Effects of morphine and naloxone on glucose metabolism in uterine strips from ovariectomized and non-ovariectomized restricted diet rats

The effect of underfeeding over glucose metabolism in uteri isolated from ovariectomized and non-ovariectomized rats subjected to a restricted diet for 25 days (50% of the normal food intake), was studied. Underfeeding decreases ¹⁴CO₂ formation from U¹⁴ C-glucose in intact animal uteri. While in ovariectomized rats (25 days), the effect is the opposite. The addition of morphine 10^-6 M to the medium does not affect rats fed ad libitum. However, ¹⁴CO₂ levels increase significantly in intact animals receiving a restricted diet. In ovariectomized rats morphine does not show any activity, regardless of the type of diet rats were subjected to. None of the rat groups seems to be sensitive to naloxone 10^-6 M. The s.c. injection of morphine (4 mg.kg^-1) increases glucose metabolism only in intact rats provided with a restricted diet, while naloxone (2.5 mg.kg^-1) produces a decrease of ¹⁴CO₂ in ovariectomized underfed animals. To conclude, morphine either 'in vivo' or 'in vitro' is active only in uteri from intact rats subjected to underfeeding. Naloxone produces a decrease in ¹⁴CO₂ production, particularly when it is s.c. injected to ovariectomized rats undergoing a dietary restriction. Since the uterus does not react to naloxone, the effect of the opiod blocker may be the result of endogenous opioids originated in other tissues.