Zalman S. Agus

The effects of glucose and insulin on renal electrolyte transport.

The effects of hyperglycemia and hyperinsulinemia on renal handling of sodium, calcium, and phosphate were studied in dogs employing the recollection micropuncture technique. Subthreshold sustained hyperglycemia resulted in an isonatric inhibition of proximal tubular sodium, fluid, calcium, and phosphate reabsorption by 8-14%. Fractional excretion of sodium and phosphate, however, fell (P is less than 0.01) indicating that the increased delivery of these ions was reabsorbed in portions of the nephron distal to the site of puncture and in addition net sodium and phosphate transport was enhanced resulting in a significant antinatriuresis and antiphosphaturia. The creation of a steady state plateau of hyperinsulinemia while maintaining the blood glucose concentration of euglycemic levels mimicked the effects of hyperglycemia on proximal tubular transport and fractional excretion of sodium and calcium. Tubular fluid to plasma insulin ratio fell, similar to the hyperglycemic studies. These results suggest that the effects of hyperglycemia on renal handling of sodium and calcium may be mediated through changes in plasma insulin concentration. In contrast to hyperglycemia, however, hyperinsulinemia cuased a significant fall in tubular fluid to plasma phosphate ratio with enhanced proximal tubular phosphate reabsorption (P is less than 0.02). This occurred concomitantly with a significant inhibition of proximal tubular sodium transport. These data indicate that insulin has a direct effect on proximal tubular phosphate reabsorption, and this effect of insulin is masked by the presence of increased amounts of unreabsorbed glucose in the tubule that ensues when hyperinsulinemia occurs secondary to hyperglycemia. Fractional excretion of phosphate fell significantly during insulin infusion but unlike the hyperglycemic studies, the fall in phosphate excretion could be entirely accounted for by enhanced proximal reabsorption.

Disorders of calcium and magnesium homeostasis

The components of calcium and magnesium balance and the factors responsible for the maintenance of the serum concentration of these cations are reviewed. Within this framework, the causes and treatment of disturbances of the serum concentration are discussed. Hypercalcemia is usually a reflection of increased bone resorption and/or gut absorption with the kidney playing a secondary role. Hypocalcemia is usually due to either a disturbance in the parathyroid hormone-adenylate cyclase system or a disturbance in vitamin D metabolism. As vitamin D is required for expression of the action of PTH at bone and as PTH is a prime regulator of vitamin D metabolism, the absence of either component results in important disturbances in calcium balance. In contrast to calcium homeostasis, the kidney plays a major role in the determination and regulation of serum magnesium. The major causes of hypermagnesemia therefore are associated with loss of renal function, and hypomagnesemia is frequently due to renal magnesium wasting.

Hypocalcemia and inhibition of parathyroid hormone secretion after administration of WR-2721 (a radioprotective and chemoprotective agent)

We investigated the causes of the hypocalcemia associated with WR-2721, an investigational drug that protects normal tissues against the toxic effects of radiation and chemotherapy. Sixteen patients with advanced cancers had serial measurements of serum calcium and other studies of blood chemistry before and after taking the compound. In all 16 patients (18 courses) the mean serum calcium level fell from 2.33 to 1.90 mmol per liter (9.33 to 7.62 mg per deciliter, P less than 0.001) after WR-2721, and in all 8 patients from whom data were obtained, serum calcium levels remained depressed at 24 hours. In 12 patients, ionized calcium levels dropped from 0.98 to 0.80 mmol per liter (P less than 0.01). Serum magnesium values fell from 0.96 to 0.74 mmol per liter (1.91 to 1.48 mg per deciliter, P less than 0.001). There were no changes in arterial pH or serum phosphate concentrations. In nine patients urinary calcium excretion rose from 3.96 to 10.68 mumol per minute (P less than 0.01) after WR-2721. Despite hypocalcemia, parathyroid hormone levels fell in all subjects. WR-2721 also inhibited the release of parathyroid hormone from bovine parathyroid cells incubated in vitro in low (0.5 mM) concentrations of calcium. We conclude that WR-2721 inhibits the secretion of parathyroid hormone and enhances calciuria, thereby leading to hypocalcemia.

Evidence for an Intrinsic Renal Tubular Defect in Mice with Genetic Hypophosphatemic Rickets

To investigate the role of parathyroid hormone (PTH) and(or) an intrinsic renal tubular reabsorptive defect for phosphate in mice with hereditary hypophosphatemic rickets, we performed clearance and micropuncture studies in hypophosphatemic mutants and nonaffected littermate controls. Increased fractional excretion of phosphate in mutants (47.2+/-4 vs. 30.8+/-2% in controls) was associated with reduced fractional and absolute reabsorption in the proximal convoluted tubule and more distal sites. Acute thyropara-thyroidectomy (TPTX) increased phosphate reabsorption in both mutants and controls with a fall in fractional phosphate excretion to congruent with7.5% in both groups indicating that PTH modified the degree of phosphaturia in the intact mutants. Absolute reabsorption in the proximal tubule and beyond remained reduced in the mutants, however, possibly because of the reduced filtered load. Serum PTH levels were the same in intact mutants and normals as was renal cortical adenylate cyclase activity both before and after PTH stimulation. To evaluate the possibility that the phosphate wasting was caused by an intrinsic tubular defect that was masked by TPTX, glomerular fluid phosphate concentration was raised by phosphate infusion in TPTX mutants to levels approaching those of control mice. Phosphate excretion rose markedly and fractional reabsorption fell, but there was no change in absolute phosphate reabsorption in either the proximal tubule or beyond, indicating a persistent reabsorptive defect in the absence of PTH. We conclude that hereditary hypophosphatemia in the mouse is associated with a renal tubular defect in phosphate reabsorption, which is independent of PTH and therefore represents a specific intrinsic abnormality of phosphate transport.

Role of antidiuretic hormone in the abnormal water diuresis of anterior hypopituitarism in man

To evaluate the role of antidiuretic hormone (ADH) in the defect in water excretion which is characteristic of glucocorticoid deficiency, the effects of hydrocortisone and ethanol upon urinary dilution during a sustained water load were studied in patients with anterior hypopituitarism. A spectrum of defects in urinary dilution was found in the seven patients with anterior hypopituitarism, and the subjects were separable into two groups. Four patients were unable to excrete a urine hypotonic to plasma (group I) while three diluted the urine (group II). In two of the group II patients, despite maintenance of hydration, urinary osmolality later rose to hypertonicity. Physiological doses of hydrocortisone improved urinary dilution in all patients. Submaximal doses of oral hydrocortisone, when given to the group I patients, converted their response to hydration to one characteristic of the group II patients, i.e., an initial hypotonic urine followed by a secondary rise to hypertonicity. Ethanol, a known inhibitor of ADH secretion, had no effect in the group I patients. When two of these patients were pretreated with sub-maximal doses of hydrocortisone, however, so that they were able to transiently dilute the urine, ethanol prevented the secondary rise in urine osmolality. Similarly, the administration of ethanol to the untreated group II patients, when the urine was hypotonic, improved diluting ability as characterized by a lowering of urinary osmolality and an increased excretion of solute-free water in all three patients. Hydrocortisone did not improve urinary dilution in three patients with complete hypophyseal diabetes insipidus and one with both anterior and posterior insufficiency receiving constant infusions of vasopressin. These data suggest, therefore, that inappropriately elevated levels of ADH play a major role in the defect in water excretion of anterior hypopituitarism. Glucocorticoids appear to be necessary for a normal neurohypophyseal response to inhibitory stimuli.

Renal tubular effects of chronic phosphate depletion.

The effects of chronic phosphate depletion on renal tubular function were evaluated by micropuncture and free water clearance studies in the dog. Proximal tubular punctures demonstrated that chronic hypophosphatemia led to a reduction in ratio of tubular fluid to plasma inulin in late superficial tubular from 1.59+/-0.08 in control animals to 1.29+/-0.06 in phosphate-depleted dogs, with proportional inhibition of calcium and sodium reabsorption. The chronic decrease in proximal tubular fluid reabsorption was confirmed by the analysis of sustained water diuresis in conscious, phosphate-depleted dogs, before and after repletion of body PO4 stores, and in control animals. Urine flow rate/100 ml glomerular filtration rate (V/GFR) was significantly higher in PO4 DEPLETION THAN CONTROL (15.8+/-1.1 VS. 10.7+/-0.82). In addition, acetazolamide infusion did not increase V/GFR in phosphate-depleted dogs (15.8+/-1.1 vs. 17.16+/-0.9), supporting the conclusion that inhibition of proximal tubular fluid reabsorption was responsible for the elevated urine flow rate. PO4 repletion over 5 days reduced V/GFR to 9.2+/-0.7 despite no change in urine osmolality and no change in GFR, further suggesting a specific reversible alteration in proximal tubular reabsorption in phosphate depletion. Although hypercalciuria was a constant finding in phosphate depletion (fractional excretion of calcium of 2.04+/-0.4% vs. 0.47+/-0.13% in controls), the enhanced distal delivery of calcium was not a crucial factor; acute phosphate infusion reduced urinary calcium excretion to control values without affecting the reduced proximal tubular reabsorption in either intact or thyroparathyroidectomized phosphate-depleted dogs the change in distal nephron calcium reabsorption was independent of parathyroid hormone (PTH) levels since infusion of PTH failed to alter urinary calcium excretion. We conclude that chronic phosphate depletion leads to a reversible, sustained inhibition in proximal tubular reabsorptive fuction as well as a specific decrease in distal nephron calcium reabsorption. This latter reabsorptive defect is sensitive to phosplate infusion but not corrected by PTH.

Normal Diluting Capacity in Hyponatremic Patients: Reset Osmostat or a Variant of the Syndrome of Inappropriate Antidiuretic Hormone Secretion

Four patients with chronic illnesses and stable hyponatremia and plasma hypotonicity had normal urinary diluting capacity, with excretion of greater than 80% of a standard water load (20 ml/kg) within 4 hours and maintenance of a urine osmolality less than 100 mosmol/kg, during sustained water diuresis. Administration of a chronic salt load did not correct the hyponatremia. However, it was stabilized after treatment of the underlying medical condition. These subjects may represent a true resetting of the osmostat or a variant of the syndrome of inappropriate antidiuretic hormone secretion.

Cardiovascular Actions of Magnesium

Intracellular magnesium is an important modulator of calcium and potassium channels in cardiac myocytes. Hypomagnesemia is common in hospitalized patients and may contribute significantly to cardiac morbidity and mortality, particularly in states associated with myocardial ischemia. Therefore, it is important to maintain the plasma magnesium concentration within the normal range in asymptomatic patients and in patients with cardiac disease as prophylaxis against the occurrence of significant arrhythmias.

Effects of two nonsteroidal anti‐inflammatory drugs, indomethacin and oxaprozin, on the kidney

Nonsteroidal anti‐inflammatory drugs (NSAIDs) have been found to cause sodium retention and to decrease glomerular filtration rate (GFR). We studied the effects of two such drugs, indomethacin and oxaprozin, a new propionic acid derivative, on renal function of awake, normal human subjects during sustained water diuresis. Although neither drug had a long‐term effect on GFR or sodium clearance (CNa), indomethacin (six subjects) but not oxaprozin (seven subjects) transiently reduced GFR and CNa. Given over the short term, oxaprozin caused a reduction in GFR from 113.7 ± 5.7 to 99.8 ± 4.7 ml/min (p < 0.01) and in CNa from 0.84 ± 0.07 to 0.61 ± 0.08 ml/min (p < 0.005). The results were much the same when an additional dose of indomethacin was given to subjects who had been receiving the drug for a week. Inference from clearance data at a time when urinary osmolality (Uosm) remained constant but urine flow per GFR (V/GFR) fell suggests that both drugs stimulated proximal tubular sodium and fluid resorption. Both suppressed renin and aldosterone levels comparably and reduced potassium excretion transiently, but only indomethacin caused a sustained change in serum potassium concentration; serum potassium rose from 4.32 ± 0.10 to 4.56 ±0.11 mEq/l (p < 0.05) after 1 wk. These disparate findings suggest that prostaglandin synthesis inhibition may not be the sole mechanism of action of NSAIDs.

Renal tubular sites of altered calcium transport in phosphate-depleted rats.

Increased calcium (Ca) excretion is characteristic of chronic phosphate (PO(4)) depletion (PD). To study the changes in tubular transport and the site of the hypocalciuric effect of PO(4) administration, clearance and micropuncture experiments were performed in intact rats pair fed either a control diet (0.5% PO(4)) or a PO(4)-depleted (PD) diet (0.01% PO(4)) plus Al(OH(3)) and in parathyroidectomized (PTX) PD rats, infused either with saline or with neutral sodium PO(4). Intact PD rats, compared with intact rats on a control diet, exhibited a lower plasma ultrafiltrable (UF) PO(4) (5.8+/-0.5 vs. 7.8+/-0.3 mg/dl), higher fractional excretion (FE) of Ca (4.1+/-1.2 vs. 0.6+/-0.1%), and reduced FE PO(4) (0.1+/-0.01 vs. 10.2+/-1.8%). Tubular fluid/plasma inulin was lower in the late proximal tubule of PD rats, associated with increases in fractional delivery (FD) from the proximal tubule of Na and Ca.The%FD of Ca to the early distal tubule of PD rats was increased (20+/-3 vs. 11+/-2%), but this difference was abolished by the late distal tubule (5.1+/-1.2 vs. 3.3+/-0.9%). In PTX-PD rats, PO(4) infusion increased plasma UF PO(4) (13.8+/-0.7 vs. 7.8+/-0.7 mg/dl). FE of Ca was reduced (1.08+/-0.35 vs. 4.59+/-1.57%) without correcting the increased Ca delivery to the late distal tubule. These data indicate that PD impairs Ca reabsorption in tubular segments before but not within the distal convoluted tubule, so that hypercalciuria is ultimately a result of decreased Ca transport either in the terminal nephron or in deeper nephrons where PO(4) infusion stimulates Ca transport independent of parathyroid hormone or changes in the filtered load of Ca.