Overdrive excitation: Onset of activity following fast drive in cardiac Purkinje fibers exposed to norepinephrine
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Purkinje fibers perfused in vitro in 5.4 mM K Tyrode remained quiescent in the presence of 8.8×10−7 M norepinephrine. When these fibers were driven at 30/min and then overdriven at 120/min for 2 min, the cessation of drive was followed by the onset of spontaneous activity (“overdrive excitation”). It was found that overdrive excitation: 1. required both overdrive and norepinephrine to occur; 2. occurred when during the overdrive there was an increase in maximum diastolic potential (Emax) but not ifEmax decreased; 3. was caused by a steepening of diastolic depolarization following overdrive; 4. was not inhibited by a burst of fast drive; and 5. subsided through a progressive decrease ofEmax and of the slope of diastolic depolarization.
It is proposed that overdrive excitation is due to a faster fall of the slowly changing potassium current to a smaller value. These changes, in turn, are caused by the combined action of: 1. a hyperpolarization due to both overdrive and norepinephrine; and 2. a shift in a depolarizing direction of the relationship between voltage and steady state slow potassium conductance due to norepinephrine.
Key wordsCardiac Purkinje Fibers Fast Rate Norepinephrine Automaticity
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