Effekte von ATP und Kalium auf die Schrittmacher-Automatie bei Frosch- und Warmblüterherzen

Summary

The effects of potassium and ATP upon the nodal tissue of frog heart and upon the Purkinje fibers of monkey heart were investigated by direct microscopic observation as well as by intracellular microelectrode recording.

It was found that:

• 1. A step-wise increase in the extracellular potassium concentration eventually leads to the disappearance of electrical activity in all portions of the frog heart, although the pacemaker region exhibits a perceptably greater potassium resistance than the non-specialized myocardial musculature. Upon raising the extracellular potassium concentration, the excitability and the degree of spontaneous activity exhibit a decrease in the following sequence; ventricular musculature, atrial musculature, sinus pacemaker, atrioventricular pacemaker, bulbus pacemaker.

• 2. If the heart is pre-treated with ATP (0.15 mg/ml), the potassium resistance of both non-specialized and pacemaker tissues is increased in essentially the same manner.

• 3. With regard to the effect upon frequency, potassium and ATP exhibit a synergistic effect in frog pacemaker tissue, since both potassium and ATP, are capable of raising the rate of spontaneous discharge. ATP acts by accelerating the slow diastolic depolarization, while the primary action of an increased extracellular potassium concentration is to decrease the pacemaker potential.

• 4. The electrical activity of the pacemaker cells, as well as the facilitating effect of potassium and ATP upon the discharge rate, is most clearly observed at those high extracellular potassium concentrations which eliminate the ability for full conducted impulses. In this respect, the intrinsic pacemaker activity, following the disappearance of action potentials, is seen as rhythmical local depolarizations which become difficult to detect at membrane potential values under 20 millivolts.

• 5. In monkey-heart Purkinje fibers, in contrast to the frog heart, ATP exhibits a depressive frequency effect. Likewise with the monkey Purkinje fibers, ATP causes a retardation of the slow diastolic depolarization.