An analysis of the effect of the rate of stimulation and adrenaline on the duration of the cardiac action potential.

1. Changes of action potential duration in cat papillary muscle have been correlated with changes of peak tension. It has been assumed that peak tension is an approximate indicator of [Ca2+]i. 2. When stimulation is commenced after a rest of several minutes, or after a decrease or increase of the stimulus rate, or after rest periods of different duration the changes of action potential duration are closely related to changes of peak tension. These results suggest that [Ca2+]i is of primary importance in determining rate-dependent changes of action potential duration, including the shortening of the action potential at high rates of stimulation. 3. The results also indicate the presence of a factor which tends to prolong the action potential at high rates of stimulation. Thus the duration of the action potential at high stimulus rates is longer than at lower rates when measured at a given value of peak tension. Furthermore in low Ca2+ there can be a prolongation of the steady state action potential at high rates. Comparison with the work of Cohen et al. (1976) suggests that this factor is responsible for the polarity of the T-wave of the ECG. 4. The action of adrenaline on action potential duration has also been analysed. It is shown to have two effects--a prolonging effect probably related to the adrenaline induced increase of Isi, and a shortening effect probably related to an increase of [Ca2+]i (as judged by the increase of peak tension).