Retention of 5-hydroxytryptamine by subcellular fractions of rat brain homogenates

After their incubation with 14C-labeled amine, microsomal fractions of rat brain homogenates in vitro retained up to 12 times the concentration of 5-HT- 14C (on the basis of their protein content) found in the corresponding mitochondrial fractions. This difference was maintained over the range of concentrations, 100–4,000 mμg of 5-HT- 14C per ml of medium. No clear evidence of saturation was seen even when the highest concentration of amine was used. While lowering the temperature of a 30-min incubation to 4° significantly reduced the retention when compared with that at 37°, similar treatment, when the duration of incubation was 10 min, failed to alter the magnitude of retention. Retention was increased (at 37°) when the duration of incubation was 30 or 60 min; however, significant retention took place even when the time of incubation was zero (i.e. when the microsomal fractions were mixed briefly with the labeled amine and immediately subjected to analysis). Microsomal fractions retained up to 12.7 times the concentration of 5-HT- 14C in the medium; the corresponding figure for 5-HTP- 14C was 3.3. It was found that microsomal as well as mitochondrial fractions of both rat and guinea pig brains contained significant amounts of endogenous 5-HT; in the case of guinea pig brains, the concentration of amine (in terms of the wet weight of fractions) in the microsomal was always higher than in the mitochondrial fractions. The physiological significance of retention of 5-HT by microsomal particles is not clear but may represent, in vitro, the process of binding of 5-HT synthesized in the brain, without the limitations on the process normally set in vivo by the presence of the cell membrane.