Natriuretic effect of naloxone in fasted, water-loaded rats

The influence of naloxone (10 mg/kg i.p.) on water and electrolyte excretion was assessed in water-loaded rats subjected to fasting or fasting and refeeding. In fed animals, naloxone had no effect on water or electrolyte excretion compared to saline injected controls. However, during fasting, which has been shown to activate endogenous opioid systems, naloxone had a marked natriuretic effect (43 +/- 10 vs. 11 +/- 4 muEq/100 g X 2 h, p less than 0.05). Naloxone increased the sodium excretion to a level not significantly different from fed animals. Potassium and water excretion were not significantly changed compared to saline injected rats. The results suggest that fasting activates a naloxone-sensitive mechanism for sodium retention, perhaps by increasing the release of endogenous opioid peptides, and that refeeding inhibits this mechanism.