Potassium-related humoral transmission of overdrive suppression

The contribution of potassium to the suppression which follows overdrive (overdrive suppression) was studied in isolated canine hearts perfused in vitro. Complete atrioventricular block was produced by ligating the His bundle. The electrical activity of the ventricles was recorded with locally implanted electrodes. Coronary sinus potassium concentration [K]cs was measured before, during and after 1 to 10 min drives. The collected coronary sinus effluent was reoxygenated and was used to perfuse in vitro Purkinje fiber strands isolated from the same ventricles. The following results were obtained: (1) during fast drive there was an initial K loss and after the drive (associated with a temporary suppression of pacemaker activity) a net K uptake; (2) Purkinje fiber strands isolated from the ventricle and perfused in vitro in all instances underwent a decrease in the spontaneous rate when exposed to the effluent collected during the first minute of overdrive; (3) when the Purkinje fibers were bathed in the coronary sinus effluent collected after the drive, spontaneous activity usually increased and in no instance decreased; (4) adding atropine to the effluent did not alter the observed changes in automatic discharge; and (5) increasing [K]_o in fresh perfusate over the range found in the coronary sinus effluent caused a similar slowing of the Purkinje fiber activity. It is concluded that an increase in [K]_o contributes to overdrive suppression which follows short but not long drives. The humoral transmission of overdrive suppression is thus only related to an increase in coronary potassium concentration.