The contribution of Na and K ions to the pacemaker current in sheep cardiac Purkinje fibres

Abstract

The ionic components of the pacemaker current are quantitatively analysed in sheep cardiac Purkinje fibres by simultaneous measurements of the intracellular Na activity (a ⁱ_Na ) and the membrane current under voltage clamp. The pacemaker current is operationally defined as the Cs inhibited membrane current (I _Cs) in Ba containing media at clamp potentials negative to −60 mV. At these potentials solutions containing CsCl (0.2–5 mM) shift the holding membrane current into the outward direction and simultaneously decreasea ⁱ_Na . The Cs effects on membrane current anda ⁱ_Na display a similar voltage dependence. A Cs inhibited Na influx contributes toI _Cs. The ratioI _Cs/(Cs inhibited Na influx in electrical units) is <1 at membrane potentials positive to the potassium equilibrium potentialE _K and >1 at potentials negative toE _K. The ratio is close to 1 atE _K suggesting Na ions to be the only carriers of the current atE _K whereas K ions contribute toI _Cs at potentials different fromE _K. The effects of Cs on the Cs inhibited Na influx andI _Cs show a very similar dose dependence. The effect is half maximum at ∼0.2 mM CsCl (in 21.6 mM K; clamp potential: −85 mV). An increase of the external K concentration augmentsI _Cs and the Cs inhibited Ca influx. Na and K ions carryingI _Cs probably cross the membrane via an identical channel. The permeability of the channel for K⁺ is about 10–20 times larger than for Na⁺. TheI _Cs reversal potential of a fibre bathed in a medium containing 5.4 mM K is estimated to be −50 to −60 mV.