A pace-maker-like current in the sheep atrium and its modulation by catecholamines.

A modified single-sucrose-gap system was used to study sheep atrial trabeculae under voltage-clamp conditions. A time- and voltage-dependent current system is described, which resembles the current if in Purkinje fibres. This current was activated at membrane potentials of between -60 and -70 mV in many fibres. The addition of Ba2+ reduced the instantaneous current (the 'jump') and thus facilitated the study of the current if. Current tails were more prominent in the presence of TTX and Mn2+. Most experiments were done in the presence of Ba2+, Mn2+ and TTX. Standard envelope tests and conductance measurements indicated that this current is an inward current, activated on hyperpolarization. We have also labelled the atrial current if. The instantaneous fully activated current-voltage relationship, i(E) was found to be linear in the activation range. Increasing the level of K+, which increased the current magnitude, also increased the slope of the i(E) curve. The current magnitude was also dependent on the level of Na+ in the medium. The current magnitude was increased by adrenaline or isoprenaline. Only a small part of the increase could be attributed to a shift in the voltage dependence of the gating kinetics. The shifts in activation curves were much smaller (3-4 mV in the depolarizing direction) than those in Purkinje fibres. Large shifts in activation curves were obtained with theophylline, indicating that the presence of Ba2+ or Mn2+ did not occlude any shifts by adrenaline. The magnitude of if was increased by theophylline, with a further increase by adrenaline. There is therefore no mutual occlusion of the two effects on if. The slope of the i(E) curve was increased by isoprenaline, indicating that there was an increase in conductance. The presence of propranolol did not prevent the increase in current amplitude by isoprenaline. A direct effect of catecholamines on the channel is suggested.