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Transmembrane action potentials and contractions were recorded from pieces of human atrial muscle stimulated electrically in Krebs solution. These preparations exhibit a new type of behavior characterized by periodic "cycling" between two phases: high-phase contractions are two to eight times those in low phase, and action potentials have two or more spikes. The phenomenon can occur in a piece of any size, and is most likely at temperatures between 20 C and 33 C. Acetylcholine weakens contractions, reduces duration of action potential (AP) plateau, and usually abolishes double spikes and cycling; this suggests relatively low K+ permeability during the long low plateau. Epinephrine shortens the cycling period and makes low- and high-phase contractions more nearly equal; additional epinephrine leads to bizarre APs and spontaneous activity. This behavior of human atrial tissue reflects intrinsic capabilities of the cell membrane not previously observed in heart. General hypotheses are proposed to account for the double-spiked APs, for cycling, and for relations between the unique contraction patterns and AP configurations.

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