The effects of noradrenaline, applied by microelectrophoresis on different cells of the rabbit's atrial pacemaker

Noradrenaline has been applied by microelectrophoresis to a small portion of the atrial pacemaker area in the rabbit's heart in order to study the local effects of this chemical transmitter separately from the ones deriving from other parts of the pacemaker area. In a first group of cells, whose action potential and location assimilate them to true pacemaker cells, noradrenaline caused a reduction in cycle length and an increase in the steepness of slow diastolic depolarization. In a second group of cells similar to latent pacemaker cells, noradrenaline caused no change in cycle length, the outstanding effect being an increase in the steepness of the slow diastolic depolarization which afterwards changed into a subthreshold oscillation. A third type of cells showed intermediate characteristics between the two previous groups. These results suggest that: a) the chronotropic effect of noradrenaline on the heart atrial pacemaker seems to be due to changes in the steepness of slow diastolic depolarization which can assume, in some instances, the shape of subthreshold oscillations; the effects on the other parameters in our preparation seem to be either less constant or less significant; b) the different effects which are obtained on various kinds of cells seem to be the result of a different degree of sensitivity to noradrenaline and to the more or less premature activation of mechanisms antagonizing the action of noradrenaline. The results are discussed on the basis of a model of spontaneous atrial pacemaking which has been recently proposed.