Incorporation of 14C-tryptophan into 14C-protein by cultured rat pineals: stimulation by l-norepinephrine.

Organ cultures of individual rat pineals incorporate (14)C-tryptophan into proteins at a nearly constant rate for at least 48 hours. Previous studies have shown that these cultures also convert (14)C-tryptophan to serotonin, melatonin, and 5-hydroxyindoleacetic acid, and release these indoles into the media. The formation of (14)C-protein from (14)C-tryptophan is accelerated by the addition to the culture medium of l-norepinephrine or related catecholamines but is not modified by serotonin, melatonin, or 5-hydroxyindoleacetic acid. One mechanism by which norepinephrine stimulates the synthesis of (14)C-protein from (14)C-tryptophan involves increasing the uptake of the (14)C-tryptophan into pineal parenchymal cells, inasmuch as (1) norepinephrine increases the intracellular content of (14)C-tryptophan as well as its conversion to its major products, (14)C-protein and (14)C-indoles; (2) norepinephrine does not stimulate (14)C-protein synthesis in pineal organs which contain a previously fixed amount of (14)C-tryptophan; and (3) norepinephrine does not stimulate (14)C-protein synthesis from (14)C-methionine or (14)C-leucine. The finding that norepinephrine, but not serotonin, can stimulate the incorporation of (14)C-tryptophan into pineal proteins is consistent with the hypothesis that norepinephrine is the neurotransmitter substance utilized by pineal sympathetic nerve endings.