Using the whole-cell recording mode of the patch-clamp technique we studied the effects of removal of extracellular potassium, [K(+)](o), on a mammalian Shaker-related K(+) channel, hKv1.5. In the absence of [K(+)](o), current through hKv1.5 was similar to currents obtained in the presence of 4.5 mM [K(+)](o). This observation was not expected as earlier results had suggested that either positively charged residues or the presence of a nitrogen-containing residue at the external TEA(+) binding site (R487 in hKv1.5) caused current loss upon removal of [K(+)](o). However, the current loss in hKv1.5 was observed when the extracellular pH, pH(o), was reduced from 7.4 to 6.0, a behavior similar to that observed previously for current through mKv1.3 with a histidine at the equivalent position (H404). These observations suggested that the charge at R487 in hKv1.5 channels was influenced by other amino acids in the vicinity. Replacement of a histidine at position 463 in hKv1.5 by glycine confirmed this hypothesis making this H463G mutant channel sensitive to removal of [K(+)](o) even at pH(o) 7.4. We conclude that the protonation of H463 at pH 7.4 might induce a pK(a) shift of R487 that influences the effective charge at this position leading to a not fully protonated arginine. Furthermore, we assume that the charge at position 487 in hKv1.5 can directly or indirectly disturb the occupation of a K(+) binding site within the channel pore possibly by electrostatic interaction. This in turn might interfere with the concerted transition of K(+) ions resulting in a loss of K(+) conduction.