Effect of increased dietary phosphate intake on dopamine excretion in the presence and absence of the renal nerves

Studies were performed to determine the relative contributions of the renal nerves and tubule synthesis to dopamine excretion during increased dietary phosphate intake. Rats underwent bilateral renal denervation (n = 5) or sham surgery (n = 5) 1 week prior to the initiation of the balance studies. All rats were placed in metabolic cages and fed 12 g/day of a low phosphate diet (LPD, 0.07%, P(i)) for 4 days, then high phosphate diet (HPD, 1.8% P(i)) for 4 days. Sodium, potassium, and chloride contents were made similar in LPD and HPD by adding sodium chloride and potassium carbonate to the food. Urine samples were collected every 24 h for determination of free dopamine and electrolyte excretions. The mean urinary phosphate excretion for 4 days of LPD was 0.17 ± 0.11 mmol/day in the group with bilateral renal denervation and 0.18 ± 0.11 mmol/day in the group with innervated kidneys. Likewise, dopamine excretion was similar in both groups in the absence (2.5 ± 0.2 μg/day) and in the presence (2.3 ± 0.1 μg/day) of the renal nerves in rats fed LPD. Increasing dietary phosphate intake from 0.07 to 1.8% significantly increased urinary phosphate and dopamine excretions. The mean urinary phosphate excretion for 4 days was similar in the denervated (4.4 ± 0.9 mmol/day) and innervated (4.1 ± 0.5 mmol/day) groups. The mean urinary dopamine excretion for the 4 days of HPD significantly increased to 4.5 ± 0.5 μg/day in the group with chronic bilateral renal denervation and 3.7 ± 0.2 μg/day in the group with innervated kidneys. Plasma 3,4-dihydroxyphenylalanine (DOPA) and dopamine levels were not changed by increased dietary phosphate intake. These observations demonstrate that increases in dietary phosphate intake result in parallel increases in urinary dopamine and phosphate excretions in the presence and the absence of the renal nerves, suggesting that increased dietary phosphate intake increases urinary dopamine excretion through synthesis by renal tubules rather than by renal nerves.