Activity-dependent changes in the electrical behaviour of sheep cardiac Purkinje fibres

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Abstract
Rate-dependent changes in the electrical activity of sheep Purkinje fibres maintained at 37.degree. C have been investigated. The duration of the action potential is maximal at a frequency of about 60 min-1. When the rate is increased above 60 min-1. When the rate is increased above 60 min-1 there is a substantial shortening of the action potential; this occurs abruptly in the first beat at the higher rate although subsequently there can be further changes in duration and these can result in a small prolongation, no change, or a small further shortening of the action potential and can take up to 10 min to reach a steady-state. When the rate is reduced from 60 min-1 there is also a shortening of the action potential but it occurs gradually over several hundred seconds. Action potential duration reaches a minimum value at a rate of about 6 min-1. 70% of preparations studied showed an increase in duration again at rates below 6 min-1 duration is always constant at frequencies below about 0.1 min-1. The maximum diastolic potential is more negative and the pacemaker potential is larger at higher rates of stimulation. When the frequency is raised these variables increase over a time course lasting several hundred seconds. At rates below 60 min-1 the slow changes in action potential duration, maximum diastolic potential and pacemaker potential, after a change in the stimulus frequency, all have similar monoexponential time courses (.tau. .apprxeq. 3 min) and are accompanied by slow changes in tension production over a similar time course. In Purkinje fibres that exhibit spontaneous activity, rapid stimulation results in overdrive excitation: an acceleration of spontaneous activity when stimulation is ceased.