Effects of Norepinephrine, Calcium, and Rate of Discharge on K Movements in Canine Cardiac Purkinje Fibers

We studied the effect of norepinephrine, calcium concentration, and rate of discharge in the presence of different |Ca * ],, on radioactive potassium movements in cardiac Purkinje fibers. The following results were obtained: (1) norepinephrine increases potassium uptake in quiescent fibers and in fibers driven at constant rate, but more in the latter; (2) norepinephrine also increases potassium uptake in quiescent fibers depolarized at the plateau; (3) increasing [Ca]0 increases potassium uptake in fibers driven at constant rate; (4) increasing [Ca],, may decrease K uptake in quiescent fibers; (5) increasing [Ca],, decreases the rate of loss of tissue radioactivity in quiescent fibers and increases it in a driven fiber; (6) increasing the driving rate increases potassium uptake in low and high |Ca|,; (7) high [Ca],, increases K* uptake, especially at low rates; (8) norepinephrine is less effective in increasing K uptake in the presence of a high |Ca|,, We conclude that: (a) norepinephrine increases potassium uptake by different mechanisms; (b) calcium affects potassium movements when it is allowed to enter the cell, presumably by affecting potassium conductance; (c) the effect of an increased rate of discharge on K' uptake may involve stimulation of active K uptake and may include a calcium-dependent component which is largest at high [Ca],, and a slow rate of drive; (d) simultaneous application of two interventions results in a summation that is smallest when one of the mechanisms is already substantially activated.