Dose-dependence of 4-aminopyridine plasma concentrations and electrophysiological effects in dogs : potential relevance to ionic mechanisms in vivo.

BACKGROUND: Previous investigators have administered 4-aminopyridine (4AP) to dogs to evaluate the role of transient outward current (I(to)) in vivo; however, plasma concentrations of 4AP were not measured, and it is therefore uncertain which cardiac ion channels were blocked at the concentrations achieved. METHODS AND
RESULTS: We applied high-performance liquid chromatography to measure 4AP concentrations produced by intravenous 4AP administration to dogs. A previously described dose regimen produced plasma concentrations that increased during the maintenance infusion but never exceeded 250 micromol/L and caused significant mortality. Whole-cell patch-clamp experiments on isolated canine myocytes showed that even the maximum 4AP concentrations achieved in vivo failed to alter ventricular I(to) and had very small effects on atrial I(to); however, concentrations achieved in vivo had a strong inhibitory effect on the dog ultrarapid delayed rectifier (I(Kur.d)), present only in atrial cells. We designed a loading and maintenance infusion regimen to produce stable 4AP plasma concentrations. At concentrations in the range of 25 and 50 micromol/L, 4AP had no effect on ventricular refractory period but increased atrial refractoriness significantly, consistent with the results of voltage clamp studies.
CONCLUSIONS: The interpretation of previous studies using intravenous 4AP administration to inhibit I(to) in dogs in vivo needs to be reevaluated in light of the fact that the infusion regimens used produce plasma concentrations that are inadequate to affect ventricular I(to). Our findings also support the concept that selective inhibition of ultrarapid delayed rectifier current can prolong atrial refractory periods without affecting ventricular refractoriness.