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

ATP dependence of Na+-driven Cl-HCO3 exchange in squid axons

Squid giant axons recover from acid loads by activating a Na(+)-driven Cl-HCO(3) exchanger. We internally dialyzed axons to an intracellular pH (pH( i )) of 6.7, halted dialysis and monitored the pH(i) recovery (increase) in the presence of ATP or other nucleotides, using cyanide to block oxidative phosphorylation. We computed the equivalent acid-extrusion rate (J(H)) from the rate of pH(i) increase and intracellular buffering power. In experimental series 1, we used dialysis to vary [ATP](i), finding that Michaelis-Menten kinetics describes J (H) vs. [ATP](i), with an apparent V(max) of 15.6 pmole cm(-2 )s(-1) and K (m) of 124 microM. In series 2, we examined ATP gamma S, AMP-PNP, AMP-PCP, AMP-CPP, GMP-PNP, ADP, ADP beta S and GDP beta S to determine if any, by themselves, could support transport. Only ATP gamma S (8 mM) supported acid extrusion; ATP gamma S also supported the HCO (3)(-) -dependent (36)Cl efflux expected of a Na(+)-driven Cl-HCO(3) exchanger. Finally, in series 3, we asked whether any nucleotide could alter J (H) in the presence of a background [ATP](i) of approximately 230 microM (control J (H) = 11.7 pmol cm(-2 )s(-1)). We found J (H) was decreased modestly by 8 mM AMP-PNP (J (H) = 8.0 pmol cm(-2 )s(-1)) but increased modestly by 1 mM ADP beta S (J (H) = 16.0 pmol cm(-2 )s(-1)). We suggest that ATP gamma S leads to stable phosphorylation of the transporter or an essential activator.

Electrogenic Na(+)/Ca(2+) exchange. A novel amplification step in squid olfactory transduction

Olfactory receptor neurons (ORNs) from the squid, Lolliguncula brevis, respond to the odors l-glutamate or dopamine with increases in internal Ca(2+) concentrations ([Ca(2+)](i)). To directly asses the effects of increasing [Ca(2+)](i) in perforated-patched squid ORNs, we applied 10 mM caffeine to release Ca(2+) from internal stores. We observed an inward current response to caffeine. Monovalent cation replacement of Na(+) from the external bath solution completely and selectively inhibited the caffeine-induced response, and ruled out the possibility of a Ca(2+)-dependent nonselective cation current. The strict dependence on internal Ca(2+) and external Na(+) indicated that the inward current was due to an electrogenic Na(+)/Ca(2+) exchanger. Block of the caffeine-induced current by an inhibitor of Na(+)/Ca(2+) exchange (50-100 microM 2',4'-dichlorobenzamil) and reversibility of the exchanger current, further confirmed its presence. We tested whether Na(+)/Ca(2+) exchange contributed to odor responses by applying the aquatic odor l-glutamate in the presence and absence of 2', 4'-dichlorobenzamil. We found that electrogenic Na(+)/Ca(2+) exchange was responsible for approximately 26% of the total current associated with glutamate-induced odor responses. Although Na(+)/Ca(2+) exchangers are known to be present in ORNs from numerous species, this is the first work to demonstrate amplifying contributions of the exchanger current to odor transduction.

Low concentrations of ouabain stimulate Na/Ca exchange in neurons

1. The effects of low concentrations of ouabain on 22Na efflux, 86Rb influx, 45Ca uptake and cyclic AMP levels were studied in snail ganglia. Ouabain, at concentrations below that which inhibits the Na-K pump as monitored by 86Rb influx, activated "reverse mode" Na/Ca exchange, as indicated by an increased 22Na efflux and 45Ca influx. 2. With electrophysiologic recordings ouabain, in the presence of K(+)-free saline to block Na/K transport, caused a membrane hyperpolarization. These concentrations of ouabain also caused elevation of intracellular cyclic AMP levels. 3. We suggest that the ouabain-induced stimulation of Na efflux is due to a stimulation of reverse Na/Ca exchange. since Na/Ca exchange is electrogenic, these observations are most consistent with ouabain stimulation of Na/Ca exchange in a reversed direction (intracellular Na for extracellular Ca). 4. The effect on Na/Ca exchange may be secondary to a rise in intracellular cyclic AMP.

Regulation of sodium-calcium exchange in intact myocytes by ATP and calcium

Regulation of Na-Ca exchange activity by ATP and by intracellular Ca (Cai) has been studied in suspensions of intact Na-loaded adult rat cardiac myocytes using 45Ca uptake and exchange of 22Na. ATP depletion of Na-loaded myocytes results in a strong inhibition of the Na-Ca exchanger, manifested as a strong inhibition of intracellular Na-dependent Ca uptake. Ca uptake by Na-loaded cells in the course of ATP depletion can be very heterogeneous because of the heterogeneity amongst cells of the extent of ATP depletion. This can result in a false measure of the dependence of exchanger activity on cell ATP content. Under conditions intended to maximize the uniformity of cell ATP content amongst cells we found a half maximal rate of Ca uptake with a cell ATP content of 1.96 nmol/mg, about 10% of the normal cell ATP level. The results suggest that ATP depletion after ischemia plus reperfusion is unlikely to limit the rate of Ca uptake by Na-Ca exchange in the whole heart if at least one quarter of the ATP is restored. Ca addition to myocytes loaded with Na in the absence of Ca results in a strong activation of the Na-Ca exchanger at an intracellular site, manifested as a large activation of Na-Na exchange activity. A similar activation of the exchanger is observed in cells with a normal level of intracellular Na, suspended in a medium containing physiological levels of Ca, when the cells are stimulated to beat by application of an electric field. This suggests that regulation of the exchanger by Cai is important physiologically, in the regulation of excitation-contraction coupling. Cells depleted of ATP show not only a strongly inhibited rate of Na-Ca exchange and Na-Na exchange, but also a strongly reduced degree of activation by Cai, even in ATP-depleted cells with no acidosis. This could result from the combined effect of ATP loss and an elevated intracellular Mg concentration on Ca binding affinity at the regulatory site.

Phosphoarginine stimulation of Na(+)-Ca2+ exchange in squid axons--a new pathway for metabolic regulation?

1. [Na+]o-dependent Ca2+ efflux (forward Na(+)-Ca2+ exchange), [32P]ATP wash-out curves and [ATP] were measured in internally dialysed squid giant axons at 17-18 degrees C. 2. We found that dialysing squid axons without ATP and with [Ca2+]i around 1 microM the basal levels of the [Na+]o-dependent Ca2+ efflux were significantly higher in the presence of N omega-phosphoarginine (PA). Phosphocreatine, a related phosphagen, is without effect. 3. PA stimulation of the Na(+)-Ca2+ exchange occurs in the complete absence of ATP (< 1 microM), being independent of, and additive to, the ATP-stimulated [Na+]o-dependent Ca2+ efflux. PA stimulation of [Na+]o-dependent Ca2+ efflux is fully and rapidly reversible with a Km around 7.7 mM. Activation by saturating [PA] is equivalent in magnitude to that of ATP. 4. PA stimulation of Na(+)-Ca2+ exchange is markedly dependent on intracellular Ca2+ and Mg2+ ions. Below 0.5 microM Ca2+i PA effect is negligible, becoming noticeable between 0.8 and 2 microM. In addition, Ca2+i considerably increases the rate at which PA activates the Na(+)-Ca2+ exchange. Although there is no absolute requirement of the PA effect for Mg2+ ions, this divalent cation largely stimulates the PA effect. 5. This work demonstrates, for the first time, the presence in squid axons of a new form of metabolic regulation of the Na(+)-Ca2+ exchange directly and solely related to PA and different from that of MgATP. This novel mechanism is likely to play a physiological role in Ca2+ extrusion through the Na(+)-Ca2+ exchanger, particularly at micromolar [Ca2+]i.

Effects of vanadate on MgATP stimulation of Na-Ca exchange support kinase-phosphatase modulation in squid axons

We have proposed that in squid axons MgATP stimulation of Na-Ca exchange involves a phosphorylation-dephosphorylation process catalyzed by a kinase-phosphatase system. In the present work, we used vanadate as a tool to gather further evidence about the mechanism of metabolic control of the Na-Ca exchanger in internally dialyzed and voltage-clamped squid axons. Vanadate, at concentrations up to 100 microM, stimulated extracellular Na (Nao)-dependent Ca efflux only in the presence of MgATP but failed to do so when the axons were dialyzed with the nonhydrolyzable ATP analogue beta, gamma-methyleneadenosine 5'-triphosphate or with CrATP, a MgATP analogue that completely abolishes MgATP stimulation of the Na-Ca exchange. In axons fully activated by Mg-adenosine 5'-O-(3-thiotriphosphate), vanadate had no effect on Na-Ca exchange. The dose-response curve for vanadate stimulation followed Michaelian kinetics with a Km of 5.6 +/- 0.4 microM and a maximum velocity of 216 +/- 10 fmol.cm-2.s-1 (intracellular Ca concentration = 0.8 microM). This coincides with the high affinity of vanadate in inhibiting the in vitro phosphatase activity of an alkaline phosphatase extracted from rat liver. In addition, vanadate increased fivefold the apparent affinity for MgATP (Km from 220 +/- 14 to 40 +/- 4 microM). Concentrations of vanadate in the millimolar range inhibited the MgATP-stimulated Na-Ca exchange (apparent Ki of 5.7 +/- 0.3 mM) and the in vitro phosphorylation by the catalytic subunit of a adenosine 3',5'-cyclic monophosphate protein kinase (apparent Ki 2.64 +/- 0.04 mM). We conclude that MgATP stimulation of Na-Ca exchange is proportional to the levels of phosphorylation that result from the balance of the activity of a kinase and a phosphatase activity.

Effects of some metal-ATP complexes on Na(+)-Ca2+ exchange in internally dialysed squid axons

1. Na(+)o-dependent Ca2+ efflux (forward Na(+)-Ca2+ exchange), and in some cases the Na(+)i-dependent Ca2+ influx (reverse Na(+)-Ca2+ exchange) were measured in internally dialysed squid axons under membrane potential control. 2. We tested the effect on the Na(+)-Ca2+ exchange of the MgATP analogue bidentate chromium adenosine-5'-triphosphate (CrATP), substrate of several kinases, and cobalt tetrammine ATP (Co(NH3)4ATP), a poor substrate of most kinases. 3. CrATP completely blocked the MgATP and MgATP-gamma-S (ATP-gamma-S) stimulation of the Na(+)o-dependent Ca2+ efflux (forward exchange) and the Na+i-dependent Ca2+ influx (reverse exchange). The analogue only blocked the nucleotide-dependent fraction of the Na(+)-Ca2+ exchange without modifying any kinetic parameters of the exchange reactions. 4. The effects of CrATP were fully reversible with a very slow time constant (t 1/2 about 30 min). 5. The MgATP stimulation of the Na(+)-Ca2+ exchange was completely saturated at 1 mM. Higher MgATP concentrations (up to 15 mM) had no additional effects. Pentalysine (internal or external), the protein kinase C inhibitor H-7 (1-(5-isoquinolinylsulphonyl)-2-methylpiperazine) and several calmodulin inhibitors did not inhibit Na(+)-Ca2+ exchange either in the absence or presence of MgATP. 6. Our results do not agree with the idea of an aminophospholipid translocase being responsible for the ATP stimulation of the Na(+)-Ca2+ exchange in squid axons; they suggest that this is due to the action of a kinase system.

ATP dependence of calcium uptake by the Na-Ca exchanger of adult heart cells

The ATP dependence of the Na-Ca exchanger was investigated in isolated adult rat heart cells to evaluate the extent to which ATP depletion after a period of ischemia plus reperfusion in whole hearts could limit calcium uptake by Na-Ca exchange. A standard state for measurement of Na-Ca exchange activity that could be used with cells depleted of ATP to different degrees was defined. This was a state of zero sarcolemmal gradient for sodium, potassium, and pH and was achieved by incubation of the cells for 5 minutes with EDTA, EGTA, ouabain, and nigericin. Heterogeneity of cell ATP levels was minimized by using a protocol of total ATP depletion by incubation under conditions similar to ischemia, followed by reoxygenation to give partial restoration of ATP levels. No ATP was regenerated when cells were reoxygenated in the presence of rotenone, and such cells showed a very low rate of calcium uptake. Without rotenone, cells showed an almost complete restoration of Na-Ca exchange activity, in spite of a restoration of ATP levels to only one third of control values. Thus, the dependence of calcium uptake on ATP was highly nonlinear under these conditions. The calculated Km for ATP was no more than 10% of normal ATP levels. We conclude that ATP depletion after ischemia plus reperfusion is unlikely to limit the rate of calcium uptake through Na-Ca exchange in the whole heart if at least one quarter of the ATP is restored. In addition, we measured the apparent ATP dependence of calcium uptake by Na-Ca exchange in cells under conditions in which we previously had concluded that cell ATP distributions were very heterogeneous: when cells undergo contracture during incubation with oligomycin and without glucose. A linear relation between calcium uptake rate and ATP was observed at all ATP levels. This can be understood if cells in contracture that are incubated with oligomycin cannot take up calcium because of low ATP, whereas rod-shaped cells are able to retain a full uptake capability. This result further supports our conclusion that the ATP level declines catastrophically to near zero in these oligomycin-incubated cells just before contracture.

ATP as a positive effector of the sodium efflux in single barnacle muscle fibers

A study has been made of the mechanism by which the injection of ATPNa2 stimulates the ouabain-insensitive Na efflux in fibers from the barnacle, Balanus nubilus. The results of this study are as follows: ATPNa2 is found to be a more potent effector of the Na efflux in unpoisoned fibers than ATPMg on an equimolar basis, but not more potent than ADPNa2. In ouabain-poisoned fibers ATPNa2 and ATPMg are equipotent but the former is more potent than ADPNa2. The magnitude of the response to ATPNa2 injection into ouabain-poisoned fibers depends on: (i) the ouabain concentration used; (ii) the concentration of ATPNa2 injected, and (iii) the external Ca2+ concentration. Ouabain is without effect when it is applied at the time of ATPNa2 injection. Responsiveness to ouabain, however, is found to return if the glycoside is applied after complete decay of the response to ATP. Under these conditions, the effect of ouabain in fibers injected with ATPNa2 is significantly less than in fibers injected with ATPMg. Preinjection of EGTA in high concentrations fails to reduce the size of the response to ATPNa2 injection. Injection of Mg2+ following peak stimulation by ATP almost completely reverses the response. The response to Mg2+ is concentration-dependent. Ryanodine but not neomycin reduces the response to ATP. ATP gamma S is not as effective as ATPNa2. Nor is AMP-PNP consistently as effective as ATPNa2. Collectively, these results support the hypothesis that the response of the Na efflux to ATPNa2 injection involves the operation of the putative Na(+)-Ca2+ exchanger in the reverse mode and that a raised Cai2+ is not an absolute requirement. They also strongly suggest that two other governing factors are the Na+ gradient across the sarcolemma and the myoplasmic pMg. Mg2+ seems to act as an inhibitor.

Control of the Na-Ca exchanger in isolated heart cells. I. Induction of Na-Na exchange in sodium-loaded cells by intracellular calcium

Isolated adult rat heart cells in suspension were loaded with sodium by incubation with ouabain in the absence of calcium for 30 minutes. Addition of low levels of calcium induced accelerated rates of sodium influx and efflux, as measured with 22Na. The magnitude of calcium-induced 22Na efflux was 50-fold greater than the net rate of calcium uptake and required extracellular sodium, but not extracellular calcium, once some calcium was taken up. Calcium did not induce 86Rb efflux. The accelerated rate of 22Na efflux was prevented by verapamil, but verapamil was ineffective when added after calcium. Addition of EGTA after calcium reversed the effect of calcium, but only after incubation. Dichlorobenzamil, unlike verapamil, both prevented and reversed the induction of sodium fluxes by calcium. We conclude 1) that intracellular calcium induces Na-Na exchange through the Na-Ca exchanger in sodium-loaded cells exposed to calcium; and 2) that Na-Na exchange can be activated by calcium that enters the cell through calcium channels. We propose that this Na-Na exchange reflects the intrinsic activity of the Na-Ca exchanger.

Sodium-calcium exchange in membrane vesicles from aortic myocytes: stimulation by endogenous proteolysis masks inactivation during vesicle preparation

Plasma membrane vesicles were purified from rat aortic myocytes by centrifugation in a discontinuous sucrose gradient. Vesicles were prepared in the presence or absence of five proteinase inhibitors (aprotinin, benzamidine, leupeptin, pepstatin A and phenylmethylsulfonyl fluoride). The proteinase inhibitors decreased the Vmax by 3.4-fold and had no effect on the Km for Ca2+ of Na+ gradient-dependent 45Ca2+ influx. The proteinase inhibitors had no direct effect on exchange activity, and they had no effect on membrane purity as indicated by 5'-nucleotidase activity. Removing the proteinase inhibitors or adding trypsin or chymotrypsin increased exchange activity approx. 2-fold. The Vmax of exchange activity in intact aortic myocytes is approx. 10-fold higher than the Vmax in plasma membrane vesicles prepared in the presence of proteinase inhibitors. Exchange activity in plasma membrane vesicles is only a sixtieth of the expected value, because the vesicles have approx. 7-fold higher 5'-nucleotidase activity and approx. 6-fold higher specific exchange activity than the crude homogenate. The large loss of exchange activity may be caused by a change in a regulatory domain of the exchanger because endogenous proteolysis restores some of the activity lost during vesicle preparation.

Energy dependence of sodium-calcium exchange in vascular smooth muscle cells

Three different types of mitochondrial poisons (oligomycin, antimycin A, and dinitrophenol) strongly inhibited Na(+)-Ca2+ exchange in aortic myocytes. Exchange activity was assayed as 45Ca2+ uptake that depended on inverting the Na+ gradient and was inhibited by 25 microM dimethylbenzamil. Glucose markedly decreased the inhibition of exchange activity by these three poisons. Glucose also prevented rotenone from inhibiting exchange and depleting cellular ATP. In the absence of glucose, rotenone decreased ATP and exchange activity with half-times of 0.8 and 0.9 min, respectively. Almost eliminating cellular ATP with rotenone maximally inhibited exchange by 80%. Repletion of ATP with glucose substantially restored Na(+)-Ca2+ exchange activity. Ca2+ uptake by organelles, subsequent to entry via exchange for Na+, does not appear to contribute significantly to exchange activity as assayed in intact myocytes. The specific activity of Na(+)-Ca2+ exchange was approximately 30 nmol.min-1.mg protein-1. These findings suggest that ATP modulates exchange activity and that there are approximately 150,000 Na(+)-Ca2+ exchangers per cell, assuming that the turnover number is 1,000 s-1.

Asymmetrical properties of the Na-Ca exchanger in voltage-clamped, internally dialyzed squid axons under symmetrical ionic conditions

In this work we have investigated whether the asymmetrical properties of the Na/Ca exchange process found in intact preparations are intrinsic to the exchange protein(s) or the result of the asymmetric ionic environment normally prevailing in living cells. The activation of the Na/Ca exchanger by Ca2+ ions, monovalent cations, ATP gamma S and the effect of membrane potential on the different operational modes of the exchanger (Nao/Cai, Cao/Nai, Cao/Cai, and Nao/Nai) was studied in voltage-clamped squid giant axons externally perfused and internally dialyzed with symmetrical ionic solutions. Under these conditions: (a) Ca ions activate with higher affinity from the inside (K1/2 = 22 microM) than from the outside (K1/2 = 300 microM); (b) experiments measuring the Cao-dependent Ca efflux in the conditions Lio-Trisi, Lio-Lii, Triso-Trisi, and Triso-Lii, show that the activating monovalent cation site on the exchanger faces the external surface; (c) ATP gamma S activates the Cao-dependent Ca efflux (Cao/Cai exchange) only at nonsaturating [Ca2+]i. Its effect appears to be on the Ca transport site since no alteration in the apparent affinity of the activating monovalent cation site was observed. The above results show that the Na/Ca exchange process is indeed a highly asymmetric transport mechanism. Finally, the voltage dependence of the components of the different exchange modes was measured over the range of +20 to -40 mV. The voltage dependence (approximately 26% change/25 mV) was found to be similar for all modes of operation of the exchanger except Nao/Nai exchange, which was found to be voltage insensitive. The sensitivity of the Cao/Cai exchange to voltage was found to be the same in the presence and in the complete absence of monovalent cations. This finding does not support the proposition that the voltage sensitivity of the Cao/Cao exchange is induced by the binding and transport of an external monovalent cation.

Vanadate and fluoride effects on Na+-K+-Cl- cotransport in squid giant axon

The effects of vanadate and fluoride on the Na+-K+-Cl- cotransporter of the squid giant axon were assessed. In axons not treated with these agents, intracellular dialysis with ATP-depleting fluids caused bumetanide-inhibitable 36Cl influx to fall with a half time of approximately 16 min. In the presence of either 40 microM vanadate or 5 mM fluoride, the decay of bumetanide-inhibitable 36Cl influx was significantly slowed; half time for vanadate-treated axons is 45 min and for fluoride-treated axons is 37 min. These agents are not exerting their effects on Na+-K+-Cl- cotransport by influencing the rate of ATP depletion of the axon, since they had no effect on the ATP hydrolysis rate of an optic ganglia homogenate. We therefore suggest that these data support the hypothesis that Na+-K+-Cl- cotransport in squid axons is regulated by a phosphorylation-dephosphorylation mechanism and that vanadate and fluoride reduce the rate of dephosphorylation by inhibiting a protein phosphatase.

The squid axon as a model for studying plasma membrane mechanisms for calcium regulation

Calcium movement across plasma membranes occurs mainly by three routes: voltage-dependent calcium channels, adenosine 5'-triphosphate-driven calcium pump, and Na+-Ca2 exchange. The regulation of the intracellular ionized calcium is the consequence of two parallel calcium transport mechanisms: a high affinity, low capacity system responsible for extruding calcium during resting conditions (calcium pump) and a low affinity and high capacity system (Na+-Ca2 antiporter). This last system is designed to extrude calcium ions when intracellular calcium rises above certain levels and also to lead calcium ions into the cell under conditions that favor the reverse mode of operation of the exchanger. This short review provides an analysis of the most conspicuous features of the two membrane transport mechanisms determined in dialyzed squid axons with special emphasis on both the complexity of the Na+-Ca2+ exchange system and its marked asymmetry.

Inhibition of calcium influx in isolated adult rat heart cells by ATP depletion

Using 45Ca, indo1, and quin2, calcium uptake was measured in isolated quiescent adult rat heart cells under different metabolic conditions. Exposure of cells in a medium containing 1 mM CaCl2 to rotenone and uncoupler resulted in adenosine triphosphate (ATP) depletion from 17.08 +/- 2.26 to 0.63 +/- 0.11 nmol/mg within 8 minutes, and the cells went into contracture. In this time, the cells lost 1.65 +/- 0.1 nmol Ca/mg of total rapidly exchangeable cellular calcium, and the level of free cytosolic calcium as measured by indo1 rose from 47.4 +/- 16.3 nM to 79.8 +/- 27.6 nM. The subsequent rate of rise of intracellular free calcium concentration was just 4 nM/min for at least 40 minutes. Therefore, we investigated the effect of ATP depletion on the rate of calcium entry. In cells loaded with sodium by ouabain treatment without calcium, the initial rate of calcium influx on calcium addition was inhibited by 82-84% when cellular ATP was depleted, as measured by 45Ca or indo1. Quin2 also showed a strong inhibition of calcium influx by ATP depletion, but itself also caused a strong inhibition of calcium influx. The rate of calcium influx declined even further in ATP-depleted cells after the initial influx: Between 1 and 12 minutes after calcium addition, the residual 45Ca uptake rate of the first minute was inhibited by an additional 90%. We conclude that ATP depletion per se does not quickly elevate cytoplasmic free calcium and that such an elevation is prevented by a very strong inhibition of the rate of calcium entry.

In squid axons, ATP modulates Na+-Ca2+ exchange by a Ca2+i-dependent phosphorylation

In squid axons ATP stimulates both the forward and reverse modes of the Na+-Ca2+ exchange by changing the affinity of the carrier towards Na+ and Ca2+ ions. Whether ATP activates the Na+-Ca2+ antiporter allosterically or is hydrolyzed during activation is still debated. The hypothesis that ATP modulates the Na+-Ca2+ exchange through phosphorylation has been tested by means of [gamma-S]ATP, an ATP analog that can act as a substrate for kinases but not for ATPases. Steady-state Ca2+ efflux was measured in squid axons dialyzed without ATP and containing either 0.7 or 100 microM Ca2+i. Addition of 1 mM [gamma-S]ATP markedly increases the Na+o-dependent component of the Ca2+ efflux. The activation by [gamma-S]ATP: requires the presence of Mg2+i, is partially reversible upon removing the analog, is greater than that caused by ATP and only operates on the exchange system since no activation of the ATP-dependent uncoupled Ca2+ efflux (Ca2+ pump) can be detected. 22Na+ experiments were used to monitor the Cao-dependent Na+ efflux (reverse Na+-Ca2+ exchange). Without Ca2+i and ATP, Na+ efflux is very small ('leak'). [gamma-S]ATP does not activate the efflux of Na+ in the absence of Ca2+i. In the presence of Ca2+i the ATP analog stimulates both the Cao- and Nao-dependent Na+ efflux components. Interestingly, neither the Na+ pump, Ca2+i-independent Na+-Na+ exchange, Nai+-Mg2+i exchange or Na+/K+/Cl- cotransport are affected by [gamma-S]ATP. The experiments indicate that a Ca2+i-dependent phosphorylation occurs during the activation of the Na+-Ca2+ exchange by ATP.

Characterization of the ATP-promoted aspect of Na+-Ca2+ exchange present in squid retinal nerve axolemma

Using an in vitro system which consists of an axolemma-rich vesicle fraction prepared from squid retinal nerve fibers, an Na+-Ca2+ exchange process has been characterized and appears identical with that reported in squid giant axon. This exchange is absolutely dependent on the establishment of an Na+ gradient, shows monovalent and divalent cation specificity and is highly sensitive to monensin, A23187 and valinomycin but not to ouabain, digitoxigenin, vanadate, pentylenetetrazole, tetrodotoxin or tetraethylammonium. Furthermore, it was found that the exchange process is enhanced by the addition of ATP. This ATP-promoted aspects of Na+-Ca2+ exchange shares many similar characteristics with Na+-Ca2+ ATP hydrolysis and may indicate a common mechanism for both activities via a protein phosphorylation-dephosphorylation event.

Plasma membrane calcium fluxes in intact rods are inconsistent with the "calcium hypothesis"

The temporal relationship between the extracellular rod photovoltage and light-induced net Ca fluxes across the rod plasma membrane is investigated. The net Ca flux measurements are derived from extracellular Ca concentration measurements at the receptor surface of the isolated bullfrog retina. As reported previously, illumination leads to a net Ca efflux, which is followed by a net influx, during which the released Ca is taken back up. However, the net Ca flux has two characteristics that are inconsistent with the hypothesis that intracellular free Ca is the intracellular messenger for phototransduction in rods. First, during maintained photovoltage saturation, the net Ca efflux is transient, declining with a stereotypic time course that is independent of stimulus intensity and duration. Second, the significant rate of net influx during Ca uptake has no correlate in the photovoltage waveform. These observations are not consistent with the "Ca hypothesis." Rather, these data corroborate recent findings suggesting that light causes a decrease rather than an increase in intracellular free Ca concentration.

Calcium transport abnormality in uremic rat brain synaptosomes

Brain calcium is elevated in patients and laboratory animals with uremia. The significance of this finding is unclear. We evaluated calcium transport in brain of both normal and acutely uremic rats (blood urea nitrogen = 250 mg/dl) by performing studies in synaptosomes from rat brain cerebral cortex. Synaptosomes are vesicular presynaptic nerve endings from brain that contain mitochondria and are metabolically active. Two mechanisms of calcium transport were evaluated using radioactive 45Ca++ as a tracer. Both mechanisms were evaluated in the absence of exogenously administered parathyroid hormone (PTH). We first evaluated Na+-Ca++ exchange in vesicles that were loaded with NaCl in an external media containing 10 microM CaCl2. Both initial rates of calcium transport and equilibrium levels of calcium accumulation in synaptosomes prepared from uremic rats were significantly greater (P less than 0.005) than in normal. To assess calcium efflux, ATP-dependent calcium uptake (1 mM ATP) was studied in inverted plasma membrane vesicles loaded with KCl. In the uremic synaptosomes, a significant increase (P less than 0.005) in ATP-dependent calcium uptake was observed as compared with the normal. These studies show that (a) Calcium accumulation via the Na+-Ca++ exchanger is increased in synaptosomes prepared from uremic rat brain. (b) Calcium influx into inverted plasma membrane vesicles from uremic rats via the ATP-dependent calcium transport mechanism is increased when compared with normal. (c) The increased calcium accumulation in uremia by both Na+-Ca++ exchange and ATP-dependent calcium transport mechanism appears to be a result of increased synaptosomal membrane permeability to calcium. Both these abnormalities of calcium transport in uremia would tend to increase brain extracellular calcium in vivo. The defects observed in uremia do not appear to be readily reversible, and the relationship to PTH is presently unclear. These abnormalities may affect neurotransmission in the uremic state.

Reduced calcium sensitivity of the sodium channel and the Na+/Ca2+ exchange system in the Kdr-type, DDT and pyrethroid resistant German cockroach, Blattella germanica

To understand the biochemical cause of nerve insensitivity to DDT and pyrethroids two cation regulating systems, the Na+/Ca2+ exchange and the Na+ channel in the membrane preparation from the central nervous system of susceptible and resistant strains of the German cockroach were examined. Both systems from resistant insects have lower affinities to Ca2+ than those from their susceptible counterparts. The resistant strains show a prominent cross-resistance to all agents affecting calcium regulation (particularly A23187) and Na+ channel operations in vivo, indicating that the basic cause of the reduced nerve insensitivity in resistant insects is related to the reduction in calcium sensitivity in these systems.

Calmodulin in the regulation of calcium fluxes in cardiac sarcolemma

Three systems mediate the fluxes of calcium across heart sarcolemma: the slow calcium channel (influx), the ATP-dependent calcium pump (efflux), and the Na+/Ca2+ exchanger (efflux, but possibly also influx). Calmodulin regulates the pumping ATPase by direct interaction and also by activating a protein kinase. The Na+/Ca2+ exchanger is modulated by calmodulin via a phosphorylation-dephosphorylation cycle. Both the kinase and the phosphatase are membrane-bound and calmodulin-sensitive. The kinase has higher Ca2+ affinity than the phosphatase.

Interactions of physiological ligands with the Ca pump and Na/Ca exchange in squid axons

We have studied the interaction of physiological ligands other than Nai and Cai with the Ca pump and Na/Ca exchange in internally dialyzed squid axons. The results show the following. (a) Internal Mg2+ is an inhibitor of the Nao-dependent Ca efflux. At physiological Mg2+i (4 mM), the inhibition amounts to approximately 50%. The inhibition is partial and noncompetitive with Cai, and is not affected by Nai or ATP. The ATP-dependent uncoupled efflux is unaffected by Mgi up to 20 mM. Both components of the Ca efflux require Mg2+i for their activation by ATP. (b) At constant membrane potential, Ki is an important cofactor for the uncoupled Ca efflux. (c) Orthophosphate (Pi) activates the Nao-dependent Ca efflux without affecting the uncoupled component. Activation by Pi occurs only in the presence of Mg-ATP or hydrolyzable ATP analogues. Pi under physiological conditions has no effect on the uncoupled component; nevertheless, at alkaline pH, it inhibits the Ca pump, probably by product inhibition. (d) ADP is a potent inhibitor of the uncoupled Ca efflux. The Nao-dependent component is inhibited by ADP only at much higher ADP concentrations. These results indicate that (a) depending on the concentration of Ca2+i, Na+i Mg2+i, and Pi, the Na/Ca carrier can operate under a low- or high-rate regime; (b) the interactions of Mg2+i, Pi, Na+i, and ATP with the carrier are not interdependent; (c) the effect of Pi on the carrier-mediated Ca efflux resembles the stimulation of the Nao-dependent Ca efflux by internal vanadate; (d) the ligand effects on the uncoupled Ca efflux are of the type seen in the Ca pump in red cells and the sarcoplasmic reticulum.

Effect of monovalent cations on Na+/Ca2+ exchange and ATP-dependent Ca2+ transport in synaptic plasma membranes

Two Ca2+ transport systems were investigated in plasma membrane vesicles isolated from sheep brain cortex synaptosomes by hypotonic lysis and partial purification. Synaptic plasma membrane vesicles loaded with Na+ (Na+i) accumulate Ca2+ in exchange for Na+, provided that a Na+ gradient (in leads to out) is present. Agents that dissipate the Na+ gradient (monensin) prevent the Na+/Ca2+ exchange completely. Ca2+ accumulated by Na+/Ca2+ exchange can be released by A 23187, indicating that Ca2+ is accumulated intravesicularly. In the absence of any Na+ gradient (K+i-loaded vesicles), the membrane vesicles also accumulate Ca2+ owing to ATP hydrolysis. Monovalent cations stimulate Na+/Ca2+ exchange as well as the ATP-dependent Ca2+ uptake activity. Taking the value for Na+/Ca2+ exchange in the presence of choline chloride (external cation) as reference, other monovalent cations in the external media have the following effects: K+ or NH4+ stimulates Na+/Ca2+ exchange; Li+ or Cs+ inhibits Na+/Ca2+ exchange. The ATP-dependent Ca2+ transport system is stimulated by increasing K+ concentrations in the external medium (Km for K+ is 15 mM). Replacing K+ by Na+ in the external medium inhibits the ATP-dependent Ca2+ uptake, and this effect is due more to the reduction of K+ than to the elevation of Na+. The results suggest that synaptic membrane vesicles isolated from sheep brain cortex synaptosomes possess mechanisms for Na+/Ca2+ exchange and ATP-dependent Ca2+ uptake, whose activity may be regulated by monovalent cations, specifically K+, at physiological concentrations.

The regulation of the Na+ -Ca2+ exchanger of heart sarcolemma

The Na+/Ca2+-exchange of calf-heart sarcolemma is activated by a treatment with ATP, Mg2+, and Ca2+, and deactivated by a treatment with phosphorylase phosphatase. The effect of the latter can be substituted by a treatment with Mg2+, Ca2+, and calmodulin. The activating treatment does not require added calmodulin, but is inhibited by calmodulin antagonists. Evidently, endogenous calmodulin is required and sufficient. Activation is half-maximal at about 2 microM Ca2+. Added calmodulin, however, decreases the Km (Ca2+) of the activating process to about 0.8 microM. Deactivation is half-maximal, at optimal calmodulin concentrations, at about 1.5 microM Ca2+. Experiments with adenosine 5'-[gamma-thio]triphosphate have shown that the activating treatment is mediated by a kinase and the deactivating treatment by a phosphatase. The concerted operation of the two enzymes is made possible by their different Ca2+ affinity. At saturating Ca2+ concentrations, the level of ATP may also influence the balance of the two enzymes.

Effects of internal sodium and hydrogen ions and of external calcium ions and membrane potential on calcium entry in squid axons

Squid giant axons were impaled with electrodes to measure pNai, pHi, Em, and were injected with either aequorin or arsenazo III to measure [Ca]i or with phenol red to measure [H]i. Depolarization of such axons with elevated [K] in sea water leads to a Ca entry that is a function of [Ca]o, [Na]i, and [H]i. With saturating [Na]i half-maximal Ca entry is produced by a [Ca]o of 0.58 mM. With saturating [Ca]o, depolarization produced by 450 mM-K+ leads to half-maximal Ca entry when [Na]i is 25 mM; entry is virtually undetectable if [Na]i is 18 mM. If [Ca]o is 50 mM, Ca entry upon depolarization as measured with aequorin is phasic with a rapid phase of light emission and a plateau; Ca entry as measured with arsenazo III shows no such phasic behaviour, absorbance vs. time is a square wave that closely follows the depolarization vs. time trace. Both detectors of [Ca]i show a square-wave response if [Ca]o is 3 mM. The introduction of 2 mM-CN into the sea water bathing the axon does not affect the response to depolarization nor does the destruction of most of the ATP in the axon following the injection of apyrase. If axons are microinjected with phenol red rather than arsenazo, the entry of Ca produces an acidification in the peripheral parts of the axoplasm. Other experiments measuring [Ca]i show that Ca entry is strongly inhibited by a decrease in pHi. Making sea water alkaline with pH buffers scarcely affects the Ca entry induced by depolarization; making axoplasm alkaline by adding NH4+ to sea water greatly enhances Ca entry by Na/Ca exchange and also enhances the ability of axoplasmic buffers to absorb Ca.

A study of calcium compartments in rat brain cortex thin slices: effects of veratridine, lithium and of a mitochondrial uncoupler

The efflux kinetics of 45Ca from rat brain cortex thin slices previously equilibrated with it, was studied in a superfusion system. Two first order kinetic components of efflux from the tissue were found: k2 = 0.0667 min-1, that was unchanged by lowering the temperature from 37 degrees C to 15 degrees, and k3 = 0.0167 min-1 at 37 degrees C, that was reduced to 0.0897 min-1 at 15 degrees C. This suggests that k2 represents efflux from the extracellular space, and k3 that from the cellular compartment. Addition of the mitochondrial uncoupler carbonyl cyanide, m-chlorophenylhydrazone (CCCP) (10(-5)M) increased the efflux fractional rate constant of 45Ca by 35%, while no change in efflux was induced by 10 mM caffeine. Veratridine (10(-5)M) drastically reduced 45Ca efflux if superfusion was with physiological salt solution (150 mM sodium present), but not if 50 mM lithium replaced an equivalent amount of sodium in the superfusion fluid. This lithium-containing solution did not affect 45Ca efflux in the absence of veratridine. These results indicate that mitochondria accumulate only a minor fraction of intracellular 45Ca; that 45Ca possibly turns over very rapidly in the endoplasmic reticulum, and that most of 45Ca is present in a different, non-mitochondrial, non endoplasmic reticular compartment, the nature of which can be only conjectured.

Ca entry at rest and during prolonged depolarization in dialyzed squid axons

Ca influx has been studied in squid axons under internal dialysis control. In axons dialyzed with "normal" physiological conditions (Nai = 40-50 mM, Cai2+ = 0.06-0.1 microM, ATP = 2 mM, Ki = 310 mM), 70% of the resting Ca influx is sensitive to external TTX (K0.5 congruent to 5 nM), 20% of it can be accounted by the reversal of the Na-Ca exchange, and the remaining fraction (10%) is insensitive to TTX, D-600, and Nai. The Ca antagonic drug D-600 (50-100 microM) has an inhibitory effect on the resting Ca influx. This compound was found to affect both the TTX sensitive and the Nai-dependent Ca influx components. In the presence of Nai and ATP, Cai2+ activates the carrier mediated Ca entry (Nai-dependent Ca influx). Most of the activation occurs in the submicromolar range of Cai2+ concentrations (K0.5 congruent to 0.6 microM). In the absence of Nai and/or ATP, no activation of Ca influx by Cai2+ was found up to about 5 microM Cai2+. Prolonged depolarization with high Ko causes an increase in Ca influx sustained for long time (minutes). Depolarizing the axons by removing Ki causes the same effect. This depolarization-induced Ca entry was only observed in axons containing Nai. In the absence of Nai, Ca influx decreases with increasing Ko. The activation of the carrier mediated Ca entry (electrogenic Na/Ca exchange) by membrane depolarization was found to be markedly dependent on the magnitude of Ca2+ i. Increasing the magnitude of Ca2+ i from 0.1 to 0.6 microM causes a ten fold increase in the extra Ca influx induced by a K-depolarization.

An ATP-dependent sodium-sodium exchange in strophanthidin poisoned dialysed squid giant axons

1. Dialysed giant axons from the squid have been used to study some of the properties of the Na+ fluxes when the Na+ pump is fully inhibited by strophanthidin. 2. In axons which had been depleted of ATP, strophanthidin had no effect on Na+ efflux. Similar negative results were obtained in axons dialysed with and without internal or external K+, and with or without 100 microM-internal Ca2+. 3. In the presence of 60 mM-internal Na+, 440 mM-external Na+ and strophanthidin, the fluxes of Na+ had the following characteristics. (i) ATP stimulated an efflux and an influx of Na+ of similar magnitude. The K1/2 for ATP, measured from its effect on Na+ efflux, was about 200 microM. (ii) The non-hydrolysable ATP analogue adenylyl(beta, gamma-methylene)-diphosphonate (AMP-PCP), at 2 mM concentration, either alone or in combination with 2 mM-internal phosphate, failed to stimulate any efflux of Na+. (iii) The ATP-dependent Na+ efflux was not affected by removal of internal or external K+, or external Mg2+ or Ca2+, and was not dependent on internal Ca2+. (iv) within the resolution of the method, all the ATP-dependent Na+ influx required internal Na+, and all the ATP-dependent Na+ efflux required external Na+. From the magnitude of the unidirectional Na+ fluxes the stoichiometry seemed to be a 1 to 1 Na+--Na+ exchange. 4. The ATP-internal Na+-dependent influx of Na+ in the presence of strophanthidin was not affected by 1 mM-vandate in the dialysis solution, a concentration which fully inhibits the Na+ efflux through the Na+ pump that is activated by external K+. 5. In the presence of external Na+, the external K+ sites of the Na+ pump are completely saturated with 100 mM-external K+. In unpoisoned axons incubated with 100 mM-external K+, replacement of external Na+ with Tris+ produced no change in the efflux of Na+. However, in axons poisoned with 50 microM-strophanthidin, replacement of external Na+ with Tris+ resulted in a reversible inhibition of Na+ efflux. This could suggest that strophanthidin poisoning might induce Na+ (cations?) fluxes which are not present in normal conditions.

The effects of ATP on the interactions between monovalent cations and the sodium pump in dialysed squid axons

1. The efflux of Na in dialysed axons of the squid has been used to monitor the sidedness of the interactions of the Na pump with Na(+) ions, K(+) ions and ATP. The axons were under conditions such that most of the Na efflux went through the Na pump by means of a complete cycle of ATP hydrolysis.2. With 310 mm-K(i) (+), 70 mm-Na(i) (+) and 10 mm-K(+) artificial sea water (ASW) more than 97% of the Na efflux was abolished by removal of ATP. The efflux of Na was stimulated by ATP with a K((1/2)) of about 200 mum. This is similar to the K((1/2)) of 150 mum found for the ATP dependence of a ouabain-sensitive Na,K-ATPase activity in membrane fragments isolated from squid optical nerves.3. A 100-fold reduction in the ATP concentration (from 3-5 mm to 30-50 mum) increased the apparent affinity of the Na pump for K(o) (+) about 8-fold. In addition, the maximal rate of K(o) (+)-stimulated Na efflux was reduced by a similar factor. Analogous results were seen in axons dialysed with 310 mm-K(i) (+) or without K(i) (+).4. The relative effectiveness of external monovalent cations as activators of the Na efflux was a function of the ATP concentration inside the axon. With 3-5 mm-ATP the order of effectiveness was K(+) > NH(4) (+) > Rb(+). With 30-50 mum-ATP the sequence was NH(4) (+) >> K(+) >> Rb(+). These results were not affected by the removal of K(i) (+).5. When the ATP concentration was 3 mm and the Na(i) (+) concentration 70 mm, the removal of K(i) (+) produced a slight and reversible increase in the total efflux of Na (15%) and no change in the ATP-dependent Na efflux. When the ATP concentration was reduced to 30-50 mum, or the Na(i) (+) concentration lowered to 5-10 mm, the removal of K(i) (+) reversibly increased the total and the ATP-dependent efflux of Na. The largest increase in Na efflux was seen when both ATP and Na(i) (+) were simultaneously reduced. The ATP-dependent extra Na efflux resulting from the exclusion of K(i) (+) was abolished by 10(-4)m-ouabain in the sea waters.6. The increase in the ATP-dependent Na efflux observed in axons dialysed with 0 K(i) (+) + 10 mm-K(+) ASW was not seen in axons perfused with 310 mm-K(i) (+) + 450 mm-K(+) ASW. However, both experimental conditions gave rise to a similar (and small) ATP-independent and ouabain-insensitive efflux of Na. This indicates that the effects on the Na pump of removing K(i) (+) are not due to the simultaneous membrane depolarization. In addition, it suggests that K(i) (+) has an inhibitory effect on the Na pump, and that that effect is antagonized by Na(i) (+) and ATP.7. The present results are consistent with the idea that the same conformation of the Na pump (and Na,K-ATPase) can be reached by interaction with external K(+) after phosphorylation and with internal K(+) before rephosphorylation. This enzyme conformation produces an enzyme-K complex from which K(+) ions are not easily released unless high concentrations of ATP are present. This also stresses a non-phosphorylating regulatory role of ATP.

Effects of ATP and vanadate on calcium efflux from barnacle muscle fibres

Calcium ions carry the inward current during depolarization of barnacle muscle fibres and are involved in the contraction process. Intracellular ionized calcium ([Ca2+]i) in barnacle muscle, as in other cells, is kept at a very low concentration, against a large electrochemical gradient. This large gradient is maintained by Ca2+ extrusion mechanisms. When [Ca2+]i is below the contraction threshold, Ca2+ efflux from giant barnacle muscle fibres is, largely, both ATP dependent and external Na+ (Na+0) dependent (see also refs 5,6). When [Ca2+]i is raised to the level expected during muscle contraction (2-5 muM), most of the Ca2+ efflux from perfused fibres is Na0 dependent; as in squid axons, this Na+0-dependent Ca2+ efflux is ATP independent. Orthovanadate is an inhibitor of (Na+ + K+) ATPase and the red cell Ca2+-ATpase. We report here that vanadate inhibits ATP-promoted, Na+0-dependent Ca2+ efflux from barnacle muscle fibres perfused with low [Ca2+]i (0.2-0.5 microM), but has little effect on the Na+0-dependent, ATP-independent Ca2+ efflux from fibres with a high [Ca]i (2-5 microM). Nevertheless, ATP depletion or vanadate treatment of high [Ca2+]i fibres causes an approximately 50-fold increase of Ca2+ efflux into Ca2+-containing lithium seawater. These results demonstrate that both vanadate and ATP affect Ca2+ extrusion, including the Na+0-dependent Ca2+ efflux (Na-Ca exchange), in barnacle muscle.

Anion transport mechanisms in neurons

Evidence has been presented and reviewed to show that chloride is often not in electrochemical equilibrium across neuronal cell membranes. An ATP- and sodium-dependent uptake mechanism has been described for the squid giant axon. Finally, the role of chloride in the maintenance of pH1 has been discussed. Present evidence favors a net chloride extrusion being involved in the acid extrusion process. Measurements of free, ionized chloride levels in several neuronal cells have suggested that the transmembrane distribution of chloride does not conform to thermodynamic equilibrium conditions. Net extrusion of Cl- against its electrochemical gradient has been measured in the giant neuron of the Aplysia abdominal ganglion. In the squid giant axon, cellular Cl- levels are higher than predicted from passive thermodynamic considerations. An ATP requirement as well as a dependence upon extracellular sodium has been demonstrated for chloride influx. Similarly, there is an ATP-, external Cl-dependent, Na+ influx. Thus, an ATP-requiring Na-Cl cotransport mechanism appears to account for the high cellular chloride content of the squid giant axon. Chloride efflux from several neurons appears to be involved in the regulation of intracellular pH (pHI). When pHi is made acidic, chloride efflux from the squid giant axon increases and this stimulation requires cellular ATP and external HCO3-. When pHi is measured following an acid load, it is found that pHi recovery toward normal values requires cellular Cl-, ATP and external HCO3-. Thus, an exchange process between cellular Cl- and extracellular HCO3- appears to play an important role in pH1 regulation. Such a process appears to be responsible for the lower-than-equilibrium levels of chloride found in certain neuronal cells.