[Effect of paired stimulation on the electrical and mechanical activity of myocardial cells in congenital and acquired heart defects].

Thin myocardial strips were dissected from the papillary muscles of patients undergoing mitral valve replacement (group I) and atrial tissue of patients undergoing the correction of congenital septal defects (group II) and mitral stenosis (group III). Electrical activity and isometric tension were measured. Paired stimulation (PS) with time intervals of 250-350 ms and 380-440 ms activated two action potentials (AP) for each potentiated contraction (80 and 70% of force increase, respectively) in bundles of groups I and III. Strips of group III had depressed level of resting potential (RP) and different types of AP including AP with additional spike activity in the plateau phase. The inotropic effect of PS depends on the level of the resting potential. If RP was more than 45 mv PS activated two AP or one doubled AP for each potentiated contraction (80-120% of force increase). If RP was less than 35 mv PS was not able to initiate doubled AP and to initiate the increase of contractility. Epinephrine (10(-6) g/ml) increased the depressed RP and AP in group III bundles and potentiated the inotropic effect of PS in groups I and III. Verapamil (2 X 10(-6) g/ml) inactivated the doubled AP and abolished the inotropic effect of PS in group III strips. It is supposed that additional inflow of Ca during excitation produces the inotropic effect of paired stimulation in human myocardium.