Autoradiographs of SDS-PAGE separated apically enriched human airway membranes which had been incubated with thiosulphate (T), sulphate (S), nitrate (N), chloride (C), bromide (B), acetate (A), iodide (I) or gluconate (G) and γ [32P]-ATP/GTP (see Treharne et al., 1994 for details). Each salt contains 50 mM Mg2+. Figure reproduced with permission from AJP.