Cellular mechanism of thrombin on endothelin-1 biosynthesis and release in bovine endothelial cell

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.

Dependence of endothelin-1 secretion on Ca2+

The role of Ca2+ and protein kinase C (PKC) activity in the release of immunoreactive endothelin-1 (ET-1) from cultured porcine aortic endothelial cells of first or second passage has been studied. ET-1 accumulation within cells and secretion into cell-conditioned medium over 3 and/or 5 hr was measured. Confluent cells incubated in medium containing 1.8 mM Ca2+ (control condition) accumulated and released ET-1 in a time-dependent way. Reducing intracellular free Ca2+ concentration ([Ca2+]i) by adding the Ca2+ entry blockers NiCl2 (0.5 mM) and amiloride (1 mM) or the Ca2+ chelator EGTA (5 mM) to the incubation medium reduced ET-1 secretion to between 50 and 30% of controls, respectively (P < 0.01). To determine the effect of high [Ca2+]i on ET-1 release, cells were incubated with thapsigargin (10-1000 nM) or Ca2+ ionophore A23187 (1 microM) which raised [Ca2+]i progressively from 190 nM (control) to > 1 microM. Both agents reduced ET-1 secretion in a concentration-dependent manner to between 50 and 20% of controls (P < 0.01). Intracellular levels of ET-1 were also reduced at both low and high [Ca2+]i (P < 0.01). In the presence of the PKC inhibitors chelerythrine (50 microM) and H-7 (60 microM), basal ET-1 secretion was reduced to below 20% of controls (P < 0.01). The PKC activator phorbol 12-myristate 13-acetate (0.4 microM) stimulated ET-1 release 1.4-fold (P < 0.01) and its effect was abolished by EGTA (5 mM). Increased [Ca2+]i stimulated the production and release of cyclic guanosine-3',5'-monophosphate, but basal ET-1 secretion rates correlated poorly with nucleotide levels. These data indicate that: (i) at resting [Ca2+]i concentrations, ET-1 release is close to maximal and is reduced at lower and higher concentrations, resulting in a bell-shaped relationship between [Ca2+]i and ET-1 release; and (ii) basal ET-1 release is largely determined by Ca(2+)-dependent PKC activity.

Characterization and functional analysis of the rat endothelin-1 promoter

To define the molecular mechanisms of endothelin-1 (ET-1) gene regulation, we cloned, sequenced, and characterized the rat ET-1 promoter. A sequence consisting of the first 1329 bp of the rat ET-1 promoter was investigated in greater detail. Sequence analysis identified putative binding sites for a number of transcriptional factors that may be involved in ET-1 gene regulation. Several of these factors have been proposed earlier to be involved in cell-specific gene regulation and may be responsible for directing ET-1 expression in vivo. For functional analysis of the ET-1 promoter, we generated a reporter gene construct using luciferase as reporter gene under control of the promoter fragment isolated. The construct was transfected transiently into bovine aortic endothelial cells, and luciferase expression was evaluated. The results indicated that the promoter segment used showed high expression in endothelial cells comparable to that induced by viral promoters. Since ET-1 is regulated by a number of vasoactive substances, we studied the effect of angiotensin II on endothelin transcription. We could demonstrate a dose-dependent transcriptional activation of ET-1 transcription by angiotensin.

Refilling state of internal Ca2+ stores is not the only intracellular signal stimulating Ca2+ influx in human endothelial cells

To further analyse the role of the refilling state of internal Ca2+ pools in the stimulation of Ca2+ influx in human endothelial cells, we investigated the combined effect of thapsigargin (TG) and histamine on cytosolic Ca2+ concentration ([Ca2+]i) and inositol polyphosphate production. At normal extracellular Ca2+ levels, TG induced a progressive and sustained elevation in [Ca2+]i which was dose-dependently prevented by pretreatment with 1-10 microM histamine. Similarly, pretreatment with 0.1 and 1 microM TG suppressed histamine-induced Ca2+ transients partially and totally, respectively. TG pretreatment did not alter the inositol triphosphate (IP3) level liberated by histamine, but modified IP3 metabolism by decreasing inositol biphosphate (IP2) and increasing inositol monophosphate (IP1) contents. In the absence of Ca2+ influx, 1 microM TG only induced a small transient increase in [Ca2+]i whereas the Ca2+ mobilization evoked by 10 microM histamine was unchanged. In both cases, the absence of any additional effect of either TG, histamine or 2 microM ionomycin indicated the complete depletion of Ca2+ stores. The re-establishment of the transmembrane Ca2+ gradient induced a transient rise in [Ca2+]i. Its amplitude differed between histamine- and TG-treated cells. It was imposed by cell pretreatment and was selectively affected by changes in the membrane potential. At 5 mM external K+, the transient rise in [Ca2+]i was more marked in histamine- than in TG-stimulated cells; this difference was suppressed by TG pretreatment. The presence of 130 mM external K+ increased Ca2+ entry in TG-treated cells but reduced it in histamine-stimulated cells. These results indicate that the refilling state of internal Ca2+ stores does not constitute the single regulator of Ca2+ influx. TG and histamine seem to activate Ca2+ influx through distinct but interdependent pathways regulated by membrane potential.

Effects of endothelin on hemodynamics, prostaglandins, blood coagulation and renal function

The interaction of the endogenous vasoconstrictors endothelin (ET), angiotensin II (Ang II) and catecholamines with the kallikrein-kinin-, prostaglandin and renin-aldosterone systems in the pathogenesis of acute renal failure (ARF) is still to be defined. In 18 anesthesized pigs the influence of i.v. bolus applications of ET (2 micrograms/kg), Ang II (10 micrograms/kg) and norepinephrine (NE; 20 micrograms/kg) on hemodynamics, plasmatic coagulation and fibrinolysis system, prostaglandins and renal function was studied. ET induced a biphasic change in blood pressure, starting with an initial short-lasting reduction followed by a long-lasting elevation of systolic and diastolic blood pressure. Endothelin bolus resulted in a significant increase of 6-keto-PGF1 alpha, PGE2 and TXB2 plasma levels (P < 0.05 against preinjection values), whereas prostaglandins remained unchanged in the Ang II and NE groups. There was a distinct correlation between the plasma ET and 6-keto-PGF1 alpha levels (r = 0.82). In contrast to Ang II or NE, ET induced a shortening of the activated partial thromboplastin time (aPTT) and increase of antithrombin III levels (ATIII), fibrin monomers (FM), prekallikrein (PKK) and factor VIII activity at the beginning. Finally a pronounced decrease of ATIII, FM and PKK occurred, indicating a consumptive coagulopathy. At the end of the experiment, elevated plasma renin activity and pCO2, significantly decreased creatinine clearance, blood pH, pO2, base excess, HCO3-, oxygen saturation (P < 0.01), a distinct glomerular proteinuria, and a final anuria were observated. These results reveal that ET activates the plasmatic coagulation system and induces an ARF accompanied by impairment of pulmonary function.(ABSTRACT TRUNCATED AT 250 WORDS)