Response of aldosterone and blood pressure to angiotensin II infusion in anephric man. Effect of sodium deprivation

Renin and renin inhibition in anephric man

Renin activity appears to be present in low concentrations in the plasma of anephric humans but could be artifactual secondary to inadvertent activation of prorenin during specimen collection and handling or from a renin-like enzyme. We studied the effects of specimen collection, storage, different assay conditions, trypsin activation, and the renin inhibitor EMD 56133 (E Merck, Darmstadt) on plasma renin activity (PRA) in anephric man. PRA was detectable in all seven bilaterally nephrectomized (BNX) patients (0.2 +/- 0.1 ng AI/ml/hr, range 0.1-0.7) but was significantly lower than normals (2.4 +/- 0.3 ng AI/ml/hr, range 1.5-3.1, p = 0.001). PRA was not different in BNX whether blood samples were collected on ice or at room temperature and assayed immediately or whether samples were frozen and assayed several days later. Prolonged cold storage of samples and five freeze-thaw cycles over six to seven months did not significantly increase PRA in normals or anephrics. However, deliberate repeated freezing and thawing over the period of a single day increased PRA 4.1-fold in BNX and 1.6-fold in normals. Renin-like activity was also detected in BNX individuals using renin concentration determinations with either excess human or sheep angiotensinogen. The inhibition of renin activity (IC-50% = 3.16 x 10(-9) molar) by EMD 56133 was not different between BNX and normals. Thus, active renin is present in the plasma of anephric humans and does not result from the inadvertent activation of prorenin due to sample handling. Although the source of PRA in BNX is unknown, the enzyme appears functionally normal as evidenced by the dose-response to a single renin inhibitor.

In vivo comparison of renal and femoral vascular sensitivity and local angiotensin generation

Experiments were conducted to compare the relative importance of the local renin-angiotensin systems in the rabbit renal and femoral vascular beds and their functional role in hemodynamic regulation. Angiotensin I (Ang I) (0.15 microgram/kg i.v.) elevated mean arterial blood pressure by 18 +/- 1 mm Hg in the renal experimental group and 19 +/- 1 mm Hg in the femoral experimental group; it decreased renal blood flow by 35 +/- 3% but increased femoral blood flow by 31 +/- 8%. All these effects were blocked by intravenous administration of captopril (2 mg/kg bolus injection plus 1 mg/kg/hr). Captopril also lowered mean arterial pressure by 17 +/- 3 and 16 +/- 2 mm Hg in the renal and femoral experimental groups, respectively, and it increased renal blood flow by 32 +/- 10% but reduced femoral blood flow by 21 +/- 4%. As a result, renal vascular resistance was decreased by 36 +/- 5%, but femoral vascular resistance remained unchanged. After captopril, plasma angiotensin II (Ang II) levels were decreased and Ang I levels increased in the two groups. The renal venous-arterial difference of Ang I was increased by captopril, but the femoral venous-arterial difference of Ang I was not, suggesting greater generation of Ang I in the kidney. In a separate group of bilateral nephrectomized rabbits, plasma Ang II levels as well as mean arterial pressure, femoral blood flow, and femoral vascular resistance were not changed by intravenous administration of captopril.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous angiotensin-aldosterone-pressure relationships during sodium restriction

The effects of moderate restriction of dietary sodium and potassium supplementation on plasma levels of renin, angiotensin II, aldosterone, and cortisol and on arterial pressure were studied in 12 patients with mild essential hypertension. To define hormone-blood pressure relationships, venous hormone levels were measured hourly and intra-arterial pressure continuously for 24 hours after 4 to 6 weeks of sodium restriction, 4 to 6 weeks of potassium supplementation, and a similar period of control diet. Our results show that compared with the control diet, moderate sodium restriction was associated with increased levels of aldosterone but no overall change in renin, angiotensin II, or cortisol levels. Further, slopes of regression lines relating log renin and log angiotensin II to aldosterone were increased, as were log cortisol/aldosterone regression lines. On the contrary, regression lines of log renin and log angiotensin II versus arterial pressure were unaltered by sodium restriction. Hormone and blood pressure relationships were not changed by the potassium supplemented diet. Although confirmatory data are needed, our findings suggest that moderate sodium restriction enhances aldosterone responsiveness to endogenous angiotensin II and adrenocorticotropic hormone without diminishing the pressor activity of endogenous angiotensin II. These results may explain in part the disappointingly small hypotensive effect of modest sodium restriction in mild essential hypertension.

The calcium antagonists PY 108-068 and verapamil diminish the effects of angiotensin II: sites of interaction in the peripheral circulation of anaesthetized cats

The sites of interaction between the vasoconstrictor angiotensin II (A II) and the calcium antagonists PY 108-068 (PY) (a dihydropyridine derivative) or verapamil (V) in different peripheral vascular beds were investigated using the microsphere method in chloralose-urethane anaesthetized open-chested cats. A II was infused intravenously into 27 cats at a rate of 0.15 microgram kg-1 min-1. Systemic haemodynamic variables and regional blood flow were measured immediately before and 10 min after the start of the infusion. While the infusion of A II continued, PY (3 micrograms kg-1 min-1), V (30 micrograms kg-1 min-1) or the vehicle was infused for 10 min into 9 cats each and the effects of this combined infusion were again measured at the end of the 10 min period. A II increased mean arterial blood pressure but decreased peripheral conductance and, to a smaller but still significant degree, cardiac output and peak acceleration of blood in the aorta (an ejection phase parameter of myocardial contractility). The calcium antagonists reversed these effects. Cardiac output and total peripheral conductance were increased even beyond the pre-A II level by PY. A II constricted the vascular beds of the kidney, small intestine, liver and skin. Arterio-venous shunt flow decreased. Vasoconstriction was also found in the stomach, spleen and in different parts of the heart with the exception of the subendocardial layer of the left ventricle, where blood flow increased and conductance remained unchanged. A II did not decrease conductance in different parts of the brain or in skeletal muscle. The vasoconstrictor effects of A II persisted or tended to be increased in most of the vascular beds of placebo treated animals. PY 108-068 and verapamil abolished the vasoconstrictor effects of A II in most of the vascular beds with the exception of the liver, the spleen, the skin and the arterio-venous shunts and caused vasodilatation in the heart. PY also induced vasodilatation in the brain and skeletal muscle, where A II had not induced vasoconstriction. The pattern of attenuation of A II effects was different from the pattern of vasodilatation induced by these and other calcium antagonists in the same cat preparation not treated with a vasoconstrictor. The sites of action of this dihydropyridine derivative (PY) on the peripheral circulation thus, appear to depend not only on the vascular bed but also on the presence of a vasoconstrictor influence at the time of investigation.

A highly sensitive cytochemical bioassay for plasma angiotensin II

A highly sensitive cytochemical bioassay has been developed for measuring angiotensin II in human plasma. The assay depends on the ability of angiotensin II to alter the reducing potency of the zona glomerulosa as measured by Prussian blue staining and microdensitometry. An inverse correlation between the intensity of the stain and the logarithm of concentration existed over the range 0.05-5.0 fmol/1 of angiotensin II. The limit of sensitivity of the assay in plasma was 50 fmol/1; the index of precision was 0.07 +/- 0.04 (mean +/- SD; n = 15); and the coefficient of variation of a quality control sample was 34%. The response was specific for angiotensin II; approximately 10(2) times more angiotensin III and approximately 10(6) times more ACTH was required to produce a similar effect. Angiotensin I had no significant activity. A significant inverse relationship existed between sodium intake and bioactive angiotensin II in 5 normal subjects studied on low, normal and high sodium diets. Extremely low levels of angiotensin II were detected in anephric subjects.

Effects of furosemide and orthostasis on active and inactive renin in normal and anephric man

We investigated active and inactive (acid-activatable) plasma renin in anephric and in normal persons. In anephric patients (n = 15) plasma concentration of active and inactive renin was 1.15 +/-- 0.2 and 40.7 +/- 7.1 microunits ml, respectively; angiotensin II (n = 13) was 14.5 +/- 1.9 pg/ml. Furosemide (n = 10), 40 mg i.v., and upright posture (n = 8) did not change active or inactive renin in the anephric state. In normal men, furosemide (n = 9) within 15 min increased active renin from 29.9 +/- 5.8 to 82.4 +/- 14.8 microunits/ml (P less than 0.001), while inactive renin slightly but not significantly decreased from 136.3 +/- 29.9 to 121.1 +/- 19.2 microunits/ml; orthostasis (n = 15) within 4 h stimulated active renin (P less than 0.001) and slightly raised inactive renin (P less than 0.05). Both furosemide and orthostasis increased (P less than 0.001 each) the proportion of active renin in normal persons. Studies in one patient within 24 h after bilateral nephrectomy indicated half-life to be 30-60 min for active and 2-4 h for inactive renin. Thus, we detected low levels of active renin and considerable amounts of inactive renin and angiotensin II in anephric patients. Our data suggest that about 30% of inactive renin in normal plasma is of extrarenal origin. The stimulation of active renin by furosemide and orthostasis is bound to the presence of the kidney. Our studies provide indirect evidence that both manoeuvres may stimulate the conversion of inactive to active renin within the human kidney.

Circulating angiotensin II and adrenal receptors after nephrectomy

Mineralocorticoid secretion is predominantly controlled by the octapeptide angiotensin II, which exerts trophic actions on the adrenal glomerulosa and acute regulatory effects on aldosterone biosynthesis. The trophic actions include stimulation of angiotensin II receptors and enzymes of the aldosterone biosynthetic pathway, with corresponding enhancement of the aldosterone secretory capacity of the adrenal gland. The positive regulatory action of angiotensin II on its adrenal receptors occurs with elevations of the circulating peptide concentration within the physiological range and probably contributes to the increased sensitivity of the adrenal during sodium deficiency. In this action, angiotensin II differs from other hormones which decrease their target-cell receptors. However, the increase in adrenal angiotensin II receptors following nephrectomy has been interpreted as evidence for a tonic down-regulating effect of angiotensin II on its adrenal receptors. To clarify these conflicting views we evaluated the effects of nephrectomy on adrenal angiotensin II receptors in relation to blood angiotensin II and plasma electrolyte levels. We show here that hyperkalaemia contributes markedly to the post-nephrectomy increase in adrenal angiotensin II receptors, and that circulating angiotensin II levels persist for an unexpectedly long period after nephrectomy, presumably due to tissue generation of the octapeptide.

Extrarenal prorenin and renin in anephric patients

Active renin was detected in 6 of 7 anephric patients (mean value: 0.72 +/- 0.27 microU/ml, n = 7; normals: 19.7 +/- 2.4 microU/ml, n = 10), using an assay method without intrinsic acid- or cryoactivation. Prorenin, measured as the difference between plasma renin concentration (PRC) before and after acid-activation in vitro, was present in considerable amounts in all patients (32.4 +/- 3.5 microU/ml; normals: 80.7 +/- 9.7 microU/ml). PRC after cryoactivation at--5 degrees C during 1 week was insignificantly lower than after acid-activation in the patients. There was a linear correlation between PRC after either activation procedure (p less than 0.01). Supine or upright posture did not influence active or inactive renin in the anephric subjects. Our studies show that low but significant levels of active renin and a considerable amount of prorenin are detectable in anephric patients. The data suggest that prorenin in normal subjects in a significant proportion is of extrarenal origin. Orthostasis has no effect on extrarenal active or inactive renin.

Effects of an angiotensin-converting enzyme inhibitor (captopril) on blood pressure in anephric subjects

Randomised, double-blind cross-over trials were performed in seven anephric patients to determine the effect of the orally active angiotensin-converting enzyme inhibitor captopril on blood pressure in fluid-depleted and fluid-replete patients. Patients were given captopril, 100 mg orally, or placebo one hour after haemodialysis, when they were fluid depleted. Their mean (+/- SEM) supine blood pressure fell from 127 +/- 12/71 +/- 6 mm Hg before captopril to 106 +/- 13/54 +/- 4 mm Hg 24 hours after the drug, while on placebo it rose from 123 +/- 11/73 +/- 5 mm Hg to 134 +/- 10/82 +/- 8 mm Hg. All patients developed orthostatic hypotension after captopril. In the fluid-replete state, two days after haemodialysis, captopril had no effect on blood pressure. The plasma concentration of active renin was extremely low and did not rise after fluid withdrawal or captopril. Thus the hypotensive effect of captopril did not appear to depend on circulating renin concentrations. The concept of "renin-dependent" hypertension, which is responsive to captopril, as opposed to "volume-dependent" hypertension, which is not responsive to captopril, may therefore be invalid.

Vascular responsiveness and body fluid status in patients on chronic hemodialysis

The vascular responsiveness and the body fluid distribution were studied in 5 hypotensive and 11 normotensive patients on long-term hemodialysis. A diminished vascular responsiveness to intravenously infused norepinephrine or angiotensin II was found in uremic patients, especially in the hypotensive group. A slightly larger, but non-significant circulating blood volume (CBV) and a less extracellular fluid volume (ECF) was found in the hypotensive group as compared with those of the normotensive group. These results suggest that with autonomic nervous dysfunction, such a diminution of vascular responsiveness may be an another factor responsible for the development of hypotension.

Post-nephrectomy changes in adrenal angiotensin II receptors in the rat; influence of exogenous angiotensin and a competitive inhibitor

3H-angiotensin binding sites have been studied in a particulate fraction prepared from rat adrenal glands. This binding is rapid and reversible, of high affinity (KD29 degrees C = 3-5 X 10(-9) M) and with demonstrable specificity for the angiotensin II octapeptide. The number of binding sites varies with endogenous angiotensin levels: nephrectomy is followed by an increase in number of binding sites. This increase can be prevented by chronic angiotensin II administration and, to a lesser extent, by administration of Sar1,Ala8-angiotensin II, a competitive antagonist of the hormone. No variation in the equilibrium dissociation constant accompanied these changes in binding capacity. The post-nephrectomy increase in capacity is time-dependent, with a lag period of 24-40 h. The observed changes in receptor concentration do not appear explicable on the basis of receptor occupancy. Accordingly, angiotensin II receptors in the rat adrenal appear to be dependent on circulating angiotensin levels as previously reported for rat uterus.