Effective antihypertensive therapy has made a major contribution to the reductions in the morbidity and mortality of cardiovascular disease that have been achieved since the 1960s. However, blood-pressure control with conventional drugs has not succeeded in reducing cardiovascular disease risks to levels seen in normotensive persons. Drugs that inhibit or antagonize components of the renin-angiotensin-aldosterone system are addressing this deficiency by targeting both blood pressure and related structural and functional abnormalities of the heart and blood vessels, thus preventing target-organ damage and related cardiovascular events.

Drugs targeting the renin–angiotensin–aldosterone system

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.

Endothelin as a regulator of cardiovascular function in health and disease.

Publisher Summary This chapter discusses the tissue distribution and the substrate specificity of peptidyl-dipeptidase A/angiotensin I-converting enzyme. Angiotensin I-Converting Enzyme (ACE) is a zinc metallopeptidase that belongs to the gluzincin family (clan MA) of metalloproteases of which thermolysin is the prototype. ACE cleaves the C-terminal dipeptide from angiotensin I to produce the potent vasopressor octapeptide angiotensin II and inactivates bradykinin by the sequential removal of two C-terminal dipeptides. In addition to these two main physiological substrates, which are involved in blood pressure regulation and water and salt metabolism, ACE cleaves C-terminal dipeptides from various oligopeptides with a free C-terminus. ACE is also able to cleave a C-terminal dipeptide-amide. The maximum expression of ACE occurs during the acrosome phase in murine species. ACE is exclusively produced in haploid germ cells and belongs to the group of proteins whose expression during definite maturation steps of spermiogenesis appears to be correlated with the unique process of germ cell differentiation. The inactivation of the ACE gene by homologous recombination leads to homozygous male mice with markedly reduced blood pressure, severe renal abnormalities and severely reduced fertility.

Peptidyl dipeptidase A : Angiotensin I-converting enzyme

BACKGROUNDThe angiotensin-converting enzyme (ACE) plays an important role in the production of angiotensin II and the degradation of bradykinin, two peptides involved in cardiovascular homeostasy. Presence of a polymorphism in the ACE gene (ACE Ss) has been postulated from segregation analysis of plasma ACE in families. This putative polymorphism, which strongly affects the plasma and cellular levels of ACE, probably by modulating ACE gene transcription, has not yet been identified at the molecular level; however, an insertion/deletion polymorphism is present in the 16th intron of the ACE gene (ACE I/D) and appears to be a very good marker for ACE Ss. The biological role of ACE suggests that the ACE gene polymorphism could affect the predisposition to myocardial infarction (MI).METHODS AND RESULTSWe have recently shown, in a large case-control study (ECTIM), that the marker allele D of the ACE gene, which is associated with higher levels of ACE in plasma and cells, was more frequent in male patients with ...

Plasma level and gene polymorphism of angiotensin-converting enzyme in relation to myocardial infarction.

In cultured endothelial cells, endothelin is produced after stimulation with angiotensin II. The effects of angiotensin II and endothelin-1 on vascular sensitivity to norepinephrine were studied in perfused rat mesenteric resistance arteries. Expression of endothelin messenger RNA (mRNA) was determined in endothelial cells obtained from the mesenteric circulation. Perfusion (5 hours) of the arteries with angiotensin II (10(-7) M) potentiated contractions in arteries with endothelium induced by norepinephrine in spontaneously hypertensive rats but not Wistar-Kyoto rats. The potentiation was inhibited by phosphoramidon and an endothelin antibody. Short-term stimulation (1 hour) with angiotensin II did not cause the potentiation. Stimulation with angiotensin I (10(-7) M; 5 hours) caused a potentiation prevented by captopril. In endothelial cells collected from the mesenteric arterial bed of spontaneously hypertensive rats, endothelin-specific mRNA was constitutively expressed, and the level of endothelin transcripts was increased by angiotensin II (10(-7) M). Threshold concentrations of exogenous endothelin-1 potentiated contractions induced by norepinephrine in arteries with and without endothelium of spontaneously hypertensive rats but not Wistar-Kyoto rats. Thus, angiotensin II stimulates the endothelial production of endothelin in situ and therapy potentiates contractions to norepinephrine in mesenteric resistance arteries of spontaneously hypertensive rats. This suggests that vascular endothelin production acts as an amplifier of the pressor effects of the renin-angiotensin system that may play an important role in hypertension.

Endothelin stimulated by angiotensin II augments contractility of spontaneously hypertensive rat resistance arteries

Previous studies from our laboratory indicate that exposure of the rat to chronic normobaric hypoxia reduces stores of active angiotensin-converting enzyme (ACE) in the lung. This study assesses directly the effects of hypoxia on ACE synthesis in cultured porcine pulmonary artery endothelial cells. Confluent cultures were exposed to hypoxia [2.5% O2 at 1 atmosphere (atm)] in a triple gas incubator; controls were cultured in normoxic conditions. After 24-, 48-, and 72-h exposure to hypoxic or normoxic conditions, followed by incubation with [35S]methionine for an additional 24 h under the same conditions, newly synthesized radiolabeled ACE was quantitated. Radiolabeled ACE was isolated by an immunobead procedure using either anti-ACE (porcine lung) immunoglobin G (IgG) or nonimmune IgG. A single radiolabeled peak (150 kDa) with the same electrophoretic mobility as purified porcine lung ACE was observed. There was a significant time-dependent increase in endothelial cell ACE antigen synthesis without a concomitant change in either cell number or total trichloroacetic (TCA)-precipitable protein in hypoxic cells compared with normoxic controls. In contrast, ACE activity, assessed by conversion of 125I-labeled angiotensin I to 125I-labeled angiotensin II was unchanged in cultures exposed to hypoxia (2.5% O2). This suggests that an inactive form of ACE is synthesized by cultured pulmonary artery endothelial cells under hypoxic conditions.

Hypoxia stimulates endothelial cell angiotensin-converting enzyme antigen synthesis.

Exposure to angiotensin-converting-enzyme (ACE) inhibitors has been associated with increased ACE activity in vivo and in vitro. In the current study, we examined the effects of the active site-directed ACE inhibitors lisinopril and captopril on ACE gene expression and activity in cultured porcine pulmonary artery endothelial cells. Exposure of endothelial cells to both lisinopril and captopril was associated with increased ACE mRNA levels and concomitant increases in ACE activity. These effects were both concentration and time dependent. ACE mRNA levels began to increase within 30 min of ACE inhibitor exposure and showed an early peak at 2 h and a higher, delayed peak at 48 h. ACE activity peaked at 24 h. Both ACE mRNA levels and activity were highest during incubation with 100 microM inhibitor. Nuclear runoff assays indicated that 48 h of exposure to 100 microM of either captopril or lisinopril increased ACE gene transcription approximately threefold relative to a tubulin control, a level comparable to the increases in ACE mRNA levels and activity observed during ACE inhibitor exposure. These findings support the hypothesis that ACE gene expression endothelial cells is stimulated by active site-directed ACE inhibitors in vitro. This provides a molecular mechanism for the observation that plasma and tissue ACE activity in vivo is increased during chronic exposure to ACE inhibitors.

S. J. King

Papers

Hypoxia stimulates endothelial cell angiotensin-converting enzyme antigen synthesis.

Converting-enzyme inhibitors increase converting-enzyme mRNA and activity in endothelial cells.