Figure 2.

Computational simulations for the mixed transport model and corresponding experimental [¹⁴C]l-serine uptake data. Left) Schematic figure outlining the experimental design for model simulations, showing the imposed direction and magnitude of the transmembrane l-serine concentration gradients generated to drive uptake of tracer (7.5 µM [¹⁴C]l-serine) into MVM vesicles (depicted as green circles). A–C) Computational simulations (with h set at 4%) for experiments 1, 2, and 5; experiments 2–4 and 4b; and experiments 5–7 of the experimental test matrix with varying substrate conditions (external: internal unlabeled l-serine concentration in micromolar). A) In the absence of added intravesicular l-serine to drive exchange, [¹⁴C]l-serine uptake was predicted to occur by facilitated transport as there is no added opposing amino acid to drive exchange (experiment 1). However, increases in the external l-serine concentration were predicted to decrease [¹⁴C]l-serine uptake in a concentration-dependent manner because of cis-inhibition (experiments 2 and 5). B) In contrast, the addition of l-serine inside the vesicles was predicted to trans-stimulate [¹⁴C]l-serine uptake (experiments 3, 4, and 4b). Interestingly, the outwardly directed concentration gradient in experiments 4 and 4b was predicted to lead to an overshoot curve (transient accumulation of [¹⁴C]l-serine above final equilibrium concentration). C) High extravesicular l-serine concentrations were predicted to reduce l-serine equilibrium concentrations compared with experiments 2, 3, and 4 because of cis-inhibition. D–F) Experimentally measured Na⁺-independent uptake of 7.5 µM [¹⁴C]l-serine into human placental MVM vesicles unloaded or preloaded with l-serine closely resembled the model simulations shown in A–C (note the difference in axis scales). Symbol numbers relate to experimental condition given by the experimental matrix. Data are shown as means + sem (n = 5–9 placentas).