| Abstract: | Pacemaker activity in atrial muscle and in Purkinje fibres is generated by a time-dependent decay of potassium current that allows the membrane to be depolarized to the threshold for action potential initiation. The kinetics of the pacemaker potassium currents in these two parts of the heart are sufficiently different to indicate that they correspond to different membrane structures. This conclusion is strengthened by the discovery that the mechanisms of acceleration produced by adrenaline are also quite different. In Purkinje fibres, the activation threshold for the potassium current is shifted in a depolarizing direction with no change in maximum amplitude. This voltage shift is adequate by itself to explain the acceleration. In atrial fibres the pacemaker potassium current is increased in amplitude with no shift in threshold. By itself, this action of adrenaline would slow pacemaker activity and the acceleration in this case is dependent on a large increase in the current attributable to calcium ions. The roles of cyclic 3',5'-AMP and of intracellular calcium ions in mediating the pacemaker actions of adrenaline will also be discussed. |
