Electrophysiology of the mechanisms of action of the yew toxin, taxine, on the heart

The effects of the sulfate salt of taxine, the alkaloid mixture of the yew (Taxus baccata L.), on membrane currents of enzymatically isolated single ventricular cells of guinea pig were studied under current- or voltage clamp conditions. It was found that the drug inhibits both the sodium (INa) and the calcium current (ICa). The mean decrease of ICa and the maximum rate of rise of the action potential (dV/dtmax), an index of INa, and their standard errors under the influence of 10(-6), 10(-5), 10(-4) g/ml of taxine (control 100%) were 87.1 +/- 2.9, 67.8 +/- 2.8, 24.4 +/- 2.0 for the amplitude of ICa and 75.4 +/- 3.7, 53.3 +/- 7.5, 9.4 +/- 1.1 for dV/dtmax, respectively. Recovery from the effects of taxine is partially possible. Under the influence of taxine, inconsistent effects were observed on the potassium outward current. In conclusion the effect of taxine on the heart results from its ICa and INa inhibiting properties similar to those of other antiarrhythmic drugs.