[³H]opiate binding: anomalous properties in kidney and liver membranes

[³H] Naloxone and [³H] dihydromorphine bind in a saturable fashion and with high affinity to membrane preparations of guinea pig kidney and liver. Binding in guinea pig kidney displays reversed stereospecificity, with pharmacologically inactive dextrallorphan being more potent than the known pharmacologically active levallorphan. Opiate agonists tend to be more potent than their corresponding antagonists in competing for [³H] opiate binding in guinea pig kidney. Unlike brain opiate receptors, in which sodium and manganese selectively decrease and increase, respectively, the binding of [³H] opiate agonists, these ions have no selective effect on the binding of [³H] opiates in guinea pig kidney and liver. The opioid peptides Met-enkephalin and β-endorphin and the opiates etorphine and diprenorphine, which have very high affinity for brain opiate receptors, have negligible effects on [³H] opiate binding in guinea pig kidney.