The hyperpolarisation-activated current,I f, is not required for pacemaking in single cells from the rabbit atrioventricular node

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Abstract
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 hyperpolarisation-activated inward current (I f) in generating pacemaker activity in morphologically normal single cells isolated from the rabbit AVN. Whole-cell patch-clamp recordings show that 80%–90% of AVN myocytes do not possessI f, but nevertheless generate spontaneous action potentials with normal pacemaker depolarisations before each action potential upstroke. We have termed this type of cell “type 1”. A small proportion (10%–20%) of spontaneously active AVN cells (type 2) do exhibitI f. A 100 nM solution of isoprenaline increased the action potential rate of type 1 cells by 31%. In these cells isoprenaline did not activate anyI f whereas in type 2 cells it clearly increased the amplitude ofI f. Manganese at 2 mM also increased the amplitude ofI f in type 2 cells, but did not revealI f in type 1 cells. We conclude that, whilstI f may play a role in modulating pacemaker activity in type 2 cells, in the majority of AVN cells (type 1) pacemaker depolarisation normally occurs in the complete absence ofI f. Furthermore, the inability of both isoprenaline and Mn to revealI f in type 1 cells suggests thatI f channels may be absent in these cells.