Figure 5. D540A phenotype and sensitivity to inhibition by Na⁺_o.

A, representative family of D540A currents elicited by a series of depolarizing test pulses in 0 [Na⁺]_o, 0 [K⁺]_o. Note the fast activation and strong rectification of current and the sizable current induced by even a modest depolarizing step (−40 mV, arrows). B, representative experiment examining Na⁺_o inhibition of D540A. Note return of current to original level after washout, which contrasts markedly with the result in D540K (Fig. 4). C, D540A inhibition placed in context of WT concentration–response curve for Na⁺_o inhibition (same data as in Fig. 2A). Summarized results from several experiments like B (n = 3–4, open circles) suggest that D540A (open circles) is quite similar to WT HERG (filled squares) in its sensitivity to inhibition by Na⁺_o.