Contractile and intracellular Ca2+ decay in potentiated contractions following multiple extrasystolic beats

Abstract Developed pressure and intracellular Ca 2+ ([Ca 2+ ] 1 ) decay in postextrasystolic beats following multiple extrasystolic contractions (ESCs) was evaluated with surface fluorometry in atrioventricular-blocked perfused rat hearts loaded with Indo-11. After priming pacing at 400 ms intervals, 1–25 ESCs were interposed with a 160 ms interval, followed by 30 postextrasystolic beats with a 400 ms interval. Both left ventricular developed pressure and the amplitude of the Indo-1 fluorescence ratio (F 400 /F 510 ; an index of [Ca 2+ ] i ) increased in a monoexponential manner with an increase in the number of ESCs. Both potentiated left ventricular developed pressure and the amplitude of F 400 /F 510 transients returned to control in a monoexponential fashion. Consistent with this exponential decay, the relationship between developed pressure or the amplitude of F 400 /F 510 transients in a postextrasystolic beat and that in the preceding beat was linear and the slope of a fitted line (recirculation fraction; RF) was evaluated as an index of rapidity of decay. The number of ESCs did not affect RF of developed pressure and the amplitude of F 400 /F 510 transients. Reducing extracellular Ca 2+ concentration (1.25 → 0.55 mM), and perfusion with an acidic solution (pH = 6.8) significantly decreased RF of both developed pressure (0.85 ± 0.06 → 0.78 ± 0.05, n = 8, P P 400 /F 510 (0.87 ± 0.06 → 0.78 ± 0.05, P P 2+ ] i decay and RF of contractile decay was an accurate estimate of [Ca 2+ ] i decay in physiologically paced isolated perfused rat hearts.