Interaction between overdrive excitation and overdrive suppression in canine Purkinje fibres

The interaction between overdrive excitation and overdrive suppression was studied in canine Purkinje fibres perfused in vitro with a solution containing noradrenaline (10(-6) mol . litre-1) and/or high calcium (8.1 mmol . litre-1). The following results were obtained: 1) in the presence of either high calcium or noradrenaline, a fast drive induced at times an acceleration of the spontaneous rhythm; 2) the simultaneous administration of high calcium and noradrenaline quite often allowed the onset of overdrive excitation that showed different patterns; 3) short (5 to 10 s) and fast (60 to 180 min) drives elicited a rhythm that was fastest immediately after overdrive, gradually slowed toward control and could be followed by suppression; 4) in other instances, overdrive caused little acceleration but failed to induce suppression; 5) drives longer than 15 s usually induced only suppression, although there could be a few beats before suppression; 6) intermittent drive led to an initial excitation and then to suppression; 7) driving at a rate slower than the spontaneous rate caused a temporary suppression of the spontaneous discharge which resumed even if the slow drive continued; 8) excitation was not induced by overdriving in low calcium, even in the presence of noradrenaline. It is concluded that many of the in vivo features of overdrive excitation can be reproduced in vitro in small strands of Purkinje fibres. The relationship between overdrive excitation and overdrive suppression involves the opposing actions of drive on the oscillatory potential and on diastolic depolarisation.