Mechanical alternans during acidosis in ferret heart muscle.

Acidosis leads to mechanical alternans (i.e., alternation of large and small contractions) in ferret papillary muscles. This alternation in the size of the contraction is paralleled by alternation in the size of the intracellular Ca2+ transient (monitored using the photoprotein aequorin). In isolated myocytes, the large contraction is accompanied by a prolonged action potential. Mechanical alternans also can be induced by acidosis in isolated myocytes during a train of voltage-clamp pulses. Thus, it appears unlikely that the mechanical alternans is secondary to changes in action potential duration; it is more likely that the observed changes in action potential duration are secondary to changes in the size of the Ca2+ transient. The observation that a Ca2(+)-activated inward current also shows alternation during mechanical alternans provides a possible mechanism for the link between Ca2+ and action potential duration. The alternation in the size of the Ca2+ transient may be secondary to the slowed mechanical restitution observed in papillary muscles during acidosis. This also could explain the observation that decreasing stimulation rate can abolish the alternans.