Effect of dietary sodium intake on the intrarenal distribution of nephron glomerular filtration rates in the rat

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

Renal adaptation to sodium deprivation. Effect of captopril in the rat

Dietary sodium restriction is associated with a rapid decrease in urinary sodium excretion and achievement of a new sodium balance within three to five days. In addition, renal vasoconstriction and progressive activation of intrarenal systems with vasoconstrictor (renin-angiotensin) or vasodilating (kallikrein-kinin and prostaglandins) properties are observed. The relationship between sodium homeostasis and the renin-angiotensin system was assessed through the use of captopril in the rat. Treatment with captopril, before and during a six-day period after suppression of dietary sodium, was associated with sodium wasting (urinary sodium always exceeded sodium intake during the observation period); in addition, the normal increase in urinary aldosterone was blunted by about 80 percent. When captopril treatment was given for six days to rats maintained on long-term sodium restriction (at least four weeks) urinary sodium increased, although transiently; at the end of the study, renal vasodilatation together with a redistribution of glomerular blood flow to nonsuperficial glomeruli was observed. These studies indicate that captopril administration markedly blunts the renal and systemic adaptations to a reduced sodium intake in the rat. They suggest that the renin-angiotensin system is probably indispensable in preventing sodium loss when dietary sodium is suppressed.

Diuretic action of metolazone in dogs

Metolazone, the sulfonamide diuretic was investigated to determine the sites of action. We used a radioactive microsphere, clearance and stop-flow method in anesthetized dogs. Urine flow and urinary excretion of sodium and potassium were increased at 5--60 min when metolazone was given intravenously at doses of 0.2--5.0 mg/kg, while total renal blood flow, distribution of cortical renal blood flow and GFR did not change. The urinary excretion rate of sodium to potassium (Na/K) increased from 5.69 +/- 0.82 to 8.07 +/- 0.76 in a dose of 1.0 mg/kg, i.v. Osmolar clearance and free water reabsorption increased almost proportionally, indicating that metolazone has little effect on the medullary portion of the ascending limb of Henle and may have a proximal site of action. In stop-flow experiments, a significantly raised U/PNa/U/Pcreatinine was observed at the dip situated distally to the ascending limb of Henle. These findings indicate that the diuretic action of metolazone may be due to the inhibition of sodium reabsorption in the distal nephron segments, in addition to the absence of modification of the cortical regional blood flow.

Renal micropuncture study of normotensive and Milan hypertensive rats before and after development of hypertension

Earlier studies of renal transplantation and of sodium metabolism indicated that the cause of high blood pressure in the Milan strain of genetically hypertensive rats (MHS) was altered renal function. To pinpoint the active factors, we used micropuncture to study several indices of renal function in normal (NR) and MHS rats at three different ages: A) 26 to 30 days, before development of hypertension (pre-MHS); B) 35 to 40 days; and C) 75 to 90 days, after the development of hypertension. The indices studied and the important differences found between the two strains were: 1) Single nephron filtration rate (SNFR) and late proximal tubular fluid delivery to the distal nephron (LPF). In group A, the pre-MHS rats had significantly lower values than did the NR (SNFR = 6.3 +/- 0.8 nl/min [MHS] vs. 8.3 +/- 1.2 [NR], P less than 0.01; LPF = 3.14 +/- 0.25 nl/min [MHS] vs. 4.1 +/- 0.35 [NR], p less than 0.05). In group C, the values in the MHS rats were significantly higher than those of the NR (SNFR = 17.3 +/- 1.4 nl/min [MHS] vs. 12.1 +/- 0.8 [NR], P less than 0.05; LPF - 7.4 +/- 0.5 nl/min [MHS] vs. 5.3 +/- 0.3 [NR], P less than 0.01). 2) Number of glomeruli. In group C only, the MHS rats had significantly fewer than did the NR rats (MHS = 55, 253 +/- 2,821 vs. NR = 64,527 +/- 2,900, P less than 0.05). 3) Glomerular filtration rate (GFR) and SNFR as a function of the mean blood pressure (MAP). In group A, the GFR of the MHS rats was lower than that of the NR rats (GFR = 0.38 +/- 0.03 ml/min . 100 g of body wt [MHS], 0.50 +/- 0.03 [NR] P less than 0.05). In group C, there was no longer any significant difference. At equal MAP, SNFR was equal in all the groups, except group A, where SNFR was significantly lower in MHS. 4) Pressure differences: Glomerular capillary pressure (gP). GP was significantly higher in MHS rats than in NR rats (group A, + 5.2 mm Hg; group C, + 6.7 mm Hg). In the pre-MHS rats, anesthesia significantly increased (P less than 0.001) the blood pressure difference between the two strains. This effect was not seen in the adult MHS rats. This may increase the differences in GP between pre-MHS and NR. 5) Afferent effective filtration pressure (EFPA). EFPA values were also higher in MHS rats (+ 2.9 mm Hg in group A, + 6.8 mm Hg in group C), but once again the effects of anesthesia probably account for the differences in magnitude seen between pre-MHS and NR. Only 22% of the absolute differences in systemic arterial pressure in the adult MHS and NR rats was transmitted to the glomerular capillary, while 33% of the difference was transmitted in the younger rats. These values suggest a reduced glomerular hydraulic conductivity, even though other explanations could not be excluded, and they are consistent with the hypothesis that the primary cause of development of hypertension in the MHS rats may be a decrease in SNFR.

Developmental changes in nephron number, proximal tubular length and superficial nephron glomerular filtration rate of rats

1. Post-natal development of single nephron glomerular filtration rate, superficial proximal tubular length, nephron number and kidney weight have been studied in Sprague Dawley and in Wistar rats. 2. Superficial tubular length is a non-linear function of body weight or age. There seems to be a rapid growth until animals weigh about 150 g in Wistar rats. In this strain, growth is slower thereafter. This difference is not as evident in Sprague Dawley rats. 3. Nephron numbers increase over the same period at which rapid tubular growth occurs. 4. Sprague Dawley rats have somewhat fewer, but longer, proximal tubules than do Wistar rats. 5. In all animals weighing more than 100 g, SNGFR is linearly related to weight. For younger, smaller Sprague Dawley rats, the same linearity holds over the age range studied--older than 20 days of age. In Wistar rats, SNGFR relative to weight is less in young animals. 6. By relating SNGFR to total kidney GFR, evidence is obtained that maturation of renal function also involves a greater increase in filtration by superficial than by juxtamedullary nephrons.

Effect of diuresis and nephron dimensional heterogeneity on the distribution of nephron filtration rates

Conflicting data in studies of the effect of natriuresis on intrarenal single nephron glomerular filtration rate (SNGFR) redistribution may arise from the interplay of hitherto largely overlooked factors. In the present work, the effect of diuresis induced by saline, glucose or mannitol, as well as the effect of anatomical nephron heterogeneity, were studied. A highly significant positive correlation was found between the logarithm of the urine flow per gram of kidney weight (log V) and the mean superficial (S) to mean juxtamedullary (JM) SNGFR ratio. The rise in S/JM SNGFR with diuresis was primarily a function of decreased JM SNGFR. Total proximal tubular length (TPL) was used as a measure of nephron size. The distribution of nephron sizes was evaluated as S/JM TPL. The effects of urine flow and anatomical heterogeneity on S/JM SNGFR were spearated by means of multiple regression analysis, which yielded the following equation: S/JM SNGFR = -0.049 + 0.179 log V + 0.818 S/JM TPL. Both slopes were highly significant (P less than 0.001). These findings indicate that S/JM SNGFR increases with urine flow, independently of sodium homeostasis, and that anatomical heterogeneity has a marked effect and must, therefore, be controlled. Conflicting data in the literature are harmonized with the present data when appropriate correction can be made for the dimensional factor. There is no evidence that SNGFR redistributions play a role in sodium homeostasis.