Faculty Bibliography

Gas chromatography is applied to improve the selectivity of sensitive organic colour tests without the use of complicated apparatus

1. The membrane currents in Purkinje fibres under voltage clamp conditions have been investigated in the range of potentials at which the action potential plateau occurs. The results show that in this range slow outward current changes occur which are quite distinct from the potassium current activated in the pace-maker range of potentials.2. The time course of current change in response to step voltage changes is non-exponential. At each potential the current changes may be analysed in terms of the sum of two exponential changes and this property has been used to dissect the currents into two components, i(x1) and i(x2), both of which have been found to obey kinetics of the Hodgkin-Huxley type.3. The first component, i(x1), is activated with a time constant of about 0.5 sec at the plateau. At more positive and more negative potentials the time constants are shorter. The steady-state degree of activation varies from 0 at about -50 mV to about 1 at +20 mV. The instantaneous current-voltage relation is an inward-going rectifier but shows no detectable negative slope. In normal Tyrode solution ([K](0) = 4 mM) the reversal potential is about -85 mV.4. The second component, i(x2), is activated extremely slowly and the time constant at the plateau is about 4 sec. The steady-state activation curve varies from 0 at about -40 mV to 1 at about +20 mV. The instantaneous current-voltage relation is nearly linear. The reversal potential occurs between -50 and -20 mV in different preparations.5. It is suggested that these currents are carried largely by K ions, but that some other ions (e.g. Na) also contribute so that the reversal potentials are positive to E(K).6. The relation of these results to previous work on delayed rectification in cardiac muscle is discussed

1. The results of the voltage clamp experiments described in a preceding paper (Noble & Tsien, 1969) have been used to reconstruct the repolarization process in Purkinje fibres.2. The results show that, at the beginning of the plateau, the instantaneous current-voltage relation has a region of net inward current which, in the absence of additional outward current, would prevent repolarization.3. The activation of the outward current, i(x1), overcomes this region of inward current within about 300 msec. This first phase of the plateau is limited mainly by the speed of activation of i(x1) and, during this time, there exists a threshold for all-or-nothing repolarization. The calculated time course of this threshold corresponds well with that recorded experimentally in uniformly polarized preparations.4. Once the net inward current has been abolished, the rate of repolarization is mainly limited by the membrane capacity, although i(x1) continues to activate until about 500 msec. The outward current, i(x1), begins to deactivate during the rapid terminal phase of repolarization, but it is not fully deactivated at the end of the action potential. A second action potential initiated at this time would therefore be shorter than the first.5. The effect of the initial outward current transient, recently identified as largely chloride current (Dudel Peper, Rudel & Trautwein, 1967 a), has been calculated. The result is a notch at the beginning of the plateau similar to that often recorded experimentally. The action potential duration is not greatly influenced by this current component.6. The role of outward currents other than i(x1) in repolarization is discussed. It is concluded that the outward current component primarily responsible for terminating the action potential may vary and depends on the action potential duration. In Purkinje fibres with action potentials of normal duration, i(x1) is the main time-dependent outward current involved