Contribution of the distal tubule to potassium excretion in experimental glomerulonephritis

Chemical evidence for the tradeoff-in-the-nephron hypothesis to explain secondary hyperparathyroidism

Background

Secondary hyperparathyroidism (SHPT) complicates advanced chronic kidney disease (CKD) and causes skeletal and other morbidity. In animal models of CKD, SHPT was prevented and reversed by reduction of dietary phosphate in proportion to GFR, but the phenomena underlying these observations are not understood. The tradeoff-in-the-nephron hypothesis states that as GFR falls, the phosphate concentration in the distal convoluted tubule ([P]_DCT]) rises, reduces the ionized calcium concentration in that segment ([Ca⁺⁺]_DCT), and thereby induces increased secretion of parathyroid hormone (PTH) to maintain normal calcium reabsorption. In patients with CKD, we previously documented correlations between [PTH] and phosphate excreted per volume of filtrate (E_P/C_cr), a surrogate for [P]_DCT. In the present investigation, we estimated [P]_DCT from physiologic considerations and measurements of phosphaturia, and sought evidence for a specific chemical phenomenon by which increased [P]_DCT could lower [Ca⁺⁺]_DCT and raise [PTH].

Methods and findings

We studied 28 patients (“CKD”) with eGFR of 14–49 mL/min/1.73m² (mean 29.9 ± 9.5) and 27 controls (“CTRL”) with eGFR > 60 mL/min/1.73m² (mean 86.2 ± 10.2). In each subject, total [Ca]_DCT and [P]_DCT were deduced from relevant laboratory data. The Joint Expert Speciation System (JESS) was used to calculate [Ca⁺⁺]_DCT and concentrations of related chemical species under the assumption that a solid phase of amorphous calcium phosphate (Ca₃(PO₄)₂ (am., s.)) could precipitate. Regressions of [PTH] on eGFR, [P]_DCT, and [Ca⁺⁺]_DCT were then examined. At filtrate pH of 6.8 and 7.0, [P]_DCT was found to be the sole determinant of [Ca⁺⁺]_DCT, and precipitation of Ca₃(PO₄)₂ (am., s.) appeared to mediate this result. At pH 6.6, total [Ca]_DCT was the principal determinant of [Ca⁺⁺]_DCT, [P]_DCT was a minor determinant, and precipitation of Ca₃(PO₄)₂ (am., s.) was predicted in no CKD and five CTRL. In CKD, at all three pH values, [PTH] varied directly with [P]_DCT and inversely with [Ca⁺⁺]_DCT, and a reduced [Ca⁺⁺]_DCT was identified at which [PTH] rose unequivocally. Relationships of [PTH] to [Ca⁺⁺]_DCT and to eGFR resembled each other closely.

Conclusions

As [P]_DCT increases, chemical speciation calculations predict reduction of [Ca⁺⁺]_DCT through precipitation of Ca₃(PO₄)₂ (am., s.). [PTH] appears to rise unequivocally if [Ca⁺⁺]_DCT falls sufficiently. These results support the tradeoff-in-the-nephron hypothesis, and they explain why proportional phosphate restriction prevented and reversed SHPT in experimental CKD. Whether equally stringent treatment can be as efficacious in humans warrants investigation.

Effect of uremia on aldosterone metabolism in the isolated perfused rat liver

The effect of uremia on hepatic metabolism of aldosterone was studied in the isolated perfused liver of female Wistar rats. Uremia was induced by five-sixths partial nephrectomy 4 weeks before experiments. Isolated livers of normal and uremic rats were perfused at a constant flow rate with a hemoglobin-free medium, to which 4-14C-D-aldosterone was added at 3 nmol/L. Aldosterone was analyzed by radioimmunoassay (RIA) and 4-14C-D-aldosterone radiometabolites in perfusate and bile were assayed by high-performance liquid chromatography (HPLC). Uremic rats had a 10% lower body weight (P < .01) and increased plasma urea, creatinine, and parathyroid hormone (PTH) levels (258%, 200%, and 208%, respectively; P < .01-.001). Blood pressure and plasma K+, Na+, and aldosterone levels were similar. Plasma renin activity was suppressed by 68% in uremic rats (P < .001). Liver wet weight and hepatic function were similar in livers of both groups of rats. Hepatic elimination of aldosterone was compatible with a first-order kinetics. Hepatic clearance of aldosterone per liver and per gram liver was similar; however, when expressed per 100 g rat body weight, a 21% higher value was observed in uremic rats (11.6 +/- 1.8 mL/min) compared with normal rats (9.6 +/- 1.5 mL/min, P < .01). Polar aldosterone radiometabolites accumulated in the perfusate to approximately 40% of the initial 14C added at 15 minutes, and were eliminated in bile at a similar rate in both groups. No qualitative difference was found in the pattern of radiometabolites of aldosterone in perfusate and bile.(ABSTRACT TRUNCATED AT 250 WORDS)

Is genetic susceptibility to pre-eclampsia conferred by homozygosity for the same single recessive gene in mother and fetus?

OBJECTIVE

To determine whether any simple, purely genetic mechanism can account for susceptibility to pre-eclampsia.

DESIGN

Six simple Mendelian models of inheritance were considered, and predictions concerning the incidence of pre-eclampsia in various family members of index cases were calculated for each genetic model. Data were then extracted from published family studies in which a suitable disease definition had been used, and compared to our theoretical expectations.

RESULTS

Only one of the genetic models considered, in which both mother and fetus must express the same recessive gene to confer susceptibility, was consistent with the observed incidence values for relatives of index cases. This model was also consistent with the putative association with HLA-DR4, but could not account for the comparative rarity of pre-eclampsia in parous women.

CONCLUSION

Homozygosity for a single recessive gene shared by mother and fetus, unlike five other genetic mechanisms tested, is consistent with published family studies in pre-eclampsia, and should be considered the best working hypothesis at present.

Sodium handling by deep nephrons and the terminal collecting duct in glomerulonephritis

The present study was designed to characterize the effects of anti-glomerular basement membrane (anti-GBM) glomerulonephritis (GN) on sodium handling by surface nephrons, deep nephrons and the terminal collecting duct segment. Studies were performed in rats during hydropenia and volume expansion. In hydropenia, the glomerular filtration rate (GFR) and sodium excretion tended to be lower in rats with GN than in controls. However, the major differences between the control and GN animals were seen in volume expansion. In the volume expanded groups fractional excretion of sodium was greater in controls (3.20 +/- 0.51%) than in GN (1.20 +/- 0.36%, P less than 0.01). Despite this, delivery to end proximal sites was similar in the two groups in absolute terms and higher in the expanded GN group compared to the expanded controls. Absolute sodium delivery to the bend of the loop of Henle in the expanded GN rats was decreased in absolute terms but increased in fractional terms compared to expanded controls. However, fractional delivery of sodium to the base of the terminal collecting duct was less in GN (3.71 +/- 1.39%) than in controls (7.19 +/- 0.96%, P less than 0.002). In both groups, fractional delivery between tip of the collecting duct fell compared to base (P less than 0.05) but delivery to the tip was again greater in controls (5.49 +/- 1.08%) than in GN (1.51 +/- 0.47%). In GN 62.6 +/- 5.0% of delivered sodium was reabsorbed between collecting duct sites, nearly twofold that of controls (28.8 +/- 9.4%, P less than 0.01). Thus, fractional sodium reabsorption in the collecting duct was enhanced by GN.

Contribution of superficial nephron segments to sodium excretion in experimental glomerulonephritis

The present study examined the contribution of individual superficial nephron segments to sodium excretion in antiglomerular basement membrane nephritis in the rat by sampling the same nephron successively from the end and beginning of the distal tubule and end of the proximal tubule. Whole kidney GFR in glomerulonephritic rats was reduced by approximately 40% from controls; absolute sodium excretion was about 25% of normal. Metabolic balance studies in the awake state had suggested that the animals were in sodium balance. Plasma renin levels before and during micropuncture were similar to controls. These findings suggest that the defect in sodium handling is intrinsic to the kidney. Glomerulotubular balance was maintained along the proximal tubule. Sodium reabsorption in the loop of Henle was reduced in absolute terms but was proportional to the load delivered. Due to the decreased absolute sodium reabsorption in the preceding segments, sodium delivery to the beginning of the distal tubule was comparable in the two groups of animals. Along the distal tubule sodium reabsorption was comparable to control animals. Therefore, the avid urinary sodium retention seen during micropuncture was due to increased sodium reabsorption by segments past the superficial proximal tubule and/or by deep nephrons.

Potassium tolerance

The maintenance of potassium homeostasis with an increased potassium intake or decreased renal function is dependent in part on the renal adaptation observed in 'potassium tolerance'. However other factors, including control of ingestion, and increased distal delivery of fluid, also play a role.