Calcium efflux from internally dialyzed squid axons: the influence of external and internal cations

The control of ionized calcium in squid axons

Measurements of the Ca content, [Ca](T), of freshly isolated squid axons show a value of 60 mumol/kg axoplasm. Axons in 3 mM Ca(Na) seawater show little change in Ca content over 4 h, while axons in 3 mM Ca(Na) seawater show little change in Ca content over 4 h, while axons in 10 mM Ca(Na) seawater show gains of 18 mumol/Ca/kgxh. In 10 Ca (Choline) seawater the gain is 2,400 mumol/kgxh. Using aequorin confined to a dialysis capillary in the center of an axon, one finds that [Ca](i) is in a steady state with 3 Ca (Na) seawater, and that both 10 Ca (Na) and 3 Ca (choline) seawater cause increases in [Ca](i). In 3 Ca (Na) seawater-3 Ca (choline) seawater mixtures, 180 mM [Na](0) (40 perecent Na) is as effective as 450 mM [Na](0) (100 percent Na) in maintaining a normal [Ca](1); lower [Na] causes an increase in [Ca](i). If axons are injected with the ATP-splitting enzyme apyrase, the resulting [Ca](1) is not loading with high [Ca](0) or low [Na](0) solutions. Depolarization of an axon with 100 mM K (Na) seawater leads to an increase in the steady-state level of [Ca](1) that is reversed upon returning the axon to normal seawater. Freshly isolated axons treated with either CN or FCCP to inhibit mitochondrial Ca buffering can still maintain a normal [Ca](i) in 1 Ca (Na) seawater.

Characterization of the ATP-dependent calcium efflux in dialyzed squid giant axons

The magnitude of the activating effect of ATP on the Ca efflux was explored at different [Ca++]i in squid axons previously exposed to cyanide seawater and internally dialyzed with a medium free of ATP and containing p-trifluoro methoxy carbonyl cyanide phenyl hydrazine. At the lowest [Ca++]i used (0.06 micron more than 95% of the Ca efflux depends on ATP. At high [Ca++]i (100 micron), 50-60% of the Ca efflux still depends on ATP. The apparant affinity constant for ATP was not significantly affected in the range of [Ca++]i from 0.06 to 1 micron. Axons dialyzed to reduce their internal magnesium failed to show the usual activation of the Ca efflux when the Tris or the sodium salt of ATP was used. Only in the presence of internal magnesium is ATP able to stimulate the Ca efflux. Nine naturally occurring high-energy phosphate compounds were ineffective in supporting calcium efflux. These compounds were: UTP, GTP, CTP, UDP, CDP, ADP, AMP, CAMP, and acetyl phosphate. The compounds 2' deoxy-ATP and the hydrolyzable analog alpha,beta-methylene ATP were able to activate the Ca efflux. The nonhydrolyzable analog beta,gamma-methylene ATP competes with ATP for the activating site, but is unable to activate the Ca efflux. The results are discussed in terms of the specificity of the nucleotide site responsible for the ATP-dependent Ca efflux.

Influence of membrane potential on the sodium-dependent uptake of gamma-aminobutyric acid by presynaptic nerve terminals: experimental observations and theoretical considerations

Sodium, potassium and veratridine were tested for their effects on the uptake of gamma-aminobutyric acid (GABA) by pinched-off presynaptic nerve terminals (synaptosomes). As noted by previous investigators, the uptake from media containing 1 mum GABA ("high-affinity" uptake) is markedly Na-dependent; the uptake averaged 65 pmoles/mg synaptosome protein x min, with [Na]0=145mm and [K]0=5mm, and declined by about 90 percent when the external Na concentration ([Na]0) was reduced to 13mm (Na replaced by Li). The relationship between [Na]0 was GABA uptake was sigmoid, suggesting that two or more Na+ ions may be required to activate the uptake of one GABA molecule. Thermodynamic considerations indicate that with a Na+/GABA stoichiometry of 2:1, the Na electrochemical gradient, alone, could provide sufficient energy to maintain a maximum steady-state GABA gradient ([GABA]i/[GABA]0) of about 104 across the plasma membrane of GABA-nergic terminals. In Ca-free media with constant [Na]0, GABA uptake was inhibited, without delay, by increasing [K]0 or by introducing 75mum veratridine; the effect of veratridine was blocked by 200 nm tetrodotoxin. The rapid onset (within 10 sec) of the veratridine and elevated-K effects implies that alterations in intra-terminal ion concentrations are not responsible for the inhibition. The uptake of GABA was inversely proportional to log [K]0. These observations are consistent with the idea that the inhibitory effects of both veratridine and elevated [K]0 may be a consequence of their depolarizing action. The data are discussed in terms of a barrier model (Hall, J.E., Mead, C.A., Szabo, G. 1973. J. Membrane Biol. 11:75) which relates carrier-mediated ionic flux to membrane potential.

Biochemical studies of the excitable membrane of Paramecium aurelia. I. 45Ca2+ fluxes across resting and excited membrane

The characteristics of Ca2+ transport across the excitable membrane of Paramecium aurelia were studied by measuring 45Ca2+ influx and efflux. The intracellular concentration of free Ca2+ in resting P. aurelia was at least ten times less than the extracellular concentration. Ca2+ influx was easily measurable at 0 degrees C, but not at 23 degrees C. The influx of 45Ca2+ was stimulated by the same conditions which cause membrane depolarization and ciliary reversal. Addition of Na+ and K+ (which stimulate ciliary reversal) resulted in a 10-fold increase in the rate of Ca2+ influx. An externally applied, pulsed, electric field (1-2 mA/cm2 of electrode surface), caused the rate of Ca2+ influx to increase 3-5 times, with the extent of stimulation dependent on the current density and the pulse width. Ca2+ influx had the characteristics of a passive transport system and was associated with the chemically or electrically triggered Ca2+ "gating" mechanism, which has been studied electrophysiologically. In contrast, Ca2+ efflux appeared to be catalyzed by an active transport system. With cells previously loaded at 0 degrees C with 45Ca2+, Ca2+ efflux was rapid at 23 degrees C, but did not occur at 0 degrees C. This active Ca2+ efflux mechanism is probably responsible for maintaining the low internal Ca2+ levels in unstimulated cells.

ATP-Dependent chloride influx into internally dialyzed squid giant axons

Measurements were made of 36C1 influx into squid giant axons whose internal solutes were controlled by means of internal dialysis. When the intracellular chloride concentration was 50mM and the internal concentration of adenosine 5'-triphosphate (ATP) was 4mM, the average chloride influx was 11.6 pmoles/cm2 x sec. When the axons were dialyzed with an ATP-free solution, the average influx fell to 5.1 pmoles/cm2 x sec. The effect was fully reversible upon the return of ATP to the dialysis fluid. Chloride-36 influx in the presence and absence of ATP was found to be inversely related to the internal chloride concentration.

Axoplasmic free magnesium levels and magnesium extrusion from squid giant axons

The free magnesium concentration in the axoplasm of the giant axon of the squid, Loligo pealei, was estimated by exploting the known sensitivity of the sodium pump to intracellular Mg2+ levels. The Mg-citrate buffer which, when injected into the axon, resulted in no change in sodium efflux was in equilibrium with a Mg2+ level of about 3--4 mM. Optimal [Mg2+] for the sodium pump is somewhat higher. Total magnesium content of axoplasm was 6.7 mmol/kg, and that of hemolymph was 44 mM. The rate coefficient for 28Mg efflux was about 2 X 10(-3) min-u for a 500-mum axon at 22-25degreesC, with a very high temperature coefficient (Q10=4-5). This efflux is inhibited 95% by injection of apyrase and 75% by removal of external sodium, and seems unaffected by membrane potential or potassium ions. Increased intracellular ADP levels do not affect Mg efflux nor its requirement for Na+/o, but extracellularl magnesium ions do. Activation of 28Mg efflux by Na+/o follows hyperbolic kinetics, with Mg2+/o reducing the affinity of the system for Na+/o. Lanthanum and D600 reversibly inhibit Mg efflux. In the absence of both Na+ and Mg2+, but not in their presence, removal of Ca2+ from the seawater vastly increased 28Mg efflux; this efflux was also strongly inhibited by lanthanum. A small (10(-14) mol cm-2) extra Mg efflux accompanies the conduction of an action potential.

Carrier-mediated sodium-dependent and calcium-dependent calcium efflux from pinched-off presynaptic nerve terminals (synaptosomes) in vitro

The influence of external cations on 45Ca2+ efflux from Ca2+ loaded synaptosomes has been examined. The synaptosomes were pre-loaded with 45Ca2+ by incubating the suspensions in potassium-rich media for 2 min. The suspensions were then diluted into "efflux" media containing a "normal" (5mM) K+ concentration; the content of Na+ and Ca2+ was varied, as noted below. Efflux of 45Ca2+ was measured for a 2-min period (except for "zero-time" samples), and was terminated by filtering the suspensions on 0.3 mum cellulose acetate filters. 45Ca2+ retained on the filters was determined by liquid scintillation spectroscopy. The difference between the 45Ca2+ in the "zero-time" samples (="Ca2+ load") and in the samples incubated for 2 min was taken as the 45Ca2+ efflux. 45Ca2+ loss into Ca2+ -free efflux media containing ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA) was markedly influenced by the Na+ concentration: nearly 80% of the 45Ca2+ was lost from the synaptosomes if the media contained 132 mM Na+, but only about 7% was lost in 2 min if 97% of the Na+ was replaced mol-for-mol by choline. In media containing 1.2 mM Ca2+ and 132 mM Na+, the 45Ca2+ uptake by synaptosomes previously loaded with 40Ca2+ was significantly less than 45Ca2+ loss from synaptosomes loaded with 45Ca2+. Thus there was a net efflux of Ca2+ from the Ca2+ -loaded synaptosomes; this efflux was, presumably, Na+ dependent. In media containing 1.2 mM 40Ca2+ and only 4 mM Na+, the 45Ca2+ efflux from 45Ca2+ -loaded synaptosomes was significantly greater if most of the external Na+ (128 mM) was replaced isomotically by Li+ rather than by choline, guanidine or glucose. This observation may be evidence for a Ca2+ -Ca2+ exchange which is promoted by Li+. Both the Na+ -dependent and the Ca2+ -dependent Ca2+ effluxes were inhibited by Mn2+. The data are consistent with a Ca2+ carrier mechanism which can extrude Ca2+ in exchange for Na+ or for Ca2+, the latter being activated by Li+. These properties bear a striking resemblance to those of a Ca2+ efflux mechanism which has been characterized in squid axons. This mechanism may there fore have evolved fairly early on in the history of the animal kingdom.

Calcium and EDTA fluxes in dialyzed squid axons

Ca efflux in dialyzed squid axons was measured with 45Ca as a function of internal ionized Ca in the range 0.005-10 muM. Internal Ca stores were depleted by treatment with CN and dialysis with media free of high energy compounds. The [Ca]iota was stabilized with millimolar concentrations of EDTA, EGTA, or DTPA. Nonspecific leak of chelated Ca was measured with [14C]-EDTA and found to be 0.02 pmol/cm2s/mM EDTA. Correction of the measured Ca efflux for this leak of chelated calcium was made when appropriate. Ca efflux was roughly linear with internal free Ca in the range 0.005-0.1 muM. Above 0.1 muM, efflux was less than proportional to concentration but did not saturate at the highest concentration studied. Ca efflux was reduced about 50% by replacement of external Na with Li at Caiota approximately 1 muM, but was insensitive to such replacement for Ca less than 0.1 muM. Ca efflux was insensitive to internal Mg in the range 0-4 mM, indicating that the Ca pump favors Ca over Mg by a factor of about 10(6). Ca efflux was reduced about 60% by increasing internal Na from 1 to 80 mM. This effect could represent weak interference of a Ca carrier by Na or a loss of driving force because of a reduction in ENa - Em occasioned by an increase in Naiota. A few measurements were made of Ca influx in intact and in dialyzed fibers. In both cases, Ca influx increased when external Na was replaced by Li.

Sodium-calcium exchange and calcium-calcium exchange in internally dialyzed squid giant axons

The influx and efflux of calcium (as 45Ca) and influx of sodium (as 24Na) were studied in internally dialyzed squid giant axons. The axons were poisoned with cyanide and ATP was omitted from the dialysis fluid. The internal ionized Ca2+ concentration ([Ca2+]i) was controlled with Ca-EGTA buffers. With [Ca2+]i greater than 0.5 muM, 45Ca efflux was largely dependent upon external Na and Ca. The Nao-dependent Ca efflux into Ca-free media appeared to saturate as [Ca2+]i was increased to 160 muM; the half-saturation concentration was about 8 muM Ca2+. In two experiments 24Na influx was measured; when [Ca2+]i was decreased from 160 muM to less than 0.5 muM, Na influx declined by about 5 pmoles/cm2 sec. The Nao-dependent Ca efflux averaged 1.6 pmoles/cm2 sec in axons with a [Ca2+]i of 160 muM, and was negligible in axons with a [Ca2+]i of less than 0.5 muM. Taken together, the Na influx and Ca efflux data may indicate that the fluxes are coupled with a stoichiometry of about 3 Na+-to-1 Ca2+. Ca efflux into Na-free media required the presence of both Ca and an alkali metal ion (but not Cs) in the external medium. Ca influx from Li-containing media was greatly reduced when [Ca2+]i was decreased from 160 to 0.23 muM, or when external Li was replaced by choline. These data provide evidence for a Ca-Ca exchange mechanism which is activated by certain alkali metal ions. The observations are consistent with a mobile carrier mechanism which can exchange Ca2+ ions from the axoplasm for either 3 Na+ ions, or one Ca2+ and an alkali metal ion (but not Cs) from the external medium. This mechanism may utilize energy from the Na electrochemical gradient to help extrude Ca against an electrochemical gradient.