J Physiol 586.17 SLC6A14 is the β-alanine carrier 4063 demonstrating that [3H]β-alanine uptake via SLC6A14 is a Na+- and Cl−-dependent process as observed with the β-alanine carrier in rabbit ileum (Munck & Munck, 1990, 1992a). Na+- and Cl−-dependent [3H]β-alanine uptake is reduced as extracellular pH becomes acidic (pH 5.5) (Fig. 2). SLC6A14-mediated β-alanine/Na+/Cl− cotransport is a rheogenic process which generates inward current under two-electrode voltage-clamp conditions (Figs 3 and 4) consistent with a stoichiometry of 2–3 Na+ : 1 Cl− : 1 amino acid, as suggested previously with other substrates for SLC6A14 (Sloan & Mager, 1999). The relative low afﬁnity of SLC6A14 for β-alanine (K m = 2.1 ± 0.9 mM, Fig. 3), compared to leucine and lysine, is consistent with Figure 2. Na+- and Cl−-dependent β-alanine uptake by SLC6A14 [3H]β-Alanine uptake (2 μM) was measured under control conditions (NaCl-containing solution, pH 7.4), in Na+-free pH 7.4 (–Na+) solution, in Cl−-free pH 7.4 (–Cl−) solution, or in a NaCl pH 5.5 solution. Under each condition, uptake in water-injected oocytes was subtracted from that in cRNA-injected oocytes to give SLC6A14-speciﬁc uptake (n = 20). ∗∗∗P < 0.001 versus control. to water-injected oocytes, being observed for the dipolar amino acid [3H]leucine and the cationic amino acid [3H]lysine (both P < 0.001 versus water) (Fig. 1). The greater uptake of [3H]leucine and [3H]lysine compared to [3H]β-alanine at tracer concentrations (Fig. 1) reﬂects the earlier observation in rabbit ileum that leucine and lysine are higher afﬁnity substrates for the β-alanine carrier (Munck, 1985). There was no signiﬁcant uptake (P > 0.05 versus water) of [14C]MeAIB into SLC6A14-injected oocytes (Fig. 1). MeAIB interacts very poorly with the β-alanine carrier in rabbit ileum (Munck, 1985). SLC6A14-mediated [3H]β-alanine uptake was abolished in the absence of either extracellular Na+ or Cl− (Fig. 2) Figure 3. Concentration-dependent β-alanine-induced current in SLC6A14-expressing oocytes β-Alanine-induced current in two-electrode voltage-clamped oocytes was measured at −60 mV following exposure to β-alanine (0.2–20 mM, 2 min) in a NaCl pH 7.4 solution. Data are expressed as percentage current produced by 20 mM β-alanine (n = 4). C 2008 The Authors. Journal compilation C 2008 The Physiological Society Figure 4. Amino acid induced inward current in SLC6A14-expressing oocytes A, example traces of amino acid-induced currents in SLC6A14-expressing and water-injected oocytes following exposure (2 min) to saturating concentrations (20 mM) of different amino acids in NaCl pH 7.4 solution. B, mean results for experiments described in A using SLC6A14-expressing oocytes where data are expressed as the percentage current induced by 20 mM leucine (n = 7).