Inhibition of the respiratory burst in mouse macrophages by ultra-low doses of an opioid peptide is consistent with a possible adaptation mechanism

The respiratory burst induced by phorbol myristate acetate in mouse macrophages was inhibited by ultra-low doses (10−15 −10−13 M) of an opioid peptide [d-Ala2] methionine enkephalinamide. The effect disappeared at concentrations above and below this range. The inhibition approached 50% and was statistically significant (P < 0.001). Increasing the time of the opioid incubation with cells brought about a shift in the maximal effect to lower concentrations of the opioid (from 10−13 to 5 · 10−15 M) and led to a decrease in the value of the effect, fully in accord with the previously proposed adaptation mechanism of the action of ultra-low doses.