BETA-ADRENERGIC REGULATION OF ACTION-POTENTIALS AND AUTOMATICITY IN YOUNG AND ADULT CANINE PURKINJE-FIBERS

To investigate developmental changes in the cellular electrophysiologi cal effects of beta(1)- and beta(2)-adrenoceptor stimulation on canine Purkinje fibers (PF), we studied the effects of isoproterenol, a nons elective beta-agonist, and salbutamol, a preponderantly beta(2)-agonis t, alone or in presence of the selective beta(1)-antagonist CGP-20712A or the beta(2)-antagonist ICI-118551. Standard microelectrode techniq ues were used to study adult and neonatal (<11 days) PF paced at a cyc le length of 1 s or allowed to beat spontaneously. In paced adult PF, isoproterenol significantly increased the maximum diastolic potential and significantly decreased action potential duration at 60 and 90% of full repolarization (APD(60) and APD(90)) in a concentration-dependen t fashion. These effects were not observed in neonatal PF, which inste ad manifested a significant increase in phase 2 amplitude and APD(30) that was not observed in adult PF. All these effects occurred as well with salbutamol but were less pronounced and required higher agonist c oncentrations. Isoproterenol decreased the automatic cycle length of a dult fibers from 4,079 +/- 796 ms during controlto 2,190 +/- 229 ms at 10(-5) M (P < 0.05) and from 1,535 +/- 105 to 1,249 +/- 90 ms in neon atal PF (P < 0.05). In both adults and neonates, > 90% of this effect was reached at a concentration of 10(-8) M. Salbutamol had the same ef fect but required higher concentrations. In both adults and neonates, the beta(2)-antagonist ICI-118551 did not modify any of the effects of isoproterenol and salbutamol, whereas the beta(1)-antagonist CGP-2071 2A significantly antagonized them. In conclusion, 1) the effects of be ta-adrenergic stimulation on transmembrane potentials of canine PF cha nge during development, both qualitatively (in paced PF) and quantitat ively (in automatic PF) and 2) the responses seen are attributable to the activation of beta(1)- and not beta(2)-adrenoceptors.