Naloxone interferes with granulocytopoiesis in long-term cultures of mouse bone marrow; buffering by the stromal layer.

Long-term cultures of mouse bone marrow cells were treated with naloxone, starting at the time of culture initiation or in the 2nd or 4th week of culture. Cell proliferation was suppressed and the ratio of immature and mature granulocytes to macrophages diminished by naloxone treatment. The effect depended on the timing of naloxone addition to the cultures and on its concentration, with a bell-shaped dose-response curve. High and low concentrations of naloxone (10(-4), 10(-6), 10(-14) M) interfered with hematopoiesis more strongly than the intermediate concentrations (10(-8) to 10(-12) M). Early cultures lacking the stromal layer were more sensitive to naloxone than the cultures with established stroma. The bell-shaped dose-response curve has been attributed to an interplay of specific (opioid-receptor-mediated) and nonspecific mechanisms. Opioidergic mechanisms apparently participate in the regulation of hematopoiesis.