The effect of 2,4-dinitrophenol on electrical and mechanical activity, metabolism and ion movements in guinea-pig ventricular muscle

ATP-sensitive potassium channels and myocardial ischemia: why do they open?

There is evidence that the "ATP-sensitive" potassium channel opens, at least during the early stages of myocardial ischemia, despite relatively high ATP levels. Thus, channel opening may partially contribute to potassium efflux and accumulation of extracellular potassium, but probably much more profoundly to electrical abnormalities associated with ischemia, including the development of lethal arrhythmias. Several factors are discussed that may promote a significant open-channel probability of the channel, in spite of relatively high levels of ATP. It is argued that, even with a very low open probability, the magnitude of total membrane current carried by these channels may be substantial (comparable to other potassium currents) because of the high density and conductance of the ATP-sensitive potassium channel. Finally, it is shown how the ATP-sensitive potassium channel may play a role in various tissue types, ranging from the physiological to the pathophysiological. This potassium channel is therefore increasingly targeted for drug development and research.

Acute effects of amiodarone on action potentials of isolated guinea-pig ventricular muscle exposed to simulated ischemic solution and metabolic inhibitors

1. Acute effects of an antiarrhythmic agent, amiodarone (AM), on action potentials from isolated guinea-pig ventricular papillary muscles exposed to simulated ischemic solution and Tyrode's solution containing NaCN or dinitrophenol (DNP) were examined. 2. In the papillary muscles exposed to simulated solution (K+ = 6 mM, glucose = 2.3 mM, pH = 7.1, PO2 less than 40 mmHg) or metabolic inhibitors (10(-3) M NaCN or 10(-4) M DNP), resting membrane potential (RMP) and maximum upstroke velocity (Vmax) of action potential were decreased with increasing time to the exposure, and action potential duration (APD) was shortened. In these conditions, addition of 4.4 x 10(-5) M AM produced transient and small increase in the APD and Vmax, followed by progressive APD-shortening and decrease in Vmax and RMP. 3. These results suggest that acute AM's antiarrhythmic effects reported are not due to APD prolonging action (Vaughan Williams Class III) of the drug, but mainly due to its local anesthetic action (Class I).

Experimentally induced defects of mitochondrial metabolism in rat skeletal muscle. Biological effects of the mitochondrial uncoupling agent 2,4-dinitrophenol

Infusion of dinitrophenol intra-arterially into rat hind limb caused an irreversible failure of isometric twitch tension and the induction of a severe progressive contracture. Metabolite analysis of muscle in which the twitch response had grossly fatigued revealed low levels of ATP and phosphocreatine together with lactate accumulation. Studies using 31P-n.m.r. confirmed the decrease in ATP and creatine phosphate concentrations and indicated a fall in intracellular pH. It is concluded that dinitrophenol-induced myopathy does not represent a good model for the human mitochondrial myopathic condition as has been previously suggested.

Lack of direct antiarrhythmic electrophysiological effects of salicylate on isolated guinea-pig myocardium

Conventional microelectrode techniques were used to study the influence of Na-salicylate, Na-benzoate, Na-2,6-dihydroxybenzoate and 2,4-dinitrophenol (2,4-DNP) on the action potential (AP) of guinea-pig papillary muscles and atria. In papillary muscles, Na-salicylate (0.19-1.87 mmol/l) concentration-dependently shortened the AP duration and the functional refractory period. The AP amplitude decreased slightly with the largest concentration, while the resting potential and the maximum depolarisation velocity (Vmax) were not affected. A concentration-dependent negative inotropic effect was also seen. All drug effects were reversible after washout. In atria, 6.24 mmol/l Na-salicylate induced a slight shortening of the AP duration, a decrease of the AP amplitude and Vmax, but no decrease of the contractile force. The effects of the uncoupling agent, 2,4-DNP (10 mumol/l), were similar to those of the largest concentration of Na-salicylate in papillary muscles and in atria. Na-benzoate and Na-2,6-dihydroxybenzoate had no significant influence on AP duration, AP amplitude, resting potential, Vmax, refractory period or force of contraction of either papillary muscles or atria. These results suggest that Na-salicylate exerts its effects on isolated guinea-pig myocardium by uncoupling the oxidative phosphorylation, whereas two other possible mechanisms of action, namely an increase of membrane surface charge and an inhibition of prostaglandin synthesis, seem to be of minor importance.

The reversible replacement of internal potassium by caesium in isolated turtle heart

1. By perfusing the isolated turtle heart with Cs solution, most of the intracellular K can be replaced by Cs. After 3--4 hr, Cs approaches a steady-state distribution with a concentration slightly below initial K concentration. 2. During the initial stages of perfusion, the heart accumulated Cs and lost K, the exchange ratio of K for Cs was estimated to be about 0.6. Subsequently perfusion yielded an equi-ionic substitution of K by Cs. 3. In presence of DNP (2 x 10(-4) M), K efflux and Cs accumulation increased but the low initial K--Cs exchange was abolished. Then, the replacement of K by Cs took place at a ratio of K:Cs of about 0.8. Ouabain (10(-5) M) suppressed uptake of Cs whereas K loss was the same as with DNP. 4. These results confirm that permeability of the cardiac sarcolemma to Cs is low, and suggest that the movement of Cs must be mainly attributed to its active transport into cells by the ionic exchange which normally transports K+ and is coupled to the extrusion of Na. The initial low net K efflux could be explained by an accumulation process which facilitates retention of K by the heart. A mechanism of this kind would be described as active reabsorption of some K present in extracellular space and would consequently reduce the uptake of Cs. 5. After loading the heart with Cs, perfusion with K solution showed the exchange of Cs for K at a ratio of Cs:K of about 0.5. K reaccumulation is reduced by ouabain (10(-5) M) and comes back to a normal steady-state distribution after 5 hr. At this time, K concentration was slightly below normal K value, but only half Cs content was eliminated. After 15 hr of perfusion, intracellular K remained constant whereas 15% of the original Cs remained.

Effect of 2-4-dinitrophenol on intercellular communication in mammalian cardiac fibres

The effect of 2-4-dinitrophenol (DNP) on cell communication, in canine Purkinje fibres, was investigated. It was found that DNP (0.5 MM) suppressed the electrical coupling in about 10 min. This effect of DNP was largely due to an increment in intracellular longitudinal resistance. The longitudinal movement of fluorescein (mol. wt. 320) along Purkinje strands, followed with the cut-end method, was also suppressed by DNP (0.5 mM). The decoupling action of DNP was related to release of Ca from intracellular stores and increase in free (Ca)i. The intracellular injection of EDTA reestablished the electrical coupling of Purkinje cells previously uncoupled by DNP. The results described in this paper indicate that cell communication in heart fibres is greatly dependent on the synthesis of high energy phosphate bonds.

Mechanically dependent changes in action potentials recorded from the intact frog ventricle

The wall of the ventricle contracts inhomogeneously during an isovolumic beat of an isolated, intact frog ventricle. Some epicardial segments actually lengthen while the pressure is rising. Almost simultaneously, the early repolarization phase of the monophasic action potential recorded from such a segment is accelerated, compared to the same phase for an isotonic beat in which the segment shortens. Segment lengthening during the isovolumic beat also may be seen during the late repolarization phase when, in contract to the above, it produces an afterdepolarization. These electrical changes disappear when isotonic contraction is restored. Corroborative findings were obtained from microelectrode and insulated gap recordings from isolated frog ventricular strip. Both electrical changes can be seen clearly when the segment is lengthened by intraventricular injections of Ringer's solution. There also is a short transition period toward the end of the action potential plateau when lengthening produces neither depolarization nor repolarization. The accelerated repolarization is manifest as a shortening of the Q-T interval in the ventricular electrogram. In all experimental preparations, the afterdepolarizations reached threshold for a propagated action potential. This mechanism may explain the generation of extrasystoles in myocardial ischemia.

Effect of anoxia and ATP depletion on the membrane potential and permeability of dog liver

1. The mechanisms responsible for the depolarization of the hepatocytes secondary to anoxia have been studied in isolated perfused dog liver. It was attempted to elucidate the role of the inhibition of the sodium pump following exhaustion of the energy reserves and of the modifications of membrane permeability. Anoxia was compared to ouabain and to a reduction of the cellular ATP level. 2. Membrane potentials were measured with micro-electrodes. Potassium, sodium and chloride were determined in plasma samples and liver tissues. Extracellular space was measured with tritiated inulin or with an electrical impedance method. Adenine nucleotides were also measured in liver biopsies. 3. The fall in membrane potential produced by administration of ouabain (0-1 mM) is greater than the effect of the redistribution of sodium + potassium ions; this suggests that the sodium pump is functioning, at least partially, electrogenically. The administration of dinitrophenol (10 mM), which causes a 74% fall in the ATP level in 15 min, produces, as does ouabain, a depolarization which also corresponds to stopping an electrogenic pump. 4. A partial reduction in the level of ATP brought about by hypoxia, by an inhibitor of cellular respiration, antimycin (10 mM), or by fructose (20 mM) results in a hyperpolarization which may be attributed to an elevation of potassium permeability (PK) since it is concomitant to a loss of K from the liver. The change in membrane permeability could be related to a rise in the free calcium in the cells which has not been documented. Other possible hypothesis include a facilitated transport for potassium. 5. The administration of amobarbitone (10 mM) produces immediately a depolarization which is independent of the progressive reduction in the level of ATP. The depolarization has been attributed to a direct effect of amobarbitone on the membrane reducing the permeability for potassium ions. 6. The depolarization observed in ischaemic anoxia is greater than that produced by ouabain for the same variation in ions concentration. In addition to a likely inhibition of the electrogenic sodium pump, changes in membrane permeability inducing a rise in the PNa/PK ratio must also occur. 7. After ischaemic anoxia for 24 hr at 3 degrees C, the ratio of PNa/PK rises to 0-68 which indicates abolishment of the selective character of membrane permeability. The augmentation in cell volume produced by anoxia might result in an opening of membrane pores, which could entail the augmentation of sodium permeability; the latter would be responsible in part for the depolarization produced by anoxia. 8. According to the severity and length of oxygen deprivation an increase in PK, a cessation of the sodium pump activity and finally an increase in PNa will occur.

Differential effects of hypoxia with age on the chick embryonic heart. Changes in membrane potential, intracellular K and Na, K efflux and glycogen

The effects of hypoxia on different parameters of cell membrane function were studied in 7 and 19 day chick embryonic hearts. The following changes were observed: 1. Transmembrane potential: A depolarization of the cell membrane and a decrease in the duration and in the overshoot of the action potential. 2. Intracellular ion concentrations: A decrease in (K)i and an increase in (Na)i. Cellular Ca-content remained constant. 3. K efflux: An increase in the rate coefficient, which was larger in stimulated preparations. These changes were more pronounced in 19 day than in 7 day hearts. The effects of hypoxia were increased by simultaneous substrate depletion and counteracted by an excess external glucose. We conclude that: 1. The 19 day hearts are more sensitive to oxygen lack than the 7 day hearts. The difference can be correlated with the observation that the younger hearts are able to consume more glycogen during hypoxia. 2. The changes of the resting membrane potential and the overshoot of the action potential correlate with changes in respectively (K)i and (Na)i. 3. An increase in the background K current may be an important factor in explaining the shortening of the action potential during hypoxia.

Hypoxia increases potassium efflux from mammalian myocardium

Hypoxia with or without simultaneous depletion of extracellular glucose increases 42K-efflux in cat and guinea-pig papillary muscles and bovine Purkinje fibres. The change observed in K efflux may be the result of an increase in K conductance at rest.

Mitochondrial uncoupling agents. Effects on membrane permeability of molluscan neurons

Agents which uncouple oxidative phosphorylation in mitochondria were applied to identified neurons in an isolated ganglion of the marine mollusc Navanax inermis. Aromatic monocarboxylic acids, acetanilides, benzamides, benzaldehydes and phenols all caused a rapid, reversible, dose-dependent increase in the membrane potential and conductance of the neurons tested. These events were due primarily to an increase in the membrane's conductance to potassium, relative to chloride. All active compounds also produced a reversible, dose-dependent decrease in the permeability of alkali-cations relative to potassium. The relative activity of congeners in each group of substances was directly correlated with the octanol-water partition coefficients of the various compounds, indicating that hydrophobicity was important in determining drug effect and suggesting that steric requirements were minimal. The results suggest that the observed changes in membrane electrical properties and cation selectivity are due to an increase in the membrane's anionic field strength caused by the hydrophobic interaction of anionic and nonionic agents with the neuronal membrane.

Membrane currents and tension in cat ventricular muscle treated with cardiac glycosides

The effect of cardiac glycosides on membrane currents and tension in cat ventricular muscle was studied using the single sucrose gap voltage clamp method. Complete tension-voltage and current-voltage relations were obtained in five preparations before and during treatment with dihydro-ouabain (DHO, 1.7 X 10(-5)M). After 1-2 minutes of DHO, the developed tension was 15% greater than control, but there was no change in either the slow inward (calcium) current (Ica) or the level of the outward current flowing at the end of a 300-msec depolarization (Iout). After 6-8 minutes of DHO, there was a 60% increase in developed tension, a noticeable increase in resting tension, a 20% decrease in Ica, and a smaller increase in Iout. It seems possible that the reduction of Ica was due to a reduced driving force. In preparations treated with ouabain (5 X 10(-7)M, 3-5 minutes), developed tension was 45-150% greater than control with no change in Ica or Iout between -45 and + 15 mv. We conclude that the inotropic action of these cardiac glycosides is not mediated by an increase in Ica.

The effects of temperature and metabolic inhibitors on the spontaneous relaxation of the potassium contracture of the heart of the frog Rana pipiens

1. The spontaneous relaxation of the potassium contracture and the relaxation induced by the removal of extracellular calcium or the restoration of the original potassium concentration, in the frog heart, show a strong dependence on temperature.2. The energy of activation of the later exponential phase of the spontaneous relaxation is 10.43 kcal mole(-1), a value close to that reported for the binding of calcium ions by isolated sarcoplasmic reticulum, but larger than that for the calcium efflux from mammalian heart.3. The use of metabolic inhibitors shows that relaxation can be sustained when glycolysis is poisoned, but the disruption of oxidative phosphorylation slows relaxation.4. Poisoning of both glycolysis and oxidative phosphorylation blocks all but the small initial part of the spontaneous relaxation of the potassium contracture and also interferes with relaxation induced by other means.5. The results are considered to favour the existence, in frog heart, of an active intracellular relaxing system which uses ATP as its substrate to lower the sarcoplasmic calcium concentration. This system is likely to be the sarcoplasmic reticulum but the mitochondria could also be involved.

DNP-induced dissipation of ATP in anoxic ventricular muscle

1. During aerobic incubation in 5 mM glucose medium, 10(-5) M-DNP reduced the action potential duration and amplitude and the developed tension of guinea-pig ventricular muscle more rapidly and to a greater extent than anoxia.2. The DNP effect on electrical and mechanical activity was even more pronounced following prolonged anoxic incubation. Since the action potential duration and developed tension of anoxic ventricular muscle have previously been shown to be dependent on glycolytic ATP, and since the effects of DNP could not be duplicated with NaCN, it was concluded that DNP was exerting an effect in addition to its uncoupling of oxidative phosphorylation.3. Anoxic muscle was incubated with 10(-4) M-IAA or with 10(-4) M-IAA + 10(-4) M-DNP. The ATP content of IAA-treated muscle was significantly lower than control but in the presence of both IAA and DNP there was a further reduction in ATP and an increased lactate production.4. Sodium azide (10(-2) M), a potent inhibitor of mitochondrial ATPase, did not prevent the reduction of ATP in DNP-treated anoxic muscle.5. Ouabain (10(-7) M) partially prevented the rapid decline of action potential duration and developed tension of DNP-treated anoxic muscle. In addition, the glycoside partially blocked the DNP-induced break-down of ATP and stimulation of lactate production.6. Oligomycin (10 mug/ml.) partially prevented the reduction in action potential duration and developed tension of DNP-treated anoxic muscle.7. It was concluded that DNP induces an ;energy leak' by actively promoting the hydrolysis of an high energy glycolytic intermediate at least one step beyond the sites of ATPase inhibition by ouabain and oligomycin.

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.