Alpha-adrenoceptor stimulation enhances automaticity in barium-treated cardiac Purkinje fibers

We used intracellular microelectrodes to study the effect of alpha-adrenoceptor stimulation on automaticity of sheep cardiac Purkinje fibers exposed to low (10(-5) - 4 X 10(-5) M) barium concentrations. Concentrations of norepinephrine (NE) (3 X 10(-8) - 3 X 10(-7) M), which did not induce spontaneous activity in the absence of barium, consistently induced or enhanced it (if already present) in the presence of barium. Similar results were obtained in the presence of practolol (10(-6) M), which in 13 out of 17 experiments did not counteract the effect of NE. However, phentolamine (3 X 10(-7) M) caused a slowing or blocking of the spontaneous activity induced by NE in barium-treated preparations either in the presence or in the absence of practolol. We conclude that alpha-adrenergic responsiveness is enhanced in barium-treated Purkinje fibers and that under these experimental conditions, alpha-adrenoceptor stimulation may influence automaticity. The possible mechanisms underlying such effects are discussed.