Metabolic depression and myocardial potassium

The ionic basis of the anti-ischemic and anti-arrhythmic properties of magnesium in the heart

The role of magnesium (Mg) in the prevention of ischemia-induced injury during cardioplegic arrest and in the treatment of cardiac arrhythmias has been considered. Although Mg possesses negative inotropic properties, potassium (K) is more effective than Mg in inducing cardiac arrest. The rationale for the inclusion of Mg in cardioplegic solutions therefore lies not in its cardioplegic properties, but in its ability to influence other cellular events such as the loss of Mg and K and perhaps to counter the detrimental effects of ischemia by antagonizing calcium (Ca) overload. Most of the Mg in the cardiac cell is complexed with high energy phosphate compounds and the loss of Mg during ischemia may restrict the repletion of ATP upon reperfusion and so impair the return of normal contractile function. The ability of Mg to limit K efflux from the cell is of importance not only in the prevention of ischemia-induced K loss but also in the treatment of digitalis-induced arrhythmias. Elevation of extracellular Mg has been shown to reduce the intracellular sodium ion activity ([Na]i) and this decline in [Na]i can be related to the negative inotropic properties of Mg. Mg may therefore exert some of its antiarrhythmic and antiischemic effects by limiting [Na]i-stimulated Ca influx (or facilitating Ca efflux) and hence preventing cellular Ca overload.

Intracellular potassium activity in guinea pig papillary muscle during prolonged hypoxia

During prolonged hypoxia, intracellular potassium concentration, [K](i) has been reported to fall by 70% with a concomitant decrease in the calculated potassium equilibrium potential, E(K). Nevertheless, resting membrane potential, V(m), declined only slightly. Because V(m) depolarized very little in relation to the calculated E(K), it was hypothesized that electrogenic Na-K pumping contributed up to 40 mV to V(m) during prolonged hypoxia. To further test this hypothesis we studied what changes prolonged hypoxia makes in the thermodynamically active fraction of cellular potassium, intracellular potassium activity, alpha(K) (i), and how change in alpha(K) (i) affects the relationship between V(m), E(K) and, by inference, the Na-K pump. Using double-barrel K-selective electrodes, V(m) and alpha(K) (i) were measured in quiescent guinea pig right ventricular papillary muscles superfused for 8 h with hypoxic Tyrode's solution. Over the 8-h period both V(m) and alpha(K) (i) decreased. However, the decline in V(m) was paralleled by a decrease in the E(K) calculated from alpha(K) (i). At no time was there hyperpolarization of V(m) beyond E(K). After 8 h the Na-K pump was inhibited by exposing the muscles to 0.1 mM ouabain. The onset of an increase in extracellular potassium activity, measured with a double-barrel electrode, was used to mark the amount of depolarization of V(m) due solely to pump inhibition. After hypoxia, V(m) depolarized 8.4+/-4.4 mV before extracellular potassium activity (alpha(K) (e)) increased. Thus, the decrease in alpha(K) (i) during hypoxia is much less than that reported for [K](i). The parallel decline in V(m) and E(K) and the small depolarization of V(m) with ouabain suggest that after prolonged hypoxia the Na-K pump continues to contribute to V(m), but the amount of this contribution is substantially less than previously reported.

Heterogeneity of intracellular potassium activity and membrane potential in hypoxic guinea pig ventricle

The relationship between membrane potential (Em) and the potassium equilibrium potential (EK) was investigated in hypoxic guinea pig papillary muscle. After more than 8 hours of hypoxia, cells with near normal Em (-86.2 +/- 0.9 mV) and action potentials were observed. However, the intracellular potassium concentration ([K+]i) based on chemical analysis and the assumption that potassium was homogeneously distributed was 41.8 +/- 4.3 mM; the apparent EK was -55.7 +/- 2.9 mV, significantly positive to Em. Measurements with potassium ion-selective microelectrodes revealed that prolonged hypoxia results in at least two populations of cells with different characteristics. The first population had an intracellular potassium activity (aiK) of 101.5 +/- 1.9 mM, and EK was 4.7 mV negative to Em. In contrast, EK was 33.4 +/- 1.3 mV negative to Em in the second population. These cells also exhibited a reduced sensitivity to changes in bath potassium, and calculations suggest aiK was about 18 mM. The existence of cell populations with a near normal and very low aiK can explain the intermediate value of [K+]i calculated assuming a homogenous potassium distribution. Cells with near normal Em and action potentials represent the population with near normal aiK. Hypoxia may also cause non-uniform changes in other cellular characteristics.

Ionic currents during hypoxia in voltage-clamped cat ventricular muscle

To explore the mechanisms underlying the shortening of the cardiac action potential in hypoxia, we studied the effect of hypoxia on the ionic currents in cat papillary and trabecular muscles using the single sucrose gap-voltage clamp technique. For potentials positive to -70 mV, hypoxia induces an increase in time-independent outward current. The changes in the tail current suggest that time-dependent outward current is not increased but, rather, reduced. Because the time course of ik remains unchanged, we concluded that the shortening of the action potential is not a result of a change in the time-dependent outward current. In the potential range of the plateau, the amplitude of the slow inward current is not affected by hpoxia. Its time constant of inactivation appears slightly decreased. The prolongation of the action potential by epinephrine during hypoxia is accompanied by an increase in the slow inward current. As a result of these studies, we conclude that the shortening of the cardiac action potential in the early stage of hypoxia results from an increase in K+ outward background current.

Metabolism and the electrical activity of anoxic ventricular muscle

1. The action potential duration of anoxic guinea-pig ventricular muscle was related to ATP generated by glycolysis. In 50 mM glucose medium the action potential duration was maintained; in 5 mM glucose medium the action potential duration shortened, the glycolytic rate declined and the ATP content was reduced.2. The action potential amplitude was related to the metabolic state of the muscle but not to the intracellular sodium concentration.3. It is suggested that changes in the action potential duration and overshoot in anoxic muscle may be due to an influence of metabolism on the slow inward current.4. Anoxic muscle incubated for 8 hr in 5 mM glucose medium had an E(m) of -77.1 mV compared to -81.1 mV in fresh muscle. The calculated E(k) of anoxic muscle was -47.4 mV.5. The resting potential of anoxic muscle was separated into two components, one dependent on potassium distribution and the other on the activity of an electrogenic sodium pump.6. The electrogenic pump component was stimulated upon raising the glucose concentration of the medium or upon raising the external potassium concentration.7. The electrogenic pump component was inhibited by ouabain or by reduction of the temperature from 35 to 8 degrees C.