Louis Tobian

Relationship of juxtaglomerular apparatus to renin and angiotensin.

IN A SYMPOSIUM devoted to angiotensin, it is appropriate to consider briefly the juxtaglomerular apparatus. Angiotensin makes its appearance in the body as a direct result of the proteolytic action of renin, and there is considerable evidence to support the hypothesis that all the renin in a kidney comes from the juxtaalomerular apparatus. Before reviewing this evidence, the anatomy of the juxtaglomerular apparatus should be considered. It consists of 2 separate cell types, the granular cells and the macula densa cells. The granular juxtaglomerular cells actually lie within the medial layer of the afferent glomerular arteriole. They have small, uniform-sized granules in the cytoplasm, which can be clearly stained with Bowie s stain1 or with osmic acid.2 When examined with the electron nmicroseope, these cells are seen to have a rich endoplasmic reticulum with an abundanee of RNA granules, a Golgi apparatus, and their specific cytoplasmic secretory granules.2 When there are abundant granules in the cytoplasm, these cells have all the appearances of actively secreting cells. One side of the granular cell abuts the intimal layer of the afferent arteriole. On the other side, the border of the cell may interdiaitate with the border of the macula densa cells. The macula densa consists of specialized cells which are part of the wall of the distal convoluted tubule. Their stainling characteristics are different from tlle usual epithelial cells lining the distal tubule. The macula densa cells are always at the very first part of the distal tubtule and are always in close proximity to the vascular pole of the glomerulus of their particular nephron. As mentioned above,

High Sodium Chloride Diets Injure Arteries and Raise Mortality Without Changing Blood Pressure

High NaCI diets often increase blood pressure and thereby accelerate lesions in arterial walls. Could high NaCI diets increase arterial lesions without raising blood pressure? To test this, 100 uninephrectomized Dahl salt-resistant (DR) rats (highly resistant to NaCI hypertension) were administered deoxycorticosterone acetate (DOCA) (250 mg/kg) in silicone implants and drinking water containing 1% NaCI for 6 weeks. Then the DOCA and saline were removed, and the rats were allowed to recover for 4 weeks. Intra-arterial mean blood pressures on all rats allowed division of the rats into two matched groups, each group with an average blood pressure of 160 mm Hg. One group continued on a 03% NaCI diet, whereas the other group began an 8% NaCI diet for 8 weeks. After 5 weeks on these two diets, the intra-arterial blood pressure averaged 158 mm Hg in both groups. Thus, the 8% NaCI diet produced no further increase in blood pressure in the DR rats. Nevertheless, after 8 weeks on the 8% NaCI diet, 53% of the rats (26 of 49) had died; whereas in the group on the 03% NaCI diet, not one rat (0 of 51) had died (p < 0.000001). After 7 more weeks on the 8% NaCI diet, all the rats in this group had died. The chief cause of death in the group of rats on the 8% NaCI diet was likely cerebral infarction because small cerebral infarcts were observed in rats that died on this diet Uremia, congestive failure, cerebral hemorrhage, and arrhythmia were not likely causes of death. Thus, it appears that a high NaCI diet in mildly hypertensive DR rats can greatly accelerate cerebral arterial disease with brain infarction and very high mortality, even when the high NaCI diet causes no increase in blood pressure whatsoever. Seemingly, salts infamy goes beyond blood pressure. It is even possible that “salt-resistant” hypertensive humans could reduce vascular complications by adhering to a low NaCI diet.

High potassium diets protect against dysfunction of endothelial cells in stroke-prone spontaneously hypertensive rats.

Two lines of evidence strongly support the hypothesis that high potassium diets protect arterial endothelial cells from hypertensive damage. Stroke-prone spontaneously hypertensive rats (SHRSP) fed normal (0.75%) K or high (2.1%) K and normotensive Wistar-Kyoto rats (WKY) were examined in an endothelial function study and a histological study. In the endothelial function study, aortic rings were suspended in tissue baths to monitor isometric tension. Rings contracted with norepinephrine were tested with acetylcholine and sodium nitroprusside. In normal K SHRSP (blood pressure, 156 mm Hg), endothelium-dependent acetylcholine relaxation was severely depressed by 49% (p < 0.001), whereas in high K SHRSP (blood pressure, 155 mm Hg), normal values were preserved. Endothelium-independent nitroprusside relaxation was virtually the same in both the SHRSP groups (high K vs normal K diet). Since indomethacin did not improve the impaired acetylcholine relaxation in normal K SHRSP, the cyclooxygenase products do not appear to have affected the endothelium-dependent relaxation in the normal K SHRSP. Thus, the endothelium-dependent relaxation response was much decreased in the normal K SHRSP and was preserved in the high K SHRSP. Thus, a high K diet appears to protect the aortic endothelium from a hypertension-induced dysfunction. In the histological study, aortic and mesenteric intimal lesions were assessed blindly under the microscope and graded from 0 to 60 for aortic and from 0 to 40 for mesenteric lesions. Aortic intimal lesion scores were 28 in normal K SHRSP (blood pressure, 209 mm Hg) and 13 in high K SHRSP (blood pressure, 207 mm Hg; −54%; p < 0.001). Mesenteric scores were 18 in rats on the normal K diet and 10 in rats on the high K diet (−45%; p < 0.001). Scores of high K SHRSP equaled those of WKY. Thus, a high K diet prevented the hypertensive intimal lesions without lowering the blood pressure. Endothelium protection by a high K diet seems a very likely partial explanation for the markedly reduced lesions in the high K SHRSP.

The question of vascular hyper-responsiveness in hypertension.

After inhibition of spontaneous tone, the contraction of spirally cut strips of aorta in response to norepinephrine was studied quantitatively in normotensive rats and in 4 different types of hypertensive rats. In no instance did the aortas of hypertensive rats show hyper-responsiveness to norepinephrine. The majority of responses were entirely normal; in a few instances there was decreased responsiveness. In vivo studies which fail to consider the critical effect of an altered “base-line” state in hypertension may demonstrate “hyper-responsiveness” which is more apparent than real.

Effect of varying perfusion pressures on the output of sodium and renin and the vascular resistance in kidneys of rats with "post-salt" hypertension and Kyoto spontaneous hypertension.

Isolated kidneys from both “post-salt” normotensive and hypertensive rats were perfused with blood from donor rats at varying pressures. At 130 mm Hg inflow pressure 15 “post-salt normotensive” kidneys put out 0.75 j*Eq Na/min/g kidney while 14 “post-salt hypertensive” kidneys put out 0.28 μEq Na/min/g (P < 0.001), a 63% reduction. They also put out 55% less water (P < 0.002). Thus, if “hypertensive” kidneys are perfused at normal pressures, they put out subnormal amounts of Na and H2O. Such Na and H2O retention maintains the hypertensive state. Normal Na output in these kidneys was only reached at hypertensive (160) inflow pressures. This shift in the “pressure natriuresis” curve explains in part how some “hypertensive” kidneys maintain hypertension. These “hypertensive” kidneys have grossly abnormal autoregulation curves, each increment of pressure actually producing progressively greater increments of blood flow. Isolated kid neys from Kyoto hypertensive and normotensive rats showed no difference in Na and H2O excretion at 130 mm Hg inflow pressure. Thus, a tendency to Na retention demonstrable in the isolated kidney is apparently not supporting Kyoto hypertension. Moreover, iso lated kidneys from Kyoto hypertensive rats released significantly lower amounts of renin at all levels of inflow pressure, averaging a 70% lower rate than Kyoto normotensive kid neys (P < 0.01). Hence, Kyoto hypertension is not supported by a supernormal renin re lease inherent in the kidney. One can speculate that Kyoto hypertensive rats normally have an elevated sympathetic tone which stimulates release of renin. The denervation which occurs in isolating kidneys might therefore produce a proportionally greater per centage loss of sympathetic influence in these kidneys, which could possibly account for some of their reduced renin release. These Kyoto hypertensive kidneys are apparently not “reset” to maintain hypertension with either Na or renin.

Prevention with thiazide of NaCl-induced hypertension in Dahl "S" rats. Evidence for a Na-retaining humoral agent in "S" rats.

SUMMARY Dahl “S” rats become hypertensive wben fed > high salt (NaCl) diet but remain normotensive on a low NaCl diet. Dahl “R” rats are normotensive on either diet. For a given perfusion pressure, isolated “S” kidneys excrete 50% less sodium than “R” kidneys. Therefore, we searched for a sodium-retaining hormone in “S” rats. Kidneys were isolated without ischemia from normal rats and were continuously perfused at 125 mm Hg with blood from Dahl “S” and “R” rats, all on low NaCl diets. All kidney and adrenal tissue had been extirpated from the perfusing rats. During 15 minutes of perfusion, the isolated “normal” kidneys excreted a mean of 164 jtEq Na/rain/100 g during 26 perfusion experiments with blood from “R” rats. The “normal” kidneys excreted a mean of 84 j*Eq Na during 24 perfusions with blood from “S” rats. Thus, the normal kidneys excreted half as much sodium wben perfused with “S” blood compared with “R” blood (p < 0.02). Seemingly, a Na-retainlng humoral agent is present in the blood of “S” rats on a low Na diet, in the absence of renal and adrenal tissue. Moreover, in these normal kidneys, perfusion with “S” blood induced a 16% higher renal vascular resistance than perfusion with “R” blood (p < 0.01), indicating vasoconstricting agents in “S” blood. However, the Na-retaining humoral effect is definitely not dependent on the vasoconstriction. The Na-retaining humoral effect in “S” blood could lead to Na retention by “S” kidneys in vivo, which could partially account for the susceptibility of “S” rats to NaCl hypertension. Furthermore, both “S” and “R” perfusing rats were continuously expanded with two-thirds blood and one-third Ringers solution at a rate of 5% of body weight per hour. This expansion induced the appearance of a natriuretic humoral agent in the blood of both “S” and “R” rats to an equal degree, so that after 45 minutes of expansion, the isolated kidneys had increased their Na excretion approximately twofold. Hypertension is readily induced in Dahl “S” rats by feeding them a high NaCl diet. However, this hypertension can be almost completely prevented by concomitant treatment with thiazide diuretics that act mainly on the kidney to facilitate sodium excretion. This result is in agreement with the hypothesis that a shift in the pressure nttriuresis curve, reducing Na excretion for a given arterial pressure, is partially responsible for the great sensitivity to NaCl hypertension in the “S” rat. The Naretaining hormone may contribute to this shift.

High Potassium Diets Reduce Vascular and Plasma Lipid Peroxides in Stroke-Prone Spontaneously Hypertensive Rats

We examined the effect of high potassium (K) diet on oxidative stress to endothelium in hypertensive rats. Five-week-old stroke-prone spontaneously hypertensive rats (SHRsp) were fed a 5% high NaCl diet containing either 0.5% normal K (n = 28) or 2.1% high K (n = 19) for 6 weeks, and lipid peroxides in the aortic intima and plasma were measured. Lipid peroxides were extracted into an organic solvent to avoid the interference of carbohydrates or glycoproteins, and malondialdehyde (MDA) produced from lipid peroxides by acid-heating was measured by its reaction to thiobarbituric acid. The antioxidant butylated hydroxytoluene prevented spurious lipid peroxide formation during the whole procedure, and optimum Fe3+ allowed a maximum MDA production from lipid peroxides. The high K SHRsp showed lower lipid peroxide levels than the normal K SHRsp both in the intima (5.6 +/- 0.3 vs. 7.2 +/- 0.4 nmol MDA/mg fatty acids, p < 0.003) and plasma (0.91 +/- 0.08 vs. 1.46 +/- 0.10 nmol MDA/ml, p < 0.001). Mean arterial pressure was slightly lower by 13 mmHg in the high K SHRsp, but these differences were still obvious even when we compared groups of rats with precisely matching blood pressures. These results indicate that high K diets reduce oxidative stress on the endothelium of high NaCl-fed SHRsp independently of blood pressure changes. This effect may be involved in the mechanism by which high K diets protect endothelium and reduce stroke incidence in hypertensive animals. Thus, we improved the method of lipid peroxide measurement and propose the protective effects of high K diet against oxidative stress to endothelium in hypertension animals.

Hypertension-producing factor in serum of hypertensive Dahl salt-sensitive rats.

To investigate whether serum in hypertensive Dahl salt-sensitive rats (S rats) contains a hypertensinogenic substance, we examined the effects of repeated injections of serum from such S rats on blood pressure (BP) and pressor responses. Serum was collected from either hypertensive or normotensive S rats (fed an 8% or 0.11% NaCl diet, respectively) and injected into uninephrectomized recipient S rats for 2 weeks (0.45 ml, twice a day, i.v.). Serum from hypertensive rats injected for 14 days significantly increased BP by 14 mm Hg (143 vs 129, p less than 0.05), pressor responses to angiotensin II (ANGII) by 45% (p less than 0.005), pressor responses to norepinephrine (NE) by 38% (p less than 0.025), and Na concentration in the aortic wall of recipient rats by 5.9% (p less than 0.05), compared to the effects of the injection of serum from normotensive S rats. These results imply that hypertensive S serum contains a hypertensinogenic substance and that this serum factor produces a mild hypertension in the recipient rats and also contributes importantly to the hypertension in donor S rats. Dahl salt-resistant rats (R rats) on either 8% or 0.11% NaCl had normal BP. Their sera produced no differences in BP or in pressor responses in recipient rats. Hence 8% NaCl, which produced no hypertension, also induced no hypertensinogenic serum factors in R rats. We sought to determine whether nephrectomy would alter these humoral factors. The BP averaged 139 mm Hg in rats receiving normotensive sham-nephrectomized S serum vs 154 in those receiving hypertensive sham-nephrectomized S serum, 15 mm Hg higher (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

A model of intervention for prevention of early essential hypertension in the 1980s.

The onset of essential hypertension early in life is indicated by the high tracking of blood pressure during adolescence; intervention in adults with mild hypertension has been found successful. How, then, can high blood pressure levels in children be modified to prevent early hypertensive cardiovascular disease in adulthood? In an entire biracial town (population 9000) we surveyed 1604 (89%) of all children aged 8--18 years for blood pressure and reexamined those in the upper decile of mean blood pressure (for each race, sex, and height) on three additional occasions. On each examination nine blood pressures were taken by trained observers. All children consistently in the top decile were randomly allocated into either a treatment (n = 50) or comparison (n = 50) group. These two groups and an additional midrange blood pressure comparison group (n = 50) were followed regularly using school facilities including community and school programs. Treatment consisted of 1) dietary guidance; 2) modifications of school lunches and snacks with healthy substitutes; 3) parental involvement; 4) a low dose diuretic and beta-antagonist given by usual standards. All study groups were monitored for blood pressure in a blind manner. In 6 months of observation, blood pressure in the treatment group remained 5 and 3 mm Hg (systolic and diastolic) less than controls (p less than 0.001 and p less than 0.01). An orchestrated community-wide attack on early-stage hypertension is feasible and seems to offer exciting potential for prevention of early hypertensive disease.

Calcium Content of Arteriolar Walls in Normotensive and Hypertensive Rats.

Summary Hypertension was produced in rats by narrowing one renal artery and removing the opposite kidney. These rats were then divided into 2 evenly matched groups. The rats in one of the groups were almost cured of hypertension; the rats in the other group remained hypertensive. The amount of calcium in the walls of the mesenteric arterioles was compared in the two groups. The hypertensive group, on the average, had 13% more arteriolar calcium than the “normotensive” group (P=0.01). Since calcium is known to promote the contraction of the muscle protein, actomyosin, the increased calcium content in the hypertensive arterioles may be partially responsible for their narrowed lumens.