Sodium currents and action potentials were recorded from myocytes of neonatal rats during acute exposure to thyroid hormone (5–20 nM). One to 5 minutes after addition of thyroid hormone to the bath, decay from peak Na current was slowed, with the fractional current flowing 20 ms after onset (relative to peak current) increasing from 6±5% to 17±13% (p<0.01, n=12). Action potential durations were increased from 55±14 to 86±36 msec (p<0.05, n=6). The effects of thyroid hormone were partially reversed by lidocaine (60 μM, n=5), a specific blocker of a slow sub-population of Na channels. Thus thyroid hormone interacts directly with myocyte membrane, probably by slowing of inactivation of Na channels.

Abbreviations and Symbols T3, 3,5,3′-triiodo-L-thyronine; INa, current carried by sodium ions; IT3, net current attributible to T3 treatment; I20, % of peak current at 20 msec after current onset; ADP90, Time required for action potential to return to within 10% of baseline level

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