Tetanus in the mammalian heart: studies in the shrew myocardium

The purpose of the present study was to test the possibility that tetanus can occur in the shrew myocardium, in which the ventricular action potential is similar to that of skeletal muscle. Action potentials and tension were recorded from the right ventricular muscle using glass microelectrodes and a force transducer. When the regular twitch (at a cycle length of 2000 ms) was followed by a train of stimuli having an internal cycle length of less than 50 to 60 ms, unfused and fused tetanus readily developed. Action potentials were generated at the frequency of the stimuli within the train. With trains of sufficient duration, the tension during the tetanus usually reached a steady state and exceeded the tension of the regular twitch preceding the train. Tetanus with similar characteristics was also observed during an arrhythmic train which was occasionally triggered spontaneously by a regular stimulus. Simultaneous recordings of the action potential and the twitch demonstrated that the shrew ventricular muscle can be tetanized, since the action potential is very short (3 to 4 ms) and terminates before any signs of mechanical activity begin. The present study demonstrates that mammalian myocardium can be tetanized when action potential duration is much shorter than the duration of the mechanical event.