We recently showed that vitamin D receptor (VDR) inactivation results in deregulated stimulation of the renin-angiotensin system (RAS). To address further the relation between RAS activation and the abnormalities in electrolyte and volume homeostasis, we studied the effect of the ANG II type I receptor antagonist losartan and the angiotensin-converting enzyme inhibitor captopril on VDR-null mice. Treatment with losartan or captopril normalized the water intake and urine excretion of VDR-null mice. However, the increase in salt excretion in VDR-null mice was not affected by either drug, suggesting that this abnormality is independent of the RAS. Northern blot and immunohistochemical analyses revealed that both drugs caused a drastic stimulation of renin expression in wild-type and VDR-null mice, but renin expression remained much higher in the treated VDR-null mice than in the treated wild-type mice, suggesting that the ANG II feedback mechanism remains intact in the mutant mice. These data firmly established a causative relation between RAS overstimulation and the abnormal volume homeostasis in VDR-null mice and demonstrated that vitamin D repression of renin expression is independent of the ANG II feedback regulation in vivo.

Effect of ANG II type I receptor antagonist and ACE inhibitor on vitamin D receptor-null mice

Publisher Summary This chapter discusses the kidney function in calcium and phosphate metabolism. Production of urine is the most conspicuous expression of kidney function but not the most significant always. The amount of phosphate excreted per day is equal to the amount of phosphate adsorbed from the diet. The average diet contains about 1500 mg phosphate per day, and the average 24-hour renal excretion of phosphate is about 600 mg. Variations in intake and urinary output are closely related. The most common cause of a low phosphate excretion rate is a phosphate-poor diet—such as low protein or low calcium diets—or administration of aluminum hydroxyde gels, which bind phosphate in the gut. Phosphate reabsorption is decreased in primary and secondary hyperparathyroidism and increased in hypoparathyroidism. A chronic high phosphate intake produces a steady state with reduced tubular reabsorption of phosphate, reduced calcium excretion, normocalcemia or slight hypocalcemia, increased immunoreactive parathyroid hormone serum level, and parathyroid hyperplasia. Chronic renal failure with decreased glomerular filtration rate is associated with bone disease.

Kidney Function in Calcium and Phosphate Metabolism

Parathyroid hormone and calcitonin regulation of renal function

Role of parathyroid hormone in the phosphaturia of extracellular fluid volume expansion

To determine if an increase in the endogenous secretion of parathyroid hormone could decrease sodium reabsorption by the proximal tubule, the ionized calcium concentration of blood perfusing the parathyroid gland of eight unilaterally thyroid parathyroidectomized dogs (TPTX) was reduced by infusion of an isotonic sodium citrate plus sodium chloride solution into the blood supply of the parathyroid gland. The fractional clearance of phosphate increased significantly (+9.3 +/- 2.8 ml/min per 100 ml GFR), while fractional sodium reabsorption by the proximal tubule decreased (-.06 +/- .02; P less than .025). In seven normal control dogs that received isotonic sodium chloride infusion, neither fractional sodium reabsorption by the proximal tubule nor the fractional clearance of phosphate was significantly altered. In five bilaterally TPTX dogs that received a sodium citrate plus sodium chloride infusion, sodium reabsorption by the proximal tubule was not significantly altered. There were no significant changes in glomerular filtration rate or renal plasma flow in any of these groups. The data demonstrate that alterations in endogenous parathyroid hormone secretion can play a significant role in the regulation of sodium reabsorption by the proximal tubule.

Effect of parathyroid hormone secretion on sodium reabsorption by the proximal tubule.

A B S T R A C T Preferential expansion of the plasma volume by infusion of salt-poor hyperoncotic albumin solution decreases sodium reabsorption by the proximal tubule. The present micropuncture studies test the thesis that albumin infusion depresses proximal reabsorption by an effect unrelated to expansion of the plasma volume, perhaps due to an effect of parathyroid hormone (PTH) on proximal sodium reabsorption. Infusion of salt-poor hyperoncotic albumin significantly decreased plasma ionized calcium, increased immunoreactive PTH (iPTH) in plasma, decreased sodium reabsorption by the proximal tubule, and increased phosphate clearance. In contrast, infusions of albumin, in which the ionized calcium was restored to normal plasma levels, had no significant effect on ionized calcium, iPTH, proximal reabsorption, or phosphate clearance in intact dogs. Similarly, in parathyroidectomized animals given a constant replacement infusion of PTH, albumin infusion had no significant effect on proximal reabsorption or phosphate clearance. Plasma volume was markedly expanded following albumin infusion in all groups of dogs. These findings (a) indicate that PTH plays a significant role in the decrease in sodium reabsorption by the renal This work was presented in part at the Fall meetings

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Role of parathyroid hormone and dissociation from plasma volume

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Role of parathyroid hormone and dissociation from plasma volume