Figure 4: Characteristics of arginine transport by TgNPT1.

(a,b) Ion-dependence (a) and pH-dependence (b) of TgNPT1-mediated arginine transport in oocytes expressing TgNPT1-HA. The uptake of 0.4?μCi?ml⁻¹ (1.1?μM) [¹⁴C]Arg was measured in the presence of 100?μM unlabelled arginine in oocytes, in either complete ND96 uptake buffer (control), media in which Na⁺, Cl⁻, K⁺, Mg²⁺, or Ca²⁺ ions were replaced (a), or media pH-adjusted to pH 5–10 (b). Uptake measured in uninjected control oocytes was subtracted from that measured in TgNPT1-HA-expressing oocytes to yield the TgNPT1-mediated uptake. All data were averaged from three independent experiments, each conducted on oocytes from a different frog (*P<0.05, **P<0.01; n.s.=not significant; ANOVA). (c) A representative trace for the arginine-induced current in a TgNPT1-HA-expressing oocyte (black) and an uninjected oocyte (grey). The red arrow indicates the point of addition of 5?mM arginine to the ND96 medium and the black arrow indicates the point at which arginine was removed. The spontaneous current relaxation following the addition of 5?mM arginine is indicated. (d) Representative traces of arginine-induced currents in TgNPT1-HA-expressing oocytes, suspended in complete ND96 medium (control) and in media lacking Na⁺, Cl⁻, K⁺, Mg²⁺ or Ca²⁺ ions. For all current traces, the resting current before arginine addition was set to 0. Red arrows indicate the points of addition of 5?mM arginine to the ND96 medium and black arrows indicate the points at which arginine was removed. The traces are representative of n=8 oocytes under each of the conditions tested. (e) pH dependence of the maximum arginine-induced current in oocytes expressing TgNPT1-HA. Oocytes were equilibrated in media of the relevant pH and the maximum amplitude of the current following the addition of 5?mM arginine was measured. For each pH, the mean arginine-induced current is shown±s.d. (n=8; **P<0.01; n.s.=not significant; ANOVA).