[Characteristics of the isometric myocardial contractions during its activation by the slow response (author's...

The characteristics of the isometric contractions of rabbit's left atria were studied during its activation by the slow response of the cardiac action potential. When compared to the conditions where activation occurs with normal action potentials it verifies that contraction amplitude decreases, becoming slowly and larger. Positive inotropic effects that enhance calcium entrance through the membrane (Ca and IPA), during action potential, increase contraction amplitude. Negative inotropic effects that depress the slow response (Ach and verapamil) also depress the contractions. Staircase becomes reversed, with contraction strength decreasing with increase in rate of beating. It was seen that adequate conduction is an important factor for the normality of contractions, and the importance of the fast component of the cardiac action potential, for this occurrence, was remembered.