AJP - Cell Physiology, Vol 234, Issue 3 90-C95, Copyright © 1978 by American Physiological Society
Properties of thyroidectomized rat extensor muscle
J. Grossie
Basic mechanical and electrical properties of rat extensor muscle were
analyzed 4--6 wk after thyroid removal. Isometric twitch tensions in
thyroidectomized (Tx) rat muscle varied considerably, with over 60% of the
muscles showing abnormally low values and the remainder showing a high
twitch force. The duration of the twitch was significantly increased from
137 to 245 ms but contraction and half-relaxation times were not
significantly changed. Tetanic force was not effected by thyroidectomy.
Electrical properties of the muscle fiber membranes were made exclusively
via intracellular techniques. The resting membrane potential was slightly
higher in thyroidectomized rats (-79 mV) as compared to sham controls (-78
mV). Both direct and indirect action potentials showed higher overshoots,
amplitudes, and rates of depolarization in thyroidectomized rats. The
threshold of the indirect action potential appeared at a higher
transmembrane potential as compared to sham-operated controls. The input
resistance, space constant, time constant, and specific membrane resistance
were all significantly increased in thyroidectomized rat extensor muscle,
whereas fiber diameter and capacitance were significantly decreased.
Estimates of specific ionic conductance show that both potassium and
chloride conductance are decreased in thyroidectomized rat muscle.
