Developmental Changes in the Effects of Beta-Adrenergic Blocking Concentrations of Propranolol on Canine Purkinje Fibers

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Abstract
We used standard microelectrode techniques to investigate the developmental changes in the effects of the beta-blocker propranolol on canine Purkinje fibers. The effects of propranolol on the transmembrane action potential were similar in neonatal and adult Purkinje fibers. In automatic Purkinje fibers we obtained concentration-response (increase in rate) curves for isoproterenol alone and with an increasing concentration of propranolol. The curve for isoproterenol in neonatal Purkinje fibers had an EC(50) of 1.0 X 10^-9 M, about 5-fold lower than in adult Purkinje fibers (EC(50) = 5.03 X 10^-9 M, p < 0.05). At both ages, propranolol caused a parallel, concentration-dependent shift of the response curves for isoproterenol, with no change in the maximum effect. Equiactive dose ratios in presence and abence of 3 concentrations of propranolol were compared by the Schild plot. The pA; values for adult and neonate Purkinje fibers were 8.32 (8.14-8.46; 95% confidence level) and 8.86 (8.69-9.03; 95% confidence level), respectively. Our results indicate significant (p < 0.05) developmental changes in the beta-blocking effects of propranolol in canine Purkinje fibers.