ON THE MOLECULAR NATURE OF THE LIDOCAINE RECEPTOR OF CARDIAC NA+ CHANNELS - MODIFICATION OF BLOCK BY ALTERATIONS IN THE ALPHA-SUBUNIT III-IV INTERDOMAIN

The mechanism of inhibition of Na+ channels by lidocaine has been suggested to involve low-affinity binding to rested states and high-affinity binding to the inactivated slate of the channel, implying either multiple receptor sites or allosteric modulation of receptor affinity. Alternatively, the lidocaine receptor may be guarded by the channel gates. To test these distinct hypotheses, inhibition of Na+ channels by lidocaine was studied by voltage-clamp methods in both native and heterologous expression systems. Native Na+ channels were studied in guinea pig ventricular myocytes, and recombinant human heart Na+ channels were expressed in Xenopus laevis oocytes. Fast inactivation was eliminated by mutating three amino acids (isoleucine, phenylalanine, and methionine) in the III-IV interdomain to glutamines or by enzymatic digestion with alpha-chymotrypsin. In channels with intact fast inactivation, lidocaine block developed with a time constant of 559+/-42 ms (n=7) at membrane potentials between -50 and +20 mV, as measured by use of twin pulse protocols. The IC50 was 36+/-1.5 mu mol/L. Control channels inactivated within 20 ms, and slow inactivation developed much later (time constant of slow inactivation, 6.2+/-0.36 s). The major component of block developed long after activated and open channels were no longer available for drug binding. Control channels recovered fully from inactivation in <50 ms at -120 mV (time constant, 11+/-0.5 ms; n=50). In 30 mu mol/L lidocaine, 49+/-3% of the current recovered with a second larger time constant of 395+/-36 ms (n=5). After removal of fast inactivation, either enzymatically or by mutagenesis, the channels were no longer blocked by 50 mu mol/L lidocaine (n=8). Much higher concentrations of lidocaine produced a tonic block (IC50, 0.4+/-0.07 mmol/L; n=13) and time dependence suggestive of open-channel block. The results indicate the importance of inactivated state block by therapeutic concentrations of lidocaine and suggest that the molecular site of action is the structural region of the channel that is responsible for inactivation.