Endothelins as regulators of growth and function in endocrine tissues

The future of endothelin-receptor antagonism as treatment for systemic hypertension

Endothelin (ET) is an endogenous peptide secreted predominantly by endothelial cells that mediates its effects via vasoconstriction and hypertrophy of vascular smooth muscle. Because the role of ET has been described in multiple pathologic processes in cardiovascular disease, including hypertension, there has been a strong interest in the development of therapeutic agents that inhibit ET receptors. ET receptor antagonists have shown much promise in disease states such as pulmonary arterial hypertension, essential hypertension, and various forms of secondary hypertension. This review serves to summarize the current role of ET and ET receptor antagonists in both the pathophysiology and the treatment of hypertension.

An endothelin-converting enzyme homologue in the locust, Locusta migratoria: functional activity, molecular cloning and tissue distribution

Endothelin-converting enzyme is the key enzyme in the process of endothelin production. Endothelin is a peptide that plays an important role in vasoconstriction and the development of neural crest-derived cells in vertebrates. Activity assays performed on membrane extracts from Locusta migratoria brain revealed the existence of a protease activity responsible for the formation of mature endothelin-1 from its precursor, big endothelin. Cloning experiments led to a cDNA sequence (Lom ECE) with an open reading frame of 727 amino acid residues displaying all the characteristic ECE features. A comparison of ECE activity levels among different tissues of the locust showed a high enzyme activity in the gonads and midgut. RT-PCR experiments showed a wide tissue distribution of Lom ECE mRNA, with transcription being most abundant in brain tissue.

Reciprocal regulation of endothelin-1 and nitric oxide: relevance in the physiology and pathology of the cardiovascular system

The endothelium plays a crucial role in the regulation of cardiovascular structure and function by releasing several mediators in response to biochemical and physical stimuli. These mediators are grouped into two classes: (1) endothelium-derived constricting factors (EDCFs) and (2) endothelium-derived relaxing factors (EDRFs), the roles of which are considered to be detrimental and beneficial, respectively. Endothelin-1 (ET-1) and nitric oxide (NO) are the prototypes of EDCFs and EDRFs, respectively, and their effects on the cardiovascular system have been studied in depth. Numerous conditions characterized by an impaired availability of NO have been found to be associated with enhanced synthesis of ET-1, and vice versa, thereby suggesting that these two factors have a reciprocal regulation. Experimental studies have provided evidence that ET-1 may exert a bidirectional effect by either enhancing NO production via ETB receptors located in endothelial cells or blunting it via ETA receptors prevalently located in the vascular smooth muscle cells. Conversely, NO was found to inhibit ET-1 synthesis in different cell types. In vitro and in vivo studies have started to unravel the molecular mechanisms involved in this complex interaction. It has been clarified that several factors affect in opposite directions the transcription of preproET-1 and NO-synthase genes, nuclear factor-KB and peroxisome proliferator-activated receptors playing a key role in these regulatory mechanisms. ET-1 and NO interplay seems to have a great relevance in the physiological regulation of vascular tone and blood pressure, as well as in vascular remodeling. Moreover, an imbalance between ET-1 and NO systems may underly the mechanisms involved in the pathogenesis of systemic and pulmonary hypertension and atherosclerosis.

Endothelin receptor antagonist prevents parathyroid cell proliferation of low calcium diet-induced hyperparathyroidism in rats

Secondary hyperparathyroidism, one of the most frequently encountered disorders of the calcium homeostasis, is characterized by an increase in parathyroid epithelial (PT) cell number, which is crucial from a functional viewpoint. However, it is still unknown what factors are involved in PT cell proliferation. Endothelin-1 (ET-1), a vasoconstrictive peptide, has been shown to act as a mitogen in a variety of cell types. Rat PT cells are reported to synthesize ET-1 and possess its receptors. To test the hypothesis that ET-1 plays a role in PT cell proliferation, we used rat test subjects fed a low calcium diet for 8 weeks (low Ca rats). The number of the proliferating PT cells, measured by proliferating cell nuclear antigen immunostaining, was significantly increased, with striking immunoreactivity of ET-1 in the low Ca rats. An endothelin receptor antagonist, bosentan (100 mg/kg.day), prevented any increase in the proliferation of PT cells in the low Ca rats (14.3 +/- 2.7/1000 PT cells with no bosentan; 2.1 +/- 1.3 with bosentan; P < 0.01). These results indicate that ET-1 is involved in PT cell proliferation in vivo and suggest that blocking of ET receptors may become one of the important therapeutic strategies for preventing secondary hyperparathyroidism.

Modulation of human colon tumor-stromal interactions by the endothelin system

Tumor neovascularization is considered to be a critical step in the development of a malignant tumor. Endothelin (ET)-1 is a powerful vasoconstrictor and mitogenic peptide that is produced by many cancer cell lines. The cellular distribution of the ET components was evaluated in human colon tumors and compared to normal colon. There was more of the ET components (preproET-1, endothelin-converting enzyme-1, and ETA and ETB receptors) in adenomas and adenocarcinomas than in the normal colon. There was overproduction of preproET-1 and endothelin-converting enzyme-1 in carcinoma cells and stromal vessels, suggesting that they are a local source of ET-1. ETA receptors were present in stromal myofibroblasts of neoplastic tissue, and there were large amounts of ETB receptors in the endothelium and myofibroblasts. There was also a redistribution of alpha-smooth muscle actin-positive cells in the vascular structures of tumors. An experimental rat model of induced colon cancer treated for 30 days with bosentan, a mixed antagonist of both ET receptors, confirmed the morphological changes observed during the tumor vascularization. Our data suggest that ET-1 and its receptor play a role in colon cancer progression, with ET-1 functioning as a negative modulator of the stromal response.

The endothelin system in normal human colon

Endothelin (ET)-1 is a potent vasoconstrictor and mitogenic peptide that has a variety of biological effects in noncardiovascular tissues. The precise cellular distribution of the ET-1 system in the wall of the normal human colon was studied to identify the physiological role of ET in the gut. In situ hybridization revealed ET-converting enzyme-1 (ECE-1) mRNA in all vessels, the colon epithelium, and macrophages. Prepro-ET-1 (PPET-1) mRNA had a similar distribution except for a scattered signal in mucosal microvessels. ET(A) and ET(B) receptor mRNAs were mainly in the lamina propria, pericryptal myofibroblasts, microvessels, and mononuclear cells, with ET(A) mRNA more abundant than ET(B) mRNA. (125)I-ET-1 binding showed ET(B) along the crypts and in nerve fibers descending from the ganglionic plexus that contained PPET-1, ECE-1, and ET(B) transcripts, whereas glia contained ET(A) receptors. The finding of the entire ET system in the normal mucosa suggests its implication in some characteristic functions of the colon and its secretion as both a neuroactive and a vasoactive peptide.

Characterization of an endothelin ET(B) receptor in the gill of the dogfish shark Squalus acanthias

Endothelins (ETs) are potent vasoconstrictive peptides that are secreted by the vascular endothelium and other tissues in vertebrates. Previous studies have demonstrated that ETs are expressed in a variety of fish tissues and contract various blood vessels. In order to determine if receptors for ET are expressed in fish gill tissue, we examined the binding kinetics of (125)I-labeled, human ET-1 to membrane fragments isolated from the gill of the dogfish shark, Squalus acanthias. (125)I-ET-1 bound at a single site, with a dissociation constant (K(d)) and binding site number (B(max)) very similar to those described in a variety of mammalian blood vessels. ET-1 and ET-3 competed equally with (125)I-ET-1, suggesting that the receptor was ET(B), which has been shown in mammalian systems to bind to both ligands equally. The ET(B)-specific agonists sarafotoxin S6c, IRL-1620, and BQ-3020 also competed against (125)I-ET-1 at a single site, supporting this hypothesis. We conclude that the shark gill expresses an ET(B) receptor with substantial homology to the mammalian receptor and that ET may play an important role in modulating such vital gill functions as gas exchange, ion regulation, acid-base balance, and excretion of nitrogen.

Cellular distribution of endothelin-converting enzyme-1 in human tissues

Endothelin-converting enzyme-1 (ECE-1) is the key enzyme of endothelin biosynthesis, catalyzing the final processing step. As shown by the targeted disruption of the ECE-1 gene, mature endothelins must be produced at specific sites for normal embryonic development. Therefore, it is important to know the exact pattern of ECE-1 gene expression. In this study we investigated the cellular distribution of ECE-1 in a variety of human tissues by in situ hybridization and immunohistochemistry. Widespread expression of the ECE-1 gene was noted, with a similar distribution pattern for mRNA and protein in normal human tissues, suggesting a major biological role for ECE-1. ECE-1 levels were particularly high in the cardiovascular, reproductive, and endocrine systems. There was strong and consistent labeling for ECE-1 in the vascular endothelial cells of all organs examined and in various nonvascular cells, especially some glandular cells. A large amount of ECE-1 protein and mRNA was detected in the Leydig cells of the testis and in the granulosa and theca cells of the ovary. In the adrenal gland, ECE-1 was detected in the cortex and medulla, with the strongest labeling in the zona glomerulosa. Therefore, ECE-1 may be involved in other systems, such as the regulation of hormone secretion, rather than exclusively generating ET-1 from its precursor. These results point out the potential side effects of ECE-1 inhibitors that are currently under development for treatment of cardiovascular diseases. (J Histochem Cytochem 47:447-461, 1999)

Endometrial endothelin: regulator of uterine bleeding and endometrial repair

1. Endothelin (ET) and its mRNA are present in endometrium. Expression of ET varies across the menstrual cycle, reaching maximal levels in the premenstrual phase, suggesting a paracrine role in endometrial bleeding and/or repair. 2. The major cellular source of ET is the epithelium, although endothelium and decidualized stroma are additional sites of production. Epithelial ET is the ET-1 isoform and this is able to contract rat thoracic aortic rings ex vivo. 3. Endothelin-1 production by cultured endometrial epithelial cells is markedly increased by serum and, to a lesser extent, by transforming growth factor-beta and interleukin-1 alpha, but not by epidermal growth factor, oxytocin, arginine vasopressin, thrombin or angiotensin II, which stimulate ET production in other tissues. 4. Endothelin-1 has mitogenic actions on endometrial stromal cells; it stimulates the uptake of [3H]-thymidine, acting via the AP-1 cis element c-jun. 5. Neutral endopeptidase (NEP), a membrane-bound ectoenzyme that is capable of degrading ET, is localized principally in endometrial stroma and immunoreactivity is maximal in the secretory phase of the cycle. 6. A potential role for ET in regulating endometrial bleeding is suggested by studies on endometrium from two groups of women who were experiencing abnormal uterine bleeding: users of the contraceptive Norplant (Leiras Co., Turku, Finland) and subjects with documented menorrhagia. In both groups, ET-1 immunoreactivity in endometrial epithelium was markedly reduced compared with the normal menstrual cycle and did not vary cyclically, while NEP immunoreactivity, particularly in the epithelium, was increased. Thus, ET may be involved in endometrial bleeding, as a vasoconstrictor before the onset of menstruation when vasoconstriction is intense and, subsequently, when it may be required in the cessation of menstrual bleeding. Furthermore, the mitogenic actions of ET may play a role in endometrial regeneration and remodelling during the menstrual cycle, particularly following menstruation.

Endothelins inhibit the mineralization of osteoblastic MC3T3-E1 cells through the A-type endothelin receptor

We examined the effects of various endothelins on the mineralization of mouse clonal preosteoblastic MC3T3-E1 cells. MC3T3-E1 cells expressed mRNAs for endothelin (ET)-1 and the A-type receptor for ET (ETA). A pharmacological study also demonstrated the predominant expression of the ETA receptor. Northern blotting analysis revealed that ETs decreased the expression of mRNA for osteocalcin, which is a marker protein for the maturation of osteoblastic cells. ET-1 also decreased in the deposition of calcium by MC3T3-E1 cells in a dose-dependent manner and it had an inhibitory effect even at 10(-11) M. The rank order of potency of ETs was ET-1 = ET-2 > ET-3. Brief treatment with 10(-7) M ET-1 on days 6-8 alone suppressed mineralization. ET-1 enhanced the rate of production of inositol 1,4, 5-trisphosphate (IP3) in MC3T3-E1 cells, but it had no effect on the rate of production of cAMP. Taken together, our data indicate that ET-1 might inhibit the mineralization of osteoblastic cells via an interaction with the ETA receptor, with generation of IP3 as the intracellular signal.

Endothelin-1 infusion inhibits plasma insulin responsiveness in normal men

OBJECTIVES

Elevated plasma endothelin (ET)-1 levels have been described in insulin-resistant states such as syndrome X, obesity, non-insulin-dependent diabetes mellitus, and in some studies in essential hypertension. To investigate whether increases in circulating ET-1 to levels observed in insulin-resistant states can modulate insulin levels and/or insulin sensitivity in humans, we assessed these variables during low, non-pressor-dose ET-1 compared with placebo infusion.

DESIGN

In a randomized, single blind, crossover design, 10 lean normotensive male subjects received either an intravenous infusion of subpressor doses of ET-1 dissolved in polygeline or a control infusion of polygeline only (placebo). Using dynamic assessment by the minimal model approach with the modified frequent sampling intravenous glucose tolerance test (FSIGT) the following and other parameters were measured: insulin sensitivity; acute insulin response to glucose (AIR(G)) calculated as the average of the three peak values between 2 and 5 min after injection of glucose from which the basal insulin levels were subtracted; the initial area under the curve (AUC(1-19)) from insulin values between time 0 and 19 min and the first-phase insulin secretion (phi1) from insulin kinetics parameters.

RESULTS

ET-1 infusion reduced AIR(G) (to 34.85 +/- 4.27 compared with 49.3 +/- 6.9 microU/ml during placebo, P=0.017) and the acute C-peptide response to glucose (to 2.33 +/- 0.41 compared with 3.1 +/- 0.44 ng/ml, P=0.018), decreased plasma insulin levels during the FSIGT compared with placebo (analysis of variance P<0.0001) and decreased the AUC(1-19) (to 2.1 +/- 0.2 compared with 2.9 +/- 0.3 U/l per 20 min, P<0.01) while phi1 tended to be lower. S1 measured during ET-1 infusion was unaltered (11.11 +/- 1.91 x 10(-4) versus 10.88 +/- 2.11 10(-4)/min per mU per l, NS).

CONCLUSIONS

These findings demonstrate that an increase in circulating ET-1 to levels observed in insulin-resistant states acutely diminishes the insulin secretory response but does not significantly modify insulin sensitivity.

Endothelin as a regulator of cardiovascular function in health and disease

The endothelins are a family of endothelium-derived peptides that possess characteristically sustained vasoconstrictor properties. Endothelin-1 appears to be the predominant member of the family generated by vascular endothelial cells. In addition to its direct vascular effects, endothelin-1 has inotropic and mitogenic properties, influences homeostasis of salt and water, alters central and peripheral sympathetic activity and stimulates the renin-angiotensin-aldosterone system. Studies with endothelin receptor antagonists have indicated that endothelin-1 probably has complex opposing vascular effects mediated through vascular smooth muscle and endothelial ET(A) and ET(B)receptors. Endogenous generation of endothelin-1 appears to contribute to maintenance of basal vascular tone and blood pressure through activation of vascular smooth muscle ET(A)receptors. At the same time, endogenous endothelin-1 acts through endothelial ET(B) receptors to stimulate formation of nitric oxide tonically and to oppose vasoconstriction. In view of the multiple cardiovascular actions of endothelin-1, there has been much interest in its contribution to the pathophysiology of hypertension. Results of most studies suggest that generation of, or sensitivity to, endothelin-1 is no greater in hypertensive than it is in normotensive subjects. Nonetheless, the deleterious vascular effects of endogenous endothelin-1 may be accentuated by reduced generation of nitric oxide caused by hypertensive endothelial dysfunction. It also appears likely that endothelin participates in the adverse cardiac and vascular remodelling of hypertension, as well as in hypertensive renal damage. Irrespective of whether vascular endothelin activity is increased in hypertension, anti-endothelin agents do produce vasodilatation and lower blood pressure in hypertensive humans. There is more persuasive evidence for increased endothelin-1 activity in secondary forms of hypertension, including pre-eclampsia and renal hypertension. Endothelin-1 also appears to play an important role in pulmonary hypertension, both primary and secondary to diseases such as chronic heart failure. The hypotensive effects of endothelin converting enzyme inhibitors and endothelin receptor antagonists should be useful in the treatment of hypertension and related diseases. Development of such agents will increase knowledge of the physiological and pathological roles of the endothelins, and should generate drugs with novel benefits.

Recombinant human endothelin-converting enzyme ECE-1b is located in an intracellular compartment when expressed in polarized Madin-Darby canine kidney cells

Endothelin-converting enzyme (ECE) is a phosphoramidon-sensitive membrane-bound metalloprotease responsible for the conversion of big-endothelins into endothelins [Yanagisawa, Kurihara, Kimura, Tomobe, Kobayashi, Mitsui, Yazaki, Goto and Masaki (1988) Nature (London) 332, 411-415]. Several distinct isoforms of ECE have been cloned and identified. ECE-1a, b and c have the same ectodomain and differ only by their cytosolic tails [Schweizer, Valdenaire, Nelböck, Deuschle, Edwards, Stumpf and Löffler (1997) Biochem. J. 328, 871-877]. The ectodomain common to ECE-1 a, b and c shares extensive sequence similarities with neprilysin, a major kidney brush border metallopeptidase. To study the sorting of ECE in polarized cells, ECE-1bcDNA was expressed by transfection in polarized Madin-Darby canine kidney (MDCK) cells. Cell-surface biotinylation and immunofluorescence studies showed that ECE-1b is not expressed on the cell-surface but was rather located in intracellular compartments that could also be labelled with anti-Rab-5 and Rab-7 antibodies and was thus tentatively identified as early and late endosomes. Similar results were also obtained when ECE-1b was expressed in non-polarized Chinese hamster ovary cells for comparison purposes. When MDCK or Chinese hamster ovary transfected cells were pre-treated with the ECE inhibitor phosphoramidon, a 3-fold increase in the level of ECE-1b was observed both by Western blotting and by enzymic activity. However, no change in the level of neprilysin or the beta-chain of meprin, two apical membrane metallopeptidases, was observed in MDCK cells transfected under similar conditions. Northern blotting showed that the increase in the level of ECE-1b was not owing to changes in the ECEmRNA transcription rate or stability. Rather, pulse-chase experiments followed by immunoprecipitation showed a decrease in the rate of degradation of ECE-1b in phosphoramidon-treated cells. Half-lives were determined to be 2.8 and 7.5 h for non-treated and phosphoramidon-treated cells, respectively. Confocal microscopy showed accumulation of ECE-1b immunoreactive material in the lysosomes of phosphoramidon-treated cells. Taken together, these results suggest that ECE-1b turns over very rapidly between endosomal and lysosomal compartments and that lysosomal degradation of the enzyme is slowed down by phosphoramidon.

Construction, expression and characterization of a soluble form of human endothelin-converting-enzyme-1

Endothelin-converting-enzyme-1 (ECE-1) belongs to the family of zinc metallopeptidases and is responsible for generating endothelin (ET) peptides from their inactive precursors the big endothelins (bigET). The enzyme is a type II integral membrane protein consisting of a short amino-terminal cytosolic domain of 56 amino acids, a single transmembrane domain and a large putative extracellular domain containing the catalytic site. Recombinant and native ECE-1 are expressed as a dimer. We have constructed a soluble form of ECE, named sECE*, by fusing the cleavable signal peptide of pro-opiomelanocortin in frame to the complete extracellular domain of human ECE-1. Stable expression of this construct in CHO cells resulted in the secretion of a fully active enzyme. In contrast to membrane-bound ECE, sECE* was expressed as a monomer, highly glycosylated, as assessed by gel filtration and Western blot. However, recombinant sECE* converted bigET-1 with similar specific activity as ECE-1a. This activity was completely inhibited by phosphoramidon, but not by thiorphan and captopril. sECE* was active in a broad range of pH, showing an optimum of 6.6-6.8 for bigET-1. Thus, the extracellular domain alone is sufficient for conferring full ECE-1 activity, inhibitors recognition and substrate specificity.

Endothelin-1 stimulates steroid secretion of human adrenocortical cells ex vivo via both ETA and ETB receptor subtypes

The role played by endothelins (ETs) and their receptor subtypes (ETA and ETB) in the regulation of steroid hormone secretion in human adrenal gland remains unclear. Therefore, we investigated the gene expression of ET-1 and its receptors in highly pure preparations of human adrenocortical cells and the effect of ET-1 on their secretory activity. Reverse transcription-PCR with primers specific for prepro-ET-1, ET-converting enzyme-1, ETA, and ETB complementary DNAs demonstrated the expression of all of these genes in human adrenocortical cells. ET-1 increased the secretion of aldosterone and cortisol by enhancing both earlier and late steps of their synthesis. The secretory response to ET-1 was partially (60%) inhibited by BQ-123 and BQ-788, which are selective antagonists of the ETA and ETB receptors, respectively. When added together, the two antagonists suppressed the secretagogue effect of ET-1. Collectively, these findings suggest that ET-1, acting via both ETA and ETB receptors, may exert an autocrine/paracrine regulation of the function of the human adrenal cortex.

A live-cell assay for studying extracellular and intracellular endothelin-converting enzyme activity

Endothelin-1 (ET-1) is formed from its precursor preproET-1 via the cleavage of the intermediate bigET-1 by endothelin-converting enzyme (ECE-1). However, the subcellular site at which this step occurs is not clear: It could occur intravesicularly along the secretory pathway or bigET-1 might be released and processed extracellularly. To address this point, we have developed an integrated autocrine system that uses a recombinant Chinese hamster ovary (CHO) luciferase reporter cell line that permanently expresses the human ET(A) receptor. Into these cells we transiently transfected human ECE-1a cDNA, either together with the human preproET-1 cDNA (as an endogenous source of bigET-1), or alone (in which case exogenous bigET-1 was added). Phosphoramidon inhibited the conversion of exogenous bigET-1 (IC50 = 5 to 30 micromol/L) much better than that of endogenous bigET-1 (IC50 > 1 mmol/L). Both conversions showed similar high yields (20% to 100%) that depended on the amount of ECE-1a expressed. Thus, ECE-1a has two equally relevant activities in this recombinant system for CHO cells: (1) an intracellular, probably intravesicular activity, corresponding to the ECE-1a-mediated step of ET-1 biosynthesis and (2) an extracellular activity at the plasma membrane. If this is also the case for endothelial cells, ECE-1a inhibitors would have to cross the plasma and vesicle membranes to be effective. The present system could be useful for screening such inhibitors.

The role of endothelins in the paracrine control of the secretion and growth of the adrenal cortex

Endothelins (ETs) are a family of vasoactive peptides (ET-1, ET-2, and ET-3) mainly secreted by vascular endothelium and widely distributed in the various body systems, where they play major autocrine/paracrine regulatory functions, acting via two subtypes of receptors (ETA and ETB): Adrenal cortex synthesizes and releases ETS and expresses both ETA and ETB. Zona glomerulosa possesses both ETA and ETB, whereas zona fasciculata/reticularis is almost exclusively provided with ETB. ETS exert a strong mineralocorticoid and a less intense glucocorticoid secretagogue action, mainly via ETB receptors. ETS also appear to enhance the growth and steroidogenic capacity of zona glomerulosa and to stimulate its proliferative activity. This trophic action of ETS is likely to be mediated mainly by ETA receptors. The intraadrenal release of ETS undergoes a multiple regulation, with the rise in blood flow rate and the local release of nitric oxide being the main stimulatory factors. Data are also available that indicate that ETS may also have a role in the pathophysiology of primary aldosteronism caused by adrenal adenomas and carcinomas.

Molecular pharmacology of endothelin converting enzymes

A critical processing step in endothelin biosynthesis is the conversion of the intermediate "big endothelin" to its biologically active product catalysed by endothelin converting enzyme (ECE). In this commentary we discuss critically the cellular location, structure, and activity of the isoforms of ECE. The current evidence supporting a metallopeptidase ECE as the physiological regulator of endothelin production is described. Its sensitivity to inhibition by the fungal metabolite phosphoramidon and subsequent cloning of the enzyme indicate it to be a type II integral membrane protein homologous with neural endopeptidase-24.11 (E-24.11), the major neuropeptide-degrading ectoenzyme in brain and other tissues. Unlike E-24.11, however, ECE exists as a disulphide-linked dimer of subunit M(r) 120-130 kDa and is not inhibited by other E-24.11 inhibitors such as thiorphan. Alternative splicing produces two forms of ECE with distinct N-terminal tails. These isoforms of ECE-1 show similar specificity converting big endothelin-1 (ET-1) to ET-1 but big ET-2 and big ET-3 are converted much less efficiently. This suggests that additional forms of ECE remain to be isolated. Immunocytochemical studies indicate a predominant cell-surface location for ECE-1, like E-24.11. This is consistent with the conversion of exogenous big ET-1 when administered in vivo and the inhibition of this event by phosphoramidon. However, mature ET-1 can be detected in intracellular vesicles in endothelial cells, suggesting that some processing occurs in the constitutive secretory pathway. This may be mediated by ECE-2, a recently cloned member of the E-24.11/ECE family which has an acidic pH optimum. Selective inhibitors of ECE may have therapeutic applications in cardiovascular and renal medicine.

Hormonal and pregnancy relationships to rheumatoid arthritis: convergent effects with immunologic and microvascular systems

OBJECTIVE

To review sex hormones and rheumatoid arthritis (RA) and the interrelationships between hormonal, immunological, and vascular systems.

DATA SOURCES

Publications detailing serum sex hormone levels and their HLA interactions, steroidogenesis, pregnancy, and therapeutic uses of sex hormones in RA.

STUDY SELECTION

Controlled studies of sex hormone levels in RA patients not previously treated with glucocorticoids.

DATA EXTRACTION

Mean (+/- SD) serum levels of dehydroepiandrosterone sulfate (DHEAS), testosterone (T), and estradiol (E2).

DATA SYNTHESIS

Mean (+/- SD) levels were collated into tables for women with pre-versus postmenopausal onsets of disease and men. Data were also ordered across all study groups by increasing mean levels of the control subjects. Pooled data were summarized statistically, and major sources of variation between the studies were identified.

CONCLUSIONS

Serum DHEAS, an adrenal androgen, was impressively decreased among women with premenopausal onset of RA. One study showed such deficiency years before disease onset. Serum T was somewhat decreased in the premenopausal onset group, but could be explained by decreased peripheral conversion of the lower levels of adrenal androgens. Women with postmenopausal onset of RA had modestly decreased serum DHEAS levels overall, but no difference in serum T, compared with controls. Male RA cases had consistently decreased serum levels of T, but not of DHEAS. Serum E2 was comparable in all RA versus control groups. The complex biology of pregnancy was interpreted as an example of vital interactions between hormonal, immunological, and vascular systems, as they may relate to the physiopathology of RA. The major age, sex, and hereditable determinants of RA were compared within a composite table of estimated relative risks. Elucidation of the interacting risk factors offers promising avenues of research in this complex disease.

Endothelin expression in the uterus

The endothelins (ETs) comprise a family of 21 amino acid peptides, ET-1, ET-2 and ET-3, first demonstrated as products of vascular endothelium. Subsequent work showed that they are also found in non-endothelial cells from a variety of tissues such as breast, parathyroid and adrenal gland. At first, the ETs were recognized for their pressor effects. However, ET administration in vivo initially caused hypotension at low concentrations by triggering the paracrine release of endothelial-derived vasodilators. The ETs exert powerful contractile actions on myometrium and other types of smooth muscle and are mitogenic, or co-mitogenic for fibroblasts, vascular smooth muscle and other cells. Demonstration of extravascular ET in endometrium has revealed a powerful vasoconstrictor which might act on the spiral arterioles to effect a powerful and sustained contraction of vascular smooth muscle. ETs might also contribute to the process of endometrial repair. In addition, the ETs appear to play a fundamental role in the control of uterine function in pregnancy. Effects on myometrial contractility have been implicated in the mechanisms governing the onset of normal and pre-term labour, and the peptides are likely to be key determinants of placental blood flow by binding to vascular smooth muscle receptors in the placenta.

Perichondrial localization of ETA receptor in rat tracheal and xiphoid cartilage and in fetal rat epiphysis

Autoradiographic studies using 125I-labeled endothelin-1 (ET-1) on sections of rat cartilage tissues, including the trachea, xiphisternum, and fetal rat epiphysis, revealed dense localization of endothelin receptors in the perichondrium. In contrast, the binding of ET-1 was not detected in the chondrocytes, cartilage matrix, and other connective tissues of the cartilage tissues tested. The perichondrial binding of 125I-ET-1 was completely abolished with BQ-123 [an endothelin receptor subtype A (ETA) antagonist] but not with BQ-3020 (an ETB agonist), and we demonstrated the perichondrial localization of ETA receptors. [3H]thymidine incorporation in vitro was significantly increased in rat xiphoid cartilage tissues exposed to ET-1. These findings suggest that the ET-1/ETA receptor system plays an important role in regulating cartilage metabolism and endochondral bone formation.

Direct and sympathetically mediated venoconstriction in essential hypertension. Enhanced responses to endothelin-1

Endothelin-1 is a potent endothelium-derived vasoconstrictor peptide. Although circulating concentrations are not increased in essential hypertension, enhanced sensitivity to endothelin-1 has been observed in animal models of hypertension. We investigated dorsal hand vein responses to local infusion of endothelin-1 and norepinephrine in 12 patients with essential hypertension who had never received treatment and in 12 age and sex matched normotensive control subjects. The maximal venoconstriction and the geometric mean of the dose of norepinephrine that caused 50% of maximal venoconstriction were similar in hypertensive (mean +/- SE; 80 +/- 4%; 31 +/- 8 pmol/min) and normotensive subjects (87 +/- 5%, 22 +/- 9 pmol/min). In contrast, mean venoconstriction to endothelin-1 was significantly greater in hypertensive (49 +/- 5%) than in normotensive subjects (27 +/- 2%; P = 0.004). Sympathetically mediated venoconstriction elicited by deep breath was substantially potentiated by endothelin-1 in hypertensive (67 +/- 7% at 90 min) but not normotensive subjects (11 +/- 3% at 90 min; P = 0.001). Venoconstriction to endothelin-1 correlated positively with mean arterial pressure in the hypertensive subjects (r = 0.82; p = 0.001) but negatively in the normotensive subjects (r = -0.58; p = 0.047). Endothelin-1 may contribute to the reduction of venous compliance occurring in the early stages of essential hypertension and to the altered systemic hemodynamics in this condition.