THE EFFECTS OF MANGANESE AND BARIUM ON THE CARDIAC-PACEMAKER CURRENT, IF, IN RABBIT SINOATRIAL NODE MYOCYTES

The isolation of ionic fluxes contributing to electric currents through cell membranes often requires block of other undesired components which can be achieved, among others, by divalent cations. Mn2+ and Ba2+ are often used, for example, to block Ca and K currents. Here we have investigated the effects of these two cations on the properties of the hyperpolarization-activated pacemaker current i(f), in rabbit sino-atrial node myocytes, as obtained by voltage clamp analysis. We find that 2 mM Mn2+ shifts the i(f) activation curve by 3.2 +/- 0.3 mV towards more positive values. However, when 1 mM Ba2+ is also added, the positive shift is more than halved (1.3 +/- 0.2 mV). We find, too, that in the absence of blocking cations the ACh-induced i(f) inhibition is slightly higher than in their presence. These results indicate that the alteration of i(f) kinetic properties by Ba2+ plus Mn2+ -containing solutions is minimal.