THE HYPERPOLARIZATION-ACTIVATED CURRENT, I-F, IS NOT REQUIRED FOR PACEMAKING IN SINGLE CELLS FROM THE RABBIT ATRIOVENTRICULAR NODE

The atrioventricular node (AVN) is vital for cardiac function. One of its properties is that it can act as a pacemaker for the ventricles if the sinoatrial node fails. This study investigates the role of the hy perpolarisation-activated inward current (I-f) in generating pacemaker activity in morphologically normal single cells isolated from the rab bit AVN. Whole-cell patch-clamp recordings show that 80%-90% of AVN my ocytes do not possess I-f, but nevertheless generate spontaneous actio n potentials with normal pacemaker depolarisations before each action potential upstroke. We have termed this type of cell ''type 1''. A sma ll proportion (10%-20%) of spontaneously active AVN cells (type 2) do exhibit I-f. A 100 nM solution of isoprenaline increased the action po tential rate of type 1 cells by 31%. In these cells isoprenaline did n ot activate any I-f whereas in type 2 cells it clearly increased the a mplitude of I-f. Manganese at 2 mM also increased the amplitude of I-f in type 2 cells, but did not reveal I-f in type 1 cells. We conclude that, whilst I-f may play a role in modulating pacemaker activity in t ype 2 cells, in the majority of AVN cells (type 1) pacemaker depolaris ation normally occurs in the complete absence of I-f. Furthermore, the inability of both isoprenaline and Mn to reveal I-f in type 1 cells s uggests that I-f channels may be absent in these cells.