No abstract

No abstract

In this preparation there was a disparity of effect on Purkinje fibre and papillary muscle. Prolongation of action potential duration and repetitive activations due to early afterdepolarisations originated in Purkinje fibres and were conducted to papillary muscle. Purkinje fibre-papillary muscle interactions are of interest in relation to torsade de pointes arrhythmias which are believed to arise from this mechanism.

No abstract

To investigate the ionic mechanisms underlying the negative chrono- and dromotropic effects of acetylcholine (ACh) on the atrioventricular (AV) node, experiments were carried out in rabbit AV node preparations with the use of double-microelectrode techniques. In current-clamp experiments (n = 6), 10(-6) M ACh hyperpolarized the resting membrane from -42.3 to -58.2 mV, decreased the specific membrane resistance from 2.54 to 1.27 k omega.cm2, decreased the space constant from 556 to 384 microns, and reduced the membrane time constant from 23.0 to 12.4 ms. Under voltage-clamp conditions, 10(-6) M ACh decreased the slow inward current by 11% and activated an ACh-sensitive outward K+ current (IACh). IACh possessed an inward rectifying property and exhibited a relaxation phenomenon with a time constant of 60-100 ms at a membrane potential of 0 to -80 mV. Its reversal potential varied with a slope factor of 56 mV per 10-fold increase in external K+ concentration. These results suggest that 1) IACh is selective for K+, 2) IACh inhibits phase 4 and phase 0 depolarizations and reduces the space constant for electronic current spread in the AV node, and 3) the negative chrono- and dromotropic actions caused by IACh may be augmented by the ACh-induced reduction in the slow inward current.