Nucleotide metabolism in cardiac activity. I. Methods

Glibenclamide specifically blocks ATP-sensitive K+ channel current in atrial myocytes of guinea pig heart

Effects of glibenclamide on the control membrane ionic currents, acetylcholine or adenosine-induced K+ current, and nicorandil-induced K+ current were examined in single atrial myocytes of guinea pig heart. The nystatin-whole cell clamp technique was used. Nicorandil evoked the time-independent K+ current which is probably the current through the ATP-sensitive K+ channel. Glibenclamide inhibited this current in a concentration-dependent fashion, although it had no effect on the other currents. We concluded that glibenclamide specifically inhibits the ATP-sensitive K+ channel current in cardiac myocytes.

Background K+ current in isolated canine cardiac Purkinje myocytes

The current-voltage (I-V) relation of the background current, IK1, was studied in isolated canine cardiac Purkinje myocytes using the whole-cell, patch-clamp technique. Since Ba2+ and Cs+ block IK1, these cations were used to separate the I-V relation of IK1 from that of the whole cell. The I-V relation of IK1 was measured as the difference between the I-V relations of the cell in normal Tyrode (control solution) and in the presence of either Ba2+ (1 mM) or Cs+ (10 mM). Our results indicate that IK1 is an inwardly rectifying K+ current whose conductance depends on extracellular potassium concentration. In different [K+]0's the I-V relations of IK1 exhibit crossover. In addition the I-V relation of IK1 contains a region of negative slope (even when that of the whole cell does not). We also examined the relationship between the resting potential of the myocyte, Vm, and [K+]0 and found that it exhibits the characteristic anomalous behavior first reported in Purkinje strands (Weidmann, S., 1956, Elektrophysiologie der Herzmuskelfaser, Med. Verlag H. Huber), where lowering [K+]0 below 4 mM results in a depolarization.

ATP-regulated K+ channels in cardiac muscle

An outward current of unknown nature increases significantly when cardiac cells are treated with cyanide or subjected to hypoxia, and decreases on intracellular injection of ATP. We report here that application of the patch-clamp technique to CN-treated mammalian heart cells reveals specific K+ channels which are depressed by intracellular ATP (ATPi) at levels greater than 1 mM. For these channels, conductance in the outward direction is much larger than the inward rectifier K+ channel which is insensitive to ATP. AMP had no effect on the ATP-sensitive K+ channel, and ADP was less effective than ATP. Thus, the ATP-sensitive K+ channel seems to be important for regulation of cellular energy metabolism in the control of membrane excitability.

Effect of exogenous adenosine triphosphate on the metabolic state of the excised hypothermic dog heart

Solutions of adenosine triphosphate (ATP) were injected into the left coronary artery of isolated nonperfused dog hearts kept for 30 to 180 minutes at temperatures varying from 4 to 34°C. The amount of ATP administered varied from 0.5 to 7 µmoles/g heart. The left ventricles of the ATP-treated hearts had a higher content of adenine nucleotides and of phosphocreatine than did the left ventricles of control hearts not exposed to ATP. This effect was temperature-dependent and was maximal at 14°C. Glycogen disappearance in the hypothermic myocardial tissue was markedly slowed in a dose-dependent fashion by the administration of ATP. Injections of adenine and adenosine were without effect.

An analysis of the intra- and extracellular distribution of simultaneously administered adenosine-8 -14 C-triphosphate and adenosine triphosphate-α- 32 P shows that the injected ATP was mainly split into ADP, AMP, adenosine, and inorganic phosphate and indicates that a minor percentage of these fission products entered the myocardial cells, some of the ADP and AMP being rephosphorylated there to ATP. The results suggest that intravascular introduction of ATP into the arrested hypothermic heart might help in the survival of the organ.

Effect of Ischemia and Reoxygenation on Glycolytic Reactions and Adenosinetriphosphate in Heart Muscle

During a 15-minute period of myocardial ischemia, glycogen concentration fell. This decrease was roughly accounted for by increases in hexosemonophosphate and lactic acid. Fructose 1,6-diphosphate, dihydroxyace-tone-phosphate and pyruvate did not accumulate. The myocardial concentration of adenosinetriphosphate declined. Reperfusion of the ischemie heart with oxygenated blood was accompanied by inconstant changes in myocardial glycogen and by rapid lowering of hexosemonophosphate. The hexosemonophosphate decrease occurred even when the ischemie myocardium was perfused with a deoxygenated buffered salt solution and is attributed in part at least to lactic acid washout. Adeno-sinetriphosphate rose steadily when the heart was perfused with oxygenated blood containing 15 mmoles KCl to prevent cardiac activity. When KCl was not added, fibrillation and irregular contractions occurred within 1 to 2 minutes. In these instances, the rise in adenosinetriphosphate was irregular and inconstant.