The amino acid sequence of hypertensin. II

Regulation of cell proliferation and growth by angiotensin II

The peptide hormone angiotensin II (AngII) has clearly defined physiologic roles as a regulator of vasomotor tone and fluid homeostasis. In addition AngII has trophic or mitogenic effects on a variety of target tissues, including vascular smooth muscle and adrenal cells. More recent data indicate that AngII exhibits many characteristics of the 'classical' peptide growth factors such as EGF/TGF alpha, PDGF and IGF-1. These include the capacity for local generation ('autocrine or paracrine' action) and the ability to stimulate tyrosine phosphorylation, to activate MAP kinases and to increase expression of nuclear proto-oncogenes. The type 1 AngII receptor, which is responsible for all known physiologic actions of AngII, has been cloned. Activation of this receptor leads to elevated phosphoinositide hydrolysis, mobilization of intracellular Ca2+ and diacylglycerol, and activation of Ca2+/calmodulin and Ca2+/phospholipid-dependent Ser/Thr kinases, as well as Ca2+ regulated tyrosine kinases. The existence of other AngII receptor subtypes has been postulated, but the function(s) of these sites remains unclear. In vascular smooth muscle, AngII can promote cellular hypertrophy and/or hyperplasia, depending in part on the patterns of induction of secondary factors that are known to stimulate (PDGF, IGF-1, basic FGF) or inhibit (TGF-beta) mitosis. Together, these findings have suggested that AngII plays important roles in both the normal development and pathophysiology of vascular, cardiac, renal and central nervous system tissues.

Recent developments in the chemistry of gastrointestinal peptides

Work carried out in different laboratories has shown that the peptide pattern of the intestinal tissue is very complex and that some of the peptides are identical to those found in the central nervous system. The best studied of the peptides are of a hormonal nature, but recently evidence has been obtained that others may primarily act as antibiotics. In addition, peptides have been isolated that are fragments of some well-known proteins that have not been viewed as being prohormones. Whether the latter peptides only represent transient degradation products of the proteins or whether, at least some of them, have a physiologically meaningful selective function of their own is not yet clear.

Local inhibition of converting enzyme and vascular responses to angiotensin and bradykinin in the human forearm

1. The function of angiotensin converting enzyme was investigated in twenty-four healthy men. Forearm blood flow was measured under basal conditions and during administration of enalaprilat (a converting enzyme inhibitor) and/or peptide substrates of converting enzyme into the left brachial artery. Blood flow was compared in the two arms. 2. Enalaprilat had no effect on basal blood flow. The concentration of enalaprilat in venous blood from the control arm was low, and plasma renin activity was not increased, indicating that systemic inhibition of converting enzyme did not occur. 3. Effects of angiotensin and of bradykinin, administered intra-arterially, were limited to the infused arm. Enalaprilat (13 nmol min-1) inhibited converting enzyme in the infused arm, in which it caused approximately a 100-fold reduction in sensitivity to angiotensin I, while having no effect on the vasoconstriction caused by angiotensin II. Enalaprilat increased vasodilatation caused by bradykinin. 4. Aspirin, an inhibitor of cyclo-oxygenase, did not inhibit vasodilatation caused by bradykinin whether infused alone or with enalaprilat, indicating that these responses are not mediated by prostaglandins. 5. We conclude that under basal conditions neither conversion of angiotensin I to angiotensin II nor degradation of bradykinin determines resistance vessel tone in the human forearm. Converting enzyme may affect vascular tone in situations in which intravascular concentrations of peptides are increased over those present under basal conditions.

The renin-angiotensin-aldosterone complex

Both the thiazide and loop diuretics have long been known to induce both potassium and magnesium wastage with resultant negative balances of these important intracellular cations. The negative potassium and magnesium balances resulting from diuretic therapy are due primarily to stimulation of the renin-angiotensin-aldosterone complex or secondary hyperaldosteronism. A long experience with an attempt to modify or reduce diuretic-induced potassium depletion by dietary sodium restriction and by the use of antikaluretics has been recorded.

The amino acid sequence of kinetensin, a novel peptide isolated from pepsin-treated human plasma: homology with human serum albumin, neurotensin and angiotensin

A novel nonapeptide with neurotensin-like immunoreactivity was isolated from pepsin-treated human plasma by dialysis, ion-exchange chromatography and high performance reversed-phase liquid chromatography. The amino acid sequence was determined by automated gas-phase sequence analysis as Ile-Ala-Arg-Arg-His-Pro-Tyr-Phe-Leu. Sequence homology with human serum albumin and with the biologically active peptides neurotensin and angiotensin is demonstrated. The name proposed for this peptide is kinetensin.

Activation of angiotensin converting enzyme by monovalent anions

The angiotensin converting enzyme catalyzed hydrolysis of furanacryloyl-Phe-Gly-Gly is activated by monovalent anions in the order C1- greater than Br- greater than F- greater than NO3- greater than CH3COO-. In the alkaline pH region, increasing anion concentrations decrease the KM but do not change the kcat. This behavior is characteristic of an ordered bireactant mechanism in which the anion binds to the enzyme prior to the substrate. At acidic pH values, however, the anion activation is a result of both a decrease in KM and an increase in kcat, implying a bireactant mechanism in which anion and substrate bind randomly. For both the ordered and the bireactant mechanisms the anion serves as an essential activator. The effect of chloride on enzyme activity was studied over the pH range 5-10 under kcat/KM conditions and demonstrates that the apparent chloride binding constant increases from 3.3 mM at pH 6.0 to 190 mM at pH 9.0. The kcat vs. pH profile exhibits two pK values of 5.6 and 9.6, while the variation of KM with pH is characterized by a pK of 8.9 and a 2-fold increase between pH 6.5 and 7.5. The chloride activation of the hydrolysis of furanacryloyl-Phe-Gly-Gly is compared with that of the physiological substrates angiotensin I and bradykinin.

Substrate specificity and kinetic characteristics of angiotensin converting enzyme

Furanacryloyl-Phe-Gly-Gly has been shown to be a convenient substrate for angiotensin converting enzyme (dipeptidyl carboxypeptidase, EC 3.4.15.1). A detailed kinetic analysis of the hydrolysis of this substrate indicates normal Michaelis-Menten behavior with kcat = 19000 min-1 and KM = 3.0 x 10(-4) M determined at pH 7.5, 25 degrees C. The enzyme is inhibited by phosphate and activated by chloride; maximal activity is observed with 300 mM NaCl. In the absence of added zinc, activity is lost rapidly below pH 7.5 due to spontaneous dissociation of the metal, but in the presence of zinc, the enzyme remains fully active to about pH 6. The pH-rate profile indicates two groups on the enzyme with apparent pK values of 5.6 and 8.4. The substrate specificity of the enzyme has been examined in terms of the fundamental specificity quantity kcat/KM as well as the separate constants by using a series of furanacryloyl-tripeptides. The activity toward furanacryloyl-Phe-Gly-Gly has been compared with that toward the physiological substrates angiotensin I and bradykinin.

Is the renin system necessary?

Numerous studies have been carried out to assess the role of the renin system in sustaining abnormally high blood pressure and in contributing to various other cardiovascular disorders such as congestive heart failure, ascites, and shock. The clinical use of potent and specific inhibitors of the renin-angiotensin system has produced important application in the treatment of high blood pressure, severe congestive cardiac failure and experimental hemorrhagic shock. Only in the state of considerable sodium depletion does blockade of the renin system produce any untoward effect, i.e. hypotension. These results are very similar to those obtained previously in patients with bilateral nephrectomy. They raise the question whether under conditions of our present salt-eating habits the renin system is really necessary.

Renal-clip hypertension in rabbits immunized against angiotensin II

The rabbit immunized against angiotensin II was shown to be a valid model for the study of renal-clip hypertension. In particular, there was a specific and complete blockade of the pressor effect of high doses of intravenous renin and angiotensin in vivo, even at angiotensin II production rates which far exceeded those associated with renal-clip hypertension. Despite this, four immunized rabbits developed hypertension after renal-artery clipping with contralateral nephrectomy, and in three of these the hypertension was severe. In four other rabbits, there was no evidence of modification of an established hypertension after immunization against angiotensin II. In both groups, the specific absence of pressor response to high doses of renin and angiotensin II after immunization was confirmed. These studies provide strong evidence that angiotensin is not the sole or even the major factor in either the initiation or maintenance of this form of hypertension.

A rapid simple method for the assay of renin in rabbit plasma

1. EDTA (10mm), 2,3-dimercaptopropan-1-ol (10mm) and chlorhexidine gluconate (0.005%, w/v) cause complete inactivation of plasma enzymes that degrade angiotensin I, but have no effect on the reaction of renin with its substrate. The reagents were termed the selective inhibitors. 2. Thus it is possible to measure renin in plasma by its ability to catalyse the release of angiotensin I. 3. Sterile plasma, treated with the selective inhibitors, is incubated with renin substrate (500-1000ng. of angiotensin content/ml.) at pH6 at 42 degrees for 6hr. 4. Under these conditions the reaction obeys first-order kinetics. Renin activity is calculated in terms of the percentage release of the angiotensin content/hr. 5. As described, the assay is sufficiently sensitive to measure renin in the plasma of all normal rabbits. By extending the length of the incubation, much lower activities can be measured.

Properties of renin substrate in rabbit plasma with a note on its assay

1. Rabbit plasma enzymes that degrade angiotensin I are inhibited completely by the combination of 2,3-dimercaptopropan-1-ol (10mm), EDTA (10mm) and chlorhexidine gluconate (0.005%, w/v). These compounds do not modify the reaction of renin with renin substrate and are termed the selective inhibitors. 2. The renin substrate concentration of plasma can be measured as angiotensin I content by incubating plasma plus the selective inhibitors with renin for a time sufficient to allow complete utilization of renin substrate. 3. This reaction obeys first-order kinetics to substrate concentrations of at least 1000ng. of angiotensin I content/ml. In general, the renin substrate concentrations of normal rabbit plasmas are less than 1000ng. of angiotensin I content/ml. Thus the time required for the complete release of angiotensin I from normal plasma is inversely related to renin activity and is independent of renin substrate concentration. 4. A method for the assay of renin substrate, taking these reaction kinetics into account, is presented.

Tachyphylaxis to alpha- and beta-angiotensin in dogs perfused with Ringer's solution or blood

Repeated injections of µ-asparaginyl-angiotensin in the hindquarters of dogs perfused either with blood or with Ringer's solution did not lead to tachyphylaxis with doses of less than 0.5 µg. Above this dose tachyphylaxis developed in preparations perfused with Ringer's solution. Tachyphylaxis occurred in blood-perfused preparations at doses of 2 µg and above. Tachyphylaxis to β-aspartyl-angiotensin appeared with doses above 0.5 µg irrespective of the perfusing medium used. It is concluded that plasma angiotensinase activity plays some part in limiting the development of techyphylaxis only to µ-asparaginyl-angiotensin; also, that removal of angiotensin from the biophase is dependent in part on the angiotensin being removed by perfusion. A dynamic model for the action of angiotensin on its receptor is proposed.

Isolation and amino acid composition of human angiotensin I

1. Angiotensin, the most powerful pressor substance known, suspected to be a causal substance in renal hypertension and previously isolated from animal sources, has now been isolated from human sources and the amino acid composition was analysed. 2. The procedures followed in the successful isolation of human angiotensin include: (a) preparation of stable materials to obtain maximum formation of human angiotensin; (b) a relatively selective adsorption of the formed angiotensin on Dowex 50W (X2); (c) gel filtration through Sephadex G-25; (d) cation-exchange chromatography on CM-cellulose; (e) anion-exchange gel filtration on DEAE-Sephadex A-25; (f) molecular-sieve chromatography through Bio-Gel P-2. 3. The homogeneity of the human angiotensin isolated was confirmed by paper and thin-layer chromatography and paper electrophoresis. 4. The biological activity observed indicates the substance isolated to be human angiotensin I. 5. The amino acid analysis suggested the following proportional composition: Asp, 1; Pro, 1; Val, 1; Ile, 1; Leu, 1; Tyr, 1; Phe, 1; His, 2; Arg, 1. This composition is similar to that of horse angiotensin I, i.e. isoleucine(5)-angiotensin I.

Development of tachyphylaxis to aspartyl and not asparginyl angiotensin II in the rat

Pressor responses to single increasing doses (0.001 to 10 µg) and constant infusions (0.01 to 5 mg/kg per hr) of synthetic aspartyl 1 angiotensin II (Asp 1 ) and asparginyl 1 angiotensin II (Aspg 1 , Hypertensin, CIBA) were measured in normal and nephrectomized rats. Mean blood pressure was recorded directly using strain gauges. Angiotensin tachyphylaxis (decline of maximal response to increasing doses, or return of blood pressure to control levels on constant infusion) was consistently seen with greater than minimal doses of Asp 1 while the phenomenon was not observed with Aspg 1 at any dose level. The results indicate that in the rat the two octapeptide analogues of angiotensin are not identical in biologic activity as has been previously held, and that the N-terminal amino acid plays an important part in determining the pressor responsiveness especially to higher doses of angiotensin peptides. The pattern of response and development of tachyphylaxis to Asp 1 closely resembles that of well known tachyphylaxis to renin. These results lend further support to our earlier findings that Asp 1 represents the natural form of angiotensin released in vivo. Renin tachyphylaxis may in its entirety be explained to depend upon the actions of Asp 1 released by renin in plasma. Tachyphylaxis to endogenous angiotensin in pathologic as well as certain physiologic conditions may be expected to occur at lower dose levels than is indicated by the available experimental work with Aspg 1 .

Effect of Angiotensin II on the Intact Forearm Veins of Man

Angiotensin II was injected into an isolated venous segment in eighteen human subjects and administered intravenously into a systemic vein in five other subjects. Injections of angiotensin II into an intact isolated venous segment produced no constriction of the segment. Intravenous systemic infusion of angiotensin II produced a slight constriction of the isolated venous segment in three subjects (mean 44 mm H 2 O) and no constriction of the segment in two subjects. The constriction in the venous segment associated with the systemic intravenous administration of the drug could be interrupted by blocking the nerves supplying the venous segment. Thus, angiotensin II has no direct effect upon the forearm veins of man. The rise in segmental venous pressure following the intravenous infusion of angiotensin II appears to originate from reflexes located outside the venous system which influence tone of the superficial veins of the forearm of man.

Angiotensin II Studies in Hypertension

1. In primary hypertension, the digital blood vessels are more reactive than normal to angiotensin II as well as to l -norepinephrine.

2. In terms of weight, the potency of angiotensin II in constricting digital blood vessels is 10 times that of norepinephrine in both normotensive and hypertensive subjects.

3. The turnover of angiotensin II-I 131 is slower than normal in patients with primary hypertension.

4. Digital vascular reactivity to both l -norepinephrine and angiotensin II is normal in "pure" renal hypertension.

5. Angiotensin II-I 131 turnover, in contrast, in the case of renal hypertension studied, was slower than in the normal group and was similar to that in the case of primary hypertension.