Effect of mercuric chloride on contractility and transmembrane potential of the guinea-pig myocardium.

1. The effect of HgCl2 in concentrations from 1 times 10(-4) to 4 times 10(-3) M on the force of contraction of the guinea-pig papillary muscles was investigated. A rapidly developing negative inotropic effect was found to be interrupted by a transient increase in contractile force. 2. In papillary muscles from reserpine-pretreated animals only the decrease of force of contraction was observed. Thus the positive inotropic effect is due to an indirect sympathomimetic action of HgCl2. 3. Mercuric chloride in concentrations above 1 times 10(-3) M raised the threshold of stimulation in more than half the muscles. 4. HgCl2 shortened the time to peak force (t1) and the relaxation time (t2); in muscles of animals pretreated with reserpine, however, t2 increased above the control value. 5. When exposed to HgCl2 all muscles developed a contracture beginning during the 5th min. 6. The time course of membrane resting potential, overshoot, and duration of action potential gradually decreased under the influence of mercuric chloride. The direct negative inotropic effect had a slightly faster onset than had the changes of the electrical parameters.