Fall In Membrane Potential Research Articles

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.

Mechanism of changes in the membrane potential in various types of muscle fibers in experimental tetanus

In early tetanus the fall in membrane potential (MP) of skeletal muscle fibers is nonspecific in character: it is connected with the development of hyperactivity of the muscles, and when this is abolished by division of the nerve, the MP of the skeletal muscle fibers returns to normal. In the late stage of tetanus poisoning the functional properties of the tetanic and tonic muscle fibers are altered to a greater degree than in early tetanus. Administration of ATP, cysteine, and phenylephrine appreciably raises the MP of the poisoned muscle fibers.

Intracellular and intercellular potentials in the early amphibian embryo.

1. The sensitivity of the membrane potential of cells of early embryos of Xenopus laevis to variations in the extracellular concentrations of sodium and potassium has been determined.2. Alterations in the extracellular sodium concentration have little effect on the membrane potential at all pregastrular stages tested.3. Up to the 32 cell stage an increase in the concentration of potassium in the bathing medium causes a fall in cell membrane potential only when membrane newly synthesized in the furrow during cleavage is exposed at the embryo surface, during the second half of the cell division cycle.4. Beyond the morula stage (48 cells) a fall in membrane potential on raising external potassium can only be demonstrated when the seal which isolates the intercellular fluid from the bathing medium is broken so that cells lining the inner face of the embryo come into contact with the high potassium solution.5. The results suggest that the egg membrane has little selective permeability whereas membrane synthesized after fertilization is highly potassium permeable.6. No evidence could be obtained for any potential difference between the intercellular fluid and the external bathing medium.7. Dinitrophenol, sodium azide and cyanide prevented normal development only if they were injected into the intercellular cavity. Embryos reared in solutions containing these poisons turned into normal tadpoles.8. The formation of the intercellular cavity could be halted by injecting ouabain into the cavity while it was still small. Embryos reared in ouabain turned into normal tadpoles.9. The results suggest that the active transfer of sodium ions from the cells to the intercellular spaces is an integral part of the formation of the intercellular fluid. A hypothesis for the mechanism of formation of the cavity is put forward along these lines.

Effects of electrogenic sodium pumping on the membrane potential of longitudinal smooth muscle from terminal ileum of guinea-pig.

1. The membrane potential of the separated longitudinal muscle of the guinea-pig terminal ileum was recorded intracellularly with glass micro-electrodes.2. In tissues kept at room temperature and then brought to 35 degrees C for 15-30 min or about 1 hr, the fall in membrane potential upon changing to potassium-free solution was 21.4 +/- 3.5 mV and 13.4 +/- 1.8 mV respectively. Ouabain (1.7 x 10(-6)M) produced a fall in membrane potential of 8.1 +/- 1.1 mV. Returning potassium to potassium-free solution, or changing from ouabain-containing to ouabain-free solution, resulted in an increase in membrane potential which was greater than the initial fall.3. Readmitting potassium to potassium-free solution produced an increase in membrane potential which began within 10 sec and reached a maximum within 15-30 sec. This response was reduced, abolished, or converted to a depolarization by ouabain. In chloride-deficient (13 mM) solution in which membrane resistance was increased, the response to readmitting potassium was increased 2(1/2)-fold so that the membrane potential sometimes exceeded -100 mV, which was probably more negative than E(K). On the basis of these results it was assumed that the response to readmitting potassium was due to the electrogenic activity of the sodium pump.4. The response to briefly readmitting a fixed concentration of potassium increased during the first 30 min in potassium-free solution. This increase was not due to an increase in membrane resistance as this fell with time in potassium-free solution. It was suggested that the increase in the response resulted from the progressive rise in internal sodium concentration which is known to occur in smooth muscle in potassium-free solution.5. Increasing the concentration of potassium over the range approximately 0.1-20 mM, increased the size of the electrogenic potential observed upon readmitting potassium to potassium-free solution. There was a fall in membrane resistance upon readmitting potassium (0.6, 5.9, or 20 mM) which was greater the larger the concentration of potassium. When allowance was made for the fall in membrane resistance, the dependency of the electrogenic response upon the concentration of potassium over the range 0.6-20 mM was much increased.6. The results indicate that the rate of electrogenic sodium pumping in this tissue is increased by increasing the external potassium concentration, and probably by increasing the internal sodium concentration. It was suggested that a rise in the latter could sensitize the pump to an increase in the former.

Action of tannic acid on frog muscle

Tannic acid, when applied to frog sartorius muscles (10 −8–10 −4 M) caused a depolarization. High concentrations (10 −6–10 −4 M) produced a more rapid and greater fall in membrane potential. Membrane resistance, measured concurrently, showed an initial rise (at 10 −7 and 10 −6 M) and subsequently fell. It is suggested that these effects are the result of the action of tannic acid on the chloride permeability of the muscle membrane, and comparisons are made with the effect of this agent on red cells and on crayfish giant axon.

The effects of hypoxia on neuromuscular transmission in a mammalian preparation.

1. The rat diaphragm-phrenic nerve preparation in vitro failed to contract in response to nerve impulses after 10-20 min exposure to solutions containing 95% N(2) and 5% CO(2) (hypoxic solutions) at temperatures between 33 and 38 degrees C. Intracellular recording revealed that end-plate potential (e.p.p.) amplitudes fell below the firing threshold for muscle fibres and then disappeared probably because of block of intramuscular nerve conduction.2. In curarized and Mg-paralysed preparations the reduction in e.p.p. amplitudes was found to be due to a fall in their quantal content. In about half of the Mg-paralysed preparations, however, and in curarized preparations after repeated exposures, there were increases in quantal content of e.p.p.s during hypoxia.3. Miniature end-plate potential (m.e.p.p. frequency increased in a cyclic fashion during hypoxia and this increase was largely suppressed in the presence of a raised extracellular Mg concentration. M.e.p.p. amplitude increased (range 0-100% of control value) after about 20 min hypoxia.4. Post-tetanic potentiation of e.p.p. amplitudes and m.e.p.p. frequency was reduced after exposure to hypoxic solutions. During hypoxia the e.p.p. amplitude potentiation was reduced but the m.e.p.p. frequency potentiation was augmented.5. There was an increase in the post-synaptic sensitivity to carbamylcholine after 20 min hypoxia which was sufficient to explain the increase in m.e.p.p. amplitude. Other post-synaptic changes were a fall in membrane potential (average 6 mV after 20 min) and a fall in membrane resistance after 30-60 min exposure to hypoxia.6. The effects of hypoxia upon neuromuscular transmission were partially explained by reduction of active transport of sodium and potassium ions and consequent depolarization of nerve and muscle.

The Control of the Membrane Potential of Muscle Fibers by the Sodium Pump

Frog sartorius muscles were made Na-rich by immersion in K-free sulfate Ringer's solution in the cold. The muscles were then loaded with Na(24) and the extracellular space cleared of radioactivity. When such Na-rich muscles were transferred to lithium sulfate Ringer's solution at 20 degrees C, Na efflux was observed to increase with time, to reach a maximum about 15 minutes after the transfer of the muscles to Li(2)SO(4), and then to decline. The decline in efflux from these muscles was proportional to ([Na](i))(8) over a considerable range of [Na](i). The membrane potential of Na-rich muscles was about -48 mv in K-free sulfate Ringer's at 4 degrees C but changed to -76 mv in the same solution at 20 degrees C and to -98 mv in Li(2)SO(4) Ringer's at 20 degrees C. By contrast, muscles with a normal [Na](i) showed a fall in membrane potential when transferred from K-free sulfate Ringer's to Li(2)SO(4) Ringer's solution. The general conclusions from this study are (a) that Na extrusion is capable of generating an electrical potential, and (b) that increases in [Na](i) lead to reversible increases in P(Na) of muscle fibers.

THE EFFECTS OF BARIUM ION ON THE RESTING AND ACTION POTENTIAL OF INTESTINAL SMOOTH MUSCLE CELL.

The effects of barium ion on the resting and action potential of the intestinal smooth muscle cell of guineapig taenia coli were examined with intracellular microelectrode. The lowest effective concentration of Ba was 0.05mM; it produced a slight increase in the frequency of spontaneous spike discharge. When the concentration of Ba was increased up to 1.4mM, the duration of action potential was increased and the prolonged depolarizations topped with multiple spikes were recorded frequently. The complete recovery was obtained by washing out Ba. A higher concentration of Ba (more than 2.2mM) produced the rapid fall in membrane potential and cessation of spike discharge. These changes caused by high concentration of Ba were incompletely reversible, even after long soaking in normal Krebs's solution.

Vagal and sympathetic effects on the pacemaker fibers in the sinus venosus of the heart.

1. Action potentials from sinus venosus and auricle fibers of spontaneously beating frog hearts have been recorded with intracellular electrodes. 2. Sinus fibers show a slow depolarization, the pacemaker potential, during diastole. The amplitude of this potential varies in different parts of the sinus. In some fibers the membrane potential falls by 11 to 15 mv. during diastole and the transition to the upstroke of the action potential is comparatively gradual. In other regions the depolarization develops more slowly and the action potential takes off more abruptly from a higher membrane potential. It is proposed that the fibers showing the largest fall in membrane potential during diastole are the pacemaker fibers of the heart, and that the rest of the preparation is excited by conduction. In auricle fibers the membrane potential is constant during diastole. 3. The maximum diastolic membrane potential and the overshoot of the action potential vary inversely with the amplitude of the pacemaker potential. The highest values were measured in auricle fibers. 4. Stimulation of vagi suppresses the pacemaker potentials. While the heart is arrested the membrane potential of the sinus fibers rises to a level above the maximum diastolic value reached in previous beats. In 28 experiments vagal stimulation increased the membrane potential from an average maximal diastolic value of 55 mv. to a "resting" level of 65.4 mv. The biggest vagal polarization was 23 mv. 5. In contrast to the sinus fibers vagal inhibition does not change the diastolic membrane potential of frog auricle fibers. 6. Vagal stimulation greatly accelerates the repolarization of the action potential and reduces its amplitude. These changes were seen both in the sinus and in auricle fibers stimulated by direct shocks during vagal arrest. 7. The conduction velocity in the sinus venosus of the tortoise is reduced by vagal stimulation. Block of conduction often occurs. 8. In the frog sinus venosus sympathetic stimulation increases the rate of rise of the pacemaker potential, accelerating the beat. The threshold remains unchanged. The rate of rise of the upstroke and the amplitude of the overshoot are increased. 9. The analogies between the vagal inhibition of the heart and the nervous inhibition of other preparations are discussed.