The effects of extracellular potassium, ouabain, and prostaglandins on intracellular potassium activity in sheep cardica Purkinje fibers

Dipyridamole suppresses catecholamine- and Ca++ influx-sensitive ventricular arrhythmias

To study the mechanism of ventricular arrhythmias, the effect of dipyridamole (DIP; 300 mg/day), an adenosine transport inhibitor, on ventricular premature contractions (VPCs) was assessed in 12 patients who showed VPCs (21312 +/- 12314/day) on Holter ECG in a controlled setting. The effects were compared with those of verapamil (240 mg/day) and bisoprolol (5 mg/day). DIP suppressed more than one-half the VPCs in 5 patients. The mean degree of reduction in these DIP-responders was 75 +/- 18%. Both verapamil and bisoprolol inhibited VPCs in all of the DIP-responders (verapamil: 71 +/- 15%, bisoprolol: 88 +/- 16%). Two of the 5 DIP-responders had sustained ventricular tachycardias (VT) that were terminated by intravenous DIP, ATP, acetylcholine, verapamil, and propranolol. In contrast, verapamil did not inhibit VPCs in any of the DIP-nonresponders. Bisoprolol also did not suppress VPCs in 3 of 6 DIP-non responders. heart rate was unaffected by DIP, but was suppressed by both verapamil and bisoprolol. In addition, DIP increased the serum concentration of adenosine (control 16.3 +/- 17.1 vs 22.3 +/- 19.0 pmol/ml after DIP, p < 0.05). The inhibitory effect of DIP may involve suppression of Ca+2 current through an extracellular increase in adenosine.

The cardioplegic solution HTK: effects on membrane potential, intracellular K+ and Na+ activities in sheep cardiac Purkinje fibres

The effects of the cardioplegic solution HTK on membrane potential (EM) and intracellular K and Na activities (aiK, aiNa) were studied in sheep cardiac Purkinje fibres by means of conventional and ion-selective microelectrodes. HTK contains (mM): Na 15, K 10, Ca 0, Mg 4, histidine 180. (1) In control conditions EM was -74.3 +/- 3.3 mV (n = 25), aiK was 116.4 +/- 4.1 mM (n = 7) and aiNa was 8.2 +/- 1.4 mM (n = 15). (2) Exposure to HTK led to a depolarization to -59.7 +/- 3.6 mV (n = 25) which exceeded by about 5-7 mV that induced in a Tyrode solution of 10 mM K and in a modified HTK solution supplemented by 2 mM Ca (n = 6). (3) Addition of 0.5 mM barium eliminated the difference in the steady-state depolarization. (4) HTK superfusion increased aiK to 120.1 +/- 4.4 mM (n = 7) and decreased aiNa to 3.9 +/- 0.9 mM (n = 15). (5) The decrease in aiNa was insensitive to amiloride (1 mM) and to external alkalization but was slightly increased by addition of 2 mM calcium. (6) When the calcium in Tyrode solution was lowered from 2.0 mM to 0.05 mM, aiNa hardly decreased during subsequent exposure to unmodified HTK and it increased in the presence of 0.1 mM dihydroouabain. We propose the hypothesis (1) that the difference in membrane depolarization between HTK and a 10 mM K-Tyrode is caused by a decrease in K conductance by the HTK solution and (2) that the aiNa decline mainly results from a coupled Ca influx via Na-Ca exchange due to a delayed washout of external calcium.

The basis for the membrane potential of quiescent cells of the canine coronary sinus

During prolonged periods of quiescence, the membrane potential of cells in the isolated canine coronary sinus, exposed to normal Tyrode solution containing 4 mM-K, declines to about -60 mV. The nature of the resting potential was investigated, in small strips of coronary sinus tissue mounted in a fast-flow system, by recording the membrane potential responses to sudden changes in the extracellular ionic environment. At extracellular K concentrations ([K]o) from 0 to 64 mM the resting potential was little affected by replacing all but 1 mM of external Cl ions with isethionate and methylsulphate ions. At [K]o levels from 4 to 150 mM the resting potential was reasonably well described by the Goldman-Hodgkin-Katz equation on the assumption that the intracellular K concentration ([K]i) was 155 mM and that the ratio of membrane permeability coefficients for Na and K, PNa/PK, was 0.07. In the presence of a high concentration of acetylcholine or carbachol (greater than or equal to 1 microM), the resting potentials at [K]o levels from 1 to 150 mM approximated K equilibrium potentials (EK) calculated on the assumption that [K]i was 155 mM. At [K]o levels less than or equal to 8 mM replacing most of the external Na with sucrose or Tris caused a substantial hyperpolarization, whereas application of 1-2 microM-tetrodotoxin caused only slight hyperpolarization. A transient hyperpolarization, due to enhanced electrogenic Na extrusion, was recorded on switching back to 4 mM-K following brief exposures to K-free solution; no transient hyperpolarization was recorded in the presence of 5 microM-acetylstrophanthidin. The acetylstrophanthidin itself caused a rapid depolarization of several millivolts. Preliminary conductance measurements made with two micro-electrodes in some smaller preparations indicate that the steady-state current-voltage relationship is N-shaped. We conclude that the low membrane potential of quiescent coronary sinus cells reflects not a low [K]i but rather a relatively high ratio PNa/PK, of about 0.07: the Na ions flow into the cells via predominantly TTX-insensitive pathways and are extruded by the electrogenic Na/K exchange pump, which thereby makes a substantial contribution to the resting potential.

The antiarrhythmic effect of PGE2 on premature ventricular beats (PVBs) in man

In 10 patients with premature ventricular beats (PVBs) we investigated the influence of PGE2 infusion on ventricular arrhythmias, arterial blood pressure, heart rate and systolic time intervals. An infusion of PGE2 in doses of 0.01, 0.02 and 0.04 microgram/kg/min, each for 10 min, did not change the blood pressure, heart rate and systolic time intervals. The incidence of PVBs decreased in a dose-dependent manner in 7 out of 10 patients, and in two of them PVBs were absolutely abolished. In one patient the incidence of PVBs was reduced dose independently, and in two patients at the beginning of infusion of PGE2 the incidence of PVBs was reduced, but the incidence of PVBs increased again although the infusion dose was doubled. In all patients the incidence of PVBs increased again when the infusion of PGE2 was stopped. Our findings demonstrate that PGE2 has antiarrhythmic properties in man.