Faculty Bibliography

Epinephrine promotes spontaneous activity in cardiac Purkinje fibers through its action on the pacemaker potassium current (i(KK2)). The mechanism of the acceleratory effect was studied by means of a voltage clamp technique. The results showed that the hormone speeds the deactivation of i(KK2) during pacemaker activity by displacing the kinetic parameters of i(KK2) toward less negative potentials. This depolarizing voltage shift is the sole explanation of the acceleratory effect since epinephrine did not alter the rectifier properties of i(KK2), or the underlying inward leakage current, or the threshold for i(NNa). The dose dependence of the voltage shift in the i(KK2) activation curve was similar in 1.8 and 5.4 mM [Ca](o). The maximal voltage shift (usually approximately 20 mV) was produced by epinephrine concentrations of > 10(-6) M. The half-maximal effect was evoked by 60 nM epinephrine, nearly an order of magnitude lower than required for half-maximal effect on the secondary inward current (Carmeliet and Vereecke, 1969). The beta-blocker propranolol (10(-6) M) prevented the effect of epinephrine (10(-7)M) but by itself gave no voltage shift. Epinephrine shifted the activation rate coefficient alpha(8) to a greater extent than the deactivation rate coefficient beta(8), and often steepened the voltage dependence of the steady-state activation curve. These deviations from simple voltage shift behavior were discussed in terms of possible mechanisms of epinephrine's action on the i(KK2) channel

1. Experiments on sheep Purkinje fibres were designed to determine whether the current mechanisms responsible for delayed rectification at the pace-maker (negative to -50 mV) and plateau (positive to -50 mV) ranges of potential are kinetically separable and independent.2. Hyperpolarizations from the plateau range were shown to produce decay of a single component of outward current within the plateau range, but two components were evident when the hyperpolarizations entered the pace-maker range.3. The time courses of recovery of the two components were too similar at -25 mV to allow temporal resolution at this potential. Clear temporal resolution was, however, possible at potentials between -55 and -95 mV. An indirect method of resolving the two components at -25 mV was used.4. The kinetic properties of the two components correspond to those previously described for the pace-maker potassium current, i(K) (2), and the outward plateau current, i(x) (1) (Noble & Tsien, 1968, 1969a).5. The instantaneous (fully activated) current-voltage relation for i(K) (2) was reconstructed from the analysed current records. It was found that this relation shows a negative slope conductance at all potentials positive to -75 mV and that the current tends towards zero at zero membrane potential.6. The results are compared with those predicted by two reaction models of the i(K) (2) and i(x) (1) mechanisms. It is concluded that i(K) (2) and i(x) (1) are kinetically separable but that it is not possible with present techniques to decide whether they are controlled by the same or completely independent membrane structures. It is also shown that the instantaneous current-voltage relation calculated for i(K) (2) does not depend on whether the controlling mechanisms are assumed to be independent or linked

1. The influence of diastolic interval on ionic currents that may determine the action potential duration in cardiac Purkinje fibres was investigated. As the diastolic interval is shortened from about 5 sec, the first effect on the action potential is to reduce and then abolish the notch at the beginning of the plateau.2. This effect corresponds to the influence of diastolic interval on the magnitude of a transient outward chloride current known as the ;dynamic current'.3. Further shortening of the diastolic interval produces a slight shortening of the action potential until intervals less than about 500 msec are used. The action potential then becomes considerably shorter. The ;time constant' of decay of this major influence of one action potential on the duration of the subsequent action potential is about 200 msec.4. This effect corresponds to the time course of decay of an outward (mainly K) current known as i(x1).5. It is shown that variations in the magnitude of i(x1) may be responsible for the alternation in action potential duration at the beginning of a train of stimuli known as ;electrical alternans'.6. The results in general are consistent with the view that i(x1) is the main current involved in determining the interval-duration relation although they cannot exclude the possibility that an inward current with a reavailability time course similar to the decay time course of i(x1) might also be involved

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