Inotropic effect of amrinone in rabbit papillary muscle: reversal of the myocardial depressant effect of halothane.

In rabbit papillary muscles, the pronounced inotropic effect of amrinone (0.4-0.8 mg/mL) observed at low stimulation frequencies (0.1-0.5 Hz) diminished as the frequency was increased to 1.0-2.0 Hz. Reversal of the negative inotropic effect of halothane (1%) by amrinone (0.8 mg/mL) was similarly dependent on stimulation frequency. The contractile response to paired-pulse stimulation in the presence of amrinone (0.8 mg/mL) was summation rather than postextrasystolic potentiation. Amrinone (0.8 mg/mL) was not effective in reversing the negative inotropic effect of halothane (1%) on potentiated state contraction generated by paired-pulse stimulation. These results suggest that amrinone increases the influx of extracellular Ca2+ across the sarcolemma but not the availability of cellular Ca2+ which accumulates, most likely in sarcoplasmic reticulum, under the condition of potentiation.