Magnesium content and net fluxes in squid giant axons

Na+ gradient-dependent Mg2+ transport in smooth muscle cells of guinea pig tenia cecum

Thin strips of guinea pig tenia cecum were loaded with the Mg2+ indicator furaptra, and the indicator fluorescence signals measured in Ca2+-free condition were converted to cytoplasmic-free Mg2+ concentration ([Mg2+]i). Lowering the extracellular Na+ concentration ([Na+]o) caused a reversible increase in [Mg2+]i, consistent with the inhibition of Na+ gradient-dependent extrusion of cellular Mg2+ (Na+-Mg2+ exchange). Curve-fitting analysis indicated that the relation between [Na+]o and the rate of rise in [Mg2+], had a Hill coefficient of approximately 3, a [Na+]o at the half-maximal rate of rise of approximately 30 mM, and a maximal rate of 0.16 +/- 0.01 microM/s (mean +/- SE, n = 6). Depolarization with 56 mM K+ shifted the curve slightly toward higher [Na+]o without significantly changing the maximal rate, suggesting that the Na+-Mg2+ exchange was inhibited by depolarization. The maximal rate would correspond to a flux of 0.15-0.4 pmol/cm2/s, if cytoplasmic Mg2+ buffering power (defined as the ratio of the changes in total Mg2+ and free Mg2+ concentrations) is assumed to be 2-5. Ouabain (1-5 microM) increased the intracellular Na+ concentration, as assessed with fluorescence of SBFI (sodium-binding benzofuran isophthalate, a Na+ indicator), and elevated [Mg2+]i. In ouabain-treated preparations, removal of extracellular Na+ rapidly increased [Mg2+]i, with an initial rate of rise roughly proportional to the degree of the Mg2+ load, and, probably, to the Na+ load caused by ouabain. The enhanced rate of rise in [Mg2+]i (up to approximately 1 microM/s) could be attributed to the Mg2+ influx as a result of the reversed Na+-Mg2+ exchange. Our results support the presence of a reversible and possibly electrogenic Na+-Mg2+ exchange in the smooth muscle cells of tenia cecum.

Specific blockage of squid axon resting potassium permeability by Haliclona viridis (Porifera: Haliclonidae) toxin (HvTX)

The action of partially purified HvTX, toxin of the marine sponge H. viridis, was explored on the giant axon of the tropical squids Doryteuthis plei and Sepioteuthis sepioidea. HvTX depolarizes the nerves dose dependently. The effect occurs after blocking sodium channels with tetrodoxin (1 microM), removing external Na+, blocking electrically excitable K+ channels with 3,4-diaminopyridine (10 mM) or internal and external application of tetraethylammonium (40 mM). Ouabain (up to 10 mM) does not modify HvTX effect. The action of HvTX occurs only when it is applied to the outer phase of the nerve membrane; microinjection of the toxin into the axons lacks depolarizing effects. HvTX reduces the dependence of membrane potential on external potassium concentration. The apparent 86Rb+ permeability (pi') was measured in axons of S. sepioidea. The value of pi' in normal artificial sea water was 80 (61,96) nm/sec (median and its 95% confidence interval, n = 8) and raised to 1030 (588, 2113) nm/sec (n = 7) when the axons were depolarized to 0 mV raising external K+ to 300 mM. In axons depolarized with HvTX (10 mM external K+) to 0 mV, pi' was 88 (55, 97) nm/sec (n = 8). HvTX could not prevent (P >> 0.05) the increase in pi' induced by 300 mM K+ when the ion concentration was raised before toxin application [pi' = 660 (354, 1876) nm/sec, n = 7]. Most of the 86Rb+ permeability increase in high K+ was prevented if HvTX was added before external K+ was raised [pi' = 298 (264, 337) nm/sec, n = 8]. All the measures of pi' were carried out in solutions containing 1 microM tetrodotoxin, 1 mM 3,4-diaminopyridine and 2 mM ouabain.

Extracellular Mg(2+)-dependent Na+, K+, and Cl- efflux in squid giant axons

An extracellular Na+ (Nao)-dependent Mg2+ efflux process that requires intracellular ATP has been proposed as the sole mechanism responsible for Mg2+ extrusion in internally dialyzed squid axons (12). We have shown that this exchanger can also "reverse" and mediate an extracellular Mg2+ (Mgo)-dependent Na+ efflux (16). We have extended these studies and found that, in the presence of ouabain, bumetanide, tetrodotoxin, and K+ channel blockers and in the absence of extracellular Na+, K+, and bicarbonate, intracellular K+ and Cl- are also involved in the Mgo-dependent Na+ efflux process. Two main observations support this view: 1) operation of the Mgo-dependent Na+ efflux requires the presence of intracellular K+ and Cl-, and 2) Mgo removal produces a reversible and nearly identical reduction in the magnitude of the simultaneous efflux of the ionic pairs K(+)-Na+ and Cl(-)-Na+. These results suggest that the putative bumetanide-insensitive Na-Mg exchanger also transports K+ and Cl-.

Mg2+ administered up to twenty-four hours following reperfusion prevents ischemic damage of the Ca1 neurons in the rat hippocampus

Inductively coupled plasma emission spectrometry analysis was applied to determine ischemia-induced changes of Mg2+ and Ca2+ in vulnerable regions of rat brain. This method can provide an accurate quantification and lower detection limits, as compared to atomic absorption spectrophotometry or several other methods. In the hippocampus, Mg2+ content significantly increases 24 h following 20 min of ischemia, followed by a gradual decrease between 48 and 72 h. Ca2+ accumulation was found at 48 and 72 h. At the cell membrane, Mg2+ plays a role as an endogenous calcium channel blocker of both the receptor-operated and voltage-dependent gates and, in the mitochondria, Mg2+ inhibits Ca2+ uptake processes. We propose that the mobilization of Mg2+ after 24 h reperfusion may counteract the process of ischemia-induced neuronal damage and that decreases of Mg2+ may be correlated with the degree of brain injury. However, in the natural concentration of Mg2+, the counteraction may not be sufficient for a neuroprotective effect. Therefore, after 24 h reperfusion, an artificial enhancement of Mg2+ is necessary for neuroprotection. In order to test the above hypothesis, MgCl2, (50 mM) was administered directly to the CA1 sector of the rat hippocampus before and at various intervals following 20 min of ischemia. Pyramidal cells were evaluated seven days later and neuronal density was determined. Consistent with the hypothesis, a neuroprotective effect was observed, even when MgCl2 was administered 24 h, but not 48 h, after the ischemic episode.

A comparison of the effect of experimental general anaesthetics on nerve impulse blockade and on a proteinaceous target site

While it has been reported that general anaesthetics inhibit the enzyme luciferase and thus reduce the light output of the reaction with luciferin, we find that in squid giant axons injected with luciferin and luciferase, treatment with experimental general anaesthetics at concentrations sufficient to block axonal conduction leads to an increase in the light production by the reaction. This potentiation of the protein activity is best observed when luciferin concentration is above the apparent association constant. Our findings raise doubts regarding the suitability of luciferase as a model for the target region of general anesthetic action.

Measurement of cytoplasmic, free magnesium concentration with entrapped eriochrome blue in nerve endings isolated from the guinea pig brain

Cytoplasmic, free Mg2+ was measured spectrophotometrically using an intrasynaptosomally entrapped Mg2+-indicator, Eriochrome blue (EB). Addition of the ionophore A23187 or disruption of the synaptosomal plasma membrane with digitonin caused an increase in absorbance of entrapped EB with a maximum at 551 nm, which is typical for the Mg2+-EB complex. A conversion of absorbance changes to levels of free Mg2+ concentrations was performed after disruption of synaptosomal plasma membranes by digitonin. The results indicated that the internal, free Mg2+ increased from 0.34 to 2.2 mM when the extracellular Mg2+ concentration was increased from 1 to 5 mM. The low values of cytoplasmic, free Mg2+ concentrations suggest the presence of effective regulatory mechanisms in the nerve endings.

Dependence of ionized and total Ca in squid axons on Nao-free or high-Ko conditions

The level of intracellular Ca in squid axons (both ionized and total Ca) was studied as a function of the experimental variables [Na]i, [Na]o, pHi, cyanide, and depolarization. Ionized Ca was measured by following the light emission of aequorin while total Ca was measured by the atomic absorption analysis of samples of axoplasm. Aequorin glow is known to be increased either by the application of Nao-free solutions or by depolarization produced by external solutions containing greater than normal K concentrations. The present results show that if [Na]i is low, the depolarization that is brought about by solutions with elevated [K] leads to a resting light emission that is decreased rather than increased, as is the case when [Na]i is high. In axons where [Na]i is varied, a comparison of the increments in light emission produced by the application first of Na-free and then of high-K solutions shows that they have an identical dependence on [Na]i, with a half-activation of Ca entry produced by an [Na]i of 25-30 mM. Changes in pHi affect the aequorin signal produced by depolarization, with acidification reducing and alkanization increasing the response. Cyanide did not greatly affect the size of the signal resulting from either Nao removal or that from depolarization.

Intracellular free magnesium in neurones of Helix aspersa measured with ion-selective micro-electrodes

Cytoplasmic free Mg2+ concentration, [Mg2+]i, was measured in identified neuronal cell bodies of the suboesophageal ganglia of Helix aspersa, using Mg2+-selective micro-electrodes. In calibration solutions, the electrodes showed significant interference from K+, but not from Na+, or Ca2+, at concentrations found intracellularly. Therefore, in order to calibrate the electrodes properly, it was necessary first to obtain an accurate value for intracellular free K+ concentration [( K+]i). The mean value for [K+]i was 91 mM (S.E. of the mean +/- 2.2 mM, n = 8), measured with K+-sensitive 'liquid ion exchanger micro-electrodes'. In seven experiments, which met stringent criteria for satisfactory impalement and electrode calibration, the mean [Mg2+]i was 0.66 mM (S.E. of the mean +/- 0.05 mM). The mean [Mg2+]i in cells that had spontaneous spike activity was not significantly different from that in quiescent cells. If Mg2+ was in electrochemical equilibrium, the ratio [Mg2+]i/[Mg2+]o would be about 55. Mg2+ is therefore not passively distributed across the neuronal membrane and an outwardly directed extrusion mechanism must exist to keep [Mg2+]i low and constant, even in cells undergoing spike activity.

The effect of temperature on veratridine action in squid giant axons

Resting membrane potential and intracellular sodium and potassium concentrations were determined at 5 and 21 degrees C in normal and veratridine-treated axons of the squid Doryteuthis plei. 300 microM veratridine produced an increase in the intracellular sodium concentration, which changed from 52 to 284 mM in 10 min of exposure at 21 degrees C, and from 76 to 260 mM at 5 degrees C. Under the same treatment the intracellular potassium concentration changed from 357 to 221 mM (21 degrees C) and from 334 to 194 mM (5 degrees C). All the changes could be prevented by adding 1 microM tetrodotoxin. Veratridine (30, 100 and 300 microM) increased the resting sodium permeability of the giant axon, and the effect was greater at 21 degrees C. The affinity of the membrane for veratridine increases when the nerves are cooled, the three concentrations tested produce maximum activation of the sodium channels at 5 degrees C. But only the higher two concentrations are saturating at 21 degrees C.

Measurements of intracellular ionized calcium in squid giant axons using calcium-selective electrodes

Ca2+-selective electrodes have been used to measure free intracellular Ca2+ concentrations in squid giant axons. Electrodes made of glass cannulas of about 20 microns in diameter, plugged with a poly(vinyl chloride) gelled sensor were used to impale the axons axially. They showed a Nernstian response to Ca2+ down to about 3 microM in solutions containing 0.3 M K+ and 0.025 M Na+. Sub-Nernstian but useful responses were obtained up to pCa 8. The electrodes showed adequate selectivity to Ca2+ over Mg2+, H+, K+ and Na+. To calibrate them properly, a set of standard solutions were prepared using different Ca2+ buffers (EGTA, HEEDTA, nitrilotriacetic acid) after carefully characterizing their apparent Ca2+ association constants under conditions resembling the axoplasmic environment. In fresh axons incubated in artificial seawater containing 4 mM Ca2+, the mean resting intracellular ionized calcium concentration was 0.106 microM (n = 15). The Ca2+-electrodes were used to investigate effects of different experimental procedures on the [Ca2+]i. The main conclusions are: (i) intact axons can extrude calcium ions at low [Ca2+]i levels by a process independent of external Na+; (ii) poisoned axons can extrude calcium ions at high levels of [Ca2+]i by an external Na+-dependent process. The level of free intracellular Ca attained at these latter conditions is about an order to magnitude greater than the resting physiological value.

The ionic mechanism of intracellular pH regulation in crayfish neurones

1. Intracellular pH (pHi) regulation in crayfish neurones was studied using pH-, Na+-, and Cl- sensitive micro-electrodes. Neuronal pH regulation has previously been studied only in molluscs. 2. The average resting pHi of crayfish neurones was 7.12 +/- 0.09, which is 1 pH unit more alkaline than that predicted were H+ ions distributed in equilibrium with the membrane potential. 3. When the cytoplasm was acidified (by NH4Cl loading, CO2 application, or HCl injection), pHi recovered towards its resting value. 4. Removal of Na+ from the external solution inhibited pHi recovery from an acid load by more than 90%. pHi recovery resumed immediately when external Na+ was reintroduced. 5. The resting intracellular Na+ concentration ([Na+]i) of crayfish neurones was 15-25 mM. During pHi recovery from an acid load, [Na+]i increased by 10-50 mM. 6. Reducing the external HCO3(-) concentration from 5 mM to 0 mM slowed pHi recovery by an average of about 45%. This slowing was appreciable even in cells in which Na+ removal almost totally blocked pHi recovery. 7. The resting intracellular Cl- concentration ([Cl-]i) was 30-40 mM, indicating that these cells actively accumulate Cl-. During pHi recovery from an acid load, [Cl-]i decreased by 3-5 mM. 8. In the presence of the anion exchange inhibitor SITS (4-acetamide-4'-isothiocyanostilbene-2,2'-disulphonic acid), pHi recovery was slowed to the rate which was normally seen in HCO3(-)-free Ringer solution. SITS abolished the dependence of pHi recovery on the external HCO3(-) concentration. 9. It is concluded that pHi regulation in crayfish neurones involves two separate mechanisms: a Na+-dependent, HCO3(-)-independent acid extrusion process, and a Cl---HCO3(-) exchange which is probably also Na+-dependent.

Calcium movement in nerve fibres

Given the existence of a difference in electrical potential between the interior of a nerve cell and the media surrounding it, where the cytoplasm is some 70 mV negative (Hodgkin, 1958), it must be expected that any positively charged ion to which the cell membrane is permeable is more concentrated in the cell interior. For monovalent cations such as Na and divalent cations such as Ca and Mg this is not the case in the majority of the cells such as the squid giant axon. In other words, nerve cells maintain a lower intracellular concentration of these ions, as compared with their concentration in the extracellular fluid. For Mg, Ca and Na ions, this lower internal concentration must, in the steady state, be effected by some membrane based mechanism which consumes energy.