Effects of renin inhibition in systemic hypertension

Direct renin inhibitors as antihypertensive agents

Hypertension, a serious disease affecting almost a billion people (25% of adults) worldwide, is a major modifiable risk factor for cardiovascular (CV) and renal disease. Despite numerous advances in the pharmacologic treatment of high blood pressure (BP) and availability of several antihypertensive drugs to treat hypertension, a significant proportion of treated hypertensive patients still have uncontrolled high BP, and thus, face serious morbidity and mortality. Furthermore, it is not sufficient to aim for optimum BP control, but to treat all CV risk factors, protect end-organ damage, prevent progression of disease, and prevent long-range adverse effects of the drugs. Therefore, new therapeutic modalities have to be developed to achieve the above objectives. Some years ago, investigators identified renin inhibition as the preferred pharmacologic approach to blockade of the renin-angiotensin system. Renin is a monospecific enzyme that catalyzes the rate-limiting step in the synthesis of angiotensin II. Amplified enzymatic activity and additional physiologic effects occur when renin and prorenin bind to the (pro)renin receptor. Until very recently, development of clinically effective renin inhibitors remained elusive but molecular modeling was used to develop aliskiren and other renin inhibitors that produce sustained suppression of plasma renin activity after oral administration with a dose-dependent BP. Additional studies will ultimately determine the place of renin inhibition in the treatment of hypertension and related CV disorders.

Differential effects of thyroid hormone on renin secretion, content, and mRNA in juxtaglomerular cells

The effects of thyroid hormone on renin secretion, renin content, and renin mRNA levels in juxtaglomerular (JG) cells harvested from rat kidneys were determined by radioimmunoassays and reverse transcriptase-polymerase chain reaction. Despite a lack of immediate effect, incubation with triiodothyronine dose dependently increased renin secretion during the first 6 h and elevated renin content and renin mRNA levels during the subsequent period. Simultaneous incubation with triiodothyronine and the calcium ionophore A-23187 abolished the increase in renin secretion and attenuated the increase in renin content but did not affect the increase in renin mRNA levels. During simultaneous incubation with triiodothyronine and the adenylate cyclase inhibitor SQ-22536 or membrane-soluble guanosine 3',5'-cyclic monophosphate (cGMP), the increases in renin secretion, content, and mRNA were similar to those observed in the presence of triiodothyronine alone, except for a cGMP-induced attenuation of the increase in renin secretion. These findings suggest that thyroid hormone stimulates renin secretion by JG cells through the calcium-dependent mechanism, whereas the stimulation of renin gene expression by thyroid hormone does not involve intracellular calcium or cyclic nucleotides.

Renin inhibition: a novel therapy for cardiovascular disease

The renin-angiotensin system (RAS) is a major contributor in the regulation of blood pressure, and pharmacologic manipulation of this system has resulted in a beneficial class of therapeutic agents, which include angiotensin-converting enzyme (ACE) inhibitors. However, ACE inhibitors are not specific for RAS, and in addition, they can affect bradykinin and prostaglandin, which can also cause changes in vascular tone. Under development are renin inhibitors that are specific for angiotensinogen and act at the initial, rate-determining step of the RAS cascade. The various pharmacologic approaches to renin inhibition include specific renin antibodies, synthetic derivatives of the prosegment of renin precursor, pepstatin analogs, and angiotensinogen analogs. The last approach is the most promising for patient therapy. Multiple studies have shown the effectiveness of the renin inhibitors in both primates and human beings. Further research is now directed toward the development of an agent with good oral bioavailability for patient treatment and as a biologic probe for helping to understand the role of the RAS in control of blood pressure and blood volume.

Haemodynamic, renal and hormonal responses to enalkiren in four patients with post-surgical oliguria

1. The haemodynamic and hormonal responses of four patients with acute post-surgical oliguria (urine output < 0.5 mL/kg per h) were measured in response to the renin inhibitor enalkiren. Enalkiren was infused at 0.01 up to 0.1 mg/kg per h for up to 4 h. 2. Enalkiren infusion was associated with a progressive fall in blood pressure, clinically significant in three of the four patients. Systemic vascular resistance fell in proportion to blood pressure fall. Cardiac output and pulse rate remained unchanged. Effective renal plasma flow rose in all four cases (236 +/- 19 to 327 +/- 38). There was no change in urine flow rate, or urinary sodium excretion. 3. Plasma renin activity (ng angiotensin I/mL per h) fell from 1.9 +/- 0.5 to 0.02 +/- 0.01 (P < 0.04), plasma angiotensin II (pg/mL) fell from 104 +/- 93 to 7.7 +/- 1.5, and plasma aldosterone (ng/dL) fell from 32 +/- 8 to 21 +/- 9 (P = 0.03) at the highest infusion dose. 4. Enalkiren inhibited plasma renin activity with reduced plasma angiotensin II and aldosterone concentrations. This was associated with vasodilation, reduced blood pressure and maintained cardiac output. There was no beneficial effect on renal function in these patients with post-surgical oliguria.

Systemic hypertension and the renin-angiotensin system in diabetic vascular complications

Antihypertensive treatment in the diabetic patient is a critical issue because hypertension has an impact on all of the vascular complications of diabetes, including nephropathy, retinopathy, atherosclerosis, and left ventricular hypertrophy. These complications are a consequence of altered endothelial-vascular smooth muscle interrelations that ultimately enhance vasoconstriction and alter the remodeling processes in the vascular wall. Several observations suggest that the renin-angiotensin system (RAS) may be an important contributor to these processes in diabetes mellitus. In both animal and human studies, angiotensin-converting enzyme (ACE) inhibitors have been demonstrated to slow the progression of glomerulosclerosis, prevent abnormal remodeling processes in the heart following injury, and slow the progression of atherosclerosis. In particular, ACE inhibitors appear to protect the kidney more than would be expected from simply the lowering of blood pressure and decreasing of intraglomerular pressure, possibly because angiotensin II has both hemodynamic and direct effects on the glomerulus. Paradoxically, however, the activity of the circulating RAS is low in diabetic patients. Part of these seemingly inconsistent observations may be due to (1) potential activity of tissue RASs, (2) increased sensitivity to angiotensin II in diabetes, or (3) an effect of ACE inhibition on other systems in addition to the RAS. Investigation of these mechanisms will be important in determining the therapeutic role of inhibition of the RAS in diabetes mellitus.

New therapeutic agents in the management of hypertension: angiotensin II-receptor antagonists and renin inhibitors

OBJECTIVE

To review the chemistry, pharmacokinetics, and clinical trials of two new classes of antihypertensive drugs, angiotensin II-receptor antagonists and renin inhibitors.

DATA SOURCES

Primary literature on angiotensin II-receptor antagonists and renin inhibitors was identified through a comprehensive medical literature search from 1961 through 1993. This search included journal articles, abstracts, and reports of both animal and human research published in the English language. Indexing terms included renin-angiotensin aldosterone system, renin inhibitors, angiotensin II antagonists, DuP 753, losartan, MK954, A-64662, and Ro 42-5892.

STUDY SELECTIONS

Emphasis was placed on clinical and pharmacokinetic studies in humans for drugs that are currently in Phase I-III research protocols in the US.

DATA EXTRACTION

All available data from human studies were reviewed.

DATA SYNTHESIS

Angiotensin II-receptor antagonists and renin inhibitors may be effective antihypertensives with few adverse effects noted in the small studies completed. Their potential advantage over angiotensin-converting enzyme (ACE) inhibitors includes a possible smaller adverse effect profile. In the past, the clinical utility of angiotensin II-receptor antagonists and renin inhibitors has been limited because of poor oral bioavailability, although newer agents are more readily bioavailable.

CONCLUSIONS

Angiotensin II-receptor antagonists and renin inhibitors may be the next new classes of antihypertensives marketed. However, definitive conclusions about their roles in the management of hypertension are not possible until larger clinical trials assessing their efficacy and safety and comparing them with ACE inhibitors are completed.

Purification and characterization of human truncated prorenin

Posttranslational processing of enzymatically inactive prorenin to an active form participates in the control of the activity of a key system involved in blood pressure regulation, growth, and other important functions. The issue is complicated because renin can be produced by a number of tissues throughout the body, in addition to the kidney, but the mechanism by which they process prorenin to renin is unknown and difficult to determine because of the small amounts of renin present. In the juxtaglomerular cell of the kidney, a 43 amino acid prosegment is cleaved from the amino terminus of prorenin to generate renin of molecular weight 44,000 [Do, Y. S., Shinagawa, T., Tam, H., Inagami, T., & Hsueh, W. A. (1987) J. Biol. Chem. 262, 1037-1043]. Using human uterine lining or a recombinant human prorenin system, we employed the same approach as that used in kidney, ammonium sulfate precipitation at pH 3.1 followed by pepstatin and H-77 affinity chromatography or gel filtration, to purify to homogeneity a 45,500-MW totally active renin. The specific activity of the active truncated prorenin was 850 Goldblatt units (GU)/mg of protein for chorion-decidua renin and 946 GU/mg of protein for recombinant renin, both similar to that reported for pure human renal renin. Both forms of renin cross-reacted with an antibody generated against 44,00-MW pure human renal renin and with an antibody generated against a peptide identical to the carboxy-terminal one-third of the prosegment.(ABSTRACT TRUNCATED AT 250 WORDS)