Methods: Multicellular preparations from nonfailing and failing human hearts or animals with cardiac hypertrophy were used to study intracellular calcium mobilization. Left ventricular muscle strips were loaded with the intracellular calcium indicator aequorin. Muscle strips were attached to a force transducer and stretched until there was no further increase in active force and stimulated to contract at varying frequencies. Muscles were placed in an oxygenated bath and studied at 30 degrees C. Pharmacological agents were used to increase intracellular sodium or intracellular calcium directly. Agents with known sites of action were then applied to define the original of resulting changes in the amplitude and shape of the caclium transient. Cellular homogenates were also used to study SR Ca(2+) ATPase activity based in a pyruvate/NADH-coupled reaction. Action potentials were also recorded from isolated muscle strips. Findings from isolated myocytes loaded with an intracellular calcium indicator are also reported.
Conclusions: In failing human cardiomyocytes, decreased SERCA2a activity contributes to abnormal calcium handling, elevated diastolic calcium concentrations, and decreased contractility at higher rates of stimulation. Enhanced sodium calcium exchanger activity when working in the reverse mode (ie, transporting calcium into the cell) can potentially worsen calcium mobilization, induce arrhythmias, and negatively impact muscle contraction. Elevated intracellular sodium concentrations can prolong the action potential duration, as well as the time course of muscle contraction, resulting in increased arrhythmogenesis.