Anode break excitation in rat and guinea pig ventricular cardiomyocytes

Excitation at the closure of the anode iso ne of the two way sto electrically elicit action potentials in single cells. Despite the fact that ‘anode break’ (AB) excitation has long been observed in cardiac myocytes, very little is known about its mechanism and implications. Using the patch clamp whole-cell technique, we challenged single rat and guinea pig ventricular myocytes with 3 ms hyperpolarizing current pulses in order to estimate the success rate of anode break excitation in the two species (83% and 58% of the tested cells respectively). We measured and compared strength-duration curves for cathodal and anodal stimulations in rat and guinea pig myocytes and found that the ratio between anodal and cathodal rheobases was much higher in guinea pig than in rat (27 vs 18). We also found that maximum rate of depolarization (dV/dtmax) increased during AB stimulation as compared with cathodal stimulation and did more so in rat than in guinea pig (+68% vs +13%). When hyperpolarizing current pulses were consecutively delivered (1 Hz) at a fixed duration (3 ms) and very close to the current threshold for excitation, action potentials were elicited at variable delays after AB. We found that, in such sequences, action potentials amplitude and dV/dtmax decreased linearly with time delay from AB. We finally measured strength-interval curves to investigate cathodal and anodal excitability in diastole and during the refractory period and found supernormal anodal excitability during repolarization phase of rat action potentials. The reported results are in good agreement with a previous study on AB excitation in ventricular myocytes (R Ranjan et al., Biophys J. 74:1850-63, 1998) and suggest for the first time that supernormal excitability of the refractory heart to AB excitation has its source at the cellular level.