[Sodium and water reabsorption in different parts of the nephron in rats with experimental hypertension]

Renal atrial peptide receptors and natriuresis in two-kidney, one clip hypertension

It has been suggested that the impaired natriuretic response of the clipped kidney in two-kidney, one clip hypertensive rats is related to downregulation of renal atrial natriuretic peptide receptors. To test this hypothesis, blood volume expansion and atrial peptide binding studies were performed in this model. Infusion of 1% and then 1.5% body weight donor blood (n = 6) caused a progressive increase in plasma immunoreactive atrial natriuretic peptide (107 +/- 26 to 168 +/- 31 to 427 +/- 154 pg/ml, p less than 0.001); the sodium excretion of the nonclipped kidney rose from 230 to 2,200 to 4,000 neq/min (p less than 0.01) but that of the clipped kidney did not rise significantly. There was a highly significant correlation between log cyclic guanosine monophosphate and log sodium excretion by the nonclipped (r2 = 0.749) but not the clipped (r2 = 0.046) kidney. Between clipped and nonclipped kidneys, the association constant (5.26 +/- 0.89 versus 5.17 +/- 0.64 x 10(9)/mol) and apparent binding site density (575 +/- 92 versus 500 +/- 74 fmol/mg protein) for atrial peptide binding in isolated glomeruli did not differ. Assay of atrial peptide-induced cyclic guanosine monophosphate release by isolated glomeruli showed that clipped and nonclipped kidneys were equally responsive. Binding affinity and receptor density did not differ in homogenates prepared from inner medullas of clipped and nonclipped kidneys. These results show that the blunted natriuretic response in clipped kidneys was not associated with any relative decrease in number or function of glomerular or papillary atrial natriuretic peptide receptors.

Effect of captopril infusion on systemic and renal haemodynamics in conscious hypertensive rats with chronic, progressive aortic ligation

The effect of the converting enzyme inhibitor captopril (5 mg (kg bwt)-1) on systemic and renal haemodynamics has been studied in conscious rats in which a progressive hypertension has been induced by progressive aortic ligation (AL) between the renal arteries, and in a sham-operated (SO) group. Cardiac output (CO), organ blood flow and vascular resistances have been measured using radioactive microsphaeres. Captopril infusion caused increases in CO in both groups of rats, but the increase was higher in SO (9.2 +/- 0.7%) than in AL rats (5.2 +/- 0.6%; P less than 0.005). Plasma renin concentrations were similar in both groups but increased more in AL (10.3 microIU +/- 1.0) than in SO (5.81 microIU +/- 0.62; P less than 0.05) after captopril. Captopril induced also a larger decrease in arterial pressure (36 +/- 4 mmHg), and of the pressure gradient across the stenosis (19 +/- 3) mmHg in AL than in SO rats (5.6 +/- 1.4 and 1.1 +/- 1.3 mmHg, P less than 0.005 for both cases). Vascular resistance of the kidney above the ligature decreased more in AL than in SO rats, but this difference was not observed in the other kidney. From these data it can be concluded that captopril has an acute hypotensive effect despite the normal renin levels of this model of chronic hypertension. In addition, blood flow to the high-pressure perfused kidney seems to be dependent on the increased renin production by the contralateral kidney.

Pressure dependence of exaggerated natriuresis in two-kidney, one clip Goldblatt hypertensive rats

This study evaluated the responses of each kidney of two-kidney, one clip Goldblatt hypertensive rats to acute volume loading to delineate the contribution of elevated perfusion pressure to the mechanisms of the exaggerated natriuresis in this model of hypertension. Eleven Goldblatt animals (0.2 mm clip 3 weeks prior to study) were studied at spontaneous blood pressure for each kidney's response to volume loading. In 11 other Goldblatt animals an aortic clamp between the renal arteries allowed reduction of perfusion pressure for the left, nonclipped kidney to normal levels. Ten normal rats served as controls. Renal function was examined during control periods and following the infusion of 3.5% body wt of 154 mM X liter-1 NaCl at 22.5 ml X hr-1. An exaggerated natriuresis was observed for the left, nonclipped kidney of the hypertensive Goldblatt rats, while the clipped kidney exhibited an attenuated natriuresis compared to either kidney of normal control rats. Reduction of perfusion pressure to the nonclipped kidney of Goldblatt animals to normal levels resulted in reduced clearance and excretory function before volume loading and attenuated its natriuretic response to levels less than those for the Goldblatt group at spontaneous, hypertensive blood pressure. The exaggerated natriuresis observed at hypertensive blood pressure was attributable to increases of filtered load of Na+ and reduced fractional absorption. These observations indicate that an exaggerated natriuresis occurs in the two-kidney, one clip Goldblatt hypertensive rat and that this phenomenon depends on the elevated renal perfusion pressure to the nonclipped kidney.

Impaired renal blood flow and cortical pressure autoregulation in contralateral kidneys of Goldblatt hypertensive rats

Experiments were conducted on two-kidney, one clip renal vascular hypertensive rats to assess the ability of the kidney contralateral to renal vascular stenosis to autoregulate renal blood flow (RBF), glomerular filtration rate (GFR), and hydrostatic pressures in cortical structures during conditions of acutely reduced renal arterial blood pressure (BP). When observed at their respective, spontaneous BPs, RBF and GFR were not different in the contralateral kidneys of the hypertensive rats (n = 11) compared to normal animals (n = 7). However, the contralateral kidneys exhibited a significantly higher renal vascular resistance (RVR), 28.9 +/- 2.8 mm Hg . min/ml than the control animals, 23.1 +/- 1.5 mm Hg . min/ml. At spontaneous BP (169 +/- 5 mm Hg), urine flow, absolute and fractional sodium excretion, and absolute and fractional potassium excretion were all significantly greater in the contralateral kidneys of hypertensive rats than in kidneys of normal rats. Hydrostatic pressures in cortical structures were similar in the two groups. When BP was reduced acutely, the kidney contralateral to the renal artery stenosis achieved only small decreases in RVR that failed to allow RBF, GFR, or cortical pressures to be maintained. In contrast, normal rats efficiently autoregulated RBF and GFR. In addition, hydrostatic pressures in proximal tubules, distal tubules, and first order peritubular capillaries were maintained during reductions in BP to as low as 100 mm Hg. Urine flow and electrolyte excretion decreased to a greater extent in the hypertensive kidneys, such that at comparable BP these indices of excretory function were not different in the two groups. These observations indicate that the capacity of the contralateral kidney to maintain hemodynamic and glomerular function at reduced BP is compromised severely and suggest the possibility that the impaired autoregulatory capability may contribute to the maintenance of hypertension observed in this model.

The juxtaglomerular apparatus in a human kidney with polar artery stenosis

Juxtaglomerular apparatuses (JGA) of a human kidney with stenosis of a polar artery from a hypertensive 18-year-old male patient were studied qualitatively and quantitatively on 2 mu thick serial sections from plastic embedded renal tissue. 11 JGA from juxtamedullary and 9 JGA from subcapsular cortical zones were photographed serially, the copies taped together and the cells and relationships within the JGA studied. On the copies the lenght of contact between the different juxtaglomerular structures and the basement membrane of the macula densa was measured. Futhermore we calculated the areas of surface contact and the macula densa basal area. In the juxtamedullary JGA affected by the polar artery stenosis all Goormaghtigh cells were transformed into epitheloid cells and the Goormaghtigh cell field was significantly larger than in the JGA of the subcapsular cortex, which obviously had been exposed to the systemic hypertension. The macula densa basal area was significantly greater in the juxtamedullary JGA than in the subcapsular JGA, but neither of these differed significantly from the macula densa in the normal JGA reported previously.

The effect of an acute increase in renal perfusion pressure on sodium transport in the rat kidney

We used micropuncture techniques to examine the intrarenal response to an acute elevation of the renal perfusion pressure. In one series of studies (epinephrine, group I) the renal perfusion pressure was acutely increased by intravenous epinephrine infusion; in another series, by bilateral carotid occlusion and vagotomy. A third series of studies (epinephrine, group II) was performed identically to the epinephrine, group I, studies except that the renal perfusion pressure was held constant during the epinephrine infusion by suprarenal aortic constriction. After epinephrine infusion (group I) and following bilateral carotid occlusion and vagotomy the renal perfusion pressure increased, from 119 +/- 1.0 (SEM) to 166 +/- 1.85 mm Hg and from 122 +/- 5.9 to 168 +/- 3.1 mm Hg, respectively. Fractional sodium excretion rose from 2.31 +/- 0.34% to 5.09 +/- 0.58% (P less than 0.001) after epinephrine and from 1.80 +/- 0.71 to 6.40 +/- 1.0% (P less than 0.01) following carotid occlusion and vagotomy. In neither study, however, did we find that the increase in renal perfusion pressure changed the glomerular filtration rate (GFR) (both kidneys) or fractional sodium delivery from the superficial cortical late distal tubule. Furthermore, we found that epinephrine infusion at a constant renal perfusion pressure (epinephrine, group II) did not affect fractional sodium excretion, although a small, but significant, decrease in the GFR and sodium delivery from the superficial late distal tubule occurred. These data suggest that the natriuresis which follows an acute elevation of the renal perfusion pressure cannot be attributed to enhanced sodium delivery from superficial nephrons but must result from (1) inhibition of sodium reabsorption in inner cortical nephrons or (2) an effect on sodium transport in the collecting system.

The hydraulic conductivity of the rat proximal tubular wall determined with colloidal solutions

The hydraulic conductivity of the rat proximal tubular wall was determined using colloidal solutions perfused in short (50--200 mum) (SMP) or long (90--200 mum) (LMP) proximal tubular segments. In SMP human serum albumin (HSA) or polyvinylpyrrolidone (PVP) was added to raffinose solutions. A Lp of 0.019 nl-min-1-mm-1-mm Hg-1 was found when high colloid concentrations were used while values of 0.055--0.092 were found when low colloid concentrations were used. In other experiments, the Lp was determined by perfusing short tubular segments with pure raffinose solutions. A value of 0.015 nl-min-1-mm-1-mm Hg-1 was found. This is twice the value found when raffinose solutions were perfused through long tubular segments and it is concluded that the short microperfusion technique overestimates Lp with a factor of two. When microperfusions of long tubular segments were conducted, PVP was added to an equilibrium solution consisting of NaCl (110 mM) and raffinose (80 mM). Lp was found to be 0.018--0.021 when high colloid concentrations were used, while a value of 0.029 was found when a low colloid concentration was used. As found in both SMP and LMP a decrease in Lp's with increasing colloid concentrations indicates that a significant influence of radial concentration differences is highly probable. It is therefore suggested that the highest Lp derived when using the lowest colloid concentrations represents the best estimate. With this Lp value (0.03--0.05 nl-min-1-mm-1-mm Hg-1) and the existing transtubular hydrostatic and oncotic pressure difference it can be calculated that these passive forces might constitute the driving force for 1/3 of the fluid reabsorbed in the proximal tubule.

Acute changes of the Na-K-ATPase-activity in plasmamembranes of the isolated, cell-free perfused rat kidney

The Na-K-ATPase-activity of the isolated, cell-free perfused rat kidney was investigated. Na-load and perfusion rate could be varied independently. The ATPase-assay was carried out on plasmamembranes (700-1200 g fraction) isolated and purified after 60 min perfusion. Both the specific Na-K-ATPase-activity and the fractional Na-reabsorption showed an indirect correlation with the Na-load as well as with the perfusion rate. The Na-K-ATPase-activity of plasmamembranes obtained from the isolated kidney at physiological Na-load fits well to control values prepared from unperfused control kidneys. The present data are in accord with the results of earlier micropuncture experiments, where acute saline loading diminished the rate of Na-absorption.

Effect of sodium restriction on renal hypertension and on renin activity in the rat

Control rats and rats with experimental renal hypertension due to unilateral stenosis of one renal artery received a standard diet (0.23% sodium) and a sodium-deficient diet (0.004% sodium), alternately, during which time blood pressure and plasma renin activity were determined. At the end of the experiment, renin content of the kidneys was measured.

In normotensive control rats, the sodium-deficient diet did not affect blood pressure, but plasma renin activity and renin content of the kidneys increased. In rats with renal hypertension, restriction of sodium supply was followed by a fall in blood pressure to normotensive levels, provided that an intact contralateral kidney was present. Similarly, sodium-deficient diet prevented the development of hypertension if given immediately after placing the clip on one renal artery. Restriction of sodium supply provoked a marked increase in plasma renin activity, whereas renin content of the ischemic kidney was only slightly, but significantly, higher than in rats with normal sodium intake. Contrary to this, in unilaterally nephrectomized rats with renal hypertension, neither hypertension nor plasma renin activity or renin content of the kidneys was affected by sodium-deficient diet. In control rats with unilateral nephrectomy, plasma renin activity was only half that of intact rats, and restriction of sodium provoked no more than an increase up to normal values of intact rats. Sodium loss by the contralateral kidney may contribute to both the antihypertensive effect of a sodium-deficient diet and the increase in plasma renin activity.

Fluid reabsorption in Henle's loop and urinary excretion of sodium and water in normal rats and rats with chronic hypertension

The function of the short loops of Henle was investigated by micropuncture technique in normal rats, in rats with spontaneous hypertension, and in the untouched kidney of rats with experimental renal hypertension. All animals received a standard infusion of 1.2 ml of isotonic saline per hr. With increasing arterial blood pressure (range from 90 to 220 mm Hg), a continuous decrease in transit time of Lissamine green through Henle's loop from 32 to 10 sec was observed. Fractional water reabsorption along the loop declined progressively from 26 to 10%, and fractional sodium reabsorption decreased from 40 to 36% of the filtered load. The fluid volume in Henle's loop calculated from transit time and mean flow rate also decreased with increasing blood pressure. There was no change in superficial single nephron filtration rate but there was a slight increase in total glomerular filtration rate (GFR). Sodium and water reabsorption in the proximal tubule remained unchanged. Urine flow rate, sodium excretion, osmolar clearance, and negative free water clearance increased with increasing blood pressure. The osmolal urine to plasma (U/P) ratio declined but did not fall below a value of 1.5. It is concluded that the increase in sodium and water excretion with chronic elevation of arterial blood pressure is caused by a decrease of sodium and water reabsorption along the loop of Henle, presumably as a consequence of increased medullary blood pressure.