Barium-induced electro-mechanical uncoupling in myocardial cells

1. 1. The heart of the adult moth Hyalophora cecropia requires extracellular calcium to maintain electrogenesis as well as tension development. In this study we ask whether the processes of autorhythmicity, driven electrogenesis and tension development require calcium specifically or whether the divalent cation Ba 2+ can be substituted for calcium to support these activities. 2. 2. Ba 2+ substituted for Ca 2+ in equimolar amounts caused a marked (25 mV) hyperpolarization, suppression both of pacemaker activity and of tension development in spontaneously beating semi-isolated heart cells. 3. 3. Heart cells bathed in Ba 2+ saline and paced by action potentials (produced by external stimuli) of greatly increased amplitude, prolonged phase 2 (plateau) and increased latency, and after 30 min, no mechanical activity was observed. These changes were completely reversible when calcium was reintroduced. 4. 4. We conclude that Ba 2+ substitution for Ca 2+ is an effective electromechanical uncoupler in moth heart cells. Although Ba 2+ can support electrogenesis, it cannot replace ‘trigger’-Ca 2+ needed to release calcium from sarcoplasmic stores to effect tension development.