Abstract

The uptake of radiolabeled serotonin as a function of concentration exhibited saturation when low (0.005-2 µM) concentrations of serotonin were used. Kinetics of the initial rate (30 s) of entry of radiolabeled serotonin into pinealocytes were examined using various concentrations of the substrate. The effects of temperature, changes in external ion concentrations, drugs, metabolic inhibitors, and other compounds on the uptake of serotonin were investigated. High concentrations (10 µM-5 mM) of serotonin appear to use amino acid transport systems, e.g., L-lysine, L-leucine, and L-alanine. Uptake is partially sodium dependent, temperature dependent, sensitive to metabolic inhibitors, and inhibited by various amino acids. Low concentrations (0.005-1 µM) of serotonin appear to use a highly specific, allosteric carrier for transport. Uptake is sodium dependent, temperature dependent, and sensitive to a variety of metabolic inhibitors.