Developmental changes in alpha adrenergic effects on canine Purkinje fiber automaticity.

Canine cardiac Purkinje fiber automaticity is modified by adrenergic agonists. Alpha adrenergic agonists slow and beta agonists increase the rate of automatic discharge. These effects vary with maturation and development. We used standard microelectrode techniques to study the effects of the alpha agonist, phenylephrine, on automaticity in isolated neonatal (less than 10 days) and adult canine cardiac Purkinje fibers. Fibers were superfused with 5 X 10(-8) M phenylephrine dissolved in Tyrode's solution. 75% (24/32) of adult and 52% (12/23) of neonatal fibers showed a decrease in rate. The magnitude of the decrease was similar at both ages and the effect was blocked during superfusion with 1 X 10(-9) M phentolamine. The remaining adult and neonatal fibers demonstrated a significant increase in rate in response to 5 X 10(-8) M phenylephrine. This effect was blocked by 5 X 10(-7) M propranolol in the adult group but not in the neonates. However, the increase in rate of fibers from 0- to 2-day neonates was blocked by phentolamine. As an indicator of adrenergic innervation of neonatal hearts, we assayed the myocardial norepinephrine concentration. There was a 6- to 8-fold increase in concentration during the first 10 days of neonatal life, suggesting that innervation was rapidly increasing during this time. In summary, neonatal Purkinje fibers can show an alpha adrenergic-induced acceleration of automatic rate. This alpha adrenergic acceleration is not seen in adults. The change in response from neonate to adult may be effected by growth and development of autonomic nerves and associated changes in receptor function.