The Effects of Carboxyl-group Specific Modification and Triiodo-L-thyronine on Cardiac Sodium Channels

Author

Samuel C. DudleyFollow

DOI

https://doi.org/10.25772/247W-H165

Defense Date

1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Physiology and Biophysics

First Advisor

Clive M. Baumgarten

Abstract

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 (γ_Na) levels. An abbreviation of mean open time (MOT) accompanied each decrease in γ_Na. 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 γ_Na 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.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

9-26-2016