In histidine-decarboxylase knockout mice as well as in their wild types R (1-30 μM) shortened AP duration at 90% level of repolarization (APD90) and increased the AP amplitude (APA) in a concentration-dependent manner. In rat ventricular papillary muscle cells R (1-30 μM) caused a significant reduction of APA and maximum velocity of depolarization (Vmax) which was accompanied by lengthening of APD90.
In single canine ventricular myocytes at concentrations ≥10 μM R decreased the amplitude of phase-1 repolarization, the plateau potential and reduced Vmax. R suppressed several ion currents in a concentration-dependent manner under voltage clamp conditions. The EC50 value for this inhibition was 25.2±2.7 μM for the transient outward K+ current (Ito), 72.3±9.3 μM for the rapid delayed rectifier K+ current (IKr), and 82.5±9.4 μM for the L-type Ca2+ current (ICa) with Hill coefficients close to unity. The inward rectifier K+ current (IK1) was not affected by R up to concentrations of 100 μM. Suppression of Ito, IKr, and ICa has been confirmed under action potential voltage clamp conditions as well.
The observed alterations in the AP morphology and densities of ion currents may predict serious proarrhythmic risk in case of intoxication with R as a consequence of overdose or decreased elimination of the drug, particularly in patients having multiple cardiovascular risk factors, such as elderly diabetic patients.