The patch clamp method was used to evaluate the effects of 3,5,3'-triiodo-L-thyronine (T3) and carboxyl modification on adult rabbit ventricular Na+ channels. In contrast to TTX-sensitive Na+ channels, Ca2+ block of cardiac Na+ channels was not prevented by selective carboxyl modification by trimethyloxonium (TMO) or water soluble carbodiimide (WSC). In 2 mM Ca2+, TMO-treated patches exhibited 3 discrete conductance (γNa) levels. An abbreviation of mean open time (MOT) accompanied each decrease in γNa. The effects on channel gating of elevating external Ca2+ differed from those of TMO pretreatment. Ensemble averages after TMO showed a shortening of the time to peak current and an acceleration of the rate of current decay. WSC caused a decrease in γNa and an abbreviation of MOT at all potentials tested. We conclude that alteration of the surface potential by a single carboxyl modification is inadequate to explain the effects of TMO and WSC. Physiological concentrations of T3 increased bursting as measured by the ratio of long events (LE) to the total number of events. In the cell-attached patch configuration, addition of 5 nM T3 to the pipette increased the %LE at all potentials examined. The increase had a biphasic voltage-dependence and peaked at -50 mV. A similar increase in the %LE occurred with 50 nM T3 suggesting saturation at ≤5 nM. LEs sometimes were grouped into runs, but the more usual pattern suggested that modal shifts occurred in ~1 s. Addition of T3 to the bath but not the pipette in cell-attached patches failed to alter the MOT, unitary current, or %LE. Na+ channel gating also was unaffected by patch excision or by addition of T3 to the cytoplasmic face of inside-out patches. Nevertheless, with T3 in the pipette, patch excision to the inside-out configuration caused a dramatic increase in the %LE, especially near the threshold potential, and an increase in the MOT. These results suggested that T3 was not membrane permeable during the time scale of the experiments and that T3's action required close proximity to the extracellular face of the Na+ channel.
| Identifer | oai:union.ndltd.org:vcu.edu/oai:scholarscompass.vcu.edu:etd-5587 |
| Date | 01 January 1991 |
| Creators | Dudley, Samuel C. |
| Publisher | VCU Scholars Compass |
| Source Sets | Virginia Commonwealth University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses and Dissertations |
| Rights | © The Author |
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