Dose related changes in tissue morphine concentration.

Rats were injected with morphine-6(3)H diluted with increasing amounts of non-radiolabelled morphine. The entry of the isotope into the brain and various tissues was measured by combustion in a tissue oxidizer. The relative distribution of morphine between the blood and brain remained constant at about 5.5:1 over the range of doses studied (0.07 - 10.0 mg/kg). No dose related differential effects on morphine uptake were evident in central tissues, with the exception of the hypothalamus which exhibited a disproportionately greater uptake. Among the noncentral tissues, kidney and liver showed the greatest dose related increases in uptake.