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Signature Currents: A Patch-Clamp Method for Determining the Selectivity of Ion-Channel Blockers in Isolated Cardiac Myocytes

Background: We describe a simple method using membrane potential ramps for rapidly determining the ion-channel selectivity of drugs that affect action-potential duration in iso lated cardiac myocytes. The method allows the simultaneous assay of compounds on a number of ionic currents in a single cardiac cell. Methods: Trains of membrane potential ramps were applied from -90 to +70 mV at 0.33 Hz to obtain a consistent "signature current," in which the major individual currents involved in the cardiac action potential could be easily identified. Confirmatory experiments were per formed using known inhibitors of these currents. Results: The identities of the currents in the signature were established by varying the concentrations of extracellular cations and by adding known ion channel blockers to super- fusion solutions. Inhibition of each current had a characteristic and reproducible effect on the overall signature current. Conclusions: The consistent current signature in the presence and absence of blockers sug gests that this method could be used for tertiary electrophysiological evaluation of com pounds, eg, in a drug discovery program focusing on antiarrhythmic agents. The ability to assay for secondary effects of novel compounds against multiple currents in the target cell type is convenient and avoids the artefacts associated with using artificial expression systems.