Angiotensin receptors in vascular tissue

Hemopexin activity is associated with angiotensin II responsiveness in humans

BACKGROUND

Hemopexin, an acute phase protein, can downregulate the angiotensin (ang) II type 1 receptor (AT1-R) in vitro. Whether hemopexin is involved in the responsiveness to ang II in vivo is unknown. Therefore, we tested whether variations in endogenous hemopexin activity are associated with the responsiveness of blood pressure to ang II in healthy volunteers.

METHOD

Healthy men (n = 33, age 26 ± 9) were studied in balance on low sodium (50 mmol Na per 24 h) and high sodium (200 mmol Na per 24 h) diet, respectively. After baseline measurements of blood pressure, ang II was infused at 0.3, 1 and 3 ng/kg per min for 1 h per dose. Hemopexin activity was measured at baseline in EDTA-plasma samples by an amidolytic assay with a chromogenic substrate suitable for hemopexin activity evaluation.

RESULTS

During high sodium the hemopexin activity was lower; 1.6 × 10 (0.6 × 10 - 4.7 × 10) versus 2.8 × 10 (1.1 × 10 - 5.1 × 10) arbitrary units (P < 0.01) and the pressor response to 3 ng ang II/kg per minute larger than during low sodium (17.6 ± 6.5 versus 14.6 ± 6.9 mmHg, P < 0.01). Hemopexin activity negatively correlated with the pressor response to ang II during either type of sodium intake (high sodium: r = 0.42, P < 0.05; low sodium: r = 0.35, P < 0.05).

CONCLUSION

These in-vivo data obtained in healthy individuals support recent in-vitro data showing that active hemopexin downregulates the availability of the AT1-R. Therefore, activated hemopexin might be considered as a factor mediating ang II effects upon blood pressure by modulating AT1-R availability.

Dietary sodium intake modulates myocardial relaxation responsiveness to angiotensin II

Dietary sodium alters renovascular responsiveness to angiotensin II (Ang II) in normal subjects. Evidence supports a connection among dietary sodium, the rennin-angiotensin system, and myocardial function. The authors hypothesized that a similar relationship would exist in the heart, and that the pattern of responses would be qualitatively similar to the renal vasculature. Thirteen healthy volunteers (aged 38.6 +/- 4 years) entered a 2 week crossover design study (week 1, high sodium diet [HS] > 200 mmol Na/day; week 2, low sodium diet [LS], < 10 mmol Na/day) to investigate the influence of dietary sodium and Ang II on myocardial relaxation and renal blood flow (RBF). At the end of each study week, the authors assessed diastolic function (myocardial relaxation velocities [E'] using tissue Doppler imaging) and RBF (para-aminohippurate clearance) at baseline and after infusion of Ang II (3 ng/kg/min x 45 min). On HS diet, E' and RBF were higher than on LS diet (E' 14.0 +/- 1.2 vs 12.6 +/- 1.0 cm/s, P = 0.02; RBF 596 +/- 24 vs 563 +/- 26 mL/min, P = 0.02). Dietary sodium significantly modulated E' and RBF responsiveness to Ang II infusion in like manner. HS was associated with increased responsiveness compared with a blunted LS response (HS DeltaE' -1.4 +/- 0.4 cm/s vs LS DeltaE' -0.1 +/- 0.3 cm/s, P = 0.02; HS DeltaRBF -135.2 +/- 13.2 vs LS DeltaRBF -62.5 +/- 10.1 mL/min, P < 0.01). The authors describe for the first time that dietary sodium modulates myocardial relaxation and responsiveness to Ang II. It is important to consider dietary sodium intake when assessing diastolic function. Ang II may play a role in the interaction between dietary sodium and myocardial relaxation. Future research will investigate whether abnormalities in these mechanisms play a role in disorders of diastolic function.

Modulation and function of extrarenal angiotensin receptors

Historically, physiological modulation of the activity of the renin-angiotensin system (RAS) was thought to be mediated only by changes in renin secretion. Hence, altered dietary sodium (Na) intake, changes in renal perfusion pressure, and/or renal adrenoreceptor activity would lead to changes in renin release and plasma angiotensin II (Ang II) concentration, which in turn contribute to regulation of blood pressure and sodium balance. Later, it became apparent that angiotensinogen availability and Ang-converting enzyme activity are also rate-limiting factors that influence the activity of RAS. Finally, over the past few years, evidence has accumulated that indicates the number of Ang II receptors and their subtypes are of great importance in regulating the activity and function of RAS. Cloning of the Ang II receptor genes, development of specific receptor-antagonist ligands, and establishment of genetically mutated animal models have led to greater understanding of the role of Ang II receptors in the regulation of RAS function and activity. This review focuses on the functions and regulation of Ang II receptors in vascular tissues and in the adrenal gland. The authors suggest that identification of control elements for Ang II receptor expression, which are tissue-specific, may provide a basis for future therapeutic manipulation of Ang II receptors in cardiovascular disease states.

Regulation of vascular type 1 angiotensin II receptor in hypertension and sodium loading: role of angiotensin II

OBJECTIVE

To test the hypotheses that a high sodium intake increases steady state messenger RNA levels of the type 1 angiotensin II receptor in the aorta and mesenteric resistance arteries, and that this increase is mediated by suppression of production of angiotensin II induced by a high sodium intake; and to test the hypotheses that angiotensin II administered at a pressor dose increases steady state messenger RNA levels of the angiotensin II type 1 receptor in the aorta and mesenteric resistance arteries, and that this increase is mediated by activation of angiotensin II type 1 receptors in these vessels.

METHODS

In experiment 1, male Wistar rats were divided into four groups and treated for 2 weeks with a (0.5%) normal sodium diet, a normal-sodium diet plus angiotensin II, a high (4%) sodium diet, or a high-sodium plus angiotensin II. We infused 25 ng/kg per min angiotensin II subcutaneously by using osmotic pumps. In experiment 2, male Wistar rats were divided into four groups and treated for 2 weeks with vehicle, 1 mg/kg per day losartan by oral gavage, 250 ng/kg per min angiotensin II by using an osmotic pump), and losartan plus angiotensin II. Angiotensin II type 1 messenger mRNA was measured with the use of quantitative reverse transcriptase-polymerase chain reaction in the presence of an angiotensin II type 1 receptor mutant complementary RNA as internal standard.

RESULTS

Results from experiment 1 show that body weight and systolic tail-cuff blood pressures did not differ among our four groups (P > 0.05). Angiotensin II type 1 messenger RNA levels of rats in high-salt diet group were 73% (aorta) and 171% (mesenteric resistance arteries) greater than those of rats in normal-salt diet group (P < 0.05). In contrast, angiotensin II type 1 messenger RNA levels both in aorta and in mesenteric resistance arteries of rats in normal-salt diet plus angiotensin II and high-salt diet plus angiotensin II groups did not differ from those of rats in normal-salt diet group. Results from experiment 2 show that systolic blood pressures in rats treated with angiotensin II and with losartan plus angiotensin II were higher than those in rats administered vehicle (P < 0.05). Mean response of arterial pressure to bolus injection of angiotensin II was suppressed in losartan-treated rats compared with that in rats administered vehicle and in rats treated with losartan plus angiotensin II compared with that in rats treated with angiotensin II (P < 0.05). Angiotensin II type 1 messenger RNA levels were higher by 73% (in aorta) and 63% (in mesenteric resistance arteries) in rats treated with angiotensin II than they were in rats administered vehicle (P < 0.05), but not in both aorta and mesenteric resistance arteries in rats treated with losartan and losartan plus angiotensin II versus rats administered vehicle.

CONCLUSION

A high-salt diet increases angiotensin II type 1 messenger RNA levels both in aorta and in mesenteric resistance arteries. This increase is completely suppressed by simultaneous nonpressor infusion of angiotensin II, suggesting that angiotensin II negatively regulates vascular angiotensin II type 1 messenger RNA in normotensive rats. Hypertension induced by pressor infusion of angiotensin II increases angiotensin II type 1 messenger RNA levels both in aorta and in mesenteric resistance arteries. This increase can be prevented by administration of losartan at a nondepressor dose, suggesting that angiotensin II positively regulates vascular angiotensin II type 1 messenger RNA via activation of the angiotensin II type 1 receptor during hypertension.

Cytosolic calcium changes induced by angiotensin II in human peripheral blood mononuclear cells are mediated via angiotensin II subtype 1 receptors

OBJECTIVE

To determine the effects of angiotensin II (AII) (1-8) on cytosolic free calcium concentrations in the absence and in the presence of the selective angiotensin subtype 1 (AT1) receptor antagonist losartan and of the selective angiotensin subtype 2-receptor antagonist P-186 in human peripheral blood mononuclear cells (PBMC). We also assessed the effect of the AII analogues AII (2-8), AII (3-8) and AII (4-8) on the cytosolic free-calcium concentration in human PBMC.

METHODS

The cytosolic free-calcium concentration was assayed in human peripheral blood mononuclear cells by measuring the fluorescence of fura-2 entrapped by these cells.

RESULTS

Administration of AII caused a concentration-dependent increase in the cytosolic free-calcium concentration in human peripheral blood mononuclear cells with a half-maximal increase at 5 x 10(-8) mol/l. Also administration of the heptapeptide AII (2-8) increased the intracellular free-calcium concentration in human PBMC, whereas AII (3-8) and AII (4-8) had no effect. The AII (1-8)-induced rise in cytosolic free-calcium concentration was blocked completely by losartan but not by P-186.

CONCLUSION

Our data demonstrate that the effects of AII on the cytosolic free-calcium concentration in human PBMC are AT1 receptor-mediated since they were abolished by the specific AII AT1 receptor antagonist losartan but not by the specific angiotensin subtype 2 receptor antagonist P-186.

The effect of angiotensin II receptor blockade on insulin sensitivity and sympathetic nervous system activity in primary hypertension

The objective of this study was to investigate the effect of Losartan (NK-954, DuP-753), a new selective angiotensin II receptor antagonist, on insulin sensitivity and sympathetic nervous system activity in patients with severe primary hypertension. Five patients with a record of diastolic blood pressure (DBP) > or = 115 mmHg, currently either untreated or with DBP > 95 mmHg on antihypertensive treatment, were examined in an open study with the euglycemic glucose clamp examination before and after being treated with Losartan for an average of 6 weeks. The glucose disposal rate increased from 6.2 +/- 2.6 to 7.9 +/- 2.6 mg/kg x min (27%, p < 0.05) during treatment with Losartan. The insulin sensitivity index (glucose disposal rate divided by mean insulin concentration during clamp) increased from 7.7 +/- 4.5 to 10.1 +/- 4.1 arbitrary units (30%, p < 0.05). Plasma noradrenaline decreased from 1.87 +/- 0.53 to 1.11 +/- 0.13 nmol/l (40%, p < 0.05), while plasma adrenaline was unchanged (0.23 +/- 0.10 vs. 0.22 +/- 0.11 nmol/l, n.s.). Mean blood pressure decreased from 132 +/- 10 to 119 +/- 13 mmHg (p < 0.05) and heart rate was unchanged during treatment with Losartan. Thus, antihypertensive treatment with the new selective angiotensin II receptor antagonist Losartan seems to improve insulin sensitivity. A decrease in plasma noradrenaline on Losartan suggests a sympathicolytic effect which together with vasodilation may explain the fall in blood pressure and the improvement in insulin sensitivity.

Myometrial angiotensin II receptor subtypes change during ovine pregnancy

Although regulation of angiotensin II receptor (AT) binding in vascular and uterine smooth muscle is similar in nonpregnant animals, studies suggest it may differ during pregnancy. We, therefore, examined binding characteristics of myometrial AT receptors in nulliparous (n = 7), pregnant (n = 24, 110-139 d of gestation), and postpartum (n = 21, 5 to > or = 130 d) sheep and compared this to vascular receptor binding. We also determined if changes in myometrial binding reflect alterations in receptor subtype. By using plasma membrane preparations from myometrium and medial layer of abdominal aorta, we determined receptor density and affinity employing radioligand binding; myometrial AT receptor subtypes were assessed by inhibiting [125I]-ANG II binding with subtype-specific antagonists. Compared to nulliparous ewes, myometrial AT receptor density fell approximately 90% during pregnancy (1,486 +/- 167 vs. 130 +/- 16 fmol/mg protein) and returned to nulliparous values > or = 4 wk postpartum; vascular binding was unchanged. Nulliparous myometrium expressed predominantly AT2 receptors (AT1/AT2 congruent to 15%/85%), whereas AT1 receptors predominated during pregnancy (AT1/AT2 congruent to 80%/20%). By 5 d postpartum AT1/AT2 congruent to 40%/60%, and > 4 wk postpartum AT2 receptors again predominated (AT1/AT2 congruent to 15%/85%). In studies of ANG II-induced force generation, myometrium from pregnant ewes (n = 10) demonstrated dose-dependent increases in force (P < 0.001), which were inhibited with an AT1 receptor antagonist. Postpartum myometrial responses were less at doses > or = 10(-9) M (P < 0.05) and unaffected by AT2 receptor antagonists. Vascular and myometrial AT receptor binding are differentially regulated during ovine pregnancy, the latter primarily reflecting decreases in AT2 receptor expression. This is the first description of reversible changes in AT receptor subtype in adult mammals.

Renal and adrenal responsiveness to angiotensin II: influence of beta adrenergic blockade

The increase in aldosterone secretion that occurs in response to Angiotensin II (AII) is enhanced when normal humans are in external balance on a low salt diet. The responsible mechanism has not been identified. Angiotensin converting enzyme inhibition reduces blood levels of AII and aldosterone, but does not decrease PRA or AI and does not modify adrenal responsiveness to AII in the sodium-depleted state. This study was designed to assess the possibility that the enhanced adrenal response reflects plasma renin activity (PRA), plasma AI concentration, or catecholamines acting via a beta adrenergic receptor. Nine healthy males were studied when in balance on a high sodium intake (200 mmol Na/day), a low sodium diet (10 mmol Na) and after 4 days of beta adrenergic blockade with either nadolol or propranolol. The adequacy of beta adrenergic blockade was assessed with a postural stimulus and significant blockade was achieved, somewhat more with nadolol (40 mg/day) than with propranolol (Inderal LA, 80 mg every 12 hrs). Beta blockade enhanced the renal vascular and pressor response to AII but did not modify the adrenal response to posture or to AII. This study confirms the role for AII levels in the modulation of renal vascular and pressor responses to AII and rules out a role for PRA, AI, or catecholamines acting via a beta adrenergic receptor in the modulation of adrenal responsiveness to AII.

The effect of angiotensin-I converting enzyme inhibition on insulin action in healthy volunteers

Acute hyperinsulinaemia, achieving insulin levels within the physiological range, induces sodium retention. At the same time an activation of the renin-angiotensin system occurs, with a rise in plasma renin activity (PRA) and angiotensin-II level but no change in plasma aldosterone. After administration of higher, pharmacological doses of insulin an increase in systolic blood pressure and heart rate can also be observed, while further increases in PRA and angiotensin-II are noted. To determine whether angiotensin-II is involved in observed insulin actions, we studied the renal and cardiovascular effects of three dosages of insulin (50 (Ins I), 300 (Ins II) and 500 (Ins III) mU kg-1 h-1) in healthy subjects after one week of treatment with the angiotensin-I converting enzyme inhibitor enalapril (10 mg twice a day), using the euglycaemic clamp technique. Control data were obtained from two previously conducted experiments in the same subjects, one with infusion of insulin and one with the insulin solvent only. The effect of insulin on fractional sodium excretion, blood pressure and heart rate was unaffected by enalapril, which precludes any involvement of the renin-angiotensin system with regard to these aspects of insulin action. Insulin sensitivity increased significantly during treatment with enalapril (with enalapril: Ins I: 11.3 +/- 3.0, Ins II: 20.0 +/- 3.4 and Ins III: 20.6 +/- 3.9 mg kg-1 min-1 glucose (mean +/- SD); without enalapril: Ins I: 8.7 +/- 2.3, Ins II: 13.7 +/- 3.0 and Ins III: 15.5 +/- 3.1 mg kg-1 min-1 glucose; P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium intake modulates renal vascular reactivity to endothelin-1 in Dahl rats

1. The systemic and renal haemodynamic responses to endothelin-1 (ET1) were evaluated and compared to Angiotensin II (AII) in anaesthetized Dahl salt-sensitive (DS) and salt-resistant (DR) rats on either low (0.1% NaCl in diet) or high (8% NaCl in diet) salt intake. 2. Baseline mean arterial pressure on low salt diet was similar in both strains, while on high salt diet it was 73 +/- 4 mmHg in DR rats and 119 +/- 8 mmHg in DS rats (P less than 0.05). Baseline renal blood flow (RBF) and renal vascular resistance (RVR) were similar in all groups. 3. AII in bolus injection induced a short, dose-dependent increase in blood pressure and renal vascular resistance and a fall in renal blood flow. The maximal pressure increase was significantly greater in DS rats on high salt diet than that in each of the other groups (P less than 0.05). The fall in renal blood flow and the increase in renal vascular resistance were attenuated in both strains on low salt diet. 4. ET1 induced an initial decrease followed by a prolonged increase in blood pressure; both phases were similar in all groups. However, renal vascular reactivity to ET1 was markedly modulated by salt intake. On low salt diet, following a bolus injection of ET1 (1 nmol/kg), RBF decreased by 34% in DR and by 20% in DS rats, while on high salt diet RBF decreased by 76% in DR and by 80% in DS rats (P less than 0.05 high vs low salt).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of acute insulin-induced hypoglycaemia on renal function in normal human subjects

The effects of acute insulin-induced hypoglycaemia on renal function were studied in 8 normal male subjects. Plasma glucose (mean (SE) fell from 4.6(0.2) to 1.3(0.2) mmol l(-1), the nadir being coincident with the acute autonomic reaction, and returned to the basal value over the following 120 min. Glomerular filtration rate declined from 118(6) to 95(4) ml min-1 at the glucose nadir (p less than 0.01), and during the recovery phase returned to 118(7) ml min-1 (NS compared with basal). Renal plasma flow fell from 625(38) to 485(27) ml min-1 (p less than 0.01), rising to 545(46) ml min-1 during recovery from hypoglycaemia (NS compared with basal). Following hypoglycaemia, urinary excretion of sodium and dopamine were reduced significantly, but the albumin excretion rate was unchanged. Plasma concentrations of adrenaline, noradrenaline, angiotensin II, and plasma renin activity increased in response to hypoglycaemia. These acute changes in renal function are probably caused by sympatho-adrenal activation and secretion of catecholamines, but other hormones, such as angiotensin II, may be contributory.

Angiotensin II receptors labelled with 125I-[Sar1, Ile8]-AII in albino rabbit ocular tissues

High affinity binding sites for the angiotensin II antagonist 125I-[Sar1,Ile8]-AII have been identified and characterized in membrane suspensions of ocular tissues of albino rabbits. Scatchard analysis of the binding indicated a single class of sites with Kd values of 186, 92, 152, 50, 102 pM for the iris + ciliary body, choroid, ciliary process, retina and cornea, respectively. The corresponding concentrations of binding sites were 22, 68, 35, 22 and 4 fmole/mg of protein. The order of potency for several AII analogs to compete with 125I-[Sar1,Ile8]-AII at its binding sites in iris + ciliary body membranes ([Sar1,Leu8]-AII = [Sar1,Ile8]-AII greater than AII = [Sar1, Ala8]-AII greater than AIII greater than AI) resembled the order of potency found for AII receptors in other tissues. The competition curves for this tissue using AII and AIII were best explained by the existence of two populations of binding sites. The addition of the guanine nucleotide, GppNHp, to the assay resulted in a 6.7-fold and 2.3-fold decrease in the respective affinities of AII and AIII for 125I-[Sar1,Ile8]-AII binding sites without a change in the slope of the competition curves. The GppNHp-induced effect was also observed in ciliary process membranes but not in retinal or choroidal membranes. These results indicate the presence of AII receptors regulated by a GTP-binding protein in both the ciliary process and the iris + ciliary body of the rabbit. They also suggest a difference in the guanine nucleotide regulation of AII receptors in different ocular tissues.

Insulin influences the renin-angiotensin-aldosterone system in humans

This study investigates whether insulin influences the renin-angiotensin-aldosterone system in humans. Six healthy male volunteers were placed on a 30-minute euglycemic insulin clamp at 160 microU/mL; euglycemia was maintained also in the following 60 minutes by means of appropriate dextrose infusion. Throughout the study, plasma renin activity, angiotensin II, aldosterone, and factors involved in the regulation of the renin-angiotensin-aldosterone system were measured: catecholamines, angiotensin-converting enzyme, sodium, and potassium. A significant increase of plasma renin activity and angiotensin II was observed, and a decrease of aldosterone was also detected. These changes can be ascribed to the effects of the rapid insulin-induced plasma potassium decrease on plasma renin activity and aldosterone secretion because they did not occur in a control clamp study with a potassium infusion.

Prolonged converting enzyme inhibition in non-modulating hypertension

Patients with normal- or high-renin non-modulating essential hypertension fail to shift their adrenal sensitivity on a low sodium diet in response to an infusion of angiotensin II (Ang II). In a prior study, 72 hours of converting enzyme inhibition (CEI) partially corrected this subnormal aldosterone response to Ang II in patients with non-modulating hypertension. Since it was uncertain whether the failure to restore normal adrenal responsiveness reflected a continued abnormality or an insufficient duration of CEI, the present study was performed wherein subjects were studied before CEI and then 72 hours and 6 weeks after CEI. Adrenal and renovascular responses were assessed in 13 subjects with normal- or high-renin hypertension in response to an infusion of Ang II (0.3, 1.0, and 3.0 ng/kg/min) in balance on a 10 meq Na+/100 meq K+ diet. Eight of 13 had a normal plasma aldosterone increment above control levels (greater than or equal to 15 ng/dl) and were classified as modulators; the remaining subjects (five of 13) were classified as non-modulators. Enalapril was then administered for 72 hours and 6 weeks, and the assessment of the Ang II dose-response relations was repeated. In the modulators, there was no change compared with levels before CEI in the aldosterone dose-response curve or threshold sensitivity to infused Ang II at either 3 days or 6 weeks after CEI administration. In the non-modulators, CEI for 72 hours partially restored aldosterone responsiveness, but more prolonged CEI for 6 weeks completely corrected the defect, restoring aldosterone responsiveness on a sodium-restricted diet to that seen in modulators and in normotensive control subjects.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional receptors for fish neuropeptide urotensin II in major rat arteries

Receptor binding of fish neuropeptide urotensin II (UII) was characterized in membranes isolated from major rat arteries. Monoiodinated UII radioligand (125I-UII) was prepared and purified using high-pressure liquid chromatography (HPLC). The contractile potency of iodinated UII (I-UII) on rat thoracic aorta strips was somewhat lower than that of native UII. The binding of 125I-UII to the membrane preparations of rat thoracic aorta was saturable, specific and time-dependent. Scatchard analysis indicated a single population of binding sites with an apparent dissociation constant of 5.9 x 10(-9) M. The calculated maximal number of binding sites was about 155 fmol/mg protein. The specific binding to the membrane preparations from the abdominal aorta and mesenteric artery was about 27 and 8%, respectively, of those in the thoracic aorta, which corresponds to the order of contractile potency of UII on rat blood vessels: thoracic aorta greater than abdominal aorta greater than mesenteric artery. The displacement of 125I-UII binding by the UII peptide or its fragments (UII-(5-12), UII-(6-12) and UII-(6-11] were also comparable to their contractile effects on rat thoracic aorta strips (UII greater than UII-(5-12) greater than UII-(6-12) much greater than UII-(6-11]. These results suggest that the fish neuropeptide, UII, can induce contraction of rat vascular tissue by interacting with its functional receptors.

Renin-angiotensin system in dogs following chronic bile-duct ligation. Relation to vascular reactivity

The pressor response to angiotensin II, blood volume, angiotensin II in arterial blood, renin substrate, renin concentration, renin activity and aldosterone in venous blood, liver function tests, kidney function tests, glucose, sodium, potassium, plasma osmolality and complete blood count were examined before and 1, 2, 3 and 5 weeks after ligation of bile ducts in nine conscious trained dogs. The pressor response to angiotensin II was markedly suppressed after bile-duct ligation, especially at 1-3 weeks postoperation. A maximal decrease in plasma renin substrate, and maximal increases in plasma renin concentration, plasma renin activity and aldosterone were noted at 1 week postoperatively. Plasma angiotensin II levels were elevated at 1 and 5 weeks postoperatively but were near normal 2 weeks postoperatively despite suppression of the angiotensin II pressor response. Endogenous levels did not correlate with suppression of the pressor response to exogenous angiotensin II.

Angiotensin II receptors in the fowl aorta

In the domestic fowl, angiotensin II (ANG II) causes an in vivo depressor response and in vitro relaxation of aortic rings which appear to be a direct action of ANG II on the blood vessels. Thus, we determined whether binding sites specific to ANG II exist in the membrane fraction of the fowl aorta. The particulate fraction of aortas from adult female fowl, Gallus gallus, exhibits high specific binding to ANG II ligand. 125I-[Ile5]ANG II (0.5 nM) binding to the above fraction (30 micrograms protein) in 50 mM Tris (pH 7.2), 10 mM MgCl2, and 0.2% bovine serum albumin at 12 degrees (1) is rapid, saturable, and reversible; (2) increases as a function of ligand or membrane concentration, time, and temperature; and (3) optimally fits to a two-site (high and low affinity) model. The equilibrium dissociation constant (0.15 +/- 0.03 nM) and binding site concentration (28.7 +/- 8.1 fmol/mg protein) of the high affinity site as well as association (0.055 nM-1.min-1) and dissociation (0.0122 min-1) rate constants are similar to those of mammalian vascular ANG II receptors. Both 125I-[Ile5]ANG II and 125I-[Val5]ANG II are competitively displaced by unlabeled ANG II. These results suggest that specific ANG II receptors exist in the fowl aorta.

Effects of angiotensin I converting enzyme inhibition and calcium channel blockade on plasma levels of active and inactive renin in conscious rabbits

The interplay of juxtaglomerular (jg) calcium fluxes and exposure to AII in the regulation of jg renin secretion, was examined in vivo. An inhibitor of angiotensin I converting enzyme (captopril), a blocker of calcium channels (verapamil) and AII amide were infused, singly or in combination, into the ear vein of conscious rabbits. The effects on arterial pressure, and on levels of active and inactive plasma renin were monitored. Captopril (50 micrograms X min-1 X kg-1) produced a greater percentage increase in renin secretion than did verapamil (20 micrograms X min-1 X kg-1), whilst the percentage fall in arterial pressure was similar. AII amide counteracted more effectively the actions of captopril than those of verapamil. When captopril was infused first, addition of verapamil did not enhance renin secretion (P greater than 0.2). When verapamil was infused first, addition of captopril greatly enhance renin secretion (P less than 0.01). However, when captopril was infused first, and its actions then suppressed by AII amide, addition of verapamil led to extremely high rates of renin secretion. The findings suggest the following: the short loop negative feedback plays in vivo an important role in the rapid modulation of jg renin secretion and the action of AII may involve up- and down-regulation at the receptor and/or post-receptor level; infused agents have rapid access to the critical sites of jg cells; exposure to raised concentrations to AII not only reduces the effectiveness of AII, but also enhances jg secretory responses to lack of AII, as well as to calcium channel blockade. Thus, at least some of the jg calcium channels appear to respond both to AII and to blockers; extreme changes in the levels of active renin are possible without changes of inactive renin levels. Secretion of the latter may be under separate control, or its secretion rate parallelled by the rate of its activation.

Differential pressor and renal vascular reactivity to angiotensin II in spontaneously hypertensive and Wistar-Kyoto rats

The suggestion has been made that the Okamoto strain of spontaneously hypertensive rats (SHR) shares some features with a subgroup of patients with essential hypertension, called nonmodulators. One feature of nonmodulators is a renal blood flow response to angiotensin II (ANG II) that is blunted on a high salt diet; the blunted renal vascular response is corrected by converting enzyme inhibition. Renal blood flow (electromagnetic flowmeter) and pressor responses to graded ANG II doses (5-300 ng) were assessed in 24 SHR and 24 Wistar-Kyoto rats (WKY) ingesting 1.6% Na. In comparison to WKY, blood pressure was higher in SHR (155 +/- 4 vs 106 +/- 2 mm Hg; p less than 0.001), renal blood flow was lower (6.9 +/- 0.5 vs 8.2 +/- 0.4 ml/min/g; p less than 0.05), and the pressor response to ANG II was enhanced, (p less than 0.0005) but the renal vascular response was blunted (p less than 0.005). Captopril (1-30 mg/kg) reduced blood pressure more in SHR than in WKY but increased renal blood flow similarly in both strains. The blunted renal vascular response to ANG II in SHR was reversed by captopril, but inhibition of converting enzyme in the kidney did not parallel systemic inhibition. Maximum blockade of converting enzyme in the kidney appears to require a larger captopril dose than is required for systemic inhibition. These results suggest that the renal blood supply in SHR also shares some of the characteristics of nonmodulators and that the action of captopril on the renal blood flow probably reflects reversal of inappropriate intrarenal ANG II formation.

Role of angiotensin II in the hormonal, renal, and electrolyte response to sodium restriction

Adrenal responses to angiotensin II (ANG II) are enhanced with restriction of sodium intake. To determine whether increased circulating ANG II levels are responsible for the enhanced responsiveness, the adrenal and blood pressure responses to ANG II in human subjects were assessed four times: in balance on a high and a low salt diet and before and after the administration of a converting enzyme inhibitor (enalapril). Before enalapril administration, sodium restriction significantly increased (p less than 0.02) plasma renin activity, ANG II, and aldosterone levels; the aldosterone response to ANG II was enhanced twofold (p less than 0.01); and the blood pressure response to ANG II infusion was reduced significantly (p less than 0.05). Despite a fixed and low plasma ANG II concentration when enalapril was employed, the adrenal response to ANG II on the low salt diet was enhanced to the same degree as that observed before administration of the converting enzyme inhibitor. Conversely, enalapril substantially altered the blood pressure response to ANG II with sodium restriction, completely preventing the reduction in responsiveness. If the subjects were first given enalapril and then sodium intake was restricted, ANG II levels did not change significantly but renal excretion of both sodium and potassium was substantially modified. The rate at which renal excretion of sodium fell to match intake was retarded strikingly (p less than 0.001); conversely, renal retention of potassium increased significantly (p less than 0.03) as low salt balance was attained. Possibly because of the potassium retention, aldosterone levels rose, but significantly less than when enalapril was absent.

Changes in adrenal responsiveness and potassium balance with shifts in sodium intake

Dietary sodium modulates the aldosterone response to angiotensin, but available evidence does not indicate whether there is a gradual change in adrenal responsiveness with intermediate sodium intakes or a sharp shift from a low to a high responsive level at some threshold sodium intake. Nine normal subjects received angiotensin II infusions while in balance on five levels of sodium intake over two orders of magnitude, 3 to 300 mEq sodium per day. Basal plasma renin activity, plasma aldosterone and plasma angiotensin II concentrations gradually fell as dietary sodium intake increased. The adrenal was quite sensitive to the state of sodium balance since a shift in sodium intake of as little as 20 mEq (e.g. 10 to 30 mEq/day) induced a significant change in basal plasma aldosterone. Adrenal responsiveness to infused angiotensin varied inversely with the log of urinary sodium excretion over the entire range of sodium intake, indicating that there is a gradual modulation of adrenal responsiveness with changes in dietary sodium. In addition, despite a constant dietary potassium intake, serum potassium gradually fell from 4.55 +/- .06 to 3.98 +/- .07 mEq/l, (P less than 0.02), as sodium intake increased, partly due to increased urinary potassium excretion (mean cumulative potassium loss of 129 mEq). Thus, changes in potassium balance may be important in modulating the sodium-associated variation in aldosterone secretion.

Contractile response of the isolated dorsal aorta of the snake to angiotensin II and norepinephrine

Vasopressor action is a phylogenetically old function of angiotensin (ANG) II. The action can be ascribed to both direct activation of vascular ANG II receptors and through catecholamine release. In the Rat snake, Ptyas korros, the possible presence of common adrenergic-ANG receptors and their involvement in this action have been proposed. In order to elucidate the vasopressor mechanism of ANG II in the snake, and to test the presence of common adrenergic-ANG receptors, the contractile response of isolated snake dorsal aorta to [Val5]ANG II and norepinephrine (NE) was studied using the cobra, Naja naja and the Rat snake, Ptyas korros. Both ANG II (3 X 10(-11) to 10(-6) M) and NE (10(-8) to 10(-5) M) produced a dose-dependent increase in tension in the aortic strips of Naja, which were more sensitive to ANG II. Phenotolamine, an alpha-adrenergic antagonist, competitively inhibited NE without altering the response to ANG II. [Sar1, Ala8]ANG II inhibited ANG II but did not affect the response to NE. A similar dose-dependent increase in tension in the aortic strips of Ptyas was seen with NE (10(-8) to 10(-5) M) but these strips did not respond to ANG II. These results suggest that functionally separate receptors for ANG II and NE exist in the dorsal aorta of the cobra and that local liberation of catecholamines from the adrenergic nerve terminals do not play a role in the expression of ANG II action. There also exist differences in the nature of ANG II action/receptors among different species of snakes and different vascular beds in the same species.

Decreased pressor reactivity to angiotensin II in cirrhotic rats. Evidence for a post-receptor defect in angiotensin action

We used a model of cirrhosis in the rat, produced by inhalation of carbon tetrachloride for 6 weeks, to investigate the mechanism of resistance to the pressor effects of angiotensin II. The pressor response to angiotensin II was significantly lower in conscious cirrhotic animals than in controls. On the other hand, cirrhotic animals had normal pressor responses to norepinephrine, indicating that a generalized defect in vascular reactivity does not cause the decreased pressor response to angiotensin II. Enhanced baroreceptor activity was not the cause of the decreased pressor response to angiotensin II, since baroreflex control of heart rate after angiotensin II was similar in cirrhotics and controls. Pretreatment with either the converting enzyme inhibitor captopril to reduce circulating angiotensin II or the prostaglandin synthesis inhibitor meclofenamate failed to normalize the response to angiotensin II. Thus, neither prior occupancy of receptors with endogenous angiotensin II nor the production of vasodilatory prostaglandins was responsible for the decreased angiotensin II response. Studies of angiotensin II binding by mesenteric artery smooth muscle particles showed that, in cirrhotic animals, receptor affinity for angiotensin II, was significantly lower than in controls (kd: cirrhosis 1.11 +/- 0.09 nM, control 0.94 +/- 0.13 nM; P less than 0.02), whereas receptor number was significantly increased (cirrhosis 315 +/- 42 fmol/mg protein, control 277 +/- 43 fmol/mg protein, P less than 0.01). However, total binding of AII by vascular receptors from cirrhotics was no different than in controls, since the decrease in affinity negated the increase in receptor number.(ABSTRACT TRUNCATED AT 250 WORDS)

Vascular reactivity in experimental acute renal failure

Vascular reactivity in-vivo and in-vitro was examined in rats with acute renal failure produced by bilateral nephrectomy or intramuscular glycerol injection. Bilaterally nephrectomized rats displayed enhanced pressor responses to noradrenaline and angiotensin. However, the contractile responses to noradrenaline, angiotensin and potassium chloride of aortic rings and portal vein segments from nephrectomized rats were not significantly different from the responses obtained in vessels from sham-operated controls. Rats with glycerol-induced ARF which were pretreated with indomethacin had significantly lower pressor responses to noradrenaline and angiotensin than similarly treated control animals. Aortic rings from glycerol-injected rats produced significantly smaller contractions to noradrenaline than preparations from controls. This difference was not abolished by incubation of vessels with indomethacin. The findings suggest that the absence of kidneys or the presence of damaged renal tissue and not uraemia itself have pronounced but opposite effects on vascular reactivity. The depression of vascular reactivity in glycerol-induced ARF does not appear to be a result of increased production of prostaglandins.

The vasopressor action of the renin-angiotensin system in the rat snake, Ptyas korros

The pressor actions of homologous kidney extract and human angiotensins I and II were studied in the conscious rat snake, Ptyas korros. A converting enzyme inhibitor (captopril), an angiotensin II analogue [( Sar1,Ala8]ANG II), an alpha-adrenergic receptor antagonist (phentolamine), and a catecholamine releaser (reserpine) were used to elucidate their actions. It was found that captopril attenuated the pressor effects of the kidney extract and angiotensin I but not that of angiotensin II. [Sar1,Ala8]ANG II and phentolamine both significantly attenuated, but did not completely inhibit the vasopressor actions of the kidney extract and angiotensins I and II. However, reserpine administration did not reduce the action of angiotensin II. These findings suggest that the renin-angiotensin system in snakes is similar to those present in mammals and other nonmammalian species, except for the mechanism of angiotensin II action. In the present study, angiotensin II was found to act partly through the alpha-adrenergic receptor which is not as specific as that in mammals and thus may respond to an agonist other than its usual ones, and partly through the vascular angiotensin II receptor.

Identification of the angiotensin II receptor in rat mesenteric artery

Specific binding sites of high affinity and low capacity for 125I-angiotensin II have been identified in a membrane fraction derived from arterial arcades of the rat mesentery. Heterogeneity of binding sites and extensive tracer degradation necessitated the use of nonlinear regression methods for the analysis of radioligand binding data. Forward and reverse rate constants for the high affinity sites obtained by three experimental approaches were in good agreement and gave a dissociation equilibrium constant (Kd) of 19-74 pM (95% confidence interval). Affinities for a number of angiotensin-related peptides calculated from competitive binding curves were in the order 125I-angiotensin II = angiotensin II greater than angiotensin III greater than [Sar1,Ile8]angiotensin II greater than [Sar1,Gly8]angiotensin II. Angiotensin I and biochemically unrelated peptides had virtually no effect on binding of tracer angiotensin II. The divalent cations Mn2+, Mg2+ and Ca2+ stimulated 125I-angiotensin II binding at concentrations of 2-10 mM, as did Na+ at 50-100 mM. In the presence of Na+ or Li+, K+ had a biphasic effect. The chelating agents EDTA and EGTA were inhibitory, as were the thiol reagents dithiothreitol and cysteine. This study defined angiotensin II binding sites in a vascular target tissue of sufficiently high affinity to interact rapidly with plasma angiotensin II at physiological concentrations.

Angiotensin receptors and the control of Na+ and K+ transport in cultured aortic smooth muscle and brain microvessel cells

We examined the effect of angiotensin on Na+ and K+ transport by cultures of aortic smooth muscle and brain microvessel cells. Angiotensin II (AII) and angiotensin III (AIII) stimulated net Na+ uptake, which was assayed in the presence of ouabain to block Na+ efflux via the Na+-K+ pump. The combination of saturating concentrations of AII and AIII produced no greater stimulation of net Na+ uptake than either angiotensin by itself. AII also stimulated ouabain-sensitive 86Rb+ uptake by cultured aortic smooth muscle and brain microvessel cells. AII was not as effective as monensin, a Na+ ionophore, in stimulating the Na+-K+ pump. In the presence of monensin, AII had no effect on ouabain-sensitive 86Rb+ uptake. These data are consistent with previous observations suggesting that AII stimulates the Na+-K+ pump by supplying it with more of its rate-limiting substrate, Na+. 125I-AII bound tightly to cultured aortic muscle cells. Approximately 5 nM unlabelled AII half-maximally inhibited 125I-AII binding. Incubation of intact cultures with 1.0 microM AII produced a time-dependent decrease in specific 125I-AII binding. This slow loss of AII binding was prevented by methylamine, a known inhibitor of receptor clustering and internalization. We conclude (1) that angiotensin increases the Na+ permeability of smooth muscle cells in culture, thereby stimulating the Na+-K+ pump; (2) that AII binds to a specific receptor on the cell surface; and (3) that the AII-receptor complex may be internalized by a methylamine-inhibited pathway.

Mechanism of decreased vascular reactivity to angiotensin II in conscious, potassium-deficient rats

Chronic potassium deficiency in the rat results in a decrease in the pressor sensitivity to exogenous angiotensin II (AII). To define the mechanism of this resistance to AII, studies were performed in conscious rats after 14-21 d of dietary potassium deficiency. The pressor response to graded doses of AII was 50% less in potassium-deficient than control animals. In contrast, the pressor response to graded doses of norepinephrine was preserved in potassium-deficient rats; therefore, the decreased response to AII was not due to a generalized defect in vascular reactivity. Pretreatment with either the converting enzyme inhibitor, teprotide, or the prostaglandin synthesis inhibitor, indomethacin, failed to normalize the response to AII. Thus, neither prior receptor occupancy with endogenous AII nor the presence of vasodilatory prostaglandins caused the decreased AII response in potassium deficiency. Since the pressor response to AII involves angiotensin interaction with its vascular receptor, binding studies of mesenteric artery and uterine smooth muscle AII receptors were performed. Scatchard analysis showed that potassium deficiency resulted in a decrease in binding affinity (50% increase in Kd) in both uterine (6.00 vs. 3.82 nM; P less than 0.05) and vascular (1.39 vs. 0.973 nM; P less than 0.005) smooth muscle. Furthermore, despite increased circulating AII, there was an increase in AII receptor number in potassium-deficient uterine (308 vs. 147 fmol/mg protein; P less than 0.005) and vascular (470 vs. 316 fmol/mg protein; 0.05 less than P less than 0.1) smooth muscle. Although potassium deficiency resulted in alterations in receptor-binding parameters, the changes in binding affinity and number were directionally opposite, so that in potassium deficiency there was either no change or an increase in total AII binding. We conclude that the decrease in angiotensin pressor sensitivity in potassium-deficient rats is mediated by a postreceptor defect since it occurs subsequent to the binding of AII to its vascular smooth muscle receptor.

Converting-enzyme inhibition corrects the altered adrenal response to angiotensin II in essential hypertension

Of patients with essential hypertension, 30% to 50% do not modulate adrenal and renovascular responsiveness to angiotensin II (AII) with changes in sodium intake. To define the role of AII in mediating these altered responses, the adrenal and renal vascular responses to AII infusion (0.3, 1.0, 3.0 ng/kg/min) were assessed on a sodium-restricted intake in 31 patients with essential hypertension and 13 normotensive controls before and after 72 hours of converting-enzyme inhibition. Forty percent of the hypertensive patients had a subnormal adrenal response to AII. There were no differences between the normal and abnormal responding hypertensive patients in a number of clinical and biochemical factors except that the "abnormal responders" had a significantly (p less than 0.03) greater control AII level (37 +/- 3 vs 29 +/- 3 pg/ml) and lower control plasma aldosterone level (14 +/- 2 vs 22 +/- 3 ng/dl) than the "normal responders." When a converting-enzyme inhibitor was administered, no change in adrenal responsiveness to AII occurred in the normotensive controls or the hypertensive normal responders. In the hypertensive abnormal responders, both the threshold sensitivity and the entire dose response curve was significantly (p less than 0.01) enhanced following short-term converting-enzyme inhibition. This increased sensitivity could not be explained by differences in AII increment with AII infusions, in basal aldosterone levels, or in blood pressure or basal AII response to converting-enzyme inhibition. Since they occurred whether captopril or enalapril (MK 421) were used, this phenomenon is likely to be a specific effect of converting-enzyme inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

The renin angiotensin system--its physiology and role in disease states

The renin-angiotensin system is one of a number of interlinked mechanisms regulating vascular resistance and blood volume. Under certain conditions it may become a predominant factor in maintaining vascular tone. Knowledge about these conditions (sodium depletion, mineralocorticoid deficiency, renovascular hypertension and iatrogenic hyperreninaemic states) is important for the safe and effective use of drugs which inhibit the renin-angiotensin system. Measurements of plasma renin activity are useful in the diagnostic assessment of hypertensive patients with hypokalaemia or evidence of renal artery stenosis. They may also have a place in the management of refractory or dialysis-resistant hypertension. Their use in the selection of antihypertensive therapy for the individual patient is controversial. Sequential measurements of plasma renin are helpful in analysing states of electrolyte depletion, and in titrating therapy for Addison's disease.

Angiotensin II does not elicit any specific prostaglandin secretion in piglet cultured endothelial cells

PGE2 and PGF2 alpha were measured in culture media from piglet aortic endothelial cells by radioimmunological analysis. Prostacyclin secretion was evaluated by radioimmunological analysis of its stable metabolite, 6-keto-PGF1 alpha after reverse-phase high pressure liquid chromatography separation. No stimulation of either prostaglandin was detectable in culture media after treatment with angiotensin II (10(-9) to 10(-6) M) for 15 to 120 min at 37 degrees C. Under the same conditions angiotensin II (10(-7) M) elicited a 2 to 3 fold increase in PGE2 and PGF2 alpha secretion when incubated with cultured piglet aortic smooth muscle cells. In addition, we failed to detect specific angiotensin receptors at the surface of intact cultured endothelial cells. Since there was a very rapid increase in prostaglandin secretion after washing or medium changes we suggested that the effects of Angiotensin II on prostacyclin production, demonstrated in perfused organs, could be due to the mechanical stimulation elicited by the contraction of the underlying smooth muscle cells.

Effects of subpressor doses of angiotensin II on renal hemodynamics in relation to blood pressure

The renal hemodynamic response to subpressor doses of angiotensin II (AII; 0.1 and 0.5 ng/min/kg) was investigated in untreated 49-year-old men (n = 50) representing a wide blood pressure range. Renal blood flow, renal vascular resistance (RVR), glomerular filtration rate (GFR), filtration fraction (FF), plasma renin activity (PRA), plasma AII, plasma aldosterone, and the urinary excretion of sodium and norepinephrine were studied. The higher the initial blood pressure the greater was the increase in RVR in response to AII infusion (p less than 0.002), indicating an increased renal vascular reactivity with increase in initial blood pressure. The AII infusion gave a significant rise in RVR in both the borderline and hypertensive group, but gave no increase in RVR in the normotensive group, implying an enhanced sensitivity of the renal vasculature in the borderline and hypertensive group. The increase in RVR was greater in the hypertensive than in the borderline group, i.e., the hypertensives had a steeper dose-response curve than the borderline group, which points to the presence of structural vascular changes in the renal vessels in the hypertensives. The increase in RVR in response to AII was positively correlated to sodium intake and plasma aldosterone concentration, indicating that these two factors might modulate the renal vascular reactivity. These factors could, however, only partly explain that RVR increased more the higher the initial blood pressure. Thus, the results indicate that there is an increased reactivity of the renal vascular bed to AII in essential hypertension. The increased reactivity seems to be mediated through an increased sensitivity of the renal vasculature to AII in mild essential hypertension and also through the presence of structural vascular changes in established hypertension. These factors may lead to a reduced excretion of sodium and water and may therefore be of importance in the development and progression of essential hypertension.

Modulation of angiotensin II pressor responsiveness by circulating levels of angiotensin II in pregnant sheep

During human and ovine pregnancy, systemic pressor responses to infused angiotensin II are decreased when compared to the nonpregnant state. An increased circulating level of angiotensin II has been proposed as one of the possible mechanisms responsible for this refractoriness. The present study was designed to determine if increased pressor responsiveness to exogenous angiotensin II occurs after circulating levels of angiotensin II in pregnant ewes are reduced. Pregnant (112 to 118 days' gestation) and nonpregnant sheep were instrumented with catheters in the femoral artery and vein. Dose-response curves to angiotensin I, angiotensin II, and norepinephrine were established prior to and during continuous short-term (2 to 3 hours' duration) and long-term (24 hours' duration) inhibition of angiotensin converting enzyme activity by either captopril or enalapril. Short-term infusions of converting enzyme inhibitors blocked the formation of angiotensin II from exogenously administered angiotensin I but did not alter pressor responsiveness to angiotensin II or norepinephrine in pregnant and nonpregnant sheep. Long-term infusions of the angiotensin converting enzyme inhibitor enalapril blocked responses to angiotensin I but did not alter pressor responses to norepinephrine in pregnant and nonpregnant sheep. In contrast, responses to angiotensin II were significantly potentiated in pregnant sheep but were not altered in nonpregnant sheep. These results suggest that increased circulating levels of angiotensin II in pregnant sheep are responsible for pregnancy-induced decreases in angiotensin II responsiveness.

Cardiovascular responsiveness to vasoactive agents in rats with obstructive jaundice

1. Pressor responses to tyramine, beta-phenylethylamine and angiotensin II were normal in urethane-anaesthetized rats 7 days after division of the common bile duct. The pressor response to noradrenaline was enhanced at the highest dose level used. 2. The hypotensive response to haemorrhage was exaggerated in bile duct divided rats. 3. The exaggerated haemorrhagic hypotension of bile duct divided rats is unlikely to be the result of impaired vascular response to noradrenaline or angiotensin II, or of accumulation of false neurotransmitters by sympathetic nerves.

Influence of cations on kinetics of angiotensin II binding to adrenal, renal, and smooth muscle receptors

A number of biological responses to angiotensin II have been demonstrated to be modulated acutely by cations, but the exact mechanism has not been elucidated. We have utilized a radioreceptor assay for angiotensin II to determine whether this acute regulatory mechanism could be related to a change in either number or affinity of angiotensin II binding to receptors. Three target tissues were used (adrenal glomerulosa, glomeruli, and uterine smooth muscle). Both mono- and divalent cations influence the kinetics of angiotensin II binding in a similar manner in all tissues. Divalent cations increase the number of receptor sites to a greater extent than did monovalent cations, while monovalent cations changed binding affinity to a greater extent than did divalent cations. These studies demonstrate that both the number and affinity to a greater extent than did monovalent cations, while monovalent cations changed binding affinity to a greater extent than did divalent cations. These studies demonstrate that both the number and affinity of angiotensin receptors can be rapidly modulated by a variety of cations.

Independent roles of prostaglandins and the renin-angiotensin system in abnormal vascular reactivity in Bartter's syndrome

To clarify the independent roles of prostaglandins and the renin-angiotensin system in the pressor resistance to angiotensin II in Bartter's syndrome, the pressor responsiveness to exogenous angiotensin II was investigated in three patients with the syndrome during the administration of indomethacin synthesis, and captopril is an angiotensin-converting enzyme inhibitor. All the patients showed high plasma renin activity, increased urinary excretion of prostaglandin E, and pressor resistance of angiotensin II. An analogue of angiotensin II that had weak agonistic properties induced a marked fall in blood pressure. Pretreatment with indomethacin (150 mg/day) decreased baseline plasma renin activity and reversed the hypotensive effect of the analogue of angiotensin II. Apparently, our data support the concept that pressor resistance ultimately results from the increase in the concentration of endogenous angiotensin II. However, the augmentation of indomethacin was significantly (p less than 0.01) greater in magnitude than the response obtained with captopril, although the concentration of plasma angiotensin II prior to each infusion of angiotensin II was the same. This observation could be explained by the finding that indomethacin suppressed both systems, but captopril inhibited only the renin-angiotensin system. Evidence presented herein suggests that the abnormalities in the vascular reactivity to angiotensin II may result from, not only the decreased number of receptor sites as a results of the increased concentration of endogenous angiotensin II, but also from the alteration of the end-organ sensitivity to angiotensin II via overproduction of prostaglandins.

Angiotensin increases Na+ entry and Na+/K+ pump activity in cultures of smooth muscle from rat aorta

Angiotensin markedly altered the Na+ permeability of smooth muscle cells cultured from explants of rat aorta. The rate of net Na+ uptake was followed in the presence of ouabain in order to block Na+ efflux via the Na+/K+ pump. Angiotensin II (AII) or angiotensin III (AIII) increased net Na+ uptake by about 3-fold. Maximal stimulation of Na+ uptake was produced by about 10 nM AII. Bradykinin and the angiotensin antagonist [Sar1, Ileu5, Ala8]AII had no significant effect on net Na+ uptake. Angiotensin also enhanced the activity of the Na+/K+ pump, which was assayed by following the rate of ouabain-sensitive 86Rb+ uptake by the cells. AII and AIII nearly doubled ouabain-sensitive 86Rb+ uptake, but bradykinin, norepinephrine, and [Sar1, Ileu5, Ala8]AII had no effect. In the presence of ouabain, 86Rb+ uptake was not significantly affected by AII or AIII, indicating that angiotensin did not alter passive permeability to Rb+. Loading the cells with Na+, either by incubation in K+-free medium or exposure to the Na+-selective ionophore monensin, markedly increased ouabain-sensitive 86RB+ uptake. This result indicates that the activity of the Na+/K+ pump is limited by the low level of Na+ that is normally in the cells. AII had no effect on the activity of the Na+/K+ pump in Na+-loaded cells. These results suggest that AII or AIII stimulates the Na+/K+ pump in cultured aortic muscle cells by increasing its Na+ supply.

Factors influencing vascular hyporesponsiveness to angiotensin II

Bartter's syndrome is characterized, in part, by hyporesponsiveness to the pressor effect of exogenous angiotensin II (AII). This has been attributed to volume contraction, hypokalemia, and/or increased prostaglandin (PG) levels. In order to investigate factors responsible for a diminished response to the pressor effect of AII, rats were made hypokalemic or volume contracted and hypokalemic (VCHK) by dietary restriction. AII sensitivity was examined by determining the dose of AII required to raise the mean arterial pressure 20 mm Hg. When compared with control rats. VCHK and hypokalemic rats were significantly less sensitive to AII. VCHK rats were significantly less sensitive to AII than hypokalemic rats. Both experimental groups were similarly hypokalemic, but plasma renin activity (PRA) of VCHK only was greater than control values. In VCHK rats, acute K+ restoration partially corrected AII hyporesponsiveness, although plasma K+ increased to normal. In VCHK rats, acute volume expansion with normal saline similarly achieved only partial correction of AII hyporesponsiveness although PRA values fell to the control range. Simultaneous K+ restoration and volume expansion to VCHK rats successfully restored AII sensitivity to the control range. Dietary sodium, chloride, and potassium restriction did not increase urinary excretion to PGE2. Indomethacin (5 mg/kg, iv) given acutely to VCHK rats did not significantly after baseline hyporesponsiveness to AII. Norepinephrine vascular sensitivity was not affected by either volume contraction or hypokalemia. These data demonstrate that volume contraction and hypokalemia individually depress exogenous AII sensitivity in the rat and do so by separate and additive mechanisms. Furthermore, these mechanisms appear to be independent of PG.

A study of angiotensin II binding sites in human placenta, chorion, and amnion

The binding of tritiated angiotensin II to 20,000 x g particulate tractions of human placenta, chorion, and amnion was investigated. Binding to particles from the three tissues reached equilibrium within 10 minutes at 29 degrees C and was displaced by the addition of 1,000-fold excess of unlabeled angiotensin II. Scatchard analysis of the data showed that two classes of binding sites were present in the placental preparation. For the high-affinity site, values of 9 x 10(-9) M and 300 fmoles/mg of protein were obtained for the dissociation constant and the binding capacity, respectively. There was little specific binding in chorion and amnion.

Biological activity of an angiotensin II--ferritin conjugate on rabbit aortic smooth muscle cells

Specific binding sites for [Asp1,Ile5]angiotensin II (angiotensin) have been demonstrated in homogenates and subcellular fractions of aortic medial smooth muscle cells, but the localization of the angiotensin receptor responsible for contraction has not been determined [Devynck, M. A., & Meyer, P. (1976) Am. J. Med. 61, 758-767]. To establish the location of this receptor, we have prepared a membrane-impermeable analogue of angiotensin by acylating its N-terminal amino group with the N-hydroxysuccinimide ester of succinylated ferritin. This angiotensin-ferritin conjugate possessed the same intrinsic activity as angiotensin but was approximately 200 times less potent in inducing contraction in rabbit aortic strips. The stability of the conjugate was investigated, and approximately 5% of the contractile activity of the angiotensin-ferritin conjugate was attributable to low molecular weight components that were present before or after exposure to aortic strips. The time required for aortic strips to reach a plateau of contraction in response to angiotensin-ferritin was significantly longer than that required by free angiotensin to produce the same level of contraction. With enzymatically dispersed aortic smooth muscle cells, however, the time taken to produce contractions by both angiotensin and angiotensin-ferritin was indistinguishable. [Sar1,-Ala8]angiotensin II, a competitive inhibitor of angiotensin, completely suppressed contractions induced by angiotensin or angiotensin-ferritin in aortic strips or dispersed aortic smooth muscle cells. These results suggest that angiotensin need not directly penetrate the plasma membrane to cause contraction and imply that the angiotensin receptor responsible for initiating contraction of aortic smooth muscle cells is located on the plasma membrane.

Angiotensin receptors in bovine umbilical artery and their inhibition by nonsteroidal anti-inflammatory drugs

1 The contractile effect of angiotensin II on bovine isolated umbilical arteries was compared to [125I]-angiotensin II binding by a subcellular fraction of that tissue. The ED50 of angiotensin was 3.1 +/- 2.8 x 10(-8) M, while the apparent dissociation constant was 4.9 +/- 1.6 x 10(-9) M. 2 Indomethacin, meclofenamate, and eicosatetraynoic acid inhibited angiotensin-induced contraction of the isolated artery and binding to a particulate fraction at comparable doses. Phenylbutazone inhibited [125I]-angiotensin binding more potently than the response. Inhibition by the first three agents was noncompetitive, whereas phenylbutazone inhibited competitively. 3 Inhibition of angiotensin activity by the nonsteroidal anti-inflammatory agents was not specific. These agents also inhibited 5-hydroxytryptamine-induced contraction, but not the contraction induced by KCl. 4 The data suggest that the angiotensin binding sites studied include receptors that mediate contraction of the isolated umbilical artery. Our data also indicate that indomethacin, meclofenamate, eicosatetraynoic acid and phenylbutazone are capable of direct inhibitory effects on receptors, as well as their well-known synthetase actions. The net effect of these activities will determine the change these agents cause in tissue responses to hormones.

Receptor binding interactions of the angiotensin II antagonist, 125I-[sarcosine1,leucine8]angiotensin II, with mammalian brain and peripheral tissues

Sarcosine1,leucine8-angiotensin II ([Sar1,Leu8]AII), an angiotensin II antagonist, binds saturably, reversibly and with high affinity (KD's of 0.03-22 nM) to calf cerebellar cortex, bovine adrenal cortex and rabbit uterine membranes. The peptide specificity of 125I[Sar1,Leu8]AII binding to brain, adrenal cortex and uterus is consistent with the labeling of physiologically relevant angiotensin receptors. A detailed study of the binding potencies of 28 angiotensin peptide analogues reveals: (1) very significant correlations between peptide binding potencies at 125I-AII compared to 125I-[Sar1,Leu8]AII binding sites, (2) many similarities between brain and uterine receptor sites and marked differences between these two tissue receptors compared to adrenal cortical receptor sites, and (3) correlations among peptide physiological potencies (AII-contracted rabbit aortic strip) and receptor binding potencies in all three tissues labeled with either 125I-AII or 125I-[Sar1,Leu8]AII. The correlations are much better for adrenal cortex than for brain or uterus, suggesting that adrenal cortical angiotensin receptors are similar to aorta angiotensin receptors.

Effects of two angiotensin II analogues on blood pressure, plasma aldosterone concentration, plasma renin activity and creatinine clearance in normal subjects on different sodium intakes

Two angiotensin II analogues (AIIA), 1-sarcosine, 8-isoleucine angiotensin II ([Sar, Ile]-AII) and 1-sarcosine, 8-alanine angiotensin II ([Sar, Ala]-AII), were infused in six normal volunteers on high, regular and low sodium diets. The agonist and antagonist activities of these AIIA on blood pressure (BP), plasma aldosterone concentration (PAC), creatinine clearance and plasma renin activity were examined. Both AIIA had agonistic pressor activities in subjects on high and regular sodium diets, [Sar, Ile]-AII being more potent than [Sar, Ala]-AII. Both AIIA caused similar elevation of PAC in subjects on high and regular sodium diets, and an equally fall in PAC in subjects on a low sodium diet. Both AIIA strongly antagonized the rise in BP, the increase in PAC and the reduction of Ccr induced by AII administration in subjects on all three sodium diets. The results indicate that both AIIA can be used to examine the activity of the renin-angiotensin system in patients with hypertension, and they also suggest that AII interaction with its receptors differs in different target tissues.

Regulation by angiotensin II of its receptors in resistance blood vessels

The sensitivity of blood vessels to the vasoconstrictor effects of the hormone angiotensin II appears to be modulated by the activity of the renin-angiotensin system. Elevation of circulating angiotensin II levels by sodium depletion or renal artery stenosis is associated with a diminished pressor response to infused angiotensin II (refs 1-3). Conversely, the vasocontrictor response to the hormone is enhanced when endogenous angiotensin II levels are reduced by sodium loading or nephrectomy. The mechanisms of these varying effects are not known, but physiological and pharmacological experiments suggest involvement of the vascular smooth receptor for angiotensin II (refs 5-8). Modification of the interaction between angiotensin II and its vascular receptor, resulting in altered responsiveness to the hormone, could occur either via 'prior occupancy' of receptors by elevated levels of endogenous angiotensin II resulting in fewer free receptors available to respond to circulating angiotensin II (ref. 5), or, elevated levels of angiotensin II could result in a decrease in receptor affinity for the hormone or a decrease in total receptor number in the vascular smooth muscle cell. We now report the first direct evidence, by radioligand binding assay, that angiotensin II regulates the number of its own receptors in resistance vasculature.