Action of external divalent ion reduction on sodium movement in the squid giant axon

Effects of divalent cations on Schaffer collateral axon function

Previously, we reported that distal Schaffer collaterals undergo biphasic changes in excitability during high-frequency stimulation (HFS), with an early hyper-excitability period followed by an excitability depression period. The extracellular divalent cations calcium and magnesium can regulate membrane excitability in neuronal tissue. Therefore, we hypothesized that altering the concentrations of extracellular calcium and magnesium would alter the biphasic excitability changes. We tested this hypothesis by recording distal Schaffer collateral fiber volleys in stratum radiatum of hippocampal area CA1 during 100 Hz HFS in artificial cerebral spinal fluid (ACSF) containing normal and altered concentrations of extracellular divalent cations. Our normal ACSF contained 2.0 mM calcium and 2.0 mM magnesium. We examined four solutions with altered divalent cation concentrations: (1) high-calcium/low-magnesium (3.8 mM/0.2 mM), (2) low-calcium/high-magnesium (0.2 mM/3.8 mM), (3) high-calcium/normal-magnesium (3.8 mM/2.0 mM), or (4) normal-calcium/high-magnesium (2.0 mM/10.0 mM), and assessed the effects on Schaffer collateral responses. Increasing or decreasing extracellular calcium enhanced or reduced (respectively) the early hyper-excitable period whereas increasing extracellular magnesium reduced the later excitability depression. Because these results might be explained by altered calcium influx through voltage-gated calcium (Ca_V) channels, we tested Ca_V blockers (ω-agatoxin IVA, ω-conotoxin-GVIA, cadmium), but observed no effects on responses during HFS. Some of the effects of altered divalent cation concentration may be explained by altered membrane surface charge. Although this mechanism does not completely explain our findings, calcium influx through Ca_V channels is not required.

The excitable system in the cell surface

A membrane model, composed of phospholipid and cholesterol, is described. The electrical resistance and hydration of this model can be controlled by manipulation of ambient ions and by current in ways strongly reminiscent of the behavior of living cells. The behavior of the model may resemble that of the membrane component of the cell. In addition, an interdependent, lipid-protein molecular structure may exist at the cell surface.

Current-voltage relations in the lobster giant axon membrane under voltage clamp conditions

The sucrose-gap method introduced by Stämpfli provides a means for the application of a voltage clamp to the lobster giant axon, which responds to a variety of different experimental procedures in ways quite similar to those reported for the squid axon and frog node. This is particularly true for the behavior of the peak initial current. However, the steady state current shows some differences. It has a variable slope conductance less than that of the peak initial current. The magnitude of the steady state slope conductance is related to the length of the repolarization phase of the action potential, which does not have an undershoot in the lobster. The steady state outward current is maintained for as long as 100 msec.; this is in contrast to a decline of about 50 per cent in the squid axon. Lowering the external calcium concentration produces shifts in the current-voltage relations qualitatively similar to those obtained from the squid axon. On the basis of the data available, there is no reason to doubt that the Hodgkin and Huxley analysis for the squid giant axon in sea water can be applied to the lobster giant axon.

Electronic measurement of the intracellular concentration and net flux of sodium in the squid axon

A unique, rapid, and non-destructive determination of the intracellular sodium concentration of a squid axon may be provided by the "voltage clamp" technique, in which the potential across the axon membrane is under electronic control. The potential at which the early component of ionic current reverses following a membrane potential step was used as an index of the intracellular sodium concentration. Several types of experiments were used to test the applicability of this method for measurement of intracellular sodium and its net flux. The concentration was found to increase from 38 mM for a fresh axon to 50 mM in about an hour. From this change, the net flux for a fresh resting axon was estimated to be 40 pmoles/cm² sec. Rapid stimulation of an unclamped axon produced a marked increase in the rate of sodium accumulation. Rapid pulsing of the membrane in a voltage clamp to potentials more positive than the sodium potential moved sodium out fast enough to produce a definite decrease in internal concentration. The agreement between the results with this method and those with more direct methods is quite satisfactory. An attractive feature of this method of intracellular sodium determination is that the physiological function of the axon is maintained and other measurements may be made concurrently.

EFFECT OF DETERGENT ON ELECTRICAL PROPERTIES OF SQUID AXON MEMBRANE

The effects of detergents on squid giant axon action and resting potentials as well as membrane conductances in the voltage clamp have been studied. Anionic detergents (sodium lauryl sulfate, 0.1 to 1.0 mM; dimethyl benzene sulfonate, 1 to 20 mM, pH 7.6) cause a temporary increase and a later decrease of action potential height and the value of the resting potential. Cationic detergent (cetyl trimethyl ammonium chloride, 6 x 10^-5M or more, pH 7.6) generally brings about immediate and irreversible decreases in the action and resting potentials. Non-ionic detergent (tween 80, 0.1 M, pH 7.6) causes a slight reversible reduction of action potential height without affecting the value of the resting potential. Both anionic and cationic detergents generally decrease the sodium and potassium conductances irreversibly. The effect of non-ionic detergent is to decrease the sodium conductance reversibly, leaving the potassium conductance almost unchanged.

Peripheral nerve reconnection: inhibition of early degenerative processes through the use of a novel fluid medium

Two pharmacologic manipulations were applied to injured nerves in the rat to minimize the secondary damage that accompanies peripheral nerve transection. It is known that calcium influx into the nerve is responsible for some of the processes that have been termed Wallerian degeneration. These disruptive effects of high intracellular calcium were retarded by chlorpromazine, a potent inhibitor of calmodulin. Our results suggested a new method for reducing posttraumatic neural disruption and supported our hypothesis regarding the involvement of calmodulin or some other Ca2+ binding protein in Wallerian degeneration. The second part of this report describes changes observed at the tips of a severed nerve and their prevention through the use of polyvinyl alcohol. Finally, we showed that neither substance produced functional deficits when injected directly into the sciatic nerve of rats and could thus be used in animal experimentation.

Mechanisms involved in differential conduction of potentials at high frequency in a branching axon

1. The ionic mechanisms involved in block of conduction of action potentials following high frequency stimulation were studied in a branching axon of the lobster Panulirus penicillatus. 2. A 2-3 mM increase in extracellular K concentration (normal concentration 12 mM) produced block of conduction into both daughter branches. 3. While conduction block induced by high frequency stimulation occurs first into the large daughter branch and only later into the smaller one, propagation into both branches is blocked simultaneously by increased extracellular K concentration. 4. Increasing extracellular K by 2-3 mM resulted in membrane depolarization, reduction in membrane resistance and reduced excitability. The latter two effects were larger than expected from the small depolarization. It appears that increase of extracellular K has direct effects on membrane excitability. 5. It is suggested that block of conduction after high frequency stimulation results from accumulation of K in the extracellular space. However, in order to account for differential conduction block in the two branches one must assume differential buildup of extracellular K concentration around the two branches during high frequency stimulation. 6. Ultrastructural studies using La and horseradish peroxidase as extracellular markers show that the space around the two branches is similar and is open to the extracellular space. Therefore differences in periaxonal volume cannot account for differential buildup of K around the two branches. 7. It is demonstrated that the lobster axon has a Na+/K+ electrogenic pump. After blocking this pump with ouabain, stimulation at high frequency resulted in a conduction block in the two branches almost at the same time. 8. Injection of Ca2+ intracellularly into the thick branch prevents or delays the appearance of conduction block after high frequency stimulation. 9. A mechanism based on these findings is suggested to explain the differential conduction block seen after high frequency stimulation in a branching axon with almost ideal impedance matching.

Na+ transport by rabbit urinary bladder, a tight epithelium

By in vitro experiments on rabbit bladder, we reassessed the traditional view that mammalian urinary bladder lacks ion transport mechanisms. Since the ratio of actual-to-nominal membrane area in folded epithelia is variable and hard to estimate, we normalized membrane properties to apical membrane capacitance rather than to nominal area (probably 1 muF approximately 1 cm2 actual area). A new mounting technique that virtually eliminates edge damage yielded resistances up to 78,000 omega muF for rabbit bladder, and resistances for amphibian skin and bladder much higher than those usually reported. This technique made it possible to observe a transport-related conductance pathway, and a close correlation between transepithelial conductance (G) and short-circuit current (Isc) in these tight epithelia. G and Isc were increased by mucosal (Na+) [Isc approximately 0 when (Na+) approximately 0], aldosterone, serosal (HCO-3) and high mucosal (H+); were decreased by amiloride, mucosal (Ca++), ouabain, metabolic inhibitors and serosal (H+); and were unaffected by (Cl-) and little affected by antidiuretic hormone (ADH). Physiological variation in the rabbits' dietary Na+ intake caused variations in bladder G and Isc similar to those caused by the expected in vivo changes in aldosterone levels. The relation between G and Isc was the same whether defined by diet changes, natural variation among individual rabbits, or most of the above agents. A method was developed for separately resolving conductances of junctions, basolateral cell membrane, and apical cell membrane from this G--Isc relation. Net Na+ flux equalled Isc. Net Cl- flux was zero on short circuit and equalled only 25% of net Na+ flux in open circuit. Bladder membrane fragments contained a Na+-K+-activated, ouabain-inhibited ATPase. The physiological significance of Na+ absorption against steep gradients in rabbit bladder may be to maintain kidney-generated ion gradients during bladder storage of urine, especially when the animal is Na+-depleted.

Local anesthetic activity of 1-(morpholino or piperidino)-2-propanol derivatives

The local anesthetic activities of 1-(morpholino or piperidino)-2-propanol derivatives were investigated. These activities were observed with nerve trunk anesthesia, rabbit's corneal anesthesia and intradermal injection anesthesia. Among these propanol derivatives, No. 24 and No. 25 which possess thioether-type sulfur were the most active and the effective potency was enhanced by increasing the dissociation constant (pKa) value in a series of the compounds. Effects on cat spinal reflex were also observed. These propanol derivatives revealed no selective depression of presynaptic depression or depression of the potential of monosynaptic reflex (MSR) and polysynaptic reflex (PSR). Procaine and lidocaine had similar actions. Effects on axonal membrane potential and interaction of the calcium on these derivatives were also investigated. These derivatives decreased the action potential without altering the resting membrane potential and when antagonized with calcium, sciatic nerve action potential was decreased. Procaine and idocaine also showed the same results.

Ionic dependence of adrenal steroidogenesis and ACTH-induced changes in the membrane potential of adrenocortical cells

1. The effects of changes of ionic environment upon corticosteroid production by rabbit adrenal glands have been investigated in vitro using a superfusion technique and on-line steroid analysis by an automated fluorescence method. In some experiments micro-electrode recordings of adrenocortical transmembrane potentials were made concomitantly with measurement of steroid output.2. Adrenocorticotrophic hormone (ACTH), 10 m-u./ml., induced a sevenfold increase in corticosteroid production rate in normal Krebs solution.3. The steroidogenic response to ACTH was not impaired after omission of [K](o) for 1 hr but was inhibited following exposure to K(+)-free medium for 3 hr. Increase of [K](o) tenfold to 47 mM increased the basal but not the ACTH-stimulated output of corticosteroid whereas raising [K](o) twentyfold to 94 mM enhanced both the basal and ACTH-stimulated steroid production rate. In K(+)-free solution the adrenocortical cells hyperpolarized from - 67 to - 86 mV; subsequently on addition of ACTH they depolarized. Reintroduction of K(+) restored the membrane potential.4. Omission of Ca(2+) partially depolarized the cells but only affected the steroidogenic response to ACTH in the presence of EDTA. A threefold increase of [Ca](o), to 7.68 mM, had no effect on either membrane potentials or steroid formation, but increasing [Ca](o) tenfold to 25.6 mM partially blocked ACTH action. Increasing [Mg](o) twentyfold to 22.6 mM had little effect on ACTH-stimulated corticosteroid output and Sr 2.56 mM, in substitution for Ca(2+), supported ACTH action, but La, 0.25 mM, completely blocked the steroidogenic effect of ACTH.5. Replacement of NaCl, 118 mM by choline chloride, 118 mM, was without effect on ACTH-induced steroidogenesis, whereas LiCl, 118 mM, reduced it by 50%. NaF, 1 and 10 mM, inhibited ACTH-induced steroidogenesis by approximately 60%.6. Nupercaine, 10(-4)M, inhibited the steroid response to ACTH with no effect upon membrane potentials: increasing the nupercaine concentration to 10(-3)M inhibited the steroid response and depolarized the cells. Ouabain, 10(-5)M, induced complete depolarization and suppression of the steroidogenic response to ACTH.7. Action-potential-like changes in membrane potential appeared in cells exposed to ACTH in a K(+)-free medium. The amplitude of the action potentials ranged from 10 to 60 mV according to cell, with a frequency up to 36/min; the frequency tended to increase with time. Tetrodotoxin, 10(-6) g/ml., did not inhibit ACTH-induced action potentials in K(+)-free medium.8. These observations are discussed in relation to the ionic requirements for the steroidogenic action of ACTH. The results further emphasize the dissociation of membrane polarization and the secretion of steroid. The mechanism of output of steroid hormone from the adrenocortical cell may thus differ fundamentally from the secretory mechanisms in other, particle-storing cells.

Permeability of a cell membrane junction. Dependence on energy metabolism

The ion permeability of the membrane junctions between Chironomus salivary gland cells is strongly depressed by treatments that are generally known to inhibit energy metabolism. These treatments include prolonged cooling at 6 degrees -8 degrees C, and exposure to dinitrophenol, cyanide, oligomycin, and N-ethylmaleimide. Intracellular injection of ATP appears to prevent depression of junctional permeability by dinitrophenol or to reverse it. Ouabain, azide, p-chloromercuriphenylsulfonic acid, reserpine, and acetazolamide fail to depress junctional permeability. Thus the ion permeability of the junctional membranes appears to depend on energy provided by oxidative phosphorylation. Possible energy-linked processes for maintaining junctional permeability are discussed, including processes involving transport of permeability-modifying species such as Ca(++).