Effect of a pause on action potentials of the working ventricular myocardium of human embryos.

The effect of a period of rest (Tp) lasting from 5 to 120 s on the action potentials (AP) of the ventricular myocardium of 9- to 11-week human embryos was studied. The result was shortening of the AP proportional to the duration of the pause and it was accompanied by a shift of the AP plateau phase to more negative membrane voltages. Shortening of the AP during the pause was more pronounced in solutions with a half extracellular calcium concentration. Recovery from the effect of the pause took place significantly more slowly in solutions with the lower extracellular calcium concentration than in the presence of a normal concentration.