Effects of α-Adrenergic Stimulation on Intracellular Sodium Activity and Automaticity in Canine Purkinje Fibers

Approximately 60% of adult canine Purkinje fibers respond to α1-adrenergic stimulation with a decrease in automaticity. Recent studies of disaggregated Purkinje myocytes have suggested that this negative chronotropic effect results from α1-adrenergic activation of the Na-K pump. In this study we evaluated 1) whether Na-K pump activation is associated with the negative chronotropic effect of α1-adrenergic stimulation in adult canine Purkinje fibers and 2) if the effect of α-agonists on the pump is direct or mediated by an increase in intracellular sodium activity (aNai). We used sodium selective microelectrodes to determine the effects of 5×10 and 5×10 M phenylephrine on aNai. Phenylephrine decreased automaticity in five of eight Purkinje fibers while an increase occurred in the other three. The rate decrease was always accompanied by a decrease in aNai (−3.9 μM;p