Effects of propafenone on calcium current in guinea‐pig ventricular myocytes

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Abstract
1 The modulation of L-type voltage-sensitive calcium channels in guinea-pig isolated ventricular myocytes by propafenone was examined by the whole cell voltage-clamp technique. 2 Propafenone, 10−7 −5 × 10−5 m, produced a concentration-dependent inhibition of Ca current (ICa) without any significant change in the current-voltage relation. Half-blocking concentration (IC50) of propafenone for the peak ICa at +10 mV was 5 × 10−6 m. 3 The voltage-dependence of ICa inactivation was shifted in the hyperpolarizing direction in the presence of 5 × 10−6 m propafenone. 4 A frequency-dependent relative block by propafenone was observed after repetitive depolarizing test pulses at a frequency of 0.5 and 1 Hz. The recovery of ICa from inactivation was prolonged by propafenone and the reactivation exhibited an additional exponential component attributed to the slow release from drug block of Ca channels. 5 These results suggest that propafenone, at therapeutic concentrations exhibits Ca channel blocking properties that may be involved in its antiarrhythmic mechanism.