Feeding but not salt loading is the dominant factor controlling urinary dopamine excretion in conscious rats

We studied urinary dopamine excretion in three different groups of rats after the following treatment regimens: normal chow and tap water (controls, CON), normal chow and 1% NaCl as drinking water (high salt, HS), and chow with low sodium content plus tap water (low salt, LS). On days 5 and 7 of the respective dietary treatment, rats were placed in metabolic cages. Using a cross over design, chow was given (fed) or withheld (fasted). Urine was collected for 24 h and analyzed for sodium, creatinine, and dopamine. Urinary dopamine excretion did not change in proportion to large differences in sodium excretion in fasted animals. Sodium excretion was enhanced (45%) due to feeding only in the CON group but not in HS and LS rats. However, there was a striking increase in renal dopamine excretion in fed compared to fasted animals, irrespective of their sodium diet: 2.5-fold in CON, 2-fold in HS, and 1.8-fold in LS rats. Urinary creatinine excretion was significantly elevated during the feeding condition compared to fasted animals in all treatment groups. Our results demonstrate that urinary dopamine excretion is dominantly influenced by feeding but not by oral sodium intake in conscious rats. We conclude that 1) the dietary state of the animals should be controlled in experiments on renal dopamine production, 2) renally formed dopamine could be involved in the functional response of the kidney to oral food intake.