Anomalous response of rabbit papillary muscles to depolarising current: the possible role of the transient outward current. A pharmacological analysis.

Rabbit papillary muscles under current depolarization generate an anomalous double action potential (AP) instead of a usual repetitive activity characteristic of myocardial fibres of different mammalian species. The mechanism of the double AP consisting of a spike-like and a delayed slow component was analysed using pharmacological approach. No changes in the anomalous double AP were observed in the presence of Cs ions. This contrasted with the inhibitory action of 4-aminopyridine (4-Apy). High sensitivity of the phenomenon to 4-APy suggests a contribution of the transient outward current, previously postulated for rabbit working myocardial fibres, to account for the double AP.