Developmental Changes of α1-Adrenergic Chronotropic Action on Human Sinus Node

The important role of α1-adrenergic receptors in the genesis of cardiac arrhythmias in ischemia and during reperfusion1,2 has recently been elucidated. On the other hand, α1-adrenergic modulation of cardiac rhythm has promoted interest in myocardial α1-adrenoceptor research. The chronotropic action of α1-adrenergic stimulation is usually reported as inhibitory rather than excitatory.3 However, in some species and in certain tissues, the positive chronotropic action of α1-adrenergic stimulation has also been reported. Moreover, there is an important developmental component to the α1-adrenergic receptor-effector system. Although α1-adrenergic stimulation decreases automaticity in the human atrium and in the ventricular conducting system of the adult rat,4,5 an increase in automaticity brought about by α1 adrenergic stimulation was reported in isolated neonatal canine Purkinje fibers.6 These findings suggest that the chronotropic response of cardiac tissues to α1-adrenergic stimulation changes from excitatory to inhibitory during development. This study clarifies the effect of α1-adrenergic stimulation on human sinus node automaticity, especially in relation to age.