Masayosi Goto

Excess lactate modulates ionic currents and tension components in frog atrial muscle

Abstract Effects of lactate, an end-product of anaerobic metabolism, on electrical and mechanical activities in the myocardium of the bullfrog atrium were studied under voltage clamp and unclamped conditions using the double sucrose-gap method. Lactate (0.1 to 20 m m ), buffered to normal pH (7.4) was introduced into the perfusate. Levels of lactate over 1.0 m m produced a negative inotropic effect accompanied by a biphasic alteration of the action potential, initial transient prolongation and late shortening of the plateau. Voltage clamp studies on the membrane currents isolated in modified Ringer solutions showed that lactate decreased the slow inward calcium current (ICa) but markedly augmented the slow inward sodium current (INas), while the delayed outward current (Ix) was gradually increased. The fast inward sodium current (INaf) and the background current (IKl) remained unchanged. As for the tension under voltage clamp, application of lactate produced an immediate decrease in the ICa-dependent tension, while ICa-independent tonic tension was not modified appreciably. These findings suggest that lactate has a specific action which modulates the ionic currents responsible for cardiac action potential, and that the mode of action of lactate differs from that of protons.

Effects of Ach on slow inward current and tension components of the bullfrog atrium

Effects of acetylcholine (ACh) on the bullfrog atrial muscle were studied by the double gap method under voltage clamped and unclamped conditions. Slight decrease in amplitude and great reduction in duration of the action potential were produced by ACh (10 −6 g/ml) without altering the resting membrane potential. Twitch tensions markedly decreased while contracture tensions due to Na-free or K-excess (100 mM) Ringer solutions were augmented. In voltage clamp experiments, ACh markedly reduced the slow inward current and the I Ca -dependent component of contraction for short depolarizing pulses (less than 100 ms) without affecting time-to-peak tension. For longer pulses, The phasic component was depressed while the tonic component was somewhat enhanced. Steady state activation and inactivation variables of the slow inward current ( d ∞ and f ∞) were not affected by ACh whereas the limiting conductance of Ca (¯g Ca ) was markedly reduced. The time constant of the activation (τd), and that of restoration of calcium conductance were not influenced. It is concluded that ACh exerts its negative inotropic effect on the myocardium not only by shortening of action potential due to increase in potassium conductance but also by decrease in calcium inward current due to reduction in ¯g Ca .

ELECTROPHYSIOLOGICAL ACTIONS OF MEXILETINE (Kö1173) ON CANINE PURKINJE FIBRES AND VENTRICULAR MUSCLE

1 The effects of mexiletine (Kö1173) were investigated in canine isolated cardiac Purkinje fibres and ventricular muscle with microelectrodes. Some Purkinje fibres were depolarized by mechanical stretch to induce spontaneous activity with slow upstroke velocity. The preparations were stimulated at rates of 1, 2, 3 and 4 Hz. The drug concentrations tested were 0.4, 2 and 10 μg/ml in Tyrode solution (KCl = 5.4 mm). 2 The ‘therapeutic’ drug concentration (2 μg/ml) shortened action potential duration and effective refractory period of Purkinje fibres, the effect being pronounced at lower stimulation rates. In ventricular fibres, action potential duration changes were not consistent while the effective refractory period was prolonged. 3 In depolarized Purkinje fibres showing automatic activity, the drug (0.4 or 2 μg/ml) depressed phase 4 depolarization and reduced the firing rate without changing maximum diastolic potential. However, when depolarized Purkinje fibres were electrically driven at a constant rate, the maximum diastolic potential became more negative with a concomitant decrease of pacemaker slope and increase of maximum rate of rise (V̇max) of action potentials. 4 Moderate (2 μg/ml) to high (10 μg/ml) concentrations of the drug depressed V̇max in Purkinje fibres stimulated at 2 Hz by 12 and 42% respectively and depressed ‘membrane responsiveness’. The decrease in V̇max depended upon the stimulation rate, being minimum at the lowest (1 Hz) and maximum at the highest (4 Hz) stimulation rate. 5 The drug (2 μg/ml) improved V̇max of the earliest propagated premature action potentials by shifting the takeoff potential to more negative levels in both Purkinje and ventricular fibres. 6 Membrane conductance in fibres mounted in a single sucrose gap chamber was increased by the drug (2 μg/ml) in both fibre types in normal and in Na+ ‐deficient solutions. This increase was attributed to an increase in membrane K+ permeability produced by the drug. 7 All these effects are similar to those of lignocaine, diphenylhydantoin or aprindine, and can explain the antiarrhythmic action of mexiletine.

Membrane currents related to configuration changes in the action potential of frog atrial muscle in Na- and Ca-free conditions

Changes in membrane currents and action potential in frog atrial muscle under Ca-free or Na-deficient conditions were investigated using double sucrose-gap techniques. In Ca-free Ringer, the instantaneous current (Ik1 and others) decreased and the delayed outward current (Ix) was markedly retarded. The slow inward current (Islow) diminished in the absence of the calcium inward current (ICa). The remaining slow sodium inward current (INa slow) was less than one fourth in amplitude of the control Islow, and showed a faster activation and a slower inactivation. In Na-deficient Ringer when LiCl or sucrose replaced NaCl, the Islow increased in amplitude, despite the elimination of INa slow. The Ik1 also increased, but the amplitude of fully activated Ix diminished although the activation of Ix appeared much faster than in the control. Thus, it became clear that in a Ca-free condition, the overshoot of the action potential (AP) decreased due to the absence of ICa, and the AP-duration was prolonged due to the pronounced delay of activation of Ix and the depression of Ik1. The INa slow was too small to contribute to the delayed repolarization of AP. In Na-deficient conditions, the AP-amplitude increased due to enhancement of ICa, and the AP-duration was shortened mainly as a result of the fast activation of Ix and due to the increase of Ik1.

Effects of exogenously applied guanosine triphosphate on membrane current and tension of bullfrog atrial muscle

The effects of exogenously applied guanosine triphosphate (GTP) were studied on bullfrog atrium by a double-gap method under voltage clamped and unclamped conditions. GTP produced an immediate augmentation of contraction showing its maximal effect within 5 to 10 min and a secondary gradual depression after longer perfusion. The positive inotropic effects of GTP were similar to those of adrenaline, although they were not prevented by the most effective dose of propranolol. Voltage clamp study revealed that GTP enhanced the slow calcium inward current (ICa) and ICa-dependent phasic tension, while inhibited the ICa-independent tonic tension. The delayed outward current was initially depressed and later augmented with time. The fast sodium inward current, the anomalous rectification and the background currents were virtually unchanged. GTP may act on the sarcolemmal level, at least on a site closely related to the movement of calcium ions through the slow channel, and on one or more unrelated sites. The exact mechanism of these effects is uncertain, but GTP exerts a positive inotropic effect concommitant with an increase in the slow inward current which was not prevented by the β-blocking agent.