THE APPLICATION OF A TRAIN OF SUPRAMAXIMAL STIMULI DURING THE ABSOLUTE REFRACTORY PERIOD OF A CARDIAC MUSCLE PREPARATION HAS TWO EFFECTS: a depression of the contraction during which it is applied, and a large potentiation of subsequent contractions. The former is ascribed to a direct effect upon the cell membrane, and is an indication of the continued control of the contractile event by this membrane. The latter is explained as a sudden liberation of norepinephrine by a stimulation of embedded nerve elements, which norepinephrine then distributes itself through the tissue and finally diffuses away.
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Brady et al. (1960) studied this question.