Deborah A. Price

Pathways for Angiotensin II Generation in Intact Human Tissue Evidence From Comparative Pharmacological Interruption of the Renin System

Multiple lines of evidence have suggested that alternative pathways to the angiotensin-converting enzyme (ACE) exists for angiotensin II (Ang II) generation in the heart, large arteries, and the kidney. In vitro studies in intact tissues, homogenates, or membrane isolates from the heart and large arteries have repeatedly demonstrated such pathways, but the issue remains unresolved because the approaches used have not made it possible to extrapolate from the in vitro to the in vivo situation. For our in vivo model, we studied young and healthy human volunteers, for the most part white and male; when these subjects achieved balance on a low salt diet to activate the renin system, the response of renal perfusion to pharmacological interruption of the renin system was studied. With this approach, we studied the renal vasodilator response to 3 ACE inhibitors, 2 renin inhibitors, and 2 Ang II antagonists at the top of their respective dose-response relationships. When these studies were initiated, our premise was that a kinin-dependent mechanism contributed to the renal hemodynamic response to ACE inhibition; therefore, the renal vasodilator response to ACE inhibition would exceed the alternatives. To our surprise, both renin inhibitors and both Ang II antagonists that were studied induced a renal vasodilator response of 140 to 150 mL/min/1.73 m2, approximately 50% larger than the maximal renal hemodynamic response to ACE inhibition, which was 90 to 100 mL/min/1.73 m2. In light of the data from in vitro systems, our findings indicate that in the intact human kidney, virtually all Ang II generation is renin-dependent but at least 40% of Ang I is converted to Ang II by pathways other than ACE, presumably a chymase, although other enzyme pathways exist. Preliminary data indicate that the non-ACE pathway may be substantially larger in disease states such as diabetes mellitus. One implication of the studies is that at the tissue level, Ang II antagonists have much greater potential for blocking the renin-angiotensin system than does ACE inhibition-with implications for therapeutics.

African-Americans on maintenance dialysis: a review of racial differences in incidence, treatment, and survival.

African-Americans are the fasting growing racial minority with end stage renal disease (ESRD) in the United States. Currently, African-Americans comprise approximately 31% of the ESRD population. African-Americans are almost a decade younger than their white (referring to non-Hispanic white) counterparts with ESRD with a mean age of 58 years old. Although African-Americans systematically receive less dialysis than whites (Kt/V of 1.05 versus 1.18, respectively), their survival is higher. The 2-year survival probability of African-Americans is 66.2% in comparison with 59.8% for whites. This improved survival with ESRD is accompanied by an improved quality of life for African-Americans. Their enhanced quality of life is reflected by a greatly decreased frequency of withdrawing from dialysis treatments. In this article, we will examine the reasons why African-Americans have an excessive incidence of selective diseases that culminate in ESRD. We will explore the factors that influence the difference in dialysis modality selection between African-Americans and whites. Lastly, we will pose and judge several hypotheses that may account for the improved survival enjoyed by African-Americans with ESRD. We contend that research to clarify the basis for these differences between African-Americans and whites with ESRD will improve outcomes for both populations and is fiscally sound health policy.

Renal Perfusion in Blacks: Alterations Caused by Insuppressibility of Intrarenal Renin With Salt

We have reported that an increased intrarenal renin-angiotensin system activity may be responsible for the reduction in renal plasma flow (RPF) in apparently healthy blacks in comparison to healthy whites during high salt balance. To ascertain whether these differences only exist in the high salt state, we performed the following study, concentrating on the manipulation of the renin system during low salt intake. We measured in 19 healthy blacks and 22 healthy whites para-aminohippurate and inulin clearances as an indication of RPF and glomerular filtration rate, respectively, on both high (200 mmol/d) and low (10 mmol/d) salt balance in random order. A subset of 11 blacks and 12 whites additionally received an angiotensin II infusion while in low salt balance (3 ng/kg per minute for 45 minutes) and captopril to assess differences in RPF response to a converting enzyme inhibitor. The 19 whites had significantly higher RPF when compared with blacks (P =0.033) when studied on high salt. However, during low salt balance, the RPFs were comparable in the 2 groups. Plasma renin activity was similar in the 2 groups on both diets. In the subset that received angiotensin II and captopril while in low salt balance, the renal vascular response was not different in whites and blacks. These data provide additional support for the concept that the intrarenal tissue renin system is more active in blacks than whites on a typical (high salt) diet and that the difference reflects primarily incomplete tissue renin suppression with an increase in salt intake. The mechanism involved may contribute to the increased susceptibility to renal injury in blacks.

Renal Hemodynamic Response to an Angiotensin II Antagonist, Eprosartan, in Healthy Men

In view of the vasodilator potential of angiotensin-converting enzyme (ACE) inhibition via prostaglandins and kinins, we asked why renin inhibition induces a larger renal vasodilator response than ACE inhibitors in healthy humans in earlier studies. One possibility was that there was a more complete blockade of the renin system, which could also be achieved by an angiotensin II antagonist, eprosartan. We measured the hormonal and renal hemodynamic responses to eprosartan doses, from 10 to 400 mg in 9 healthy young men in balance on a 10-mmol/d sodium intake. The threshold eprosartan dose to influence renal perfusion was <10 mg, and the 100-mg dose induced a near-maximal vasodilator response of 135+/-19.7 mL x min(-1) x 1.73 m2. When the dose was increased to 400 mg, there was a modest additional increase of 147+/-57 mL x min(-1) x 1.73 m(-2). A highly significant dose-related fall in arterial blood pressure occurred (r=-.97; P<.001), with no indication of a maximal response at 400 mg. In 6 additional subjects, we compared responses to eprosartan on a high salt and a low salt diet. The renal response to 200 mg eprosartan on a high salt diet, 26.0+/-6.6 mL x min(-1) x 1.73 m(-2), was significantly less than that seen with the low salt diet (P<.001). There was no renal partial agonist angiotensin-like effect of eprosartan. Eprosartan reduced sharply the pressor, renal vascular, and hormonal responses to exogenous angiotensin II. The renal vasodilator response to the angiotensin II antagonist eprosartan closely resembles responses to renin inhibition and exceeds previously reported responses to ACE inhibitors. Thus, eprosartan probably exerted its effect via the angiotensin receptor. More complete blockade of the renin system can be achieved by pharmacological interruption at this level, a finding that could have therapeutic implications.

The state and responsiveness of the renin-angiotensin-aldosterone system in patients with type II diabetes mellitus

We have recently reported a combination of renal features that suggests independent angiotensin-mediated control of the renal circulation in the majority of hypertensive patients with type II diabetes. To ascertain whether other tissue elements of the renin-angiotensin-aldosterone system (RAAS) also were activated, we examined the adrenal response to angiotensin II (AngII) infusion on a low salt diet. We assessed also the renin response to the upright position in patients on a low salt diet and renin suppression in patients on a high salt diet. We compared responses in 42 hypertensive patients with type II diabetes (53.1 +/- 1.4 years, mean +/- SEM), 25 healthy controls (52.6 +/- 4.4 years); and 137 essential hypertensives without diabetes (43.3 +/- 1.2 years). A low renin state, defined as a plasma renin activity (PRA) <2.5 ng angiotensin I (AI)/mL/h after 5 to 7 days on a 10-mmol Na diet and 2 h of upright posture, was found in 21% of the essential hypertensives, but in only 14% of patients with type II diabetes. On this diet, PRA increased from 2.7 +/- 0.4 supine to 10.1 +/- 1.3 ng AI/mL/h upon standing in healthy subjects. In patients with type II diabetes, PRA was 3.6 +/- 0.4 and increased to 9.1 +/- 1.0 ng AI/mL/h. On a high salt (200 mmol) diet, healthy subjects showed the expected PRA suppression (0.3 +/- 0.1), but in patients with type II diabetes the PRA was less suppressed (1.2 +/- 0.3 ng AI/mL/h; P = .003). Thus, in most hypertensive patients with type II diabetes the RAAS shows normal activation, but is poorly suppressible. AngII infused intravenously to assess adrenal responsiveness in patients on a low salt diet caused an essentially identical increase in aldosterone concentration in patients with type II diabetes (21.1 +/- 1.7 to 44.0 +/- 5.9 ng/dL) and in essential hypertension (20.6 +/- 1.4 to 43.7 +/- 2.8 ng/ dL). The frequency of nonmodulation assessed as a blunted adrenal response to AngII infusion was identical in type II diabetes (47%) and in essential hypertension (46%) after exclusion of the low renin patients. Thus, at the level of one tissue renin system, the adrenal glomerulosa, responses were unaltered in patients with type II diabetes. The relative unresponsiveness of the renal blood supply to infused AngII in type II diabetes in association with an enhanced renal vasodilator response to angiotensin converting enzyme inhibition probably reflects local, intrarenal actions secondary to the diabetic state. The infrequency of a low renin state, and the inappropriately high renin levels on a high salt intake, provide a rational basis for pharmacologic interruption of the renin system to treat patients with type II diabetes.

Effect of Angiotensin II Antagonist Eprosartan on Hyperglycemia-Induced Activation of Intrarenal Renin-Angiotensin System in Healthy Humans

We have previously reported that hyperglycemia in healthy human subjects increased the renal vasodilator response to the angiotensin-converting enzyme inhibitor captopril. This observation raised intriguing possibilities relevant to the pathogenesis of nephropathy in patients with diabetes mellitus. To ascertain whether the effect of captopril was indeed mediated by a reduction in angiotensin II (Ang II) formation, we performed another study in which an Ang II antagonist, eprosartan, was used in place of captopril. Nine healthy subjects were studied in high sodium balance (ie, sodium intake 200 mmol/d). On the first day, the subjects received 600 mg eprosartan orally, and renal plasma flow (RPF) and glomerular filtration rate (GFR) were measured. Glucose was infused intravenously on the second and third study days to increase plasma glucose to a level below the threshold for glycosuria ( approximately 8.8 mmol/L). Eprosartan at a dose of 600 mg or placebo was administered randomly on the second or third study day 1 hour after initiation of glucose infusion. RPF increased (by 76+/-7 mL. min(-1). 1.73 m(-2), P<0.01) in response to sustained moderate hyperglycemia and then increased further (by 147+/-15 mL. min(-1). 1. 73 m(-2), P<0.01) when eprosartan was administered during hyperglycemia. Eprosartan, conversely, did not affect RPF and GFR in normoglycemic subjects. GFR was not affected by either hyperglycemia or eprosartan. Neither plasma renin activity nor plasma Ang II concentration changed during hyperglycemia, suggesting that the hormonal responses responsible for the enhanced renal vasodilator response to eprosartan occurred within the kidney. The enhancement of the renal vasodilator effect of eprosartan during hyperglycemia is consistent with activation of the intrarenal renin-angiotensin system.

Volume sensitive hypertension and the digoxin-like factor reversal by a fab directed against digoxin in DOCA-salt hypertensive rats

Although volume and vasoconstriction have been considered polar elements in a useful pathogenetic hypertension model, many observations suggest that vasoconstriction is involved in volume-dependent hypertension, reflecting the effect of a digitalis-like factor. To examine that possibility, we assessed the depressor responses to Digibind, an antibody Fab directed against digoxin, in a volume-dependent model--DOCA-salt-induced hypertension in rats. Digibind (10 mg/kg, intravenously) induced a gradual blood pressure fall over 2 h that was sustained for 4 h (P < .001). Blood pressure did not fall with Digibind when DOCA was administered without salt or a high-salt intake was provided without DOCA. The intracellular sodium content of the rat aorta, measured by atomic absorption spectroscopy after cold choline wash, was increased in the DOCA-high-salt rats (23.3 +/- 2.7 mEq/L) compared to control rats (12.1 +/- 0.8 mEq/L; P < .001). Aorta sodium content, in parallel with blood pressure, was not increased either by dietary salt supplementation without DOCA, or by DOCA with a low-salt diet. Sodium pump activity was measured as 86Rb uptake into vascular smooth muscle (VSM). Both ouabain-sensitive and ouabain-resistant 86Rb uptake were significantly higher in VSM from DOCA-high-salt animals (P < .01). Despite its effectiveness in reducing blood pressure in this model, Digibind influenced neither VSM sodium content nor 86Rb uptake. The results are consistent with a role for a circulating digitalis-like factor in this volume-dependent model, but events at the VSM level are complex.

Hyperglycemia and angiotensin-mediated control of the renal circulation in healthy humans

Type 1 and type 2 diabetics have an enhanced renal vasodilator response to angiotensin-converting enzyme (ACE) inhibition despite suppressed plasma renin activity (PRA), indicating possible activation of the intrarenal renin angiotensin system. To investigate the role of hyperglycemia, we evaluated the renal hemodynamic response to ACE inhibition in 9 healthy subjects in high-salt balance after steady-state hyperglycemia (8.4+/-1 mmol/L) was achieved via intravenous glucose administration. Renal plasma flow (RPF) and glomerular filtration rate (GFR) responses to captopril and to angiotensin II (Ang II) were measured as paraminohippuric acid and inulin clearances. Hyperglycemia produced a significant increase in RPF of 117 mL. min-1. 1.73 m-2 after 90 minutes but not GFR. Administration of captopril at a dose of 25 mg during glucose infusion led to an increase in RPF of 173+/-24 mL. min-1. 1.73 m-2 (P<0.01) but did not significantly change RPF in the absence of hyperglycemia (7+/-21 mL. min-1. 1.73 m-2). Captopril did not alter GFR in the presence or absence of hyperglycemia. Ang II infusion during hyperglycemia decreased RPF by 45+/-16 mL. min-1. 1. 73 m-2, and this was significantly enhanced by captopril (-98+/-26 mL. min-1. 1.73 m-2, P<0.05). In contrast, there was no enhancement of the vasoconstrictor response to Ang II in the absence of hyperglycemia. PRA did not change with hyperglycemia. Enhancement of renal vasodilation during hyperglycemia by captopril without alteration of PRA suggests activation of the intrarenal renin angiotensin system.

Sodium Pump Isoform Specificity for the Digitalis-Like Factor Isolated From Human Peritoneal Dialysate

We have isolated a labile, specific sodium pump inhibitor or digitalis-like factor from the peritoneal dialysate of volume-expanded renal failure patients whose levels correlated closely with volume status and blood pressure. This study characterizes the inhibitory profile of this agent compared with that of ouabain against the three alpha-isoforms of the sodium pump. We prepared microsomal Na,K-ATPase from rat tissues representing the highest proportion of one of the alpha-isoforms. Both Northern and Western blot analyses confirmed that kidney had predominantly the alpha1-isoform, skeletal muscle the alpha2-isoform, and fetal brain the alpha3-isoform. Ouabain (5 x 10(-6) mol/L) produced little inhibition of kidney Na,K-ATPase (3.4+/-2.0%) but significant inhibition of skeletal muscle (37.2+/-3.7%, P<.001) and fetal brain (38.8+/-3.5%, P<.001) activity. In contrast, the labile digitalis-like factor, causing comparable inhibition of fetal brain Na,K-ATPase activity (33.3+/-4.7%), produced markedly greater inhibition of kidney (42.5+/-5.6%, P<.001) and moderately greater inhibition of skeletal muscle pump activity (57.7+/-6.3%, P<.05). In addition, the labile digitalis-like factor produced a marked concentration-dependent inhibition of the alpha2- and alpha3-isoforms (r=.79, P=.00005). Experiments combining the labile digitalis-like factor and ouabain confirmed that digitalis-like factor, unlike ouabain, was an effective inhibitor of all three isoforms in rat, in particular alpha2. The different pattern of isoform sensitivity displayed by the labile digitalis-like factor and ouabain further differentiates the two agents and raises some interesting possibilities about the functional implications of the endogenous factor.

Development of gastrointestinal β2-microglobulin amyloidosis correlates with time on dialysis

Dialysis-associated beta2-microglobulin (beta2m) amyloidosis affects predominantly musculoskeletal tissue, but visceral involvement also occurs. To evaluate the clinical significance and prevalence of gastrointestinal beta2m amyloidosis, we studied hemodialysis patients admitted for gastrointestinal-related complaints. Hemodialysis patients (excluding those with non-beta2m amyloidosis) who were admitted with gastrointestinal complaints from 1984 to 1994 were identified. Gastrointestinal tissues from patients with available autopsy or surgical specimens were examined using hematoxylin and eosin stain, Congo red stain, and beta2m immunostain. Each case was evaluated independently by two pathologists and scored for quantity and location of beta2m amyloid and associated pathology. Of 24 patients, eight (four men and 4 women) had beta2m amyloid deposits within the gastrointestinal tract. Acute clinical presentation ranged from abdominal pain to gastrointestinal bleeding and was not significantly different for patients with or without gastrointestinal beta2m amyloid deposits. However, the mean time on dialysis of 15.3 +/- 5.7 years (range 6-24 years) for patients with gastrointestinal beta2m amyloidosis was significantly greater than that of patients without gastrointestinal beta2m amyloidosis (10.5 +/- 7.0 years, range <1 to 22 years, p < 0.05). Vascular histopathology ranged from mild focal thickening of vessel walls to massive vascular beta2m amyloid deposition with thrombosis. Extravascular beta2m amyloid ranged from mild to severe with marked expansion of the submucosa. Mucosal pathology ranged from none to severe ulceration. The degree of beta2m amyloid and the associated pathology tended to increase in severity with time on dialysis. Gastrointestinal beta2m amyloid deposition is an underappreciated complication of chronic hemodialysis that is significantly associated with increased time on dialysis. Gastrointestinal beta2m amyloidosis should be considered in any patient on hemodialysis 10 years or more who has gastrointestinal symptoms and can be identified in resection specimens as well as some biopsy specimens. Congo red stain and beta2m immunostains may be necessary for sensitive histopathologic evaluation of gastrointestinal beta2m amyloidosis.