An effect of noradrenaline on resting potential and Na activity in sheep cardiac Purkinje fibres.

Noradrenaline (NA; 10(-9) to 10(-6) M) depolarizes the cell membrane of quiescent sheep Purkinje fibres at resting potential level. A corresponding inward shift of the holding current occurs in voltage clamped preparations in a potential range between ca. -40 and -100 mV. The depolarizing effect is present in phentolamine (1.5 X 10(-6) M) containing solution, mimicked by isoprenaline, blocked by propranolol (5 X 10(-6) M) and is therefore supposed to be beta-adrenoceptor mediated. The inward shift of the holding current is half maximum at 60 nM noradrenaline. The shift is accompanied with an increase of the intracellular Na activity (aiNa) of the fibres as measured by Na sensitive microelectrodes. Both, the inward shift of the holding current and the accompanying aiNa increased are strongly inhibited by 2 mM CsCl. It is concluded that the depolarizing action of noradrenaline is mainly caused by the known catecholamine induced shift of the steady state activation curve of the pacemaker current (if). The shift increases if in a potential range between -40 and -100 mV and augments thereby aiNa.