Fig. 4. Stable expression of Lumbricus terrestris OAT transporter RNA in transformed Sf9 insect cells. The relative affinity of LutOAT for neurotransmitter monoamines (MA) was determined by exposing transformed Sf9 cells to radiolabelled isotopes of the phenolamines tyramine (TA) and octopamine (OA), and the catecholamines dopamine (DA) and norepinephrine (NE) in vitro. Cells expressing LutOAT take up all four monoamines with high-affinity and in a Na⁺-dependent manner. TA and OA uptake saturates at mid-nanomolar substrate concentrations, whereas DA and NE uptake saturates at lowmicromolar concentrations. Uptake under non-saturating substrate concentrations is log-linear (as indicated by the slope of the broken lines). There is an inverse relationship between affinity and transport capacity. LutOAT has the highest affinity and the lowest transport capacity for TA; the opposite holds for NE. The estimated kinetic values are given in Table 1. Each point represents the mean ± s.d., N=3-5 independent trials.