IN VITRO STUDIES ON THE DIPHASIC ACTION OF METHADONE

¹ Department of Pharmacology and Experimental Therapeutics, University of California Medical Center, San Francisco 22, California

It is quite evident that the diphasic response produced by many agents on the respiration of brain slices is not limited to a specific class of drugs and since oxygen consumption is such an inclusive response, one cannot assume a common biochemical mechanism of action. It is interesting that the concentrations of l-methadone which produced maximum stimulation and 50 per cent inhibition of rat brain slice respiration (table 1) were practically identical to those having a similar effect on pig heart cytochrome-c oxidase activity (Wang and Bain, 1953). However, with brain slices, the concentrations for diphasic activity were the same for both of the optical isomers, while the latter authors found the l-isomer more potent with the pig heart preparation. The significance of the response is unknown although it suggests a means to classify drug action on a cellular level and represents a positive response at concentrations lower than those required to produce classical inhibition of respiration.

Three aspects of the diphasic response of methadone have been investigated. The results of the structure action studies indicated that the amine portion of the methadone molecule was responsible for this response, while the nonpolar portion tended to reduce the concentration required for activity. Additional structural requirements necessary for stimulation by amines have been found. The information obtained in the potassium study showed that although respiration is increased by methadone, it does not maintain the potassium gradient. At all concentrations of methadone used, cellular potassium ion was progressively lost, preceding the respiratory inhibition that followed with time.

Methadone binding and the stimulation of oxygen uptake by methadone can be reversed by washing, but the effect of inhibitory concentrations of methadone on respiration is irreversible. Uptake is unaltered by lack of glucose substrate, anerobic conditions, or N-allylnormorphine.