INTERACTIONS BETWEEN SPONTANEOUSLY PACING AND QUIESCENT BUT EXCITABLEHEART-CELLS

The authors previously developed a technique for studying a mathematic al model cell with spontaneous activity, namely, a 'real time' simulat ion of a rabbit sinoatrial node (SAN) model cell that is simultaneousl y electrically coupled via a 'coupling clamp' circuit to a real, isola ted ventricular myocyte. This technique was applied to investigate the effects of coupling conductance (G(c)), cell size and modulation of m embrane potential by elevated extracellular potassium ion concentratio ns on the ability of an ectopic focus, represented by the SAN model ce ll, to successfully drive a ventricular cell. Values of G(c) and the r elative sizes of the two cells define three possible outcomes: spontan eous pacing of the SAN model cell but not driving of the ventricular c ell; cessation of spontaneous pacing; or pacing of the SAN model cell and driving of the ventricular cell. Below a critical size of the SAN model cell, only the first two outcomes are possible. Above this criti cal size, there is a range of G(c) that allows successful operation of the system as an ectopic focus. Elevation of extracellular potassium ion concentrations from 4 to 8 mM increases both the lower and upper b oundaries of G(c) for this range. Elevation of extracellular potassium ion concentrations, commonly observed in myocardial ischemia, may aff ect either inhibition or release of inhibition of an ectopic focus.