ORAL PHOSPHATE LOAD UNMASKS UNDERLYING

RENAL PHOSPHATE LEAK IN SYMPTOMATIC AND

ASYMPTOMATIC MEMBERS OF FAMILY WITH

IDIOPATHIC HYPERCALCIURIA

DAVID JUAN, M.-D.

From the Department of Medicine, Northwestern University Medical School, Chicago, Illinois

ABSTRACT-The effects of a 2 Gm oral phosphorus load in a family with idiopathic hypercal- ciuria (IH) consisting of 3 symptomatic (DT, CS, DS) and 2 asymptomatic (MS, PD) members were compared with 12 normal control subjects. Biochemical parameters measured included: total and ionized calcium, phosphorus, intact and carboxyl-terminal parathyroid hormone, urinary calcium, phosphorus, and sodium. Water loading had no effect on these parameters. After the phosphorus load, serum phosphorus rose 1.60 mg/dl in the control subjects but only 1.34 mg/dl in the IH family at the end of one hour. Basal tubular reabsorption of phosphate (TRP) were comparable in the control subjects and the IH family. After the phosphorus load, the TRP in the control subjects fell (average 9.2 %) accompanied by a significant (P < 0.02) rise in the carboxyl-terminal parathyroid hormone. Except for DT who had been taking hydrochlorothiaxide, the TRP fell dramatically in the rest of the IH family (DS 25 % , CS 12 % , PD 26 % , MS 50 %) in the absence of any perturbations in either the intact or carboxyl-terminal parathyroid hormone. A hypocalciuric effect was observed in the IH family but not in the control subjects after phosphorus loading. The oral phosphorus challenge unmasked a parathyroid hormone independent renal phosphate leak in both sympto- matic and asymptomatic members in a family with idiopathic hypercalciuria.

A tendency to hypophosphatemia in patients with idiopathic hypercalciuria (IH) was first noted by Albright and his colleagues in 1953.’ Since then, the role of a primary renal phos- phate leak as a pathogenetic mechanism in IH has received much attention.2-5 However, low serum phosphorus values in the fasted state are not an invariable finding in IH. Investigators like Pak and Wilkstrom have failed to show sig- nificant differences in serum phosphorus be- tween patients with calcium nephrolithiasis and age-matched controls.6 Tschope, Ritz, and Schmidt-Gayk7 provided cogent arguments against the existence of a simple fixed perma- nent tubular phosphate leak in patients with absorptive hypercalciuria. They showed evi- dence for the maintenance of serum phosphate in the IH patients during fasting.

The genetic influence in the pathogenesis of calcium nephrolithiasis is not well understood. Resnick, Pridgen, and Goodman8 showed a polygenic influence among kidney stone form- ers with a lesser risk for females. In addition, Thomas and colleagues9 showed that the risk of developing calcium oxalate stone among the first degree relative is four times higher than those with calcium phosphate stone. If the genetic factor is important, we reasoned that both symptomatic and asymptomatic relatives might show disturbances in mineral metabo- lism only when challenged with either a cal- cium or phosphorus load.

There is a paucity of data concerning the metabolic status of asymptomatic relatives of patients with IH. The author was fortunate to have encountered such a family in which three

22 UROLOGY i JULY 1985 / VOLUME XXVI, NUMBER 1

members were totally asymptomatic at the time of this study. When given an oral calcium chal- lenge, both symptomatic and asymptomatic members of this family showed higher calcemic and phosphatemic responses accompanied by a fall in carboxyl-terminal parathyroid hormone and urinary cyclic AMPlO The purpose of the present study is to compare the responses in nor- mal subjects with members of this family with IH when challenged with an oral phosphate load.

Material and Methods Seven males and 7 females (aged 21-51 years;

mean 30) participated in this study. Twelve sub- jects participated in the phosphate loading study and 2 in the water loading study. They were all healthy and on regular diets with nor- mal intake of calcium and phosphorus (Ca SOO- 1,000 mg/day; P 800-1,200 mg/day). They were not taking medications or vitamins known to affect mineral metabolism.

The family tree is shown in Figure 1. The eldest son, LS, lived in another state and was reported to have had calcium oxalate stones currently being treated with a thiazide diuretic. The mother of this family, DT, and her other 2 sons (DS, CS) all had documented calcium oxa- late stones in the past. CS and DS have had three kidney stones each in the last five years but have not been treated with any medica- tions. MS, the only child of CS, had recurrent unexplained lower abdominal pains. DT’s two daughters, PD and RS, have never had calcium stones. Because RS had inflammatory bowel disease, she was excluded from this study. DT was treated with hydrochlorothiazide (25 mg a day) which was discontinued twenty-four hours before the phosphorus study. None of the mem- bers of the IH family had ingested vitamin D or been known to have disorders of calcium me- tabolism such as sarcoidosis, hypercalcemia, os- teoporosis, or hyperthyroidism . Their physical examinations were completely normal. Daily intake of calcium in this family was estimated to be between 600-1,000 mg a day and phos- phorus intake of 800-1,000 mg a day. Based on two to four twenty-four-hour urine collections, urinary excretion of calcium in the IH family was: DT 164 mg/day; CS 363 mg/day; DS 371 mglday; PD 270 mg/day; RS 308 mg/day, and MS 250 mg/day, A consent form was obtained from each subject before any study.

All subjects were told to take nothing except water by mouth after 7 PM the previous eve-

DT (5SYrn)

-.

0 /

A L v d A :&Y/a DS

mYID) ZSYltN EY/D) Pi& Y/C4 MS (BY/o)

FIGURE 1. Pedigree of family with idiopathic hy- percalciuria: circle = female; square = male; solid = symptomatic; open = asymptomatic.

ning. After arriving in the laboratory, they were told to void and then to start drinking three cups of tap water every hour until the comple- tion of the study. Urine was collected at 11:OO and 13:00 hr (0 and 120 minutes) after the wa- ter or phosphorus load. Blood was drawn at ll:OO, 12:00, and 13:00 hr (0, 60, and 120 min- utes) after the water or phosphorus load. The 2- Gm phosphorus load consisted of a mixture of NaHB PO, = Nae HPOr (9:l) in 200 ml of wa- ter. All subjects had to lie flat for thirty minutes prior to each venipuncture. In between blood samples, their activities were limited to bed, chair, and bathroom.

Total calcium (TCa) was measured by atomic absorption spectrophotometry,” ionized cal- cium (Ca+ ‘) by the calcium specific electrode (Applied Medical Technology, Palo Alto, CA), and serum phosphorus (Pi) by the Fiske and Subbarrow method. l2 Both intact and carboxyl- terminal parathyroid hormones (PTHi PTH,) were measured by Dr. Charles Hawker at the Upjohn Company (Kalamazoo, MI) by es- tablished methods. l3 The reported interassay coefficients of variation for both PTH assays are between 7 to 12 per cent.

In the tables and figure, results are given as mean f standard error of the mean (SEM). Student’s t test for paired observations was used to analyze for statistical significance. The tubu- lar reabsorption of phosphate (TRP) is calcu- lated as: TRP = (1 - Urinary phosphate x

Serum creatinine/Serum phosphate x Urinary creatinine) x 100 (%). Normal ranges reported are 73 to 98 per cent.’

Results

As shown in Table I, water loading had caused TRP to rise slightly in 2 normal subjects. In normal control subjects given the phosphorus load, TRP uniformly fell in all averaging 9.2 per cent. In the IH family, TRP fell dramati- cally in both symptomatic (CS 12 % ; DS 25 %) and asymptomatic members (PD 26% ; MS

UROLOGY I JULY 1985 / VOLUME XXVI, NUMBER 1 23

TABLE I. Effect of water and 2-Gm phosphorus loads on tubular reabsorption of

phosphate in 11 normal subjects and family with idiopathic hypercalciuria

Basal After Subject Load (%) Load (%)

Normal 1 Hz0 80.1 82.2 2 Hz0 85.4 90.3 3 P 88.4 72.6

: P P 95.6 87.2 87.9 74.1 6 P 96.1 81.1 7 P 93.8 88.1 8 P 92.7 86.6 9 P 89.9 83.4

10 P 90.6 82.4 11 P 97.0 91.8

Idiopathic hypercalciuria cs P 88.0 76.0 DS P 95.4 70.0 PD P 94.5 68.5 DT P 94.0 90.0 MS P 95.0 45.0

50 %). DT who had been taking hydrochlo- rothiazide showed a suboptimal fall of 4 per cent.

After the phosphorus load, mean serum phos- phorus rose 1.60 mg/dl in the normal subjects and 1.34 mg/dl in the IH family at the end of one hour. At the end of the second hour, serum phosphorus in both groups tended to fall to- ward baseline values. The phosphorus values for MS at 0 hr, 1 hr, and 2 hrs were 7.4,8.3, and 6.3 mg/dl, respectively.

Basal total and ionized calcium values were comparable in both groups (Table II). After the phosphorus load, there was a tendency for both the total calcium (A 0.15 mg/dl P < 0.05) and ionized calcium (A 0.46 mg/dl at 1 hr; 0.22 mg/ dl at 2 hr P < 0.02) to drop in the normal sub- jects but not in the IH family. Basal calcium ex- cretion was higher in the IH family (0.18 mg/mg creatinine) than in the control subjects (0.09 mg/mg creatinine (Table III).

After the phosphorus load, a distinct hypo- calciuric effect was observed in the IH family (P < 0.05) but not in the control subjects. Basal intact parathyroid hormone levels in the control subjects and the IH family were comparable (Table II). Even though there was a tendency for the intact parathyroid hormone values to rise in both the control subjects as well as in the family with IH, these changes were not signifi- cant. Basal carboxyl-terminal parathyroid hor- mone levels were slightly higher in the control subjects than in the IH family. After the phos- phorus load, there was a significant rise in the carboxyl-terminal parathyroid hormone values at 1 hr and 2 hrs (63 and 81 pg Eq/ml, P < 0.02) in the control subjects but not in the IH family. The control subjects had higher sodium excretion rates than the IH family (Table III). Phosphorus loading led to a rise in sodium ex- cretion rates in both groups. However, these changes were not statistically significant.

Comment Low serum phosphorus in the fasted state is

not an invariable finding in patients with idiopathic hypercalciuria. Wilkstroms and co- workers showed no significant differences in the

TABLE II. Effect of oral phosphorus load on calcium, phosphorus, intact and carboxyl-terminal parathyroid hormone in 12 normal subjects and family with idiopathic hypercalciuria*

Biochemical Parameters Group 0

Time (Min) ‘/ 60 120

TCa (mg/dI) (8.8-10.0)

g;++; @g/dl) . *

P b-4~) (2.5-4.5) PTHi (pgEq/ml) (163-347)

PTH, (pgEq/ml) (150-375)

Normal IH Normal IH Normal IH Normal IH Normal IH

9.47 + 0.07 9.43 + 0.09 4.38 + 0.07 4.20 f 0.13 3.56 + 0.18 4.09 + 0.33 232 f 10 225 + 14 249 * 15 226 + 6

9.49 f 0.08 9.37 + 0.09 3.92 -t 0.30 4.10 If: 0.07 5.16 + 0.25**** 5.43 f 0.55*** 243 f 8 239 f 9 312 + 21*** 245 + 24

9.28 Ii 0.10** 9.44 f 0.12 4.16 f 0.05*** 4.32 f 0.10 4.90 f 0.16**** 4.79 f 0.31 242 f 6 234 + 9 330 + 19*** 228 + 38

*Serum phosphorus values of MS not included. tAl1 values are mean f SEM in hormal subjects and the family with IH. Subscripts indicate that using paired Student’s t test, these

values are different from the control period: **p c 0.05, ***p -z 0.02, ****p < 0.01.

24 UROLOGY / JULY 1985 / VOLUME XXVI, NUMBER 1

TABLE III. Effect of oral phosphorus load on calcium and sodium excretion in 9 normal subjects and family with idiopathic hypercalciuria’

Before Load After Load Uca/cr Uca/cr

Subject (mg/mg) cllE,N/“,i*) (mg/mg) (pE$in) Normal 0.09 f 0.02 137.8 f 19.3 0.06 f 0.02 151.8 f 24.8 IH 0.18 f 0.03 83.5 f 10.4 0.09 f 0.02** 87.3 i 13.9

*MS not included. All values are mean + SEM in normal subjects and in the family with idiopathic hypercalciuria. Subscript indicates that using the paired Student’s t test the value is different from the control period;-•*P < 0.05.

serum phosphorus values between stone form- ers and age-matched controls in a population survey of middle-aged men. The family with IH in this study had higher fasted serum phos- phorus values than control subjects (Table II), evidence in support of Tschope’s argument that patients with IH do not have a fixed permanent tubular phosphate leak.’ After the phosphorus load, normal subjects showed a higher rise in serum phosphorus (mean 1.6 mg/dl) than the IH family (mean 1.3 mg/dl). Four of 5 subjects in the IH family showed a dramatic fall in the TRP (12 to 50 % ) when compared with the con- trol subjects (mean 9.2%) when challenged with a phosphate load (Table I). DT who had been taking hydrochlorothiazide showed a sub- optimal response to the phosphate load (TRP 4%).

The effect of hydrochlorothiazide on phos- phate excretion is unique in that phosphate ex- cretion occurs despite extracellular fluid vol- ume contraction.” It has been reported that prolonged treatment of patients with IH with this drug leads to persistent reduction in TmP/ GFR and a fall of serum phosphorus as seen in DT, and these effects may persist after the diuretic is discontinued.i5 Even though DT stopped taking the drug for twenty-four hours, this may not have been long enough to obviate its renal tubular effect. Nonetheless, the subop- timal response to the oral phosphorus is difficult to explain.

The most dramatic change in TRP occurred in the son of CS, the eight-year-old MS. Children are reported to have higher serum phosphate values and a relatively decreased phosphaturia.le MS’s basal serum phosphate value of 7.4 mg/dl was appropriate for his age. However, when challenged with the phos- phorus load, his serum phosphate rose by 0.9 mg/dl only at the end of the first hour followed by a precipitous drop to a level below baseline value (6.3 mg/dl). This fall in serum phosphate was accompanied by a dramatic decline in the

UROLOGY I JULY 1985 I VOLUME XXVI, NUMBER 1

TRP from 95 to 45 per cent (Table I). Whether or not these changes are peculiar to him as an inherent trait of the IH or a characteristic com- mon to children of the same age and sex is un- clear at the present time.

In addition to serum phosphorus, parathy- roid hormone is another important regulator of TRF? Could the observed changes in TRP be ex- plained on the basis of phosphorus-induced per- turbation of parathyroid hormone levels? As shown in Table II, the oral phosphorus load in- duced no appreciable changes in the intact parathyroid hormone levels in the normal sub- jects or IH family. Since intact parathyroid hor- mone has a short half-life (< 5 min), it is not surprising to find no change in this study since blood was sampled every sixty minutes. Reiss and coworkersl’ gave a l-Gm phosphorus load by mouth to normal subjects and sampled blood every ten minutes and reported definite rise in biologically active parathyroid hormone. How- ever, the carboxyl-terminal parathyroid hor- mone levels rose significantly in the normal control subjects but not in the IH family when given the phosphorus load (Table II).

It is known that the liver as well as the kidney possesses specific proteases that can cleave in- tact parathyroid hormone into inactive carboxyl-terminal fragmentsl*Je The changes seen in this study most likely reflect accumula- tion of inactive fragments. Thus, the fall in TRP in the normal control subjects after the phosphorus load was in part due to phosphorus- induced parathyroid hormone release. In the IH family, on the other hand, carboxyl- terminal parathyroid hormone levels remain unchanged after the phosphate challenge.

It has been well established that the mecha- nism by which oral or parenteral phosphorus lowers serum calcium is by the formation of CaHP04 complex. eo Kaplan and coworkerszl reported that a rise in 1 mg/dl of serum phos- phate was accompanied by a drop of 0.13 mg/ dl of ionized calcium in normal and uremic

25

dogs. In our normal control subjects, a rise of 1.60 mg/dl in serum phosphate was accompa- nied by a fall of 0.26 mg/dl of ionized calcium (Table II). Even though the rise in serum phos- phorus in the family with IH was slightly less than the control subjects (1.30 vs 1.60 mgldl), ionized calcium failed to drop after the phos- phorus load. Thus, the intact and carboxyl- terminal parathyroid hormone levels in the IH family showed no changes. Nonetheless, the TRP in the IH family fell dramatically after the phosphorus load in the absence of any pertur- bation of parathyroid hormone levels. Unlike the normal control subjects, the change in TRP in the IH family was due to a parathyroid hor- mone independent mechanism, namely, renal phosphate leak.

The hypocalciuric effect of the oral phos- phorus load was most evident in the IH family (Table III). Van derBerg and co11eaguesz2 showed that the hypocalciuric effect of chronic oral phosphate treatment involved decreased synthesis of 1,25 dihydroxy-vitamin D3 inde- pendent of parathyroid function. Since it has been shown repeatedly in man and animals that it would take many hours (> 4 hr) before any detectable rise in serum 1,25 dihydroxy-vitamin D3 could be demonstrated after parathyroid hormone administration, it is unlikely that the acute hypocalciuric effect seen in this study in- volved the vitamin D endocrine system,23

In conclusion, this study shows that basal twenty-four-hour urine is useful in detecting hypercalciuria even in the asymptomatic rela- tives of patients with documented IH. Since both the basal excretion of phosphate or the TRP in symptomatic and asymptomatic sub- jects with IH are indistinguishable from control subjects, the inherent tendency to waste phos- phate could only be unmasked by the oral phos- phorus challenge. This parathyroid hormone independent renal phosphate leak eventually leads to phosphate depletion and hypercalciuria with calcium nephrolithiasis. The clinical im- plication of this study is that once the asympto- matic relatives are identified prophylactic measures could be instituted to delay or even avert the development of renal stone.

Clinical Pharmacology Center 303 E. Superior Street

Chicago, Illinois 60611

ACKNOWLEDGMENTS. To Dr. J. E. Zerwkh and Dr. J. Arruda for their invaluable assistance during the prepara-

tion of this manuscript. This research was supported by the Douglas Foundation of Toledo, Ohio.

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