MECHANICAL RESPONSE OF FROGS AND MAMMALIAN MYOCARDIUM TO MODIFICATIONS OF ACTION POTENTIAL DURATION BY CONSTANT CURRENT PULSES

In isolated preparations of the frog's and the mammalian ventricular myocardium (papillary muscles of cat, rabbit, guinea pig, and rhesus monkey) the duration of the action potential was altered by constant current pulses applied through a sucrose gap. The isometric mechanical response of the polarized muscle section was recorded simultaneously. The following results were obtained: In the mammalian myocardium the contractile response to alteration of one single action potential lasts for several contraction cycles. Shortening of the action potential by anodal current polarization reduces the peak tension of the corresponding contraction only by shortening of the time to peak. The amplitude of the following contraction, however, initiated by a normal action potential, is even more depressed, due to a decreased rate of tension development. Prolongation of the action potential by cathodal pulses has an opposite effect. If the duration of the action potential is altered repeatedly, summation of the delayed negative or positive inotropic responses occurs. Independent of the frequency of stimulation, 5 to 7 beats are necessary for the development as well as the restitution of a steady state. With increasing extracellular Ca++ concentration (up to 5,4 mM/l) the inotropic effects are more pronounced with the first beat, bur are decreased in the steady state. In the frog's myocardium, the contractile response to alterations of the duration of the action potential is limited to the corresponding contraction. The mechanical time to peak and the rate of tension development are influenced simultaneously. The differences between the mechanical response of mammalian and frog's myocardium may explain the different contractile reactions accompanying alternating changes of the duration of the action potential as well as paired pulse stimulation. They are considered as a functional manifestation of some differences of the ultrastructure of the sarcoplasmic reticulum in both tissues. © 1969 Springer-Verlag.