Harvey C. Gonick

Lead-Induced Hypertension: Interplay of Nitric Oxide and Reactive Oxygen Species

An elevation of mean blood pressure was found in rats treated with low lead (0.01% lead acetate) for 3 months, as contrasted to paired Sprague-Dawley control rats. In these rats, measurement of plasma and urine endothelins-1 and -3 revealed that plasma concentration and urinary excretion of endothelin-3 increased significantly after 3 months (plasma: lead group, 31.8+/-2.2, versus controls, 23.0+1.7 pg/mL, P<.001; urinary excretion: lead group, 46.6+11.7, versus controls, 35.6+6.7 pg/24 h, P<.05), whereas endothelin-1 was unaffected. Plasma and urinary nitric oxide (NO) and cyclic GMP concentrations were not significantly changed. However, assay of plasma and kidney cortex malondialdehyde by high-pressure liquid chromatography, as a measure of reactive oxygen species, was elevated in lead-treated rats compared with that in control rats (plasma: lead group, 4.74+1.27, versus controls, 2.14+.49 micromol/L, P<.001; kidney cortex: lead group, 28.75+3.46, versus controls, 16.38+2.37 nmol/g wet weight, P<.001). There was increased NO synthase activity in lead-treated rat brain cortex and cerebellum. In lead-treated rat kidney cortex, the endothelial constitutive NO synthase protein mass was unaffected, whereas the inducible NO synthase protein mass was increased. These data suggest a balance between increased NO synthesis and degradation (by reactive oxygen species) in lead-treated rats, which results in normal levels of NO. Thus, the hypertension may be related to an increase in the pressure substances, endothelin-3 and reactive oxygen species, rather than to an absolute decrease in nitric NO.

Lead promotes hydroxyl radical generation and lipid peroxidation in cultured aortic endothelial cells

Early studies by our group have shown that lead-induced hypertension (HTN) is closely related to enhanced activity of reactive oxygen species (ROS). In addition, we have found indirect evidence that hydroxyl radical may be the most likely culprit in lead-exposed animals. In the present study, rat aortic endothelial cells were incubated in the presence of 0, 0.01, 0.1, 0.5, and 1.0 ppm lead acetate for 1, 24, and 48 h. At the conclusion of the incubation period cells were harvested and the media were collected. Lipid peroxidation products were measured as malondialdehyde-thiobarbituric acid (MDA-TBA) in the medium and hydroxyl radical was measured as 2,3-dihydroxybenzoic acid (2,3 DHBA) in the cells. After exposure to lead for 48 h, MDA-TBA generation and 2,3 DHBA formation were significantly increased. These data clearly demonstrate that lead exposure promotes hydroxyl radical generation and induces oxidative stress in isolated endothelial cells, mimicking the effects observed in lead-exposed animals. Enhanced inactivation of endothelium-derived nitric oxide by locally produced oxygen free radicals could contribute to endothelial dysfunction and HTN in lead-exposed animals.

Simultaneous Measurement of Marinobufagenin, Ouabain, and Hypertension-associated Protein In Various Disease States

We have previously demonstrated that a 12 kD hypertension-associated protein (HAP) is elevated in essential hypertension and that this protein has the characteristics of natriuresis, inhibition of Na-K-ATPase, displaces 3H-ouabain from binding sites, and is vasoconstrictive in vitro. In the present study, plasma from 101 patients were examined [25 normals (N)age 50, 7 with acute congestive heart failure (CHF), 24 with chronic renal failure (CRF), on dialysis, 5 with idiopathic hyperaldosteronism (PA) and 27 with essential hypertension, untreated (EHT)]. Plasma was extracted with 32% acetonitrile, then analyzed by DELFIA for marinobufagenin and ouabain. In addition, from 32 patients (6 N <50, 6 N >50, 5 CHF, 5 CRF, 6 EHT, and 4 PA) SDS gradient gels were obtained. The 12 kD bands were extracted, analyzed for Na-K-ATPase inhibition, marinobufagenin and ouabain, and compared to 14 kD and 21 kD bands. Marinobufagenin was found to be elevated in CRF, EHT, PA and CHF. Ouabain was increased only in PA. When the relative optical densities of the 12 kD and 21 kD bands were contrasted, CRF, PA, and EHT were found to be increased and CHF to be decreased in the 12 kD band, with no discernible changes in the 21 kD bands. Following extraction of the bands, Na-K-ATPase inhibitory activity measured 38% in 18 pooled 12 kD bands, with essentially no activity found in the 14 kD or 21 kD bands. Thus only the 12 kD HAP band possessed all of the attributes of natriuretic hormone.

Experimental model of lead nephropathy. II. Effect of removal from lead exposure and chelation treatment with dimercaptosuccinic acid (DMSA)

Male Sprague-Dawley rats were exposed to high-dose (0.5%) lead acetate for periods ranging from 1 to 9 months; then lead exposure was discontinued, and animals were sacrificed after 12 months. Controls were pair-fed. Two additional groups of low-dose (0.01%) and high-dose (0.5%) rats were exposed to lead for 6 months, then lead was discontinued and the rats were treated with three 5-day courses of 0.5% DMSA (dimercaptosuccinic acid) over the next 6 months. Controls were rats exposed to lead for 6 months, then removed from exposure for 6 months without receiving DMSA. Low-dose lead-treated rats showed no significant pathological changes with or without DMSA treatment, but exhibited a significant increase in GFR after DMSA. High-dose lead-treated animals showed no functional or pathological changes when lead exposure was discontinued after 1 month. However, when duration of exposure was 6 or 9 months, GFR was decreased and serum creatinine and urea nitrogen were increased as compared to controls. Tubulointerstitial disease was severe. Administration of DMSA resulted in an improvement in GFR and a decrease in albuminuria, together with a reduction in size and number of nuclear inclusion bodies in proximal tubules. However, tubulointerstitial scarring was only minimally reduced. It may be concluded that, except for brief initial exposure, discontinuation of high-dose lead exposure fails to reverse lead-induced renal damage. Treatment with the chelator, DMSA, improves renal function but has less effect on pathological alterations. As GFR improved after DMSA treatment in both low-dose and high-dose lead-treated rats, irrespective of the degree of pathological alterations, it may be concluded that the DMSA effect is most likely mediated by hemodynamic changes.

Functional impairment in chronic renal disease. III. Studies of potassium excretion.

Abstract: The ability to respond to an acute oral potassium tolerance test was compared in 19 normal subjects and in 44 patients with a wide spectrum of renal disorders and renal function (15 with predominantly glomerular pathology and 29 with predominatly tubular pathology). Glomerular filtration rate was used to estimate the number of residual nephrons. Following two 1-hour control periods, 0.75 mEq KCI/kg body weight was administered orally, and urine and blood specimens were obtained over three subsequent 2-hour periods. Baseline plasma potassium did not exceed 6.0 mEq/liter in any of the renal patients. However, the increment in potassium excretion after potassium loading was blunted and slower in appearance in the renal patients when compared to the normals. Plasma potassium tended to remain elevated. The maximum rate of potassium excretion and the per cent of administered potassium excreted over the 6-hour experimental period were highly correlated with the glomerular filtration rate (p< 0.001). No differences in plasma potassium values or urinary excretion patterns were discernible between the two patient subgroups.

Lead-induced hypertension. III. Increased hydroxyl radical production

Lead-induced hypertension has previously been shown to be closely associated with an increase in reactive oxygen species in low lead (100 ppm)-treated rats. The present study has attempted to define the specific moiety involved by noting the blood pressure (BP), reactive oxygen species (MDA-TBA), hydroxyl radical, and nitrotyrosine responses to infusion of the reactive oxygen species scavenger dimethylthiourea. Dimethylthiourea, a reputed scavenger of hydroxyl radical, normalized BP and MDA-TBA in the lead-treated rats but had no effect in normal control animals. MDA-TBA, hydroxyl radical, and nitrotyrosine, the tissue end product of peroxynitrite, were reduced to or toward normal by dimethylthiourea. The results, therefore, are consistent with the suggestion that either hydroxyl radical or peroxynitrite may be the reactive species affected by lead.

Erythrocyte lead-binding protein after occupational exposure: I. Relationship to lead toxicity

Abstract Lead content was determined in various fractions of red blood cell (RBC) hemolysates from normal controls and from three groups of lead-exposed workers: without toxicity, toxicity associated with high blood lead levels, and toxicity associated with low blood lead levels. The most significant finding was a decrease in the lead bound to a 10,000 molecular weight protein in the group of workers with toxicity at low blood lead levels (43 to 54 μg%). These results suggest that workers who have diminished capacity for synthesizing this low-molecular-weight lead-binding protein are at increased risk for developing lead toxicity at relatively low blood lead levels.

Effect of Extracellular Volume Expansion on Renal Na-K-ATPase and Cell Metabolism

In the present study the reduction in renal fractional reabsorption of sodium from 99.6 ± 0.2 to 86.0 ± 3.3 % following ECV expansion with isotonic saline in the rat was associated with a small decrease in renal cortical Na-K-ATPase activity from 12.2 ± 0.5 to 10.8 ± 0.6 µmol Pi/mg protein/h and an increase in medullary enzyme activity from 23.3 ± 1.3 to 28.6 ± 1.8 µmol Pi/mg protein/h. In addition, an increase in cortical ATP concentration from 1.21 ± 0.12 to 1.55 ± 0.10 µmol/g wet weight was observed. These alterations resemble renal metabolic changes induced by low dose ouabain and favor the hypothesis that decreased tubular reabsorption of sodium following ECV expansion may be associated with alterations in active sodium transport.

In vitro Inhibition of Na-K-ATPase by Trace Metals: Relation to Renal and Cardiovascular Damage

The potential role of trace metals in the pathogenesis of various disease states, especially of renal and cardiovascular disease, has been recognized increasingly. Moreover, altered membrane transport was recently incriminated to play a role in renal tubular syndromes, such as the Fanconi syndrome, as well as in the pathogenesis of volume dependent hypertension. We therefore investigated the possible in vitro effects of various trace metals on Na-K-ATPase, the biochemical correlate of active cellular transmembrane sodium and sodium-dependent transport. To more closely mimic the in vivo situation, we deliberately chose as enzyme source renal tissue homogenate, which may contain protective agents. Under these experimental conditions, the metals studied inhibited the enzyme quantitatively in the following order: Hg greater than Pb greater than Cd greater than Ur greater than Cu greater than Zn greater than Mn greater than Ba greater than Ni greater than Sr. Enzyme kinetic studies showed that Hg, Pb, and Cd competitively, and Cu noncompetitively, inhibited the enzyme. In general, Mg-ATPase was significantly less sensitive to the trace metals. Accumulation of these metals may present serious health hazards by producing a general defect in cell membrane transport. From the other metals studied, i.e., Mn, Ba, Ni and Sr, some may have toxic effects via other mechanisms, whereas some may exert a protective role against toxicity of other agents including metal ions.

Effect of Continuous Ambulatory Peritoneal Dialysis on Blood Pressure Control

To assess the efficacy of blood pressure control in continuous ambulatory peritoneal dialysis (CAPD), blood pressure was examined sequentially in 63 CAPD patients transferred from hemodialysis (HD), and in 97 patients started de novo on CAPD (NEW), over periods ranging from 3 to 63 months. Blood pressure changes were related to changes in body weight, hematocrit, and treatment with recombinant human erythropoietin (rHu-EPO), as well as to changes in antihypertensive drug requirements. Both groups of patients showed an immediate improvement in blood pressure control at 1 month, as manifested by an absolute decrease in blood pressure in HD patients (-4.3% +/- 2.1% [SEM], P < 0.05) and by a decrease in antihypertensive drug requirements in NEW patients (from 78% to 43.3%). This early improvement in blood pressure appeared to be volume-related, as reflected by changes in body weight. Both groups showed an additional decrement in blood pressure at approximately 6 months (-7.8% +/- 2.6% [SEM], P < 0.05, HD group; -3.4% +/- 2.4% [SEM], P < 0.05, NEW group). Treatment of anemia with rHu-EPO in 22 of the CAPD patients had no effect on blood pressure. CAPD thus appears to be more effective than HD in controlling blood pressure.