Dopamine Enhances the Phosphaturic Response to Parathyroid ...

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(PTH) is phosphaturic and inhibits phosphate transport by the proximal tubule .... artery for monitoring mean arterial pressure and collection of blood samples.
Dopamine Parathyroid Jorge

Isaac,

Enhances Hormone

Theresa

J. Berndt,

the Phosphaturic Response to in Phosphate-Deprived Rats’

Sharon

L. Chinnow,

G.M.

Tyce,

T.P. Dousa,

and

Franklyn

G. Knox2

for day J.

Isaac,

T.J.

Dousa,

F.G.

Knox,

Departments cine, Mayo (J. Am.

Berndt,

S.L.

Chinnow,

Nephrology

Research

of Physiology and Clinic and Foundation,

Soc.

Nephrol.

1992;

G.M.

Tyce.

T.P.

and

1; 28.4 ± 1.4 versus 7.1 ± 3.6%for day 2; and ± 2.8 versus -0.2 ± 0.2% for day 3; N= 5 for all

groups). phosphate

Laboratories,

Biophysics Rochester,

10.7

MediMN

sis is low phaturic

2:1423-1429)

Key Words:

ABSTRACT

Phosphate phosphaturic mine

deprivation results in a resistance effect of parathyroid hormone.

is phosphaturic

proximal

and

tubule,

parathyroid

the

is synthesized

nephron

hormone

inhibits

to the Dopa-

by

kidney

subsegment

where

phosphate

transport.

Thus, to test the hypothesis that phosphate deprivation Is associated with low intrarenal dopamine synthesis and that dopamine infusion will overcome the resistance to the phosphaturic response to parathyrold

hormone,

The effect dopamine mine

the

of dietary synthesis,

excretion,

was

following

study

was

performed.

phosphate intake on intrarenal as reflected by urinary dopadetermined.

Rats

were

placed

In metabolic cages (N = 5) and were fed a lowphosphate diet (0.07% P) for 4 days and then a highphosphate diet (1 .8% P) for 4 days. Twenty-four-hour urinary dopamine excretion was significantly lower in rats fed a low-phosphate diet (2.53 ± 0.06 versus 4. 10 ± 0.30 ,g/day). Further, the effect of dopamine Infusion

on

the

blunted

phosphaturic

response

to

parathyroid hormone was studied in rats fed a lowphosphate diet for 1, 2, and 3 days. Control clearances were taken 2 h after thyroparathyroidectomy; then, parathyroid hormone (33 U/kg plus I U/kg/mm), dopamine (25 g/kg/min), or parathyroid hormone plus dopamine were infused for 60 mm. Changes in the fractional excretion of phosphate were significantly greater in rats fed a low-phosphate diet infused with parathyroid hormone plus dopamine than In rats fed a low-phosphate diet infused with parathyroid hormone alone (t.27.9 ± 5.8 versus I 1.2 ± 2.6%

I

2

ReceIved June Correspondence

Mayo

Clinic

and

24, 1991. Accepted to Dr. F.G. Knox, Foundation,

November Department

200 First Street,

14, 1991. of Physlolo9y SW.,

Rochester,

1046-6673/0209-1423$03.00/0 Journal of the American Society of Nephrology CopyrIght C 1992 by the American Society of Nephrology

Journal

of the American

Society

of Nephrology

and

Biophysics,

MN 55905.

It was concluded that, deprivation, intrarenal and dopamine infusion response to parathyroid Dopamine.

phosphate.

during short-term dopamine syntheenhances hormone.

parathyroid

the

phos-

hormone

P

anathyroid hormone (PTH) is phosphaturic and inhibits phosphate transport by the proximal tubule (1 .2). Rats fed a low-phosphate diet (LPD) exhibit a well-established resistance to the phosphaturic effect of PTH by mechanisms not yet fully understood (3.4). Extraneunal dopamine (DA) is synthesized by kidney proximal tubular cells (5). the same nephron subsegment where PTH inhibits phosphate transport. Aromatic L-amino acid decarboxylase. the enzyme responsible for the conversion of L-dopa to DA. has been webb characterized in the kidney of the rat, and its’ activity has been demonstrated in the proximal convoluted tubule and pars recta subsegments of the rat nephron (6). DA infusion has been shown to increase phosphate excretion. This effect was reproduced by the infusion of L-dopa. and a role for intranenal DA in the regulation of phosphate reabsorption was suggested (7). We propose a role for intrarenally synthesized DA in the resistance to the phosphatunic effect of PTH during short-term phosphate deprivation. Therefore. this study was performed to test the hypothesis that intrarenab DA synthesis is bow in rats fed a LPD and that DA infusion enhances the phosphatunic response to PTH administration in short-term phosphate-deprived rats.

METHODS Effect of Changes in Dietary on Urinary DA Excretion

Phosphate

Intake

Metabolic balance studies were performed in conscious male Sprague-Dawley rats (N = 5) that were fed LPD (0.07% P1; ICN Pharmaceutical) for 4 days and then changed to a high-phosphate diet (HPD; 1 .8% P) for 4 days. The HPD was prepared by supplementing the standard LPD with a mixture of sodium and potassium phosphate (ratio of monobasic:dibasic salts of 1 :4), to a final content of 1.8%

1423

Dopamine

and

Low-Phosphate

Diet

P1. For rats fed the LPD, sodium and potassium were supplemented to the same concentration as in the HPD. Rats were monitored to assure a daily ingestion of 1 2 g of the respective diet. Urine samples were collected every 24 h in a container in dry ice with 5 mL of 33% acetic acid to prevent catechobamine degradation. For measurements of urinary-free DA, the samples were filtered and purified by a modification of the method of Sharpless et at. (8). Quantification of urinary DA was then determined by HPLC with electrochemical detection (9). Free urinary DA excretion is a well-established reliable index of intrarenab DA synthesis (1 0- 1 2). Recoveries of added standards in all of the extractions averaged 80 to 95%. Urinary sodium concentrations were measured by an ionselect ive electrode (Beckman E2A; Beckman Instruments, Fullerton, CA). Phosphate concentrations were determined by the method of Chen et at. (13).

Twelve groups of rats on phosphate excretion

RATS

(A)

on the of PTH

Resistance

to the

Male Sprague-Dawley rats weighing 200 to 300 g. from Harlan-Sprague-Dawbey (Indianapolis. IN), fed 1 2 to 1 5 g/day of LPD for 1 . 2, and 3 days (0.07% P; ICN Pharmaceutical) or normal phosphate diet (NPD; 0.7% P). with free access to water, were used in this study. The NPD was prepared by supplementing the standard LPD with sodium and potassium phosphate. The ratio of monobasic-to-dibasic salts was 1 :4, giving a final concentration of 0.7% P. For animals fed LPD, sodium and potassium contents were supplemented by NaCl and KC1 to the same concentration as in NPD. Rats were anesthetized by a 100mg/kg body wt i.p. injection of 5-sec-butyl-ethyb-2thyobarbitunic acid (Inactin; BYK Golden, Konztanz, Germany) and were placed on a heated table to maintam body temperature between 36 and 38#{176}C. Rats were acutely thyropanathyroidectomized by heat cautery. and then a tracheostomy was penformed. Polyethylene catheters (PE-50) were placed in the left and night jugular veins for infusions and in the left carotid artery for monitoring mean arterial pressure and collection of blood samples. A PE-90 catheter was placed in the bladder for collection of urine samples. An i.v. infusion of a solution of 3% inubin in saline (0.9% NaCb) and of saline (0.9% NaC1), each at a rate of 1 mL/100 g body wt. were started. A 1-h equilibration period was allowed after the surgery. Control clearances of 30 mm were begun 2 h after thyroparathyroidectomy. After the control clearance was taken. infusion of an appropriate experimental solution was started. Experimental clearances of 30 mm were initiated an hour later. Animals infused with DA also received a volume replacement with saline solution (0.9% NaC1), at a rate of 1 .5 mL/h.

1424

Effect

(B)

Effect

studied, and determined.

the

effects

A NPD

of PTH (N

After the control turic dose of PTH 33 U/kg. followed of 1 U/kg/mm.

=

5)

clearance, a maximally was given with an initial by a continuous infusion

of PTH in the

presence

phosphabobus of at a rate

of DA (N

5)

=

PTH was given as described in protocol A. In addition, a simultaneous infusion of a pharmacological dose of DA (25 g/kg/min) was given.

(C)

Effeci of DA Infusion Phosphaturic Effect

FED

were were

Effect

of DA (N=

5)

DA was infused as described in protocol B. The same experimental infusions and doses in protocols A. B. and C, for rats fed a NPD, followed for rats fed a LPD for 1 2, and 3 days.

used were

,

RATS FED LPD FOR I DAY (A)

Effect

of PTH (N

(B)

Effect

of PTH in the

(C)

Effect

of DA (N=

=

5) presence

of DA (N

=

5)

of DA (N

=

5)

5)

RATS FED LPD FOR 2 DAYS (A)

Effect

of PTH (N

(B)

Effect

of PTH in the

(C)

Effect

of DA (N

=

=

5) presence 5)

RATS FED LPD FOR 3 DAYS (A)

Effect

of PTH (N

(B)

Effect

of PTH in the

(C)

Effect

of DA (N =4)

=

5) presence

of DA (N = 5)

Inubin concentrations in plasma and urine were measured by the anthrone method (14). Sodium and potassium concentrations in plasma and urine were measured with ion-selective electrodes (Beckman E2A analyzer; Beckman Instruments). Phosphate concentrations in plasma and urine were determined by the method of Chen et at. (13). The GFR was calculated on the basis of the clearance of inulin. All values are reported as mean ± SE. One-way analysis of variance was used to compare the changes in fractional excretion of phosphate (FEp). If analysis of variance indicated overall group differences, then group comparisons were made by the Bonferroni method ( 1 5). Other comparisons were

Volume

2



Number

9

.

1992

Isaac

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