Bursting of cardiac sodium channels after acute exposure to 3,5,3'-triiodo-L-thyronine.
Physiological concentrations of 3,5,3'-triiodo-L-thyronine (T3) acutely increased burst-mode gating of Na+ channels in rabbit ventricular myocytes. Bursting was measured as the ratio of long events to the total number of events multiplied by 100 (%LE); a long event was defined as a set of openings or a single opening with a total duration greater than or equal to five times the control mean open time (MOT) for cell-attached patches. In the cell-attached configuration, adding either 5 or 50 nM T3 to the pipette increased the %LE. %LE had a biphasic voltage dependence and peaked at -50 mV, although the largest percentage change from control occurred between -30 and -40 mV. Neither unitary conductance nor the overall MOT was altered by T3-induced bursting. However, the MOT of openings within bursts increased, implying a kinetically distinct mode of channel gating during bursts. Long events sometimes were grouped into runs, but the more usual pattern suggested that modal shifts occurred in approximately 1 second. Similar behavior was observed with triiodothyroacetic acid, a T3 analogue that does not elicit protein synthesis. To investigate involvement of soluble second messengers, cell-attached recordings were made with and without T3 in the bath. Placed outside the pipette, 50 and 100 nM T3 failed to alter MOT, unitary current, or %LE. Na+ channel gating also was unaffected by patch excision and by exposing the cytoplasmic face of inside-out patches to 50 nM T3. Nevertheless, excision to the inside-out configuration with 5 nM T3 in the pipette dramatically increased the %LE and lengthened MOT. These results suggest that T3 induced Na+ channel bursting by an extranuclear mechanism that requires proximity of T3 to the extracellular face of the Na+ channel. Furthermore, T3 was not membrane permeant on the time scale of these experiments. Na+ channel bursting may contribute to the propensity for arrhythmias in hyperthyroidism and to the positive inotropic effect of acute T3 administration in the stunned and ischemic myocardium.
Department of Physiology, Medical College of Virginia, Richmond 23298-0551.
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Dudley
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Baumgarten
D000818:Animals; D004594:Electrophysiology; D015640:Ion Channel Gating / Q000187:drug effects; D009206:Myocardium / Q000378:metabolism*; D015290:Second Messenger Systems; D015222:Sodium Channels / Q000187:drug effects* / Q000502:physiology*; D013997:Time Factors; D014285:Triiodothyronine, Reverse / Q000494:pharmacology*
D015222:Sodium Channels; D014285:Triiodothyronine, Reverse