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Electromechanical relationships of rabbit papillary muscle under interpolated extrasystole conditions and after a
Abstract:
The aim of the study was to describe and attempt to explain certain specific features of electromechanical coupling in the rabbit myocardium. Electromechanical correlations in the papillary muscles of the right ventricle of adult rabbit hearts were studied by a programmed stimulation technique. The duration of action potentials (AP) was measured in the plateau phase (Do, ms) and at -80 mV level (D80, ms), together with the intensity of the corresponding isometric contractions (MG, arbitrary units). After twenty AP of 1 Hz frequency, we interpolated an extrasystole with a variable interval (TE = 100-900 ms) and measured D0 and MG of the premature AP and the first AP of the subsequent cycle. When a steady state at 1 Hz frequency had been reached, we interpolated pauses (Tp) of 5 to 600 s and read D80 and MG of the first to the tenth contraction after the pause. The extrasystole D0 attained the maximum at TE = 260 ms and then fell abruptly. The MG of extrasystoles with a longer TE grew from the lowest value (0.3), attained 1 at TE = 700 ms and then remained stable. The first contraction after extrasystole displayed distinct postextrasystolic potentiation (MG = 2), while D0 was unwontedly short. Prolongation of TE was accompanied by an increase in its D0 value and by a steep drop in MG (to as little as 0.2). D80 of the first AP evoked after the pause fell proportionally to log Tp and then, from Tp = 60 s, gradually rose. The MG value of the first contraction after the pause fell proportionally to log Tp. The AP recovered much more rapidly than contractility from the effect of the pause. In the discussion, an attempt is made to explain the found correlations on the basis of differences in the behaviour of the calcium current channel system in the rabbit myocardium and a commentary on electromechanical correlations is based on the hypothesis that the free sarcoplasmic calcium level determines both membrane electrogenesis and the inotropic state.