AMERICAN Vol. 230,

JOURNAL OF PHYSIOLOGY No. 3, March 1976. Printed

Intrarenal

in U.S.A.

calcium

in phosphate

J. L. CUCHE, C. E. OTT, G. R. MARCHAND, Nephrology Research Laboratory, Departments Foundation, Rochester, Minnesota 55901

CUCHE, J. L., C. E. OTT, G. R. MARCHAND, J. A. DIAZBuxo, AND F. G. KNOX. Intrarenal calcium in phosphate handling. Am. J. Physiol. 230(3):790-796. 1976. -The intrarenal role of plasma ionized calcium (Cai) on fractional phosphate excretion (FE PO,) was investigated in dogs with control of parathyroid hormone (PTH). In series 1, acute thyroparathyroidectomy was immediately followed by a constant infusion of bovine PTH (0.01 U/kg per min). Subsequent calcium chloride infusions increased Cai and plasma phosphate and decreased the percentage of ultrafiltrable phosphate. A 20% increase in Cai significantly increased FE PO, by +3.82 t_ 0.97% (P < 0.01) when infused intravenously and by +2.62 t 1.06% (P < 0.05) when infused in the renal artery. In contrast, a 75% increase in Cai did not significantly change FE PO,. In series 2, dogs were thyroparathyroidectomized 18 h before experiments, and no PTH infusion was initiated. A bolus of bovine PTH (30 U/kg) increased FE PO, + 8.9 t 0.9% (P < 0.001) in hypocalcemic dogs, +19.1 t 4.4% (P < 0.001) in normocalcemic dogs, and + 15.5 t 1.5% (P < 0.001) in hypercalcemic dogs. We conclude that increases in plasma calcium potentiate the phosphaturic effect of PTH. This potentiating effect is attenuated in marked hypercalcemia by superimposed hemodynamic and/or metabolic changes.

parathyroid phosphate;

hormone; fractional

plasma excretion

ionized calcium; ultrafiltrable of phosphate; calcium chloride

THE RENAL REGULATION of phosphate excretion is a multifactorial system. Three factors are closely interrelated - parathyroid hormone (PTH), plasma phosphate, and plasma calcium. The intrarenal effects of the first two factors are well known (13, 14), but the possible role of the third remains unclear. That calcium has an indirect role in renal phosphate regulation, through regulation of PTH release, is well established (14). Although increases in plasma calcium should decrease PTH release and therefore decrease phosphate excretion, data conflicting with this prediction have been reported in man and monkey (2, 11, 18). Levitt et al. (11) infused calcium gluconate in five normal men and observed an increase in fractional phosphate excretion in each subject. This increase could be due to an osmotic effect of calcium gluconate, to the observed 25% increase in plasma phosphate concentration, or to a direct effect of calcium (4, 12). A direct effect of calcium to increase phosphate excretion received support from a study of hypoparathyroid patients in which a chronic calcium chloride infusion increased phosphate excretion, although plasma phosphate concentration decreased 25% (5). On the other hand, infusions of calcium chloride in the renal artery of dogs decreased phosphate

handling

J. A. DIAZ-BUXO, AND F. G. KNOX of Physiology and Medicine, Mayo Clinic and

excretion; however, glomerular filtration decreased during the infusion (10). In vitro studies suggest that calcium is a prerequisite for the physiologic effect of PTH (15). In renal tubule preparations, PTH-induced gluconeogenesis is higher in the presence of calcium than in the presence of EGTA. On the other hand, calcium inhibits this response when concentrations are increased in the medium (15). This is in agreement with the concept that calcium can inhibit cyclic AMP formation both in vitro and in vivo (1, 3, 16,

19). In the present study, a possible intrarenal calcium in phosphate handling was investigated presence and absence of PTH.

role of in the

PROTOCOLS

Effects of Calcium Chloride Phosphate Excretion

Infusion

on Urinary

Effect of increased plasma calcium concentration when plasma PTH level is controlled. Plasma PTH level was controlled in dogs by a constant infusion of bovine PTH1 (0.01 U/kg per min) initiated immediately after surgical thyroparathyroidectomy. Plasma ionized calcium (Ca,> concentration was increased by an infusion of 25 pmol/min of calcium chloride, hereafter referred to as low dose. Values from three control clearance periods were compared to three experimental periods obtained 50 min after beginning infusion. In additional studies, to minimize any possible systemic effect, calcium chloride was infused into the renal artery. A 20-gauge curved needle was inserted against the flow of blood into the left renal artery, and a cannula was placed in the left renal vein for blood sampling. A probe for an electromagnetic flowmeter was placed on the left artery between its aortic origin and the 20-gauge needle. In this later experimental condition, 10 min were allowed after starting calcium infusion before three experimental periods were obtained. Two higher doses of calcium chloride also were studied. An intravenous infusion of 65 pmol/min was given to four dogs and one of 95 pmol/min was given to four others. Because responses to the two higher doses showed no significant differences, the data were pooled. Effect of increased plasma PTH concentration when plasma calcium level is controlled. The role of calcium l TCA extracts of bovine parathyroid kindly provided by Dr. Claude Arnaud.

glands

(25 U/mg>

were

790

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PLASMA

CALCIUM

AND

FRACTIONAL

PHOSPHATE

791

EXCRETION

in renal phosphate handling was investigated in absence of PTH and then in the presence of the hormone. In this protocol all animals were surgically thyroparathyroidectomized 18 h before experiments. On the next morning, two 15min collection periods were obtained during the hypocalcemic phase. These animals were either kept hypocalcemic or made normocalcemic or hypercalcemic; this was achieved by an infusion of calcium chloride (45 pmol/min) for either 60 or 150 min. Two collection periods were then obtained to measure urinary phosphate excretion and evaluate a possible direct role of calcium in the absence of PTH. Finally, a bolus of PTH (30 U/kg; bovine PTH came from the same batch for all dogs but one) was infused, and urinary phosphate was measured in three collection periods 20 min after the bolus.

tion rate, the effect of changes in renal hemodynamics on phosphate excretion was studied in five dogs with control of PTH concentrations. A clamp was placed on the left renal artery between an electromagnetic flowmeter probe and a needle for renal perfusion pressure recording. Dogs were volume loaded with Ca++-free Ringer solution (1 ml/kg per min for 20 min, then 0.5 ml/ kg per min for the next 40 min) and made hypercalcemic by --a sustained infusion of calcium chloride (65 ,umol/ min). Three lo-min control clearance periods were obtained. The renal artery was then progressively occluded, and 5-10 min after renal blood flow was reduced, three experimental periods were obtained. The clamp was then released, and 30 min were allowed before three postocclusion periods were begun. METHODS

Effects of Calcium Chloride Fraction of Phosphate

Infusion

on Ultrafiltrable

Since the percentage of ultrafiltrable phosphate is important in the calculation of fractional excretion of phosphate, the effect of plasma calcium on ultrafiltrable phosphate was studied in detail. Calcium chloride was infused in two groups of dogs, and both total and ultrafiltrable phosphate were measured. Six dogs were acutely thyroparathyroidectomized, and a replacement infusion of bovine PTH (0.01 U/kg per min) was started immediately. After two control samples were obtained during sodium chloride infusion, a 45-pmol/min calcium chloride solution was infused and blood samples were drawn each 20 min for the next 2 h. Six other dogs were surgically thyroparathyroidectomized the day before experiments. Eighteen hours later, blood samples were obtained during sodium chloride infusion for control measurements and each hour for the next four h during an infusion of 45 pmol/min of calcium chloride. Ultrafiltrable phosphate was obtained by centrifugation of plasma and placed in a filter cone (Amicon) in an atmosphere of 5% CO, and 95% 0,. The ultrafiltrate was then processed for phosphate measurement. Effect of Sodium Phosphate Phosphate Excretion

Infusion

on Fractional

Since acute hypercalcemia is associated with an increase of plasma phosphate concentration, the effect of sodium phosphate infusion on fractional phosphate excretion was investigated in two groups of hydropenic dogs: plasma PTH level was controlled in the first group (acute parathyroidectomy plus bovine PTH infusion), and PTH was absent in the second (surgical parathyroidectomy 18 h before experiments). After two 15-min collections during sodium chloride infusion, sodium phosphate (0.25 M, 1 ml/min) was infused, and three additional collections were obtained. Plasma phosphate concentration was increased comparable to that during calcium chloride infusion. Effect of Decreased Renal Phosphate Excretion

Blood

Since high doses of calcium

Flow

on Urinary

chloride

decreased

filtra-

The dogs were fed a sta ndard pell et diet providing approximately 1% calci urn and 0.8% phosphate. They -were allowed- free access to water, and food was withThey 18 h before the experiment. held approximately were anesthetized with sodium pentobarbital (30 mg/kg body wt) and the trachea was intubated. Cath .eters were placed in a jugular vein and in a femoral artery and vein for infusions and blood sampli .ng. Ureters were cannulated. Sixty minutes before the initial clearance periods, all dogs received a priming dose of inulin and PAH, and a maintenance infusion was administered at 1 ml/min throughout the experiments to maintain inulin and PAH concentrations of 0.25 and 0.01 mg/ml, respectively. Blood samples were collected at the midpoint of urine collections. Blood for ionized calcium was collected with a syringe and then injected into heparinized 5-ml Vacutainers (heparin content, 143 U). Plasma was withdrawn anaerobically into a tuberculin syringe through the rubber stopper of the Vacutainer after centrifugat-ion at 3,000 rpm for 10 min. For measurements of plasma ionized calcium, a calcium activity electrode ( 20) was used as follows: 1) standards were prepared in Vacutainers containing the same amount of heparin as .mide was add .ed the samples; 2) no trypsin or triethanola weekly, and 3) to the standards, which were prepared the membrane was primed with pooled normal plasma before the daily standard curve was run. Plasma and urine concentrations were measured as follows: inulin by the anthrone method (6), PAH by the method of Harvey and Brothers (8), calcium by atomic absorption spectroscopy, and phosphorus by the method of Young cm . RESULTS

Effects of Calcium Chloride (UF) Fraction of Phosphate

Infusion

on Ultrafiltrable

A calcium chloride solution was infused in two groups of dogs. In the first group, animals were thyroparathyroidectomized 18 h before the experiment; in the second group, acute parathyroidectomy was followed immediately by a sustained infusion of bovine PTH. Table 1 shows that the increase of plasma calcium concentration

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792

CUCHE

1. Effect

TABLE

of calcium 18 h PTX

Time,

min

Protein, g/100 ml

Total

calcium, mM

chloride Dogs

(n,

infusion

on ultrafiltrable

fraction

of plasma

6)

Acutely

Phosphate s Total

PTX

phosphate Dogs

+ PTH

Replacement

2.78 +.07

6.35 k-12

2.44 LO7

1.55

+.lO

%

UF

Time,

Protein, g/100 ml

min

Total

calcium, mM

60

120

180

240

300

1.91 2.06

1.49 513

Before thyroparathyroidectomy 96.7 t7.1

1.86 2.11

chloride

97.7 k2.1

0

20

6.14

2.43

2.04

2.01

98.9

+.ll

509

+.lO

~08

k2.1

5.92

2.31 k.12

2.10

Cakium 91.9

~08

3.23 2.06

~08

5.77 k.14

3.65

2.61 k.12

2.38

92.6

+.ll

k.08

+6.8

5.70 ~23

3.89 +.lO

2.93 ~18

2.44 LO7

84.8 +5.4*

80

5.92

4.02 k.15

3.23

2.39 +.ll

75.8 k7.2-t

100

k.19

~26

2.69

6.09 ~23

~08

6.04 ~23

1.47 k.12

LO9

1.46 LO8

1.47 k.12

40

26.8

5.98 k.22

2.83 k.05

1.52

1.60 k.05

105.4

t.04

5.94

3.08 ~06

1.61 2.08

1.70

105.3

k.05

k5.5

1.73 k.05

k5.5

60

2.65

k.22

*P < 0.05.

Infusion

k3.5

97.9

3.29

1.78

~06

LO8

5.86 k.20

3.41

1.92

1.85

96.3

LO9

k.07

k.05

k3.3

5.84 ~18

3.70

2.08

1.89

91.5

+.lO

k.10

k.05

+3.8

3.76 k.13

2.31 ~18

2.13 LO8

k6.6

5.80

94.5

-tP < 0.01.

is associated with an increase of total phosphate concentration in plasma as reported by others (2, 7, 11,Zl) (Fig. 1). The increase of total phosphate concentration results in an increase of ultrafiltrable phosphate concentration. This increase, however, is progressively smaller than that of total phosphate, as shown by the decrease in the percentage ultrafilterability in both groups of dogs, but reaching a statistically significant level in chronic PTX dogs only. Decreased ultrafilterability of phosphate as a function of plasma calcium has been reported both in vivo and in vitro (9, 21). Figure 2 shows that the decrease in the percentage of ultrafiltrable phosphate is closely related to the calcium-phosphate product. This empiric correlation was used to estimate the ultrafiltrable phosphate from the total phosphate in dogs given calcium chloride infusion. Effects of Calcium Chloride Phosphate Excretion

100 k4.7

5.90

k.19 + SE. UF, ultrafiltrable.

~15.6

k.22

140 are means

101.8

1.44 k.07

chloride

120

Values

%

UF

Total mM

Sodium 0

(n, 6)

Phosphate

mM

6.25 2.15

ET AL.

on Urinary

Effects of increased plasma calcium concentration when plasma PTH level is controlled. The effects of increased plasma ionized calcium concentration during calcium chloride infusion are reported in Table 2. A lowdose intravenous infusion of calcium chloride increases both total and ionized calcium concentrations by about 20%. This increase is associated with a significant increase in absolute and fractional excretion of phosphate in 10 out of 12 dogs. Blood pressure was increased by +6.7 t 1.5 mmHg (P < O.Ol), but renal hemodynamics were unchanged; clearances of PAH and inulin were,

J

Y s

18 hr PTX

dogs

Q4-

2 2 3 2

3-

-2-

5

f =+0.973

T

1

I

I

I

I

I

2

3

4

5

6

Total calcium,

mM/L

1. Effect of calcium chloride infusion on plasma phosphate concentrations in dogs 18 h after thyroparathyroidectomy (18 hPTX dogs) or in acutely thyroparathyroidectomized animals given a constant infusion of PTH (PTX - PTH dogs). FIG.

2

b

110

r. .. PTX + PTH dogs 18hr PTX dogs y=111.9-2.45x 2

4

6

8

Calcium-phosphate

10

12

14

product

FIG. 2. Relationship between ultrafiltrable fraction of plasma phosphate (UFIT x 100) and product of total calcium and phosphate concentrations, each expressed as millimoles per liter.

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PLASMA

CALCIUM

2. Effect

TABLE

AND

FRACTIONAL

of calcium

PHOSPHATE

chloride

infusion

793

EXCRETION

on urinary Calcium

phosphate

excretion

in dogs with control

of PTH

Chloride Control

Low

dose High

iv (n,

P. calcium, Total

RA

(n,

iv (n,

8)

NaCl

iv (n,

1‘2)

10)

mM

Ionized

P. phosphate, Total -UF T

12)

dose

x 10ob

-UF mM

C E

2.48 + 2.99 +

.05 .06d

2.35 + 2.91 t

.06” .12d

2.48 2 4.03 Ik

.09 .Hd

C E

1.12 + 1.36 in

.03 .07d

1.11 + 1.38 +

.02” .Oijd

0.95 k 1.67 +

.06 .16

C E

1.86 + 2.12 *

.12 .13d

1.67 rt 1.92 t

.09 .08d

1.68 iI 2.66 t

.lO .21d

2.43 -f 2.37 +

.03 .05

2.09 t 2.19 Ik

.12 .ll

mM

C E C E

100.5 96.5

+ 0.8 + O.gd

1.86 2 0.11 2.03 + O.lld

102.2 98.2

101.6 85.2

+ 0.5 t l.Od

Ik 0.7 2 2.gd

1.70 * 0.09 2.22 iI O.lod

1.70 rt 0.07 1.88 + 0.07d

99.5 99.1

+ 0.7 * 0.7

2.07 + 0.10 2.16 + 0.08

UV phosphate, pmol/min

C E

11.8 16.2

+ 2.4 + 2.8’

4.8 6.8

k 1.6 I!I 1.7

10.9 15.2

k 2.5 + 3.7

12.7 15.3

k 3.1 f. 3.9

FE phosphate,’ %

C E

16.0 19.8

+ 2.9 + 2.6d

8.1 10.8

k 3.1 IL 3.5’

23.5 23.2

IL 5.6 Ik 4.9

15.7 15.5

t 3.2 k 2.8

Values phosphate < 0.05.

are means + SE. RA, renal artery. a Renal vein measurements. from Fig. 2. (’ FE (fractional excretion) of phosphate was calculated

respectively, 127 t 16 and 38.6 t 4.5 ml/min before and 135 t 19 and 40.9 t 5.1 ml/min during calcium infusions. A comparable observation was made during infusions into the renal artery. This low dose of calcium chloride increased fractional excretion of phosphate by +2.62 t 1.06% (P < 0.05) in 8 out of 10 dogs. No significant changes were detected in blood pressure or renal hemodynamics; clearances of PAM and inulin were 114 t 8 and 29.6 -+ 1. 6 before and 104 t 8 and 28.9 t 1.5 ml/min during calcium infusions, respectively. In addition to clearance of PAH, renal blood flow was measured with an electromagnetic flowmeter and was 242 t 35 before and 233 t 19 ml/min during calcium chloride infusion. The third column in Table 2 summarizes the effect of intravenous infusion of the high dose of calcium chloride. The blood pressure was significantly increased from 113.8 t 3.0 to 133.8 -+ 6.0 mmHg (P < O.Ol>, and the clearance of PAH was significantly decreased from 121 t 9.0 to 82 t 6.0 ml/min (P < O.Ol), but the glomerular filtration rate remained unchanged, 29.8 t 3.0 before and 29.2 t 2.9 ml/min during calcium infusion. For comparison, sodium chloride instead of calcium chloride was infused in 12 dogs. The mean difference in fractional excretion of phosphate was -0.39 t 1.49%, NS. This group also served as a time control. Effect of increased plasma PTH concentrations when plasma calcium level is controlled. The foregoing studies indicate that a small increase in plasma Cai concentration increases the excretion of phosphate when the plasma PTH level is controlled. However, this phosphaturic effect is small. Further, it cannot be determined whether the effect requires the presence of parathyroid hormone. This series of experiments was designed to verify the phosphaturic effect of calcium in the presence

b UF/T x 100, ultrafiltrable fraction of total plasma (’ P d P < 0.01. using the estimated UF phosphate.

of PTH and to investigate the role of calcium in the absence of PTH. Data from dogs thyroparathyroidectomized for 18 h and given a bolus of PTH are reported at three different periods during the experiment: 1) during infusion of sodium chloride (C 1), 2) during infusion of either sodium chloride or calcium chloride (C2), and 3) during infusion of sodium chloride or calcium chloride plus PTH (PTH) (Table 3). In dogs kept hypocalcemic and given sodium chloride throughout experiments (hypocalcemia), PTH increased the fractional excretion of phosphate +8.8 t 2.9% (P < 0.01) (the mean difference between C, and PTH). There were no changes in blood pressure, clearance of PAH or inulin, and fractional excretion of sodium. In the second group of dogs, a calcium chloride infusion was started, after C, collections were obtained, to restore plasma calcium concentration to a level comparable to the preparathyroidectomy level (normocalcemia). PTH increased fractional phosphate excretion +19.1 t 4.4%, P < 0.001, a response more than twice as large as in hypocalcemic dogs. Blood pressure increased slightly from 121.2 _+ 3.9 mmHg during C, to 129.8 t 4.5 (P < 0.05) and 128.4 t 5.4 mmHg (P < 0.05) during C, and PTH, respectively. Clearances of PAH and inulin were unchanged. The third group of dogs was made hypercalcemic by continued infusion of calcium chloride (hypercalcemia). Hypercalcemia (C,) significantly increased fractional phosphate excretion, +5.85 t 1.29% (P < 0.001). However, it is important to note that hypercalcemia increased plasma phosphate from 2.10 t 0.12 to 3.00 t 0.14 mM. The possible contribution of the increased plasma phosphate concentration in this increase in urinary phosphate excretion is apparent from the next series of

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794

CUCHE

TABLE

3. Effect of increased

plasma

PTH

on urinary

phosphate

Hypocalcemia

P. calcium

total,

P. phosphate

-UF

total,

mM

x 100, %

T -UF,

mM

UV phosphate,

FE phosphate,

G

2.23 505

2.20 k.07

2.06 t.11

1.93 k.15

100.6 k.5

101.4 +l.O

2.07 2.08 pmollmin

%

in dogs with control

ofplasma

Normocalcemia PTH

Cl

mM

excretion

1.95 k.14

ET AL.

calcium

Hypercalcemia PTH

Cl

C*

PTH

Cl

G

2.18 2.06

2.38 +.08 *

2:76 +.lO *

2.87 +.lO *

2.00 2.23 *

3.81 ~18 *

4.04 i1.18 *

1.78 4.16

2.15 ~26

2.19 ix.25

2.02 k.20

2.10 k.12 *

3.00 k.14 *

2.77 k.15 *

102.3 k1.0

1.81 k.14

99.5 k1.3

2.12 +.23

97.3 k1.3

101.4 +1.6 *

97.9 a.1

2.11 k.22 ,

83.8 k1.9 *

2.12 LO9 *

1.97 k.17

2.50 ~08 *

84.4 k2.0 * 2.33 LO9 *

3 1 . k1.5

5.6 23.0 *

18.3 +6.1 *

13.0 k3.7

12.6 26.1 *

52.5 d6.7 *

10.4 t3.7 *

23.8 23.6 *

35.2 k4.0 *

1.3 k.5

3.5 t2.1 *

12.2 +4.8 *

6.1 t1.6

4.2 a.9 *

23.2 +6.2 *

5.5 k1.7 *

13.3 k2.4 *

28.9 +2.4 *

Values are means + SE. C1, 2 control periods during NaCl. Ce, 2 control periods during either NaCl or CaCl. PTH, 3 experimental periods obtained 20 min after a bolus of bovine PTH (30 U/kg). Plasma calcium and phosphate concentrations (mM) before thyroparathyroidectomy were, respectively: 2.67 + 0.09 and 1.40 + 0.09 in hypocalcemia, 2.79 k 0.09 and 1.52 + 0.18 in normocalcemia, and 2.73 + 0.17 and 1.47 + 0.10 in hypercalcemia. ‘“P < 0.01.

studies. PTH increased the fractional excretion of phosphate + 15.5 t 1.5% (P < 0.001). The response was not statistically different from the response in normocalcemia. Hypercalcemia significantly altered hemodynamics. Blood pressure increased from 122.3 t 3.4 mmHg during C, to 138.2 t 2.8 and 137.5 t 2.5 mmHg (P < O.Ol), respectively, during C, and PTH. Clearances of PAH and inulin decreased from 337 t 29 and 87.9 t 8.4 ml/min during C, to 238 t 23 and 74.9 t 5.0 ml/min (P < 0.01) and 182 t 8 and 53.0 t 5.0 ml/min (P < 0.001) during C, and PTH. Effect of Sodium Phosphate Phosphate Excretion

Infusion

N

60

-PTX ----v8hr

on Fractional

Sodium phosphate solution (0.25 M, 1 ml/min) was infu.sed in five dogs with controlled PTH levels. The effect on fractional phosphate excretion is shown in Fig. 3. There were no significant changes in glomerular filtration rate, total calcium, and ultrafiltrable calcium concentrations during sodium phosphate infusions. Similar experiments were performed in dogs in the absence of PTH. Sodium phosphate infusion increased plasma phosphate concentration and fractional phosphate excretion as shown in Fig. 3. There were no significant changes in glomerular filtration rate. Plasma total calcium concentration significantly decreased when compared to that in the first period with infusion of sodium chloride (2.29 t 0.11 mM); it was 2.20 t 0.12, P < 0.05, during the second control period, and 2.17 t 0.12 (P < 0.02), 2.11 t 0.12 (P < O.Ol), and 2.02 t 0.12 mM (P < 0.001) during infusion of sodium phosphate. However, ultrafiltrable calcium remained constant (0.94 t 0.97 and 1.01 t 0.04 mM during sodium

0

+ PTH dogs (n=5) PTX dogs (n=6)

I

I

1

2

Ultrofiltrable

I

I

4 5 3 phosphate, mM/L

FIG. 3. Relationship between ultrafiltrable phosphate concentration and fractional phosphate excretion during sodium chloride (open symbols) and sodium phosphate infusions (closed symbols).

chloride and 1.00 t mM during infusion

0.04, 0.95 t 0.04, and 0.97 t 0.05 of sodium phosphate).

Effect of Decreased Renal Phosphate Excretion

Blood

Flow

on

Large increases in plasma calcium concentrations were associated with either a decrease in the clearance of PAH or both. the clearances of PAH and inulin. Experiments were designed, therefore, to study the effects of renal hemodynamic alterations on fractional phosphate excretion (Table 4). When blood pressure in the renal artery was decreased by 50%, both renal blood flow and glomerular filtration rate decreased by 40%, and fractional phosphate excretion decreased by 20%.

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PLASMA

CALCIUM

AND

FRACTIONAL

PHOSPHATE

TABLE 4. Effect of renal hemodynamic alterations hypercalcemic dogs with control of PTH

on urinary Before

Renal blood pressure, Renal blood flow, Glomerular Plasma

Plasma total UF/T, % UF, mM Filtered

ml/min

filtration

total

mmHg

calcium,

rate,

ml/min

phosphate,

load of phosphate,

mol/min

excretion

in five 30 Min

At Clamping

in 7.6

205

t 16 +

8.1

3.38 k

2.0

1.73 rt 97.5 2 1.68 k

mM

phosphate

Clamping

141

38.1

mM

795

EXCRETION

74

2 9.7*

140

?I 9.1

121

* 8.0*

164

Ik 17

~fr 4.6-j.

32

22.4

3.41 Ik

.08 1.3 .06

62.5 k 11.4

Postclamping

+

5.5

.21

3.58 2

.18

1.95 +: .04 95.5 + 1.1 1.86 t .04

2.07 + 93.8 + 1.93 iI

.lO 0.8 .08

61.5 + 10.0

41.1 of: 8.4t

UV phosphate,

mol/min

17.6 +

2.8

9.9 + 2.8*

20.5 k

2.4

FE phosphate,

%

31.2 +

5.0

25.9 + 5.0*

34.7 k

3.9

Values

are means

+ SE.

*P

< 0.01.

$ P < 0.05.

DISClJSSION

The results of the present studies indicate a phosphaturic effect of mild hypercalcemia. Two different but complementary studies support this conclusion. In the first, plasma PTH level was controlled and kept within physiologic limits, and plasma calcium concentration was increased about 20% (Table 2). A small but consistent increase in phosphate excretion was observed. This effect was of intrarenal origin, since the same observations were made during the intrarenal infusion of calcium chloride with no significant change in phosphate excretion by the contralateral kidney (Fig. 4). In the second study, plasma calcium was controlled, and the response to a pharmacologic dose of PTH was measured. The phosphaturic response in thyroparathyroidectomized dogs was larger in normocalcemia than it was in hypocalcemia (Fig.-5). Thus, a normal plasma calcium concentration is necessary to observe the full phosphaturic response to PTH. A comparable observation has been reported in a patient with type 2 pseudohypoparathyroidism (17). A role for calcium in the phosphaturic effect of parathyroid hormone is consistent with the concept that calcium is required to observe the physiologic response to parathyroid hormone as detailed in the introduction. The role of calcium on phosphate excretion was also investigated in the absence of parathyroid hormone. Calcium chloride infusions restored plasma calcium concentration to normal levels without an effect on phosphate excretion. In dogs with hypercalcemia, fractional phosphate excretion increased significantly (Table 3). However, plasma phosphate significantly increased, and this hyperphosphatemia can quantitatively account for the increase of phosphate excretion (Fig. 3). Thus, we were unable to detect a direct effect of altered plasma calcium on renal phosphate excretion in the absence of parathyroid hormone. In contrast to the findings with mild hypercalcemia, marked hypercalcemia was not phosphaturic. In thyroparathyroidectomized dogs given a constant infusion of bovine PTH, a 75% increase in ionized calcium concen-

Calcium chloride Low dose High dose

NaCl

Total -3% calcium change +6 -

+20% +24%

+62 %

Fractional

FIG. 4. Response (A, experimental minus control measurements) of fractional phosphate excretion at different levels of plasma calcium _concentration. stripped bars, intra_ _ _ Open bars, intravenous; renal artery infusions.

+25

Normocalcemia iypocalcemia

+20

T

Hypercalcemia

Fractional phosphate ti5 excretion response to a bolus 40 of PTH +5 0 t- ROOOIf .

LNSt . .

5. Response (experimental minus control measurements) of fractional phosphate excretion to a bolus of PTH when plasma calcium concentrations are controlled at 3 different levels in 18 h thyroparathyroidectomized dogs. FIG.

tration was not associated with an increase in fractional excretion of phosphate. The absence of an increase in phosphate excretion is striking if we take into consideration that plasma phosphate concentration is increased by 30%. Referring to Fig. 3, the 30% increase in plasma phosphate would be predicted to increase the fractional

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796 excretion of phosphate about +5%. These results suggest that marked hypercalcemia masks the phosphaturic role of calcium. The effect of marked hypercalcemia could be due to intrarenal hemodynamic alterations or to metabolic changes or to both. In the present study, marked hypercalcemia decreased the clearances of PAH or inulin or both, depending on the level of plasma calcium concentration. Table 4 shows that decreased glomerular filtration rate is associated with a significant decrease in fractional excretion of phosphate. This effect could account for Lavender and Pullman’s (10) observation of decreased fractional phosphate excretion during renal arterial infusions of calcium in dogs. In regard to possible metabolic effects of marked hypercalcemia, several reports indicate that calcium can inhibit cyclic AMP both in vitro and in vivo (1,3,16,19). Beck et al. (1) have reported a marked inhibition of PTH mediated cyclic AMP and phosphate excretion in hypercalcemic rats.

CUCHE

ET AL.

In conclusion, the present study demonstrates an intrarenal role for calcium in renal phosphate handling. The phosphaturic effect of calcium requires the presence of PTH. Further, calcium potentiates the phosphaturic effect of PTH. In markedly hypercalcemic dogs, this phosphaturic effect is either absent or decreased, probably due to additional hemodynamic and/or metabolic effects. The authors gratefully acknowledge the technical assistance of l Rita Semerad, John Haas, and Terry Berndt and the contributions of Randall Wilkening, Karen Nelson, Alrundus Hart, and Robert Barnett as well as the secretarial assistance of Carma Jean Fink. J. L. Cuche is a Minnesota Heart Research Fellow, Drs. Ott and Marchand are National Institutes of Health Postdoctoral Fellows, and J. A. Diaz-Buxo is a National Kidney Foundation Postdoctoral Fellow. F. G. Knox is National Institutes of Health Career Development Awardee HL-18518. This investigation was supported by Public Health Service Grant HL-14133. Received

for publication

19 May 1975.

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Intrarenal calcium in phosphate handling.

The intrarenal role of plasma ionized calcium (Ca), on fractional phosphate excretion (FE PO4) was investigated in dogs with control of parathyroid ho...
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