Inhibition of carbonic anhydrase by parathyroid hormone and cyclic AMP in rat renal cortex in vitro

Three Siblings With a Rare Familial Hyperphosphatemia Syndrome: A Case Series

Hyperphosphatemia familial tumoral calcinosis (HFTC) and hyperphosphatemia hyperostosis syndrome (HHS) are rare autosomal recessive disorders caused by mutations in the polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3), fibroblast growth factor 23 (FGF23), or klotho (KL) genes. They are characterized by hyperphosphatemia and recurrent episodes of bone lesions with hyperostosis and/or soft tissue calcinosis. Management options include phosphate-lowering therapies, anti-inflammatory medications, and surgical excision of the calcified masses in significantly disabled cases. We describe three cases from a consanguineous family who were found to have the same genetic mutation caused by a homozygous mutation in intron eight of GALNT3 c.1524+1 G>A (IVS8+1). The first case had a presentation similar to chronic osteomyelitis, while the second one presented with a calcified mass in her gluteal area. The third case presented with left leg pain. Being a rare disease, the findings of tumoral calcinosis/ bony abnormalities, along with elevated phosphate levels, should raise the possibility of this entity. Family history and biochemical findings can help reach the diagnosis.

A case of familial tumoral calcinosis/hyperostosis-hyperphosphatemia syndrome due to a compound heterozygous mutation in GALNT3 demonstrating new phenotypic features

SUMMARY

A new case of familial tumoral calcinosis (FTC)/hyperostosis-hyperphosphatemia syndrome (HHS) due to a novel compound heterozygous mutation in N-acetylgalactosaminyltransferase 3 (GALNT3) and with new phenotypic findings is presented. The response in serum phosphate and fibroblast growth factor 23 (FGF23) to medical treatment is detailed. This case expands the genotype and phenotype of FTC/HHS and gives insight into its treatment and pathophysiology.

INTRODUCTION

FTC and HHS are caused by mutations in FGF23, GALNT3, or KLOTHO. They are characterized by hyperphosphatemia, increased phosphate reabsorption, and elevated or inappropriately normal serum 1,25-dihydroxyvitamin D(3) (1,25-D(3)); FTC is associated with calcific masses, and HHS with diaphyseal hyperostosis.

METHODS

A 36-year-old woman presented with abnormal dental X-rays at age 12 and was hyperphosphatemic at 22. She underwent radiographic, biochemical and genetic testing, and medical treatment.

RESULTS

Serum phosphorus was 7.3 mg/dL (2.5-4.8), TmP/GFR 6.99 mg/100 mL (2.97-4.45), 1,25-D(3) 35 pg/mL (22-67). Radiographs revealed tooth anomalies, thyroid cartilage calcification, calcific masses in vertebral spaces, calcification of the interstitial septa of the soft tissue in the lower extremities, and cortical thickening of the long bones. Her total hip Z score was 1.9. C-terminus serum FGF23 was 1,210 RU/mL (20-108), but intact FGF23 was 7.4 pg/mL (10-50). DNA sequencing determined she was a compound heterozygote for mutations in GALNT3. Treatment with niacinamide and acetazolamide decreased TmP/GFR and serum phosphate, which was paralleled by a decrease in serum C-terminus FGF23.

CONCLUSIONS

This case broadens the spectrum of phenotypic and genotypic features of FTC/HHS and suggests treatments to decrease renal phosphate reabsorption in the setting of a low intact FGF23.

Neutral phosphate-induced renal tubular metabolic alkalosis

A severely burned patient receiving neutral phosphate supplement developed renal tubular alkalosis. This phenomenon is compared with the results of experimental observations on animals, reported in the literature. The physiologic mechanism, including the possible role of parathyroid hormone, is illustrated.

Inhibition of bicarbonate absorption by peptide hormones and cyclic adenosine monophosphate in rat medullary thick ascending limb

In vitro microperfusion experiments were performed to examine the effects of peptide hormones on bicarbonate and ammonium transport by the medullary thick ascending limb (MTAL) of the rat. Arginine vasopressin (AVP; 2.8 X 10(-10) M in the bath) reduced bicarbonate absorption by 50% (from 7.8 to 3.7 pmol/min per mm). AVP caused a similar reduction in bicarbonate absorption in tubules perfused with 10(-4) M furosemide to inhibit net NaCl absorption. Glucagon (2 X 10(-9) M in the bath) also reduced bicarbonate absorption (from 11.7 to 7.6 pmol/min per mm). The inhibition of bicarbonate absorption could be reproduced with either exogenous 8-bromo-cAMP or forskolin. With 8-bromo-cAMP (10(-3) M) in the bath, addition of vasopressin to the bath did not significantly affect bicarbonate absorption. PTH significantly inhibited bicarbonate absorption, but the extent of inhibition was less than that observed with either AVP or glucagon. Vasopressin had no effect on net ammonium absorption in MTAL perfused and bathed with 4 mM NH4Cl. These findings indicate that: (a) vasopressin, glucagon, and PTH directly inhibit bicarbonate absorption in the MTAL of the rat; (b) this inhibition occurs independent of effects on net NaCl absorption and appears to be mediated in part by cAMP; and (c) HCO3- and NH4+ absorption can be regulated independently in the MTAL.

Regulation of the renal Na+-H+ exchanger by protein phosphorylation

Starting from observations in intact cells and extending to studies in native membranes and solubilized membrane proteins, a significant body of evidence has been accumulated to indicate that some of the short-term regulatory influences on the Na+-H+ exchanger in the apical membrane of the proximal convoluted tubule act via protein phosphorylation mediated by specific protein kinases. Protein phosphorylation mediated by PKA inhibits the Na+-H+ exchanger while that mediated by PKC stimulates activity. The effect of PKA and PKC on the Na+-H+ exchanger in native membranes and in solubilized brush border membrane proteins appears to be consistent with most of the published observations in intact cells. Further studies using solubilized, renal brush border membrane proteins indicated that protein phosphorylation mediated by CaM-kinase II inhibited the activity of the Na+-H+ exchanger. The physiologic significance of this observation in intact cells remains to be determined. It is hoped that the types of experimental approaches outlined in this review will yield additional insights into the structure of the Na+-H+ exchanger and to a clearer understanding of its physiologic regulation.

Calcium-regulating hormones modulate carbonic anhydrase II in the human erythrocyte

The effect of calcitonin (CT) and parathyroid hormone (PTH) on carbonic anhydrase (carbonate hydrolyase, EC 4.2.1.1.) activity was tested in human erythrocyte hemolysates and with purified carbonic anhydrases I and II. The most important effect was on carbonic anhydrase II: CT showed a 2-fold increase and PTH showed a 50% decrease of carbonic anhydrase activity. This effect was observed at low hormonal concentrations and suggests the importance of CT in regulating carbonic anhydrase activity in the two important sites of CO2 exchange, erythrocyte and lung.

Metabolic acidosis in patients receiving anticonvulsants

Blood pH, bicarbonate, PCO2, serum calcium, alkaline phosphatase and red cell carbonic anhydrase were measured in 37 selected patients receiving anticonvulsants. Patients with metabolic acidosis showed a high incidence of hypocalcemia with increased alkaline phosphatase and a significant reduction of carbonic anhydrase-B activity. High iPTH levels were found in 13 patients, but this was not correlated with acid-base balance status. Anticonvulsant drugs seemed to inactive carbonic anhydrase-B activity. Metabolic acidosis might be one of the factors causing a disturbance of calcium metabolism in these patients.

Effect of cholera toxin and prostaglandins on the rat choroid plexus in vitro

The present study was performed to measure the effects of cholera toxin (CT) and prostaglandins (PG) on choroid plexus cyclic AMP accumulation. Choroid plexuses, isolated from Sprague-Dawley rats, were incubated in Krebs-Ringer bicarbonate buffer containing either CT (10 microgram/ml), heat inactivated CT, prostaglandins, or appropriate controls. After a minimum incubation period of 60 min with CT, cyclic AMP accumulation (pmol/mg protein) in treated incubates (58.10 +/- 6.43, n = 6) was 4 times that in controls (14.-5 +/- 1.4, n = 6). This large increase was only seen when theophylline (10 mM) was in the incubation mixture 3--10 min prior to the end of the incubation period. The increase in cyclic AMP accumulation was dose responsive and was irreversible after 5 min of incubation. Of the 5 prostaglandins studied, only PGE2 resulted in an increase in cyclic AMP accumulation. Cyclic AMP levels were 29.64 +/- 2.5 in controls and 57.57 +/- 3.5 in plexus which had been incubated with PGE2 (20 microgram/ml) for 1 min, and this increase was dose responsive.

Prostaglandin-mediated hypercalcemia: a paraneoplastic syndrome

Evidence has been presented for prostaglandin-mediated hypercalcemia and bone resorption in malignancies of both, experimental animals and man. Occurence of hypercalcemia in cancer patients is known for a long time, but its pathogenesis has been poorly understood so far. Besides ectopic parathyroid hormone secretion by tumors, an osteoclast-activating factor released from leukocytes and direct bone destruction by tumor cells, prostaglandins of the E series have to be considered as one of the candicates involved in the pathomechanism of hypercalcemia and osteoclastic osteolysis in cancer patients. This new concept on the pathophysiology of cancer-associated hypercalcemia has implications for the diagnosis and management of this common complication of neoplastic disease.

Diuretics and the renal adenylate cyclase system

1 The relationship between the diuretic effectiveness and the effect on the renal adenylate cyclase of three diuretics, acetazolamide, frusemide and ethacrynic acid, was examined. The hypothesis that acetazolamide and parathyroid hormone (PTH), inhibit renal carbonic anhydrase by a cyclic adenosine 3',5'-monophosphate (cyclic AMP)-dependent mechanism was also tested.2In vitro, acetazolamide, frusemide and ethacrynic acid at high concentrations (10(-3)M) all produced some inhibition of basal and stimulated rat kidney plasma membrane adenylate cyclase. The effect of acetazolamide was much less than that of frusemide and ethacrynic acid. These plasma membrane effects were reproduced in studies of cyclic AMP formation in isolated kidney tubules of rats.3 Intravenous injections of acetazolamide did not change the total cyclic AMP content of the kidneys of rats killed by microwave irradiation.4 Acetazolamide produced a diuresis in the rat and a slight inhibition of the antidiuretic effect of Pitressin. Frusemide produced a diuresis and greatly reduced the antidiuretic response to Pitressin. Ethacrynic acid was ineffective as a diuretic in the rat and actually enhanced the antidiuretic response to Pitressin.5 In investigating the possible influence of diuretics and PTH on the activity and state of phosphorylation of carbonic anhydrase it was found that: there was no correlation between the ability of diuretics to inhibit carbonic anhydrase activity and to inhibit carbonic anhydrase phosphorylation; neither PTH nor cyclic AMP (in the presence of adenosine triphosphate, Mg(2+), K(+) and incubation at 37 degrees C) inhibited rat cortex homogenate carbonic anhydrase activity.6 It seems unlikely that any of the tested diuretics exerts its pharmacological effect by means of changes in kidney cyclic AMP metabolism.

Failure of parathyroid hormone and cyclic AMP to inhibit renal carbonic anhydrase

It has been suggested that the parathyroid hormone and cyclic AMP produce their bicarbonaturic effects through inhibition of renal carbonic anhydrase. In the present study, the incubation of renal carbonic anhydrase with parathyroid hormone or cyclic AMP in presence of ATP, Mg++ and K+ ions, did not produce any inhibition of the enzyme when the pH of the solution was maintained above 7. It is concluded, that parathyroid hormone and cyclic AMP produce urinary bicarbonate excretion by a mechanism independent of carbonic anhydrase inhibition.

Influence of bicarbonate on parathyroid hormone-induced changes in fluid absorption by the proximal tubule

Segments of the proximal tubule of the rabbit kidney were perfused in vitro in order to examine the influence of bicarbonate on the reduction in fluid absorption that occurs following the acute administration of parathyroid hormone (PTH). Studies were performed using either normal ultrafiltrate as perfusion fluid and normal rabbit serum as bath or low bicarbonate ultrafiltrate as perfusion fluid and low bicarbonate rabbit serum as bath. Low bicarbonate fluids were prepared by replacement of bicarbonate with chloride. In the presence of normal concentrations of bicarbonate, the addition of PTH to the bath (1 U/ml) resulted in a decrease in the fluid absorption rate (Jv) from 1.13 +/- 0.08 to 0.60 +/- 0.04 nl/mm X min (p is less than 0.001) in 23 convoluted segments and from 0.64 +/- 0.05 to 0.46 +/- 0.05 nl/mm - mm (P less than 0.01) in ten straight portions. Simultaneous with the PTH-induced reduction in Jv, the chloride concentration in the collected fluid changed from 119.0 +/- 2.0 to 113.4 +/- 1.1 mEq/liter (P less than 0.01) in the pars convoluta and from 117.7 +/- 0.6 to 114.0 +/- 1.9 mEq/liter (P less than 0.01) in the pars recta. However, there was no change in the net flux of chloride which averaged 42.58 +/- 5.00 pEq/mm - min during the control periods. Additional studies were performed in eight convoluted segments during perfusion on a randomized basis with low bicarbonate fluids as well as during perfusion with fluids having normal levels of bicarbonate. As before, in the presence of normal levels of bicarbonate, PTH reduced Jv from 1.16 +/- 0.15 to 0.68 +/- 0.07 nl/mm - min (P less than 0.001) and the chloride concentration in the collected fluid ([Cl]o) from 118.6 +/- 2.9 to 111.6 +/- 1.3 mEq/liter (P less than 0.005). Substitution of low bicarbonate fluids for normal bicarbonate fluids resulted in a decrease in Jv from 1.16 +/- 0.15 to 0.74 +/- 0.10 nl/mm - min (P less than 0.001). In the presence of low bicarbonate fluids, the addition of PTH resulted in no further decrease in Jv (0.74 +/- 0.10 vs. 0.72 +/- 0.10 nl/mm - min). These data indicate that in the proximal tubule the PTH-induced reduction in fluid absorption may be mediated by changes in bicarbonate absorption.

Bicarbonate reabsorption in the dog with experimental renal disease

Renal bicarbonate reabsorption (expressed per unit of glomerular filtration rate, GFR) has been reported to be diminished in uremic man and uremic rats. Both the increases in parathyroid hormone concentrations and in natriuretic forces have been considered to play a role in this change. The increased kaliuresis per nephron observed in chronic uremia could theoretically also contribute to inhibition of bicarbonate reabsorption. Despite the common use of normal dogs in studying bicarbonate reabsorption and of uremic dogs in studying alterations of renal function in disease, few studies of bicarbonate reabsorption in uremic dogs have been performed. In the present studies we have examined bicarbonate reabsorption in normal dogs and in dogs with experimental renal disease using a conventional bicarbonate titration technique. In unanesthetized normal dogs, the threshold for bicarbonaturia was 24.8 mEq/liter of GFR. A maximal reabsorptive rate (Tm/GFR) of 34.0 mEq/liter of GFR was obtained. In a second group of dogs, GFR was decreased to one-fifth normal. FENa was increased 16.9-fold over normal values: UKV/100 GFR and FEP were increased 5.8-fold and 10.9-fold, respectively. The threshold for bicarbonaturia in these dogs was increased to 30.5 mEq/liter of GFR and the maximal reabsorptive rate was increased to 41.2 mEq/liter of GFR. Thus, the capacity to reabsorb bicarbonate was increased despite the presence of high fractional excretion rates for sodium, potassium and phosphate. This increased reabsorptive capacity could not be accounted for by the effects of other known determinants of bicarbonate reabsorption.

Effect of parathyroid hormone on phosphate reabsorption in the presence of acetazolamide

The hypothesis that parathyroid hormone and carbonic anhydrase inhibitors have a common mechanism or site of action on phosphate reabsorption by the renal tubule was tested by administration of parathyroid hormone in the absence and presence of acetazolamide in thyroparathyroidectomized dogs. Re-collection micropuncture and electron probe microanalysis methodologies were utilized. In the absence of acetazolamide, parathyroid hormone increased fractional delivery of phosphate (and volume) from the proximal tubule from 25 +/- 2 to 38 +/- 3%, P less than 0.025, and increased fractional phosphate excretion from 3.8 +/- 1.2 to 19.9 +/- 3.7%, P less than 0.005 (eight dogs). In the presence of acetazolamide, parathyroid hormone increased fractional delivery of phosphate (but not volume) from the proximal tubule from 50 +/- 4 to 58 +/- 5%, P less than 0.025, and increased fractional excretion of phosphate from 8.7 +/- 2.2 to 31.0 +/- 4.3%, P less than 0.001 (12 dogs). Thus, the effects of parathyroid hormone were additive to the effects of maximal inhibition of carbonic anhydrase indicating that parathyroid hormone and carbonic anhydrase inhibitors have different mechanisms of action on phosphate reabsorption by the renal tubule. In addition, phosphate reabsorption beyond the point of micropuncture in the late proximal tubule was much more markedly inhibited by parathyroid hormone than by acetazolamide.

Acute effects of parathyroid hormone on proximal bicarbonate transport in the dog

Re-collection micropuncture and simultaneous clearance studies were performed in thyroparathyroidectomized (TPTX) dogs to evaluate the effects of the acute administration of parathyroid hormone (PTH) on bicarbonate reabsorption. The i.v. administration of PTH from 74 to 94 U/hr reduced proximal fractional reabsorption (FRHCO3) from 0.28 +/- 0.03 to 0.14 +/- 0.03 (P less than 0.005) and absolute bicarbonate reabsorption (THCO3) from 556 +/- 126 to 255 +/- 73 pmoles/min (P less than 0.05), whereas there were no changes in PCO2 (37.0 +/- 1.4 leads to 37.2 +/- 1.4 mm Hg, P greater than 0.90), plasma bicarbonate (PHCO3) (18.5 +/- 0.4 leads to 18.3 +/- 0.4, P less than 0.60), single nephron glomerular filtration rate (102.2 +/- 15;9 leads to 90.1 +/- 10.3 nl/min, P greater than 0.40), serum ultrafilterable phosphate concentration (SUFp) (1.71 +/- 0.13 leads to 1.83 +/- 0.12 mmoles/liter, P greater than 0.25), or serum ultrafilterable calcium (SUFCa) (1.85 +/- 0.05 leads to 1.88 +/- 0.05 mEq/liter, P greater than 0.60). PTH also reduced proximal fractional fluid (and sodium) reabsorption (0.40 +/- 0.04 leads to 0.28 +/- 0.08, P less than 0.05) while TFHCO3 did not change (20.5 +/- 0.4 leads to 20.8 +/- 0.4 mmoles/liter) indicating a rejection of bicarbonate proportional to the inhibition in tubular fluid transport. The invariable reduction in proximal bicarbonate reabsorption did not uniformly result in an increased urinary bicarbonate concentration.

Renal phosphate reabsorption in the rat: effect of inhibitors

The maximum rate of inorganic phosphorus (P1) reabsorption (Tmp1) was measured during phosphate infusion in chronically parathyroidectomized rats. In either initially hydropenic rats or others which underwent moderate preliminary saline loading, Tmp1 tended to rise during early phosphate infusion and then decline, so that steady-state Tmp1 values were similar to the P1 reabsorption rates at the animals' own evelated endogenous plasma P1 concentrations. These results indicate that the "self-depression" of Tmp1 during phosphate infusion is not dependent upon the progressive stimulation of endogenous parathyroid hormone secretion. Parathyroid hormone and dibutyryl cyclic adenosine monophosphate (AMP) decreased Tmp1 substantially, whereas acetazolamide inhibited P1 reabsorption at lower filtered P1 loads but did not diminish Tmp1.