Calcium-sodium Exchange Research Articles

Hydrogeochemical characterization of the thermal springs in northeastern of Los Cabos Block, Baja California Sur, México.

The existence of hot springs in the northeastern part of Los Cabos Baja California Sur (BCS), is known from pre-Hispanic times, but their hydrochemical composition had not been previously described. Several springs are located within the watershed of Santiago, and the objective of this study was to define the hydrogeochemical composition of the thermal springs and to characterize the geothermal reservoir. A total of 16 water samples were taken in 11 geothermal manifestations under dry (June 2014) and humid (March 2015) conditions. A geothermal system of low enthalpy and low mineralization was found along the San José del Cabo Fault (FSJC), with an average salinity (TDS) of 261mg/L and an alkaline pH (8.5-9.5). The hydrogeochemical composition corresponds to the sodium-bicarbonate type, and geothermometers (silica and Na-K) indicate temperatures ranging from 70 to 115°C for the deep thermal reservoir in conditions of equilibrium. The thermal springs with these hydrogeochemical characteristics differ in respect to the hydrochemical composition of the springs, formally described on several sites of BCS. Br/Cl and B/Cl ratios as well as the enrichment factor (EF) indicate that rainwater with a seawater component represents the source of the thermal spring water. In the springs, a mixture between thermal water and surface water is observed, combined with a relatively deep water circulation, allowing a calcium-sodium exchange, according to the host rock geochemistry. The higher temperatures found at some hot springs are related to the main trace of the San José del Cabo Fault.

Combined Effect of Sodicity and Organic Matter on Soil Properties under Long‐Term Irrigation with Treated Wastewater

Core Ideas We observed increasing sodicity hazard and a decreasing saturated soil hydraulic conductivity from treated wastewater irrigation. All of the data on sodicity hazard could be incorporated into a single Gapon constant for calcium‐sodium exchange that depends on the ratio of soil DOC concentration to calcite content. All the data on Ks could be incorporated in a single power‐law relationship involving the ratio of ESP to soil DOC concentration. The increasing reuse of treated wastewater (WW) for irrigation brings with it a need to reconsider irrigation water quality criteria because of the expected lower quality of WW. In particular, the impacts of higher sodium and dissolved organic carbon (DOC) concentrations on soil permeability must be evaluated in practical field settings over long periods of WW reuse. Here we report the long‐term impact of WW reuse for irrigation on soils at three different semiarid‐zone field sites under row crop or orchard agriculture. The soils contain about 60% clay, dominated by smectite, and present an order of magnitude variation in calcite content (1–11%). In two of the sites, parcels irrigated with freshwater (FW) are available for comparison. Our results show an increasing sodicity hazard and a decreasing saturated soil hydraulic conductivity (Ks) from WW irrigation, although the depth profiles of soil chemical and physical properties were highly site‐specific. Despite this spatial variability, all of the data on sodicity hazard, represented by the relationship between exchangeable sodium percentage (ESP) and the soil sodium adsorption ratio (SAR), could be incorporated into a single Gapon constant for calcium–sodium exchange whose values depended uniformly on the ratio of soil DOC concentration to calcite content. Moreover, all of the data on Ks, for both FW and WW irrigation, could be incorporated into a single power‐law relationship involving the ratio of ESP to soil DOC. These two relationships unify complex interactions between sodicity and organic matter (OM) that influence soil permeability to yield simple correlations with predictive power. The main detrimental effect of WW application was related to sodicity hazard. Therefore, effort should be invested in reducing the SAR of WW for irrigation. This could be achieved, for example, by mixing WW with FW, including desalinized water, when and if available.

Polymer‐surfactant nanoparticles for sustained release of water‐soluble drugs

Poor drug encapsulation efficiency and rapid release of the encapsulated drug limit the use of nanoparticles in biomedical applications involving water-soluble drugs. We have developed a novel polymer-surfactant nanoparticle formulation, using the anionic surfactant Aerosol OT (AOT) and polysaccharide polymer alginate, for sustained release of water-soluble drugs. Particle size of nanoparticles, as determined by atomic force microscopy and transmission electron microscopy, was in the range of 40-70 nm. Weakly basic molecules like methylene blue, doxorubicin, rhodamine, verapamil, and clonidine could be encapsulated efficiently in AOT-alginate nanoparticles. In vitro release studies with basic drug molecules indicate that nanoparticles released 60-70% of the encapsulated drug over 4 weeks, with near zero-order release during the first 15 days. Studies with anionic drug molecules demonstrate poorer drug encapsulation efficiency and more rapid drug release than those observed with basic drugs. Further studies investigating the effect of sodium concentration in the release medium and the charge of the drug suggest that calcium-sodium exchange between nanoparticle matrix and release medium and electrostatic interaction between drug and nanoparticle matrix are important determinants of drug release. In conclusion, we have formulated a novel surfactant-polymer drug delivery carrier demonstrating sustained release of water-soluble drugs.

Effect of Cosolvents on Ca-Na Exchange onto Wyoming Bentonite

AbstractCalcium-sodium exchange on Wyoming bentonite in methanol, ethanol and acetone-water systems were investigated at 0.03 N Cl and at room temperature. Calcium-sodium exchange isotherms were plotted at cosolvent concentrations ranging from 0 to 70% wt./wt. using Ca and Na ionic activities before and after correction for CaCl+ formation. In both cases and in all treatments, a greater selectivity of bentonite surfaces for Ca ions was observed. When compared to water, different trends were observed among and within cosolvents. These trends varied depending on whether or not CaCl+ formation was accounted for. Ignoring the formation of CaCl+, the preference of bentonite for Ca increased in methanolwater systems with increased percent methanol at low equivalent Ca fraction (<0.2). At higher Ca fractions, this preference matched that of water. In ethanol-water, no increased preference of the surface for Ca was observed. In acetone-water, increasing cosolvent concentration decreased the preference of the surface for Ca. The magnitude of this decrease was larger at low equivalent Ca fraction (<0.2). After correction for CaCl+, both in solution and on the surface, the preference of bentonite for Ca2+ was larger in methanol- and ethanol-water systems. In acetone-water, increased surface preference for Ca was only apparent at low acetone fractions (<50%). At higher acetone fractions, there was evidence of increased Na loading but no increase in Ca2+ selectivity. Clearly, ion-pair formation and its effects on Ca-Na exchange reactions cannot be ignored in mixtures of aqueous-organic solvents. After accounting for this effect, Ca-Na exchange in the studied solvents appears to be more of a surface- than a solution-controlled phenomenon that involves both electrostatic and specific solvent-surface types of interactions that have not been elucidated.

Rhabdomyolysis After Correction of Hyponatremia in Psychogenic Polydipsia Possibly Complicated by Ziprasidone

To report a case of rhabdomyolysis related to correction of hyponatremia secondary to psychogenic polydipsia, possibly complicated by the use of ziprasidone. A 50-year-old white man treated for 3 weeks with ziprasidone 40 mg twice daily for chronic paranoid schizophrenia was admitted to the intensive care unit after a witnessed generalized seizure. Marked hypotonic hyponatremia was present secondary to psychogenic polydipsia. After correction of hyponatremia with intravenous NaCl 0.9%, he developed a substantial elevation in the creatine kinase level without any evidence of muscle trauma, stiffness, or swelling or any signs of neuroleptic malignant syndrome. Renal failure or compartment syndrome did not complicate the clinical picture. It is well known that severe hyponatremia can cause neurologic complications such as stupor, seizures, and even coma. Hyponatremia from water intoxication (n = 28) and its correction with intravenous fluids (n = 2) may cause non-neurologic complications such as rhabdomyolysis. An explanation may lie within the calcium-sodium exchange mechanism across the skeletal myocyte or the failure of cell volume regulation secondary to extracellular hypo-osmolality. Neuroleptic medications have been linked to the development of rhabdomyolysis, with antipsychotics being the primary offenders. As of August 2005, there has been only one reported case of rhabdomyolysis related to correction of hyponatremia complicated by an atypical antipsychotic (clozapine). It is possible that ziprasidone, like clozapine, may enhance muscle cell permeability leading to rhabdomyolysis under similar conditions. Psychiatric patients treated with atypical antipsychotic medications should be closely monitored for rhabdomyolysis during correction of hyponatremia, thus permitting prompt therapy to limit its complications.

Sodium withdrawal contractures in developing and regenerating rat extensor digitorum longus muscles.

During postnatal development of extensor digitorum longus (EDL) muscle, sodium withdrawal contractures were observed during the first 6 days after birth, and not after this time. In regenerating EDL muscles, zero-Na contractures were demonstrated: (1) 7 days after bupivacaine injection, but not 14 or 90 days after this injection; (2) 7, 14, and 90 days after autotransplantation; and (3) 7, 14, and 90 days after the intervention in sliced muscles. The present findings emphasize the role of the denervation in the development of zero-Na contractures in the regenerating muscles and suggest that a calcium-sodium exchange across the sarcolemma may appear in these muscles.

Acute effects of prolactin on passive calcium absorption in the small intestine by in vivo perfusion technique

The acute effect of intraperitoneally administered prolactin (0.2, 0.4, and 0.6 mg/kg body weight) on passive calcium transport in duodenum, proximal jejunum, and ileum of sexually mature female Wistar rats was investigated by using an in vivo perfusion technique. Test solution containing (in mM) NaCl, 100; KCl, 4.7; MgSO4, 1.2; CaCl2, 20; D-glucose, 11; sodium ferrocyanide (Na4Fe(CN)6), an index of net water transport, 20; and 0.7 microCi 45CaCl2 (1 Ci = 37 GBq) was perfused through the 10-cm intestinal loop for 60 min. Results showed that 0.4 mg prolactin/kg body weight significantly increased duodenal net Ca absorption (net Ca) from 23.81 +/- 1.84 to 30.56 +/- 1.57 mmol/g dry weight (p < 0.05) by stimulating the lumen to plasma calcium flux (CaL-P). The jejunum responded to 0.2, 0.4, and 0.6 mg prolactin/kg body weight by reversing from net Ca absorption of 18.60 +/- 1.70 mmol/g dry weight to net secretion of -3.30 +/- 1.56, -10.39 +/- 2.21, and -11.79 +/- 2.04 mmol/g dry weight (p < 0.01), respectively, as a result of a dose-dependent increase in plasma to lumen calcium flux (CaP-L). Calcium fluxes in the ileum on the other hand did not respond to prolactin. There was a close correlation between net water flux and net calcium flux in all three intestinal segments under basal condition regardless of the luminal sodium concentration. However, this correlation was lost after prolactin administration, which while having no effect on net water flux, altered the duodenal and jejunal calcium fluxes. By varying the luminal concentration of sodium, it was found that the stimulatory effect of 0.4 mg prolactin/kg body weight on the duodenal CaL-P was reduced when compared with control, i.e., 17.84 +/- 0.91 vs. 26.64 +/- 1.05 mmol/g dry weight at a sodium concentration of 180 mM, and 14.48 +/- 0.99 vs. 20.12 +/- 1.34 mmol/g dry weight at a sodium concentration of 140 mM. At a sodium concentration of 80 mM, the prolactin effect was absent. Since duodenal Na(+)-K+ ATPase activity was increased by prolactin from 3.77 +/- 0.16 to 4.95 +/- 0.30 mumol Pi.mg-1 protein.h-1 (p < 0.05), sodium dependency of the prolactin-enhanced lumen to plasma calcium flux may be related to both sodium-induced water flow and calcium-sodium exchange across the basolateral membrane. Thus, it was postulated that under basal condition, net calcium transport in the small intestine occurred with the sodium-induced water transport along the paracellular pathway. However, after prolactin administration, this association was lost. Prolactin-enhanced lumen to plasma calcium flux in the duodenum was sodium dependent and involved the Na(+)-K+ ATPase activity. In the proximal jejunum, prolactin stimulated plasma to lumen calcium flux, but the mechanism was not known.

Acute effects of prolactin on passive calcium absorption in the small intestine by in vivo perfusion technique

The acute effect of intraperitoneally administered prolactin (0.2, 0.4, and 0.6 mg/kg body weight) on passive calcium transport in duodenum, proximal jejunum, and ileum of sexually mature female Wistar rats was investigated by using an in vivo perfusion technique. Test solution containing (in mM) NaCl, 100; KCl, 4.7; MgSO4, 1.2; CaCl2, 20; D-glucose, 11; sodium ferrocyanide (Na4Fe(CN)6), an index of net water transport, 20; and 0.7 microCi 45CaCl2 (1 Ci = 37 GBq) was perfused through the 10-cm intestinal loop for 60 min. Results showed that 0.4 mg prolactin/kg body weight significantly increased duodenal net Ca absorption (net Ca) from 23.81 +/- 1.84 to 30.56 +/- 1.57 mmol/g dry weight (p < 0.05) by stimulating the lumen to plasma calcium flux (CaL-P). The jejunum responded to 0.2, 0.4, and 0.6 mg prolactin/kg body weight by reversing from net Ca absorption of 18.60 +/- 1.70 mmol/g dry weight to net secretion of -3.30 +/- 1.56, -10.39 +/- 2.21, and -11.79 +/- 2.04 mmol/g dry weight (p < 0.01), respectively, as a result of a dose-dependent increase in plasma to lumen calcium flux (CaP-L). Calcium fluxes in the ileum on the other hand did not respond to prolactin. There was a close correlation between net water flux and net calcium flux in all three intestinal segments under basal condition regardless of the luminal sodium concentration. However, this correlation was lost after prolactin administration, which while having no effect on net water flux, altered the duodenal and jejunal calcium fluxes. By varying the luminal concentration of sodium, it was found that the stimulatory effect of 0.4 mg prolactin/kg body weight on the duodenal CaL-P was reduced when compared with control, i.e., 17.84 +/- 0.91 vs. 26.64 +/- 1.05 mmol/g dry weight at a sodium concentration of 180 mM, and 14.48 +/- 0.99 vs. 20.12 +/- 1.34 mmol/g dry weight at a sodium concentration of 140 mM. At a sodium concentration of 80 mM, the prolactin effect was absent. Since duodenal Na(+)-K+ ATPase activity was increased by prolactin from 3.77 +/- 0.16 to 4.95 +/- 0.30 mumol Pi.mg-1 protein.h-1 (p < 0.05), sodium dependency of the prolactin-enhanced lumen to plasma calcium flux may be related to both sodium-induced water flow and calcium-sodium exchange across the basolateral membrane. Thus, it was postulated that under basal condition, net calcium transport in the small intestine occurred with the sodium-induced water transport along the paracellular pathway. However, after prolactin administration, this association was lost. Prolactin-enhanced lumen to plasma calcium flux in the duodenum was sodium dependent and involved the Na(+)-K+ ATPase activity. In the proximal jejunum, prolactin stimulated plasma to lumen calcium flux, but the mechanism was not known.

Effect of boron on the ultrastructure of parathyrocytes and atrial cardiomyocytes

Ultrastructural shifts in parathyrocytes and atrial cardiomyocytes induced by long-term treatment with boron-containing water (250 mg/liter) were found against the background of boron accumulation. These changes are indicative of enhanced secretory activity of these cells. It is hypothesized that boron modulates the effect of parathyroid hormone and atrial natriuretic factor and the rate calcium-sodium exchange.

Interaction of alkali, alkaline-earth and sulphate ions with clay minerals and sedimentary rocks

Ion-exchange isotherms of kaolinite and montmorillonite for calcium, barium and sulphate ions in a solution at constant pH near neutral have been examined. The ionic strength of the solutions ranged from seawater to formation waters from oil reservoirs in the North Sea. 45Ca-, 133Ba- or 35SO 4-tagged solutions with added NaCl as supported electrolyte were used for adsorption measurements. From distribution coefficients found for CaCl 2 NaCl or BaCl 2 NaCl in the clay suspensions, the adsorbed quantities of divalent cations can be predicted for sedimentary rocks with a given mineralogical composition. These results show that the equivalent ionic fractions of alkaline-earth ions adsorbed on clay pretreated with seawater, or formation water, fit the normalized isotherms obtained for the calcium-sodium exchange on the clay within uncertainties of ± 10%. The same correlation was not found for sulphate adsorption from seawater because the clay minerals pretreated with seawater were less reactive for sulphate adsorption. These results suggest that in addition to adsorption due to electrostatic forces, some ions were chemisorbed to the clay minerals during the pretreatment with seawater. The lower sulphate adsorption may be due to ion pair formation.

Diastolic dysfunction and myocardial energetics

Myocardial relaxation is an energy-dependent process. Indeed, adenosine triphosphate (ATP) is required to pump free myoplasmic calcium back into the sarcoplasmic reticulum. It is also necessary to extrude the calcium ions which enter the cell during the plateau phase of the action potential. The calcium-sodium exchange mechanism does not seem to require energy in itself, but sodium exchanged for calcium eventually needs to be extruded via sodium/potassium ATPase and there is also an ATP-dependent calcium pump. Thus, when ATP production is limited, calcium may remain fixed to troponin for part or for the whole of diastole, resulting in a slower rate of isovolumic relaxation and reduced distensibility of the myocardium. Alterations in diastolic function caused by inadequate energy production occur in the high-demand type of myocardial ischaemia. There is also growing evidence that most forms of heart failure are accompanied by a state of energy depletion. Alterations in mitochondrial density and enzymatic activity are common in the failing myocardium and may partially explain the reduction in ATP production. Inadequate growth of the capillary network in hypertrophied myocardium, impaired subendocardial perfusion due to increased diastolic wall stress and/or coronary artery disease, probably also contribute to an imbalance between energy production and utilization. As relaxation is intrinsically a much slower process than activation and since changes in ATP concentration may also affect calcium efflux by allosteric effects, impaired relaxation and reduced diastolic distensibility are almost universal in chronic congestive heart failure. Optimal therapy of heart failure should, therefore, also aim at improving this phase of the cardiac cycle.

Stimulation of calcium-sodium exchange in dog red blood cells by hemolysis and resealing

Osmotic hemolysis and resealing greatly increase calcium influx in dog red blood cells. The resealed ghosts show a saturable calcium entry pathway with complex kinetics. As expected for a calcium-sodium exchanger, calcium uptake is stimulated by internal sodium and inhibited by external sodium. Compared to fresh, intact red cells the resealed ghost calcium-sodium exchanger is less responsive to quinidine and to alterations in medium tonicity. The differences in calcium uptake rate among cells from different donors are minimized in the ghost preparation. There are several ways to stimulate sodium-dependent calcium movements in these cells, of which hemolysis-resealing is the most potent. The results of these and previous studies suggest that dog red blood cells have a latent capacity for calcium-sodium exchange.

Effect of calcium-sodium exchange and calcium channel inhibition on calcium release from bone in an in-vitro bone perfusion system.

The regulation of calcium release from bone is likely to be at least in part, dependant on cellular activity. In recent years it has been shown that the intracellular calcium concentration is important in regulating the activity of many cells from a wide range of tissues including those of the endocrine system and bone. There are several mechanisms involved in regulating cell calcium including calcium sodium (Ca-Na) exchange, calcium-magnesium sensitive ATPase and calcium transport via slow calcium channels1. Thus it is possible that by regulating intracellular calcium, some of these mechanisms could also regulate the cell mediated component of the transfer of calcium from bone to the extracellular fluid.

The effects of electrolyte concentrations on calcium-sodium exchange equilibria in two soil samples of india

The effects of total electrolyte concentrations of the equilibrium solutions (t.e.c.) on Ca 2+-Na + exchange equilibria in two soil samples (high and low in organic matter, clay content and CEC) were studied. Homoionic (Na +-saturated) soil samples were equilibrated with solutions having a large range in sodium adsorption ratio (SAR) at 25, 50, 75 and 100 meq. 1 -1 t.e.c. The exchange equilibria data were analysed, using a thermodynamic approach and the selectivity coefficients of Gapon (1933), Vanselow (1932) and Krishnamoorthy et al. (1948) (K G, K V and K KDO). At a given proportion of Ca 2+: Na + in the equilibrium solution, the development of the exchangeable sodium percentage (ESP) in both soil samples increased with the increase in t.e.c. At a given SAR, the effect of t.e.c. on the development of ESP was less on a soil sample with high organic matter (O.M.), clay content and cation exchange capacity (CEC) than on a soil sample with low O.M., clay content and CEC. The values of exchange selectivity coefficients decreased with the increase in t.e.c, and did not remain constant throughout the exchange isotherm for any of the t.e.c. tried.

Some physico-chemical properties of an exposed B horizon of a red-brown earth treated with gypsum

Physico-chemical properties of a red-brown earth B horizon (Goulburn clay loam) were studied for three years following exposure and treatment with gypsum. In the first year irrigation and rain caused considerable leaching of soluble salts to 80 cm in both the control and gypsum treated plots. Gypsum was found to enhance the leaching of chloride by increasing the hydraulic conductivity in the upper soil profile. After three years soluble salts had returned to within 30 cm of the surface in the control plots, due to the occurrence of a perched water table. No similar increase occurred in the gypsum plots. Calcium-sodium exchange due to gypsum was slow to occur, and was mainly limited to the 0-10 cm layer. The exchange of calcium for magnesium was the dominant exchange reaction in this layer.

Calcium antagonists in hypertension: relation to abnormal sodium transport.

Leucocyte sodium efflux rate constants and intracellular electrolyte contents were estimated in 13 patients with untreated essential hypertension. There was no correlation between intracellular sodium or potassium content or efflux rate constant and blood pressure. The patients were then treated with oral nifedipine and blood pressure controlled. Sodium efflux rate constants and electrolyte contents were estimated one and three months after the start of treatment. There was a significant fall in blood pressure, but mean sodium efflux rate constant and intracellular sodium content were unchanged. There was no correlation between the fall in blood pressure, initial sodium efflux, or intracellular sodium content. These data do not support the hypothesis that the sodium pump and intracellular sodium content have a direct role in generating raised blood pressure, or that treatment of hypertension with calcium antagonists corrects a fundamental alteration of calcium-sodium exchange across the cell membrane.

Miscible displacement of reactive solutes in saline-sodic soils under varying flow conditions

SummaryThe paper reports a laboratory study on miscible displacement of reactive solutes during leaching a highly saline-sodic soil rich in soluble sodium carbonate with solutions of calcium chloride. The effect of varying flow velocities, initial soil moisture content and concentration of calcium in the leaching water on the extent of carbonate precipitation and replacement of sodium by calcium from the exchange positions was studied in soil columns. The results showed that precipitation of soluble carbonates in the soil increased with increasing flow velocity, initial soil moisture content and concentration of calcium in the leaching solution. The results further showed that the calcium exchange capacity of the soil was reduced significantly and nearly proportionally with decrease in the degree of water saturation as a consequence of decreased water application rate and initial soil moisture content. In view of the results obtained, a simple model to describe calcium-sodium exchange under unsaturated flow conditions in saline-sodic soils under consideration has been proposed. Possible implications of the results in actual field situations is discussed.

A model for prediction of calcium-sodium exchange reactions in sodic soils

The material balance equation that describes the miscible displacement involving cation exchange was solved numerically for prediction of calcium-sodium exchange during reclamation of sodic soils. This exchange process was studied for Fatehpur loamy sand and Bhabiana loam. Four columns of each soil were packed and saturated with sodium by leaching with 0.1 M sodium chloride solution. The adsorbed sodium was displaced by 0.50 M calcium nitrate solution. Different volumes of calcium nitrate solution were passed to cause differential removal of exchangeable sodium. Nonlinear exchange isotherms were developed for the soils and incorporated in the convective-dispersive transport equation, which was solved by finite difference technique. Good agreement between the computed and experimental profiles of soluble and exchangeable calcium following input of different volumes of calcium nitrate solution was obtained. Last traces of exchangeable sodium which were difficult to remove from Bhabiana loam were not predicted by the model. Reasons for this discrepancy are enumerated.

Regulation of calcium transport in isolated periosteal cells, effects of hormones and metabolic inhibitors.

Calcium transport was studied in cells isolated from cultured periostea of 18-day-old chick embryos. Net calcium uptake was stimulated by iodoacetate (IAA) and inhibited by dinitrophenol (DNP). Calcium efflux from the intracellular compartment was inhibited by IAA. Changes in the extra- or intracellular sodium concentration had only minor effects on calcium transport. This indicates that calcium efflux from periosteal cells is probably directly dependent on ATP hydrolysis, whereas calcium-sodium exchange is of less importance. The cation ionophore A23187 stimulated calcium uptake during short incubations but was inhibitory in long incubations. The possible involvement of the mitochondria in this effect is discussed. Parathyroid hormone (PTH) and calcitonin (CT) stimulated the net uptake of calcium in relatively low doses (both 0.01 U/ml or higher). The effects of the hormones on net calcium uptake were not additive. Calcium efflux was not changed in the presence of PTH or CT.

Biochemical aspects of the visual process. XXXVI. Calcium accumulation in cattle rod outer segments: Evidence for a calcium-sodium exchange carrier in the rod sac membrane

Accumulation of calcium has been studied in bovine rod outer segments (rods), isolated by sucrose density gradient centrifugation. Calcium-depleted rods are obtained by having ethyleneglycol-bis-( β-aminoethylether)- N, N′-tetraacetic acid (EGTA) present during isolation. Rods thus isolated have a leaky plasma membrane, as shown by the effects of ionophore A23187 and by their light-induced phosphorylation behaviour. The accumulation of 45Ca, determined by incubation followed by a single fast washing-filtration procedure, thus represents translocation across the rod sac membrane. Accumulation in non-depleted rods is independent of the external calcium level and of ATP, suggesting exchange of 40Ca by 45Ca. In depleted rods in the presence of ATP there is net uptake, sigmoidally increasing with the external calcium concentration to the level attained in non-depleted rods. This net uptake is abolished by omission of ATP, its replacement by β,γ-methylene ATP and lowering the temperature to 0° C, suggesting involvement of enzymatic hydrolysis of ATP. Replacement of KCl by NaCl in the medium causes marked inhibition of 45Ca uptake, both net uptake and exchange. Oligomycin, ruthenium red, lanthanum and ouabain do not inhibit accumulation. Efflux of 45Ca from pre-loaded rods is slow in a KCl medium ( t 1 2 ∼30 min at 25° C), but is greatly accelerated by addition of NaCl or Ca 2+ ( t 1 2 10 s at 25°C). It is concluded that the rod sac membrane contains a carrier system, which is sensitive towards Ca 2+ and Na + and which requires ATP for net uptake of Ca 2+ but not for exchange transport of Ca 2+ with Ca 2+ or Na +.