In vitro experiments on the metabolism, uptake and release of 5-hydroxytryptamine in bovine retina

The accumulation, metabolism and potassium-induced release of 5-hydroxytryptamine (5-HT) in bovine retinae was studied. When isolated retina pieces were incubated at 37 degrees C in a medium containing [14C]5-HT, tissue:medium ratios of about 5 were obtained after only 10 min. Within this time the accumulated amine is hardly metabolized and even after a 60 min period, only approximately 12% of the amine is metabolized to form 5-hydroxyindoleacetic acid. The process responsible for this accumulation showed properties of an active transport system: it was temperature-sensitive, sodium-dependent and inhibited in particular by Lilly 110140, chlorimipramine and desimipramine. The uptake of [14C]5-HT was saturated with increasing 5-HT concentrations and could be accounted for by two saturable processes; a high-affinity system with a Km1 of 1.3 X 10(-7) M and Vmax1 of 0.65 pmol/mg/10 min, and a low affinity system with a Km2 of 1.2 X 10(-5) and Vmax2 of 29 pmol/mg/10 min. A rapid efflux of [14C]5-HT from tissue loaded with [14C]amine was observed when exposed to 70 mM KCl. This release was inhibited when the calcium content in the medium was replaced by sucrose. Cobalt ions added to the incubation medium also counteracted the effect of KCl, while chlorimipramine added to the medium enhanced the release of 5-HT caused by KCl. Bovine retinae also possess the enzymes necessary to produce 5-HT from tryptophan. These data generally support the idea that 5-HT is a transmitter in the retina, although as this study shows, the amine is present only in minute quantities.