Norepinephrine, potassium and overdrive suppression

The influence of norepinephrine on ventricular overdrive suppression and attendant potassium shifts has been studied in isolated perfused canine hearts with complete atrioventricular block. It was found that: 1) there is a potassium loss during the drive and a potassium uptake after the drive); 2) reducing the driving rate from 240 to 120/min decreases potassium loss; 3) norepinephrine increases potassium uptake in spontaneously beating ventricles and during the recovery from 120/min drive; 4) norepinephrine enhances K loss during and after a 240/min drive; 5) norepinephrine shortens the overdrive pause under all the conditions tested; 6) in ventricles driven at a constant rate, norepinephrine causes a small loss of potassium; 7) reserpinized hearts show a small potassium loss during drive and a larger potassium uptake after drive; yet, the suppression is longer; 8) norepinephrine increases K loss with drive and decreases overdrive suppression in reserpinized hearts; 9) norepinephrine enhances the increase in oxygen consumption caused by overdrive; and 10) norepinephrine antagonizes the depressant effect of high [K]_o on automaticity. It is concluded that norepinephrine shortens the pause independently of potassium levels and antagonizes the inhibitory influence of high K. The effect of norepinephrine on K movements depends on the ventricular rate and such rate-dependence is related to oxygen availability with respect to the increased metabolic demand.