The Action of β-Adrenoceptor Antagonist Penbutolol on the Pacemaker Current of Cardiac Purkinje Fibres

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The action of β-adrenoceptor antagonist Penbutolol on the current underlying pacemaker activity in cardiac Purkinje fibres was analysed using the voltage clamp technique described by DECK et al.(1964).
After the application of adrenalin, β-blockers are able to counteract the well known shift of the s-kinetics of the pacemaker current. However, without any prior application of adrenalin the β-blocker Penbutolol has no effect on these kinetics except for a small depression of the am-plitude of the pacemaker current tails. The rectifier properties of the pacemaker current and the negative slope of the fully activated current vol-tage relationship of i_K2 are unchanged.
Penbutolol is able-even after a longer period of washout (about 60-90 min was necessary)-to protect the β-adrenoceptors from the action of adrenalin (HASHIMOTO et al., 1979). These findings suggest that β-blockers are competitive inhibitors of β-stimulators and further support the notion that the pacemaker current in cardiac Purkinje fibres is controlled by β-adrenoceptors.