Action of sex steroids on the electrical and mechanical properties of rat atrium

¹ Department of Pharmacology, University of Southern California School of Medicine, Los Angeles, California

The simultaneous effects of estradiol, testosterone, and progesterone on the electrical and mechanical characteristics of atrial cells were examined. Intracellular potentials were determined using microelectrodes; the mechanical activity was recorded from a strain gauge. Testosterone from 10^–6 to 10^–5 m produced a marked stimulation in normally contracting atria; while similar concentrations of progesterone and estradiol produced only contractile depression. All three sex steroids exerted a definite effect on the electrical characteristics, namely a reduction in the rates of depolarization and repolarization, without altering the magnitudes of the resting or action potentials appreciably. As a result, there is an increase in the duration and area of the action potential. They also slowed the conduction rate and increased the latent period, effects presumably associated with the slowed depolarization. The marked reduction in the rates of depolarization and repolarization is believed to be due to decreases in the ionic fluxes brought about by reducing the ionic permeability of the cardiac cell membranes. The effects are similar to those produced by antidysrhythmic drugs and may explain certain observations on the relation of cardiac excitability to steroid levels.