Procaine hydrochloride as a monoamine oxidase inhibitor: implications for Geriatric therapy.

The results of in vitro experiments showed that inhibition by procaine hydrochloride of monoamine oxidase (MAO) from either rat brain or liver was substrate-dependent. Procaine was more effective in inhibiting serotonin oxidation than phenylethylamine oxidation and had an intermediate effect on tryptamine oxidation. MAO activity in tissue homogenates from rats treated with procaine (150 mg/kg intraperitoneally) was inhibited most in liver, less in heart, and only very slightly brain for a duration of up to 8 hours. Procaine injected in that dose did not alter brain norepinephrine levels and elevated only slightly the brain serotonin levels. It did not protect against the degradation of exogenous radioactive tryptamine in brain. These data confirm and extend prior observations on in vitro inhibition of MAO by procaine and suggest that in high doses procaine may inhibit MAO weakly in vivo. If the reported usefulness of procaine preparations in treating geriatric patients indeed depends upon MAO inhibition, more effective inhibitors would seem to be available.