The ionic basis of concentration‐related effects of noradrenaline on the action potential of calf cardiac purkinje fibres.

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Abstract
Noradrenaline [norepinephrine] has paradoxical concentration-dependent effects on the Purkinje fiber action potential. At concentrations greater than 500 nM, it shortens the action potential; at lower concentrations it prolongs the action potential. The dose-response relation for Isi [slow inward current] to noradrenaline in Purkinje fibers and found that a noradrenaline concentration of 126 nM produces a half maximal effect. A maintained component of Isi had a similar response to noradrenaline. The dose-dependence of noradrenaline-induced changes in Ix [slowly activating outward current] was determined. The half maximal noradrenaline concentration for this relationship was 133 nM. The dose-response relationships for Ix and Isi were not altered by variation in extracellular Ca2I+. Ix appears to be aproximately twice as responsive as Isi to saturating doses of noradrenaline. These data were incorporated in a computer-generated reconstruction of the effects of noradrenaline on the action potential and are sufficient to account for the paradoxical effects of noradrenaline.