-independent contraction of the pig coronary arteries, independently of an inhibition of EDRF effects

Hemoglobin is widely used as an inhibitor of EDRF effects. Hemoglobin contracts pig coronary arteries in vitro. However, during this contraction, effects of substance P and bradykinin which act via the EDRF are not inhibited. This means that the hemoglobin contraction is not caused by inhibition of the EDRF. This contraction is caused by a substance released from the endothelium, and by eicosanoïds released from the smooth muscles.

The direct observation of arteriolar constriction induced by endothelin in vivo

The effect of endothelin on the microvasculature of the hamster cheek pouch was observed directly using intravital microscopy. Endothelin caused a dose-dependent arteriolar constriction without affecting adjacent venules. Endothelin was less potent than vasopressin, but responses to endothelin were longer lasting (48.9 +/- 11.1% reduction in arteriolar diameter was still observed 30 min after topical application of 3 pmol endothelin). The results demonstrate that endothelin induces a prolonged increase in local arteriolar tone after a single application in vivo.

Mediators and the anti-thrombotic properties of the vascular endothelium

This review discusses the role of three mediators synthesized by the vascular endothelium, which are involved in maintaining the surface of the endothelial cells in a non-thrombogenic state. Prostacyclin, discovered in 1976, is a product of arachidonic acid metabolism. This labile prostanoid, with a chemical half life of approximately three minutes, relaxes vascular smooth muscle and inhibits the aggregation of blood platelets. Endothelium-derived relaxing factor (EDRF), discovered in 1980, is even more labile than prostacyclin with a half life counted in seconds. It also relaxes smooth muscle and inhibits the aggregation and adhesion of platelets. Recently, it has been identified as nitric oxide. Prostacyclin and EDRF are released together following stimulation of receptors on endothelial cells and cooperate to inhibit platelet aggregation and adhesion. 13-HODE, acts from inside the cell to make the endothelial surface less adhesive and is not released. These mediators act together to form the endothelial defence mechanism against adhering blood cells. Underproduction can lead to diseases such as hypertension or atherosclerosis. A mainly fish diet, rich in eicosapentaenoic acid alters the prostacyclin/thromboxane balance in favour of prostacyclin-like activity. This type of diet may provide protection against atherosclerosis and myocardial infarction.

The failure of endothelin to displace bound, radioactively-labelled, calcium antagonists (PN 200/110, D888 and diltiazem)

The effect of endothelin, a potent vasoconstrictor polypeptide, on three types of calcium antagonist binding sites was examined, in rat cardiac membrane fragments. Endothelin 10 nM affected neither the affinity nor density of dihydropyridine binding sites. At concentrations of 10(-12)-10(-7) M, endothelin failed to displace bound (+)-[3H]-PN 200/110, (-)-[3H]-D888 and (+)-cis-[3H]-diltiazem. These results suggest that the calcium antagonist binding sites associated with L-type calcium channels are not the primary site of action of endothelin.

Molecular cardiology: new avenues for the diagnosis and treatment of cardiovascular disease

This review summarizes some of the major advances in the investigation of molecular mechanisms underlying both normal and abnormal cardiovascular function. Four major areas are highlighted including cardiac muscle, the blood vessel, atherosclerosis and thrombosis/thrombolysis. The remarkable strides in understanding multifactorial diseases such as atherosclerosis, and the development of innovative new therapies such as the use of thrombolytic agents produced by recombinant deoxyribonucleic acid (DNA) technology, are noted. Moreover, it is concluded that the past decade of basic research has provided a solid framework for improvements in the diagnosis and therapy of other forms of cardiovascular disease as well. An evaluation of current trends in basic cardiovascular research suggests that diagnostic and therapeutic approaches to disease will increasingly target specific molecular processes underlying the pathophysiologic state.

Endothelin stimulates phospholipase C, Na+/H+ exchange, c-fos expression, and mitogenesis in rat mesangial cells

A recently described peptide hormone, endothelin, is a potent vasoconstrictor, but it is unclear whether endothelin has other biological actions. These experiments extend the range of biological actions of endothelin to stimulation of mitogenesis. Endothelin at low concentrations (0.1-10 nM) induced mitogenesis by quiescent rat glomerular mesangial cells in culture. Mitogenesis induced by endothelin was accompanied by activation of phospholipase C with increased inositol phosphate turnover and increments of intracellular [Ca2+]. Endothelin also activated Na+/H+ exchange, causing cytosolic alkalinization, and enhanced transcription of the c-fos protooncogene, additional biochemical signals closely linked to proliferation. In addition to being a vasoconstrictor, endothelin thus also functions as a mitogen, presumably through activation of phospholipase C.

Endothelial and myocardial injury during ischemia and reperfusion: pathogenesis and therapeutic implications

Early reperfusion remains the most effective way of limiting myocardial necrosis and improving ventricular function in experimental models and human patients. However, the introduction of oxygen and cellular elements, especially the neutrophil, into the ischemic zone may initiate a deleterious cascade of events that limits myocardial salvage after reperfusion. Although the pathogenesis of reperfusion injury remains controversial, recent studies have suggested that the endothelium may play a critical role. Endothelial cells maintain flow in the microcirculation by secreting a number of vasodilatory compounds and substances that prevent plugging of capillaries by inhibiting neutrophil adherence and platelet aggregation. Reperfusion of ischemic myocardium accelerates structural and functional changes in endothelial cells, resulting in a progressive decrease in microcirculatory flow ("no reflow" phenomenon). Numerous studies suggest that activated neutrophils mediate vascular damage by releasing reactive oxygen species and potent proteolytic enzymes. The administration of therapeutic agents that limit endothelial disruption and neutrophil plugging has shown promising results in limiting myocardial reperfusion injury in experimental models.

Potent contractile effect of endothelin in isolated guinea-pig airways

Endothelin produced a concentration-dependent (1 nM-0.3 microM) contraction of isolated guinea-pig airways (trachea and main bronchi). The response was unaffected by tetrodotoxin (1 microM) and slightly depressed by indomethacin (5 microM) but promptly abolished by isoprenaline (1 microM) or EDTA (3 mM). In the bronchi, the response to endothelin was enhanced by removal (rubbing) of the epithelium. The response of the trachea or bronchi to endothelin (0.3 microM) was unaffected by NiCl2 (0.1 mM) or omega conotoxin (0.1 microM) but was partially inhibited by nifedipine (1 microM).

Effects of endothelin on regional hemodynamics in conscious rats

Endothelin is a potent vasoactive peptide in anesthetized rats and isolated vascular smooth muscle. This study was performed to describe the hemodynamic effects of endothelin in conscious, freely moving rats. Endothelin (0.067-2 nmol/kg i.v.) produced long-lasting, dose-dependent increases in arterial pressure, mesenteric and, to a lesser degree, hindquarters vascular resistances and decreases in heart rate. We suggest that endothelin may play an important role in regulation of arterial pressure by modulating peripheral vasomotor tone.

Activation of phospholipase A2 by endothelin in cultured vascular smooth muscle cells

The ability of endothelin to promote phospholipid hydrolysis has been studied in myo-[2-3H]-inositol-, [3H]-arachidonic acid- or methyl-[3H]choline chloride-prelabelled cultured vascular smooth muscle cells (VSMC) from rat and bovine thoracic aortae and human omental vessels. The biochemical responses to endothelin were comparable between the different VSMC isolates. Endothelin promoted the accumulation of glycerolphospho[3H]inositol and concomitant loss of [3H]-inositol label from phosphatidylinositol. Exposure of [3H]choline-labelled VSMC to endothelin resulted in a loss of radioactivity from phosphatidylcholine that was inversely parallelled by an increase in water-soluble [3H]-choline metabolites. In [3H]-arachidonic acid ([3H]-AA)-labelled VSMC, endothelin induced extracellular release of [3H]-AA which derived from both phosphatidylcholine and phosphatidylinositol. Half-maximally effective concentrations of endothelin for all these responses were approximately 2-7 nM and did not vary between VSMC types. Endothelin-induced release of [3H]-AA into VSMC medium-overlay was inhibited by quinacrine and nordihydroguaiaretic acid but not by neomycin or indomethacin. The data herein implicate activation of phospholipase A2 by endothelin with subsequent metabolism of arachidonic acid via the lipoxygenase pathway.

Endothelin action on vascular smooth muscle involves inositol trisphosphate and calcium mobilization

Cultured endothelial cells release a potent vasoconstrictor peptide, endothelin. Cumulative addition of synthetic endothelin to isolated rabbit aortic rings elicited a concentration-dependent increase in contractile tension which was endothelium-independent. In cultured rabbit vascular smooth muscle cells loaded with the fluorescent dye fura 2, endothelin induced a concentration-dependent increase in [Ca2+]i over the range of 0.01 to 100 nM. Moreover, in the absence of extracellular Ca2+, endothelin could still induce an increase in [Ca2+]i. In addition, endothelin stimulated 45Ca2+ efflux from preloaded vascular smooth muscle cells in the presence and absence of extracellular Ca2+, as well as stimulating 45Ca2+ influx in a concentration-dependent manner. Measurement of inositol phosphates in [3H]-myoinositol-labelled vascular vascular trisphosphate. Unlabelled endothelin inhibited (125I)-endothelin binding to cultured rabbit vascular smooth muscle cells in a concentration-dependent manner. Binding was not inhibited by other vasoactive hormones or calcium channel ligands, suggesting cell surface receptors specific for endothelin. We conclude that one of the initial membrane events in the action of endothelin is to induce phospholipase C-stimulated PIP2 hydrolysis and that this signalling mechanism is initiated by endothelin/receptor interaction at the plasma membrane.

Endothelin-sensitive intracellular Ca2+ store overlaps with caffeine-sensitive one in rat aortic smooth muscle cells in primary cultures

We made use of quin2 microfluorometry to determine the effects of endothelin (ET) on cytosolic free Ca2+ concentrations [Ca2+]i) in rat aortic smooth muscle cells in primary culture. In Ca2+-containing medium, ET induced a rapid and sustained elevation of [Ca2+]i. In the latter component, in particular, the elevation of [Ca2+]i was inhibited by diltiazem. In Ca2+-free medium, ET induced a rapid and transient [Ca2+]i elevation, which was not inhibited by diltiazem. When the caffeine-sensitive intracellular Ca2+ store was practically depleted by repeated treatment with caffeine in Ca2+-free media, ET did not elevate [Ca2+]i. Thus, it was suggested that ET induces [Ca2+]i elevation not only by extracellular Ca2+-dependent, mechanisms but also by releasing Ca2+ from the intracellular store, and that the ET-sensitive Ca2+ store may overlap with the caffeine-sensitive one, in cultured vascular smooth muscle cells.

Endothelial thromboxane receptors: biochemical characterization and functional implications

We have identified thromboxane specific receptors in membrane preparations of bovine pulmonary artery endothelial cells using a potent thromboxane specific antagonist, [125I]-PTA-OH in a binding assay. The binding was specific and saturable. Neither thromboxane B2, prostaglandin D2 nor prostaglandin F2 alpha displaced the ligand (0.1 nM) at concentrations up to 10 microM. However, binding was displaced by IPTA-OH greater than SQ29548 greater than U46619. In addition, we observed that thromboxane mimetic U46619 significantly lowered the basal production of prostacyclin and also markedly suppressed bradykinin-stimulated prostacyclin released by endothelial cells. We propose that an important biological effect of thromboxane on vascular endothelial cells may be the suppression of prostacyclin production.

Competitive interaction between endothelin and sarafotoxin: Binding and phosphoinositides hydrolysis in rat atria and brain

Binding studies with the structurally similar vasoconstrictor peptides 125I-endothelin and 125I-sarafotoxin b, the former of mammalian origin and the latter derived from snake venom, reveal their mutually exclusive binding to rat atrium and various regions of the rat brain. In these tissues endothelin, like sarafotoxin, induces phosphoinositide hydrolysis which is in part Ca2+-independent. It is suggested that endothelins and sarafotoxins share common binding sites and mechanisms of action.

Endothelin action: Inhibition by a protein kinase C inhibitor and involvement of phosphoinositols

Endothelin tightly bound to rabbit aortic strips and caused a prolonged vasoconstriction both in the presence and absence of extracellular Ca2+, although only partial constriction (20-30%) developed in the latter case, indicating that its action may not be limited to the opening of a calcium channel. The endothelin-induced constriction was reversed by the protein kinase C inhibitor, 1-(5-isoquinolynylsulfonyl)-2-methylpiperazine (H-7). In contrast to the observation of Hirata et al (1), endothelin caused a robust phosphatidylinositol breakdown producing inositol mono-, bis-and trisphosphates in cultured rat vascular smooth muscle cells. It showed no effect on cyclic nucleotide levels in the same cultured cells. These results indicate that phosphatidylinositol turnover and protein kinase C activation are involved in endothelin-induced vasoconstriction.

Sarafotoxin receptors mediate phosphoinositide hydrolysis in various rat brain regions

Sarafotoxin-b, a potent snake vasoconstrictor peptide homologous to the mammalian endothelial vasoconstrictor endothelin, induces phosphoinositide (PI) hydrolysis in various brain regions of the rat. Sarafotoxin-b induced PI hydrolysis is largely independent of extracellular Ca2+ and is detected in all brain regions where toxin-binding sites are found. These results point to the existence of a hitherto undetected neuroreceptor associated with the PI cycle.

Mesangial cell, glomerular and renal vascular responses to endothelin in the rat kidney. Elucidation of signal transduction pathways

We investigated the actions of endothelin in anesthetized rats and cultured mesangial cells. Intravenous infusion of endothelin (10 pmol/min) decreased renal blood flow by 44% at 20 min without changing arterial pressure, which subsequently rose significantly from 124 +/- 3 to 133 +/- 4 mmHg over 60 min. Micropuncture during the nonhypertensive period revealed increases in afferent (65%) and efferent (82%) arteriolar resistances, thereby reducing nephron plasma flow rate. The glomerular ultrafiltration coefficient (Kf) fell from 0.097 +/- 0.035 to 0.031 +/- 0.011 nl/(s.mmHg) as did single nephron filtration rate (41 +/- 3 to 19 +/- 3 nl/min). Addition of 5 nM endothelin to mesangial cells plated on a silicone rubber substrate increased the intensity and number of tension-generated wrinkles, and caused their reappearance in forskolin prerelaxed cells. 20-30 s following exposure of fura-2 loaded mesangial cells to 10 nM endothelin, single cell intracellular calcium concentration ([Ca]i) increased from a mean baseline value of 66 +/- 11 (SE) to a peak of 684 +/- 250 nM (P less than 0.05) followed by a sustained elevation at 145 +/- 42 nM. Anion exchange HPLC revealed rapid (15 s) and dose-dependent stimulation of inositol 1,4,5-trisphosphate (IP3) generation following exposure of [3H]myoinositol preloaded mesangial cells to 10-100 nM endothelin. Endothelin also led to intracellular alkalinization of 2'7'-bis(2-carboxy-ethyl)-5(and-6)carboxyfluorescein (BCECF)-loaded mesangial cells and its addition was associated with dramatic augmentation of mitogenic activity. Thus, endothelin exerts potent constrictor effects on renal arterioles which precede its systemic hypertensive action. It lowers Kf and contracts mesangial cells, likely through stimulation of IP3 generation and elevation of [Ca]i. It is a potent mesangial cell mitogen. These studies define functional responses and signal transduction pathways for endothelin in the rat kidney and propose a potential role for this peptide in the control of mesangial cell function, glomerular filtration rate, and renal vascular tone.

Endothelin, its diverse biological activities and mechanisms of action

Endothelin, originally identified as a vasoconstrictive peptide derived from vascular endothelial cells, is now known to exert diverse biological effects on a wide variety of tissues and cell types through its own receptor(s). One of the outstanding actions of endothelin is a cell growth promoting activity which is demonstrated in several cell types including cultured vascular smooth muscle cells, fibroblasts, glomerular mesangial cells and osteoblasts. The mitogenic effect is likely mediated by stimulation of phospholipase C via receptor-G-protein coupling, and subsequent activation of protein kinase C. The effect of endothelin may contribute to the cell-proliferation response under various physiological and pathological conditions, such as wound healing and development of atherosclerosis and glomerulonephritis. Recently, three distinct endothelin-related genes have been cloned, suggesting that mammals, including humans, produce three members of this peptide family, endothelin (ET)-1 (the 'classical' endothelin), ET-2 and ET-3, which may act on distinct subtypes of endothelin receptor to induce different cellular responses.

Alpha-adrenoceptor subtypes in blood vessels: physiology and pharmacology

The existence of alpha 1- and alpha 2-adrenoceptors subtypes is now well established. Presynaptically, alpha 2-adrenergic autoreceptors modulate norepinephrine release. In vascular smooth muscle both alpha 1- and alpha 2-adrenoceptors exist postjunctionally, the alpha 1 subtype being innervated preferentially in most vascular beds. alpha 2-Adrenoceptors may be present on the smooth muscle cell as well as the endothelial cell where they may participate in the release of endothelium derived relaxing and constricting factors. Developing insights into the role of alpha-adrenoceptor subtypes in the cardiovascular system, and into the classification of subtypes of alpha 1- and alpha 2-adrenoceptors may be expected to have, if confirmed, significant therapeutic implications for drug discovery in the cardiovascular system.

Endothelin: a potent vasoconstrictor associated with coronary vasospasm

Endothelin, administered into the coronary arteries of anesthetized dogs, produced a profound and long-lasting reduction in coronary blood flow with electrocardiographical evidence of myocardial ischemia. Coronary angiography revealed delayed filling of the distal branches and, in some cases, cessation of the blood flow distal to the epicardial portions of coronary arteries. The coronary vasoconstriction induced by endothelin subsided after intracoronary administration of nitroglycerin. Pretreatment with the Ca2+-channel antagonist, nitrendipine, suppressed endothelin-induced vasoconstriction. These findings suggest that endothelin, produced by vascular endothelial cells, may contribute to the pathogenesis of coronary vasospasm.

The interaction between oxygen and vascular wall

In vascular strips of canine carotid arteries stepwise lowering of oxygen tension from hyperoxic levels of 550 mmHg to 20 mmHg caused in preparations with endothelium a dose-dependent hyperpolarization and relaxation of smooth muscle cells when oxygen tensions between approximately 150 mmHg and 35 mmHg were attained. Pronounced hypoxia with oxygen tensions below 30 mmHg induced a depolarisation and an increase in force generation. During comparable investigations on vessel preparations without endothelium only a slight hyperpolarization and relaxation of the smooth muscle were observed when decreasing the oxygen tension from 550 mmHg to approx. 35 mmHg. In the presence of indomethacin (10(-5) M) a small but significant reduction in the hypoxia-induced hyperpolarization and decrease in smooth muscle tone was found in intact vascular strips with endothelium. Depolarisation and contraction occurred at oxygen tensions below approx. 50-60 mmHg.

Dilator actions of endothelin in coronary resistance vessels and the abdominal aorta of the guinea pig

Endothelin has been characterized as a potent constricting factor. The purpose of this study was to investigate possible dilator effects of this peptide and to examine whether dilator responses occur through an endothelium-mediated mechanism in guinea pig coronary resistance vessels and isolated aortic rings. Changes in perfusion pressure after bolus injections of endothelin were measured using a constant-flow modified Langendorff preparation with a transducer between the flow pump and the heart. An immediate fall in perfusion pressure, averaging 6 mmHg, was observed after injection of endothelin (10(-14)-10(-12) moles). This effect was maximal at 1 minute and tended to return toward baseline levels within 4 minutes. In response to endothelin (10(-9) M), isolated aortic rings relaxed 35% after being contracted with prostaglandin F2 alpha (10(-7) M). In both preparations, dilation was converted to constriction after endothelium damage by oxygen radicals or endothelium removal (mechanical rubbing). Dilator responses to endothelin were blocked by pretreatment for 30 minutes with indomethacin (14 microM) in the presence of an intact endothelium in coronary resistance vessels, whereas in the abdominal aorta they were not. We conclude that endothelin has significant dilator properties and that this effect is opposed by its constrictor action at higher doses. In addition, dilator responses to endothelin require an intact endothelium in both coronary vessels and abdominal aorta. Finally, endothelin-induced dilation in coronary resistance vessels appears to occur through a cyclooxygenase product-mediated mechanism.

The endothelium-derived vasoconstrictor peptide endothelin inhibits renin release in vitro

The effect of endothelin, a newly identified endothelium-derived vasoconstrictor peptide, on renin release from rat kidney cortical slices was examined. Endothelin produced a concentration-dependent inhibition of renin release and this inhibitory effect was dependent on extracellular calcium. The dihydropyridine calcium channel blockers nifedipine and nicardipine did not antagonize the inhibitory effect induced by endothelin. On the other hand, nifedipine completely antagonized the extracellular high potassium- or Bay K 8644-induced inhibition of renin release. The endothelin-induced inhibition of the release was markedly blocked by the addition of Co2+. Similar blocking effects of Co2+ were also observed with extracellular high potassium or Bay K 8644. Thus, endothelin exerts an inhibitory action on renin release in vitro, in a calcium-dependent manner. This inhibition may be mediated by the increased calcium influx through dihydropyridine-insensitive calcium channels.

Comparative effects of endothelin and phorbol 12-13 dibutyrate in rat aorta

The vasoconstrictive properties of endothelin (ET-1) and the protein kinase C activator, phorbol 12-13 dibutyrate (PDB) were comparatively investigated in isolated rat aorta. ET-1 (0.3-100 nM) and PDB (10 nM-3 microM) induced a slowly developing sustained contraction in endothelium denuded aorta. Maximal contractions induced by ET-1 and PDB were unaffected by diltiazem (10 microM). Substantial contraction to ET-1 (30 nM) and PDB (0.1 microM) remained in calcium-free medium. Contractions of ET-1 and PDB in calcium-free medium were unaffected by intracellular calcium depletion induced by phenylephrine. Following the response to ET-1 and PDB in a calcium-free medium, an additional sustained contraction was observed after calcium (2.5 mM) was added to the bath. The protein kinase C inhibitor, H7 (100 microM) was more potent in inhibiting contractions induced by phenylephrine and KCl than the ones elicited by ET-1 and PDB. The other protein kinase C inhibitors i.e. staurosporine (50 nM) and phloretin (100 microM) inhibited to a similar extent all the agonists tested. These results suggest that protein kinase C may play an important role in mediating the contraction to ET-1 in rat aorta.

Role of the endothelium and arginine peptides on the vaso-motor response of porcine internal mammary artery

The long-term patency of the internal mammary artery (IMA) graft is of considerable interest owing to its extensive use in myocardial revascularization. The aim of the present study was to elucidate the role of endothelium in modulating the responses of the porcine IMA to several vasoactive drugs. Isolated ring segments of porcine IMA contracted in a reproducible and dose dependent manner to phenylephrine, potassium chloride and the thromboxane mimic U46619, but the responses to serotonin, histamine and ATP were significantly less prominent. Both acetylcholine and bradykinin elicited endothelium-dependent relaxation which was not inhibited by indomethacin, but by methylene blue, an inhibitor of soluble guanylate cyclase. These two endothelium-dependent drugs and two endothelium-independent relaxing drugs, nitroprusside and nitroglycerin relaxed the IMA in a dose dependent manner which was associated with an elevation of cyclic GMP. The endothelium dependent vasodilator peptides such as bradykinin contain L-arginine in their sequence. Benzoyl derivatives of L-arginine but not L-arginine relaxed the IMA in a dose dependent manner. These data confirm and extend exploratory studies performed with a simpler vascular model which indicate that the precursor of endothelium derived relaxing factor (EDRF) is an arginine moiety.

Activation of sodium channels is not essential for endothelin induced vasoconstriction

The role of sodium and calcium ions in the vasoconstrictor response of isolated rat aorta and protal vein to synthetically prepared endothelin is investigated. Contractile responses to endothelin, unlike those induced by the sodium channel activator veratridine, are unaffected by tetrodotoxin or by the removal of sodium chloride from the solution bathing the tissue. The responses are the same whether sodium chloride is replaced iso-osmotically with either sucrose or potassium chloride. The endothelin responses in all media are entirely dependent on the presence of extracellular calcium, and can be blocked by 1 microM nitrendipine. These findings offer no support to the idea that voltage activated sodium channels are the primary site of action of endothelin as suggested by sequence homologies to scorpion alpha-toxins, but are entirely consistent with the possibility that the site of action of endothelin is closely coupled to the calcium channel (Yanagisawa et al, 1988).

Calcium ion uptake, somatotropin release, and fine structure of somatotrophs in cultures of the rat anterior pituitary upon the action of an oligopeptide (Boc-Gln-Leu-Lysinal)

Cultured cells from the anterior pituitary glands of adult rats were treated with the tripeptide aldehyde proteinase inhibitor, Boc-Gln-Leu-Lys-H. The oligopeptide had a profound releasing effect on growth hormone, whereas the prolactin release remained unchanged at 10(-3) mol/l drug concentration after an incubation for 2 h. In the presence of the oligopeptide a time- and dose-dependent calcium influx into cultured cells has been shown which was proved to be almost completely antagonized with magnesium ions but not with Nifedipine. In addition, radioactive calcium ions could be detected in a number of cells by light microscopic autoradiography when cultures were treated with Boc-Gln-Leu-Lys-H for short periods. The selective Gel action of the oligopeptide on growth hormone producing cells has been demonstrated also in fine structural investigations: multigranular and single exocytotic profiles have been observed. Accordingly, we have postulated that Boc-Gln-Leu-Lysinal mimics the effects of the known ionophores. Its mode of action needs, however, further studies especially on isolated somatotrophs.

Why are converting enzyme inhibitors vasodilators?

1. The primary action of the converting enzyme inhibitors to prevent the formation of angiotensin II can explain a decrease in peripheral vascular resistance in patients with elevated, but not in those with normal or reduced plasma renin levels. 2. The inhibition of the breakdown of bradykinin will potentiate the vasodilator properties of the endogenously produced peptide. These include direct relaxation of certain vascular smooth muscle, production of vasodilator prostanoids and release of endothelium-derived relaxing factor(s). The greater release of the latter in the kidney could exert a negative feedback on the release of renin. 3. In addition, converting enzyme inhibitors may directly (by a prejunctional effect) and indirectly (by curtailing the production of angiotensin II) reduce the release of noradrenaline in the blood vessel wall. 4. Converting enzyme inhibitors may also directly reduce the responsiveness of vascular smooth muscle to vasoconstrictor stimuli (e.g. alpha-adrenoceptor activation). 5. The different effects of these therapeutic agents may concur to induce peripheral vasodilatation.

The interplay between platelet and vessel-wall mediators in coronary artery occlusion

Myocardial infarction (MI) is associated with platelet aggregation and coronary vasospasm. Endogenous mediators produced by platelets and the vessel wall alter platelet function and smooth muscle tone and may be involved in the infarctive process. The synthesis and actions of these mediators is largely determined by interactions between platelets and the vessel-wall. MI occurs at sites of endothelial cell damage where the balance of mediators is shifted in favour of aggregation and vasospasm. Therapeutic intervention should aim at restoring the balance of mediators, and this will involve manipulation of endothelium-derived nitric oxide or its intracellular second messengers.