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

Developed pressure and intracellular Ca2+ ([Ca2+i) decay in postextrasystolic beats following multiple extrasystolic contractions (ESCs) was evaluated with surface fluorometry in atrioventricular-blocked perfused rat hearts loaded with Indo-1. 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 (F400/F510: an index of [Ca2+]i) increased in a monoexponential manner with an increase in the number of ESCs. Both potentiated left ventricular developed pressure and the amplitude of F400/F510 transients returned to control in a monoexponential fashion. Consistent with this exponential decay, the relationship between developed pressure or the amplitude of F400/F510 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 F400/F510 transients. Reducing extracellular Ca2+ 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 +/- P < 0.05 and 0.85 +/- 0.07 --> 0.78 +/- 0.06, n=8, P < 0.05, respectively) and the amplitude of F400/F510 (0.87 +/- 0.06 --> 0.78 +/- 0.05, P < 0.05, and 0.89 +/- 0.08 --> 0.78 +/- 0.07, P < 0.05, respectively). This study confirmed that, in all conditions evaluated, contractile decay was determined by [Ca2+]i decay and RF of contractile decay was an accurate estimate of [Ca2+]i decay in physiologically paced isolated perfused rat hearts.