Adrenocorticotropic hormone and dibutyryl adenosine cyclic monophosphate-mediated Ca uptake by rat adrenal glands

The effect of adrenocorticotropic hormone and dibutyryl cyclic AMP on the uptake of⁴⁵Ca²⁺ by the rat adrenal gland has been investigated. After injection of ⁴⁵Ca²⁺ and adrenocorticotropic hormone into rats, the adrenal ⁴⁵Ca²⁺ concentration was significantly enhanced 90 to 180 min following hormone administration. The rise in adrenal ⁴⁵Ca²⁺ content was accompanied by a marked increase of the serum corticosterone levels. During incubation of rat adrenal glands in the presence of ⁴⁵Ca²⁺, adrenocorticotropic hormone and dibutyryl cyclic AMP caused significant accumulation of adrenal ⁴⁵Ca²⁺ and increased corticosterone synthesis. The degree of stimulation of both adrenal ⁴⁵Ca²⁺ uptake and corticosterone synthesis by adrenocorticotropic hormone or dibutyryl cyclic AMP was dependent upon the concentration of calcium in the incubation medium and upon the amount of adrenocorticotropic hormone or dibutyryl cyclic AMP added. Theophylline mimicked the stimulatory effect of adrenocorticotropic hormone and dibutyryl cyclic AMP and increased the uptake of ⁴⁵Ca²⁺ by rat adrenal glands in vitro. Determination of calcium by atomic absorption spectroscopy showed that the adrenocorticotropic hormone-mediated adrenal ⁴⁵Ca²⁺ uptake was due to a net accumulation of calcium in the tissue and not only to an increased rate of exchange of extracellular ⁴⁵Ca²⁺ with the intracellular calcium pool. Adrenocorticotropic hormone-stimulated adrenal ⁴⁵Ca²⁺ uptake was not observed when steroidogenesis was inhibited with elipten. Both adrenocorticotropic hormone-mediated corticosterone synthesis and adrenal ⁴⁵Ca²⁺ uptake were abolished after treatment of rats with cycloheximide but not after treatment with actinomycin D, indicating that adrenal ⁴⁵Ca²⁺ uptake and steroidogenesis have similar requirements for de novo protein synthesis, but not RNA synthesis.     

Large scale preparation of rabbit aortic smooth muscle cells for use in calcium uptake studies

Summary Rabbit aortic smooth muscle cells were prepared by enzymatic digestion of the aortic smooth muscle layer. The cells were subcultured up to Passage 22 starting from a cryogenically preserved stock (approximately 10¹⁰cells, Passage 8) and characterized morphologically and for⁴⁵Ca⁺⁺ uptake. Microscopically the cells demonstrated the characteristics of vascular smooth muscle cells.⁴⁵Ca⁺⁺ uptake by the cells plated on tissue culture flasks (25 cm²) was determined at 25°C in physiological salt solution (PSS) containing⁴⁵Ca⁺⁺ in low (5 mM) or high (50mM) KCl concentrations. At the end of the incubation period (0 to 30 min), PSS was aspirated and the cells quickly washed, digested with 0.5N NaOH, and counted for⁴⁵Ca⁺⁺. High K⁺ increased the⁴⁵Ca⁺⁺ uptake by 100% or more compared to the low K⁺ uptake of⁴⁵Ca⁺⁺. This K⁺-induced⁴⁵Ca⁺⁺ uptake was eliminated in osmotically shocked cells, and inhibited by nifedipine, verapamil, and diltiazem, in a dose-dependent manner. The extent of⁴⁵Ca⁺⁺ uptake and the inhibitory activity of nifedipine were retained up to Passage 22. It is concluded that the developed methodology for scaled-up cultures of rabbit aortic smooth muscle cells provides morphologically intact and biochemically functioning cells suitable for calcium channel studies.     

Acidosis and Ca2+ distribution in myocardial tissue of flounder and rat

Summary The effect of acidosis on the myocardial Ca²⁺ distribution was examined at 15°C in ventricular strips of the flounder (Platichthys flesus) and at 30°C in atrial strips of the rat (Rattus norvegicus).Lowering the Ringer pH from 7.6 to 6.9 by increasing its CO₂ (flounder 2% to 12%, rat 4% to 14%), resulted in an elevated Ca²⁺ efflux in resting strips as well as in strips stimulated (12/min) to contraction. A decrease in pH of the Ringer used for the flounder myocardium by a lowering of bicarbonate (30 mM to 5 mM) also resulted in an elevation of the Ca²⁺ efflux, but the effect was smaller than that produced by an increased CO₂.With 11 mM Ca²⁺ and 10 mM EGTA added to the Ringer to reduce the amount of⁴⁵Ca²⁺ bound to extracellular sites, an increased CO₂ with a concomitant drop in Ringer pH resulted in an increased Ca²⁺ efflux in both myocardia. The Ca²⁺ efflux was only marginally elevated in the flounder myocardium and unchanged in that of rat when the same drop in Ringer pH was produced with a lowering in bicarbonate.In a nominally Ca²⁺-free Ringer with 0.1 mM EGTA the⁴⁵Ca²⁺ efflux was stimulated for both myocardia by an increase in CO₂.The flounder myocardium was exposed to high CO₂ in a nominally Na⁺, Ca²⁺-free Ringer and again the⁴⁵Ca²⁺ efflux increased. After a return to Na, Ca and low CO₂ in the Ringer, a higher efflux persisted in the strips being subjected to a high CO₂ than in the controls.The Ca²⁺ uptake rate was about the same at high and low CO₂ for both myocardia.Based on these results the measured increase in Ca efflux following an increase in CO₂ or a decrease in bicarbonate probably results from an elevated cytoplasmatic Ca²⁺ activity. It seems unlikely that an increased uptake rate of Ca²⁺ or a direct stimulation of Ca²⁺ transporting mechanisms in the cell membrane are responsible for the change.     

Effects of adenosine diphosphate on Ca2+ fluxes and Ca2+ accumulation of sarcoplasmic reticulum

Ca²⁺ transport by sarcoplasmic reticulum vesicles was examined by incubating sarcoplasmic reticulum vesicles (0.15 mg/ml) at 37°C in, either normal medium that contained 0.15 M sucrose, 0.1 M KCl, 60 μM CaCl₂, 2.5 mM ATP and 30 mM Tes at pH 6.8, or a modified medium for elimination of ADP formed from ATP hydrolysis by including, in addition, 3.6 mM phosphocreatine and 33 U/ml of creatine phosphokinase. In normal medium, Ca²⁺ uptake of sarcoplasmic reticulum vesicles reached a plateau of about 100 nmol/mg. In modified medium, after this phase of Ca²⁺ uptake, a second phase of Ca²⁺ accumulation was initiated and reached a plateau of about 300 nmol/mg. The second phase of Ca²⁺ accumulation was accompanied by phosphate uptake and could be inhibited by ADP. Since, under these experimental conditions, there was no significant difference of the rates of ATP hydrolysis in normal medium and modified medium, extra Ca²⁺ uptake in modified medium but not in normal medium could not be explained by different phosphate accumulation in the two media. Unidirectional Ca²⁺ influx of sarcoplasmic reticulum near steady state of Ca²⁺ uptake was measured by pulse labeling with ⁴⁵Ca²⁺. The Ca²⁺ efflux rate was then determined by subtracting the net uptake from the influx rate. At the first plateau of Ca²⁺ uptake in normal medium, Ca²⁺ influx was balanced by Ca²⁺ efflux with an exchange rate of 240 nmol/mg per min. This exchange rate was maintained relatively constant at the plateau phase. In modified medium, the Ca²⁺ exchange rate at the first plateau of Ca²⁺ uptake was about half of that in normal medium. When the second phase of Ca²⁺ uptake was initiated, both the influx and efflux rates started to increase and reached a similar exchange rate as observed in normal medium. Also, during the second phase of Ca²⁺ uptake, the difference between the influx and efflux rates continued to increase until the second plateau phase was approached. In conditions where the formation of ADP and inorganic phosphate was minimized by using a low concentration of sarcoplasmic (7.5 μg/ml) and/or using acetyl phosphate instead of ATP, the second phase of Ca²⁺ uptake was also observed. These data suggest that the Ca²⁺ load attained by sarcoplasmic reticulum vesicles during active transport is modulated by ADP accumulated from ATP hydrolysis. ADP probably exerts its effect by facilitating Ca²⁺ efflux, which subsequently stimulates Ca²⁺ exchange.     

Adrenocorticotropic hormone and dibutyryl adenosine cyclic monophosphate-mediated Ca2+ uptake by rat adrenal glands

The effect of adrenocorticotropic hormone and dibutyryl cyclic AMP on the uptake of ⁴⁵Ca²⁺ by the rat adrenal gland has been investigated. After injection of ⁴⁵Ca²⁺ and adrenocorticotropic hormone into rats, the adrenal ⁴⁵Ca²⁺ concentration was significantly enhanced 90 to 180 min following hormone administration. The rise in adrenal ⁴⁵Ca²⁺ content was accompanied by a marked increase of the serum corticosterone levels. During incubation of rat adrenal glands in the presence of ⁴⁵Ca²⁺, adrenocorticotropic hormone and dibutyryl cyclic AMP caused significant accumulation of adrenal ⁴⁵Ca²⁺ and increased corticosterone synthesis. The degree of stimulation of both adrenal ⁴⁵Ca²⁺ uptake and corticosterone synthesis by adrenocorticotropic hormone or dibutyryl cyclic AMP was dependent upon the concentration of calcium in the incubation medium and upon the amount of adrenocorticotropic hormone or dibutyryl cyclic AMP added. Theophylline mimicked the stimulatory effect of adrenocorticotropic hormone and dibutyryl cyclic AMP and increased the uptake of ⁴⁵Ca²⁺ by rat adrenal glands in vitro. Determination of calcium by atomic absorption spectroscopy showed that the adrenocorticotropic hormone-mediated adrenal ⁴⁵Ca²⁺ uptake was due to a net accumulation of calcium in the tissue and not only to an increased rate of exchange of extracellular ⁴⁵Ca²⁺ with the intracellular calcium pool. Adrenocorticotropic hormone-stimulated adrenal ⁴⁵Ca²⁺ uptake was not observed when steroidogenesis was inhibited with elipten. Both adrenocorticotropic hormone-mediated corticosterone synthesis and adrenal ⁴⁵Ca²⁺ uptake were abolished after treatment of rats with cycloheximide but not after treatment with actinomycin D, indicating that adrenal ⁴⁵Ca²⁺ uptake and steroidogenesis have similar requirements for de novo protein synthesis, but not RNA synthesis.     

Simultaneous recording of cytosolic Ca2+ levels inDidinium andParamecium during aDidinium attack onParamecium

Summary The carnivorous ciliateDidinium nasutum captures prey such asParamecium by discharging extrusomes, known as toxicysts, while the attackedParamecium defensively discharges trichocysts. Several authors have suggested that both discharges, the toxicysts ofDidinium and the trichocysts ofParamecium, are evoked by the rise in cytosolic Ca²⁺ level in each cell. However, these putative increases in cytosolic Ca²⁺ levels have not as yet been recorded simultaneously in these cells during aDidinium attack onParamecium. We injected the fluorescent Ca²⁺ indicator Ca-Green 1 dextran into bothDidinium andParamecium, and simultaneously observed the cytosolic Ca²⁺ levels in these cells asDidinium attackedParamecium. When aParamecium came into contact with theDidinium proboscis, theDidinium showed a significant rise in cytosolic Ca²⁺ in the basal portion of the proboscis. One video frame (33 ms) after the onset of the Ca²⁺ rise inDidinium, theParamecium also showed an increase in cytosolic Ca²⁺. This is the first simultaneous recording of changes in the Ca²⁺ level during a predator-prey interaction in ciliates. The possible roles of these Ca²⁺ increases are discussed in relation to the discharge of toxicysts during theDidinium attack and of trichocysts as a defensive behavior ofParamecium.Abbreviations AED aminoethyldextran - P_i inorganic phosphate - FITC fluorescein isothiocyanate     

Effects of adenosine diphosphate on Ca fluxes and Ca accumulation of sarcoplasmic reticulum

Ca²⁺ transport by sarcoplasmic reticulum vesicles was examined by incubating sarcoplasmic reticulum vesicles (0.15 mg/ml) at 37°C in, either normal medium that contained 0.15 M sucrose, 0.1 M KCl, 60 μM CaCl₂, 2.5 mM ATP and 30 mM Tes at pH 6.8, or a modified medium for elimination of ADP formed from ATP hydrolysis by including, in addition, 3.6 mM phosphocreatine and 33 U/ml of creatine phosphokinase. In normal medium, Ca²⁺ uptake of sarcoplasmic reticulum vesicles reached a plateau of about 100 nmol/mg. In modified medium, after this phase of Ca²⁺ uptake, a second phase of Ca²⁺ accumulation was initiated and reached a plateau of about 300 nmol/mg. The second phase of Ca²⁺ accumulation was accompanied by phosphate uptake and could be inhibited by ADP. Since, under these experimental conditions, there was no significant difference of the rates of ATP hydrolysis in normal medium and modified medium, extra Ca²⁺ uptake in modified medium but not in normal medium could not be explained by different phosphate accumulation in the two media. Unidirectional Ca²⁺ influx of sarcoplasmic reticulum near steady state of Ca²⁺ uptake was measured by pulse labeling with ⁴⁵Ca²⁺. The Ca²⁺ efflux rate was then determined by subtracting the net uptake from the influx rate. At the first plateau of Ca²⁺ uptake in normal medium, Ca²⁺ influx was balanced by Ca²⁺ efflux with an exchange rate of 240 nmol/mg per min. This exchange rate was maintained relatively constant at the plateau phase. In modified medium, the Ca²⁺ exchange rate at the first plateau of Ca²⁺ uptake was about half of that in normal medium. When the second phase of Ca²⁺ uptake was initiated, both the influx and efflux rates started to increase and reached a similar exchange rate as observed in normal medium. Also, during the second phase of Ca²⁺ uptake, the difference between the influx and efflux rates continued to increase until the second plateau phase was approached. In conditions where the formation of ADP and inorganic phosphate was minimized by using a low concentration of sarcoplasmic (7.5 μg/ml) and/or using acetyl phosphate instead of ATP, the second phase of Ca²⁺ uptake was also observed. These data suggest that the Ca²⁺ load attained by sarcoplasmic reticulum vesicles during active transport is modulated by ADP accumulated from ATP hydrolysis. ADP probably exerts its effect by facilitating Ca²⁺ efflux, which subsequently stimulates Ca²⁺ exchange.     

Kinetic studies on sodium-dependent calcium uptake by myocardial cells and neuroblastoma cells in culture

Kinetic analyses were made on intracellular Na⁺-dependent Ca²⁺ uptake by myocardial cells and neuroblastoma cells (N-18 strain) in culture. Cells loaded with various concentrations of Na⁺ could be prepared by incubating them in Ca²⁺-free medium containing various concentrations of Na⁺. Cells pre-loaded with various concentrations of Na⁺ were incubated in medium containing Ca²⁺ and ⁴⁵Ca. The resulting ⁴⁵Ca uptake by the two types of cell depended greatly on the initial intracellular concentrations of Na⁺. Lineweaver-Burk plots of the initial rate of Ca²⁺ uptake against the external concentration of Ca²⁺ fitted well to straight lines obtained by linear regression (r > 0.95). This result shows that Ca²⁺ uptake by the two types of cell was achieved by a carrier-mediated transport system. This Na⁺-dependent Ca²⁺ uptake was accompanied by Na⁺ release and the ratio of Na⁺ release to Ca²⁺ uptake was close to 3 : 1. A comparison of the kinetic data between myocardial cells and N-18 cells suggested that N-18 cells possess a carrier showing the same properties as that of myocardial cells, i.e.: (1) a similar dependency on the intracellular concentration of Na⁺; (2) the coincidence of the apparent Michaelis constants for Ca²⁺ (0.1 mM); (3) the similarities of the K_i values for Co²⁺, Sr²⁺ and Mg²⁺ (Co²⁺ < Sr²⁺ < Mg²⁺) and (4) a similar dependency on pH. However, the maximal initial rate, V, of N-18 cells was about $\text{1}\text{100}$ that of myocardial cells. The rate of Na⁺-dependent Ca²⁺ uptake by non-excitable cells was much lower than that by myocardial cells.     

Adenine and pyridine nucleotide levels during calcium-induced conidiation in Penicillium notatum

Concentrations of adenine and pyridine nucleotides and the associated charge values were examined in extracts of mycelium of Penicillium notatum during vegetative growth and reproductive development promoted by the addition of Ca²⁺ (10 mmol dm^-3). The significant increase in adenylate energy charge promoted by Ca²⁺ was due to a fall in intracellular AMP and a concomitant rise in ATP concentration. Intracellular concentrations of NADH and NAD fell within 1 h of the addition of Ca²⁺. The catabolic reduction charge was unchanged by Ca²⁺ whilst the anabolic reduction charge increased in Ca²⁺-induced mycelium due to lowered intracellular NADP concentration. Reduced concentration of NADPH in Ca²⁺-induced mycelium, relative to the vegetative controls, lowered the phosphorylated nucleotide fraction. The results are discussed in relation to metabolic economy during morphogenesis in P. notatum.     

Sodium-calcium exchange in renal epithelial cells: Dependence on cell sodium and competitive inhibition by magnesium

Summary Kinetic properties of Na⁺–Ca²⁺ exchange in a renal epithelial cell line (LLC-MK₂) were assessed by measuring cytosolic free Ca²⁺ with fura-2 and⁴⁵Ca²⁺ influx. Replacing external Na⁺ with K⁺ produced relatively small increases in free Ca²⁺ and⁴⁵Ca²⁺ uptake unless the cells were incubated with ouabain. Ouabain markedly increased cell Na⁺ and strongly potentiated the effect of replacing external Na⁺ with K⁺ on free Ca²⁺ and⁴⁵Ca²⁺ uptake.⁴⁵Ca²⁺ influx in 140mm K⁺ or N-methyl-d-glucamine minus influx in 140mm Na⁺ was used to quantify Na⁺–Ca²⁺ exchange activity of Na⁺-loaded cells. The dependence of exchange on cell Na⁺ was sigmoidal; theK _0.5 was 26±3 mmol/liter cell water space, and the Hill coefficient was 3.1±0.2. The kinetic features of the dependence of exchange on cell Na⁺ partly account for the small increase in Ca²⁺ influx when all external Na⁺ is replaced by K⁺. Besides raising cell Na⁺ ouabain appears to activate the exchanger. Magnesium competitively inhibited exchange activity. The potency of Mg²⁺ was 8.2-fold lower with potassium instead of N-methyl-d-glucamine or choline as the replacement for external Na⁺. Potassium also increased theV _max of exchange by 86% and had no effect on theK _m for Ca²⁺. The exchanger does not cause detectable²²Na⁺–Mg²⁺ exchange and does not appear to require K⁺ or transport⁸⁶Rb⁺. Although exchange activity was plentiful in the epithelial cells from monkey kidney, others from amphibian, canine, opossum, and porcine kidney had no detectable exchange activity. All of the measured kinetic properties of Na⁺–Ca²⁺ exchange in the renal epithelial cells are very similar to those of the exchanger in rat aortic myocytes.     

Effects of noradrenaline on45Ca2+ efflux from isolated rat islets of Langerhans

Noradrenaline caused a prompt but transient increase in the rate of⁴⁵Ca²⁺ efflux from isolated rat islets of Langerhans perifused in Ca²⁺ depleted medium. The response was modest in size and was unaffected by isosmotic replacement of NaCl with choline chloride or by inclusion of 0.5 mM dibutyryl cAMP in the perifusion medium, suggesting that it was not mediated by Na+: Ca²⁺ exchange nor by lowered cAMP. Despite its effect on⁴⁵Ca²⁺ efflux, noradrenaline treatment did not alter the kinetics of⁴⁵Ca²⁺ efflux in response to the muscarinic agonist, carbamylcholine, nor did it change the magnitude of the response to this agent. Simultaneous introduction of 20 mM glucose with noradrenaline prevented a rise in⁴⁵Ca²⁺ efflux and indeed resulted in inhibition of⁴⁵Ca²⁺ efflux. The data suggest that noradrenaline does not directly activate the mechanisms which regulate Ca²⁺ extrusion from islets cells, and they do not support a primary role for the Ca²⁺ efflux response in mediating adrenergic inhibition of insulin secretion.     

Tentacle contraction inDiscophrya collini: The effects of ionophore A23187 and ruthenium red on Ca2+-induced contraction and uptake of extracellular calcium

Summary The tentacles of the suctorian protozoonDiscophrya collini are stimulated to contract by externally applied Ca²⁺. The role of extracellular Ca²⁺ in tentacle contraction was studied by monitoring⁴⁵Ca²⁺ uptake, using ionophore A23187 to facilitate membrane transport of calcium and ruthenium red (RR) as an inhibitor of transport. The degree of tentacle retraction was dependent upon external Ca²⁺ concentration and studies with⁴⁵Ca²⁺ using scintillation counting indicated a linear relationship between external Ca²⁺ concentration and Ca²⁺ uptake. Uptake of Ca²⁺ was enhanced in the presence of the ionophore while RR caused little inhibition.⁴⁵Ca²⁺ uptake was only partially inhibited by RR when cells were subjected to a Ca²⁺, ionophore and RR mixture. Grain counts from light microscope autoradiographs after treatment of cells with⁴⁵Ca²⁺/ionophore,⁴⁵Ca²⁺/RR or⁴⁵Ca²⁺ alone showed heavy, light and intermediate labelling respectively. In all instances the grains were evenly distributed within the cell.These observations are interpreted as supporting the suggestion that the ionophore enhances both the uptake of extracellular Ca²⁺ and release of Ca²⁺from an internal source, while the RR could only partially prevent movement of Ca²⁺ through the plasma mebrane. A model is presented suggesting that tentacle retraction is mediated by cytosolic Ca²⁺ levels which are determined by the fluxing of Ca²⁺ across the plasma membrane and the membrane of elongate dense bodies which act as internal Ca²⁺ reservoirs.     

Comparison of 45Ca2+ uptake activity by microsomes from control and stimulated mouse pancreatic acini

The ability of microsomal preparations to transport ⁴⁵Ca²⁺ was studied in preparations of control and secretagogue-stimulated pancreatic acini. ATP-dependent ⁴⁵Ca²⁺ uptake activity was present in the pancreatic post-mitochondrial supernatant and microsomes but little activity was present in the postmicrosomal supernatant. Treatment of acini with the secretagogues cholecystokinin (CCK) and carbamylcholine (CCh) prior to cell fractionation increased the subsequently measured microsomal ⁴⁵Ca²⁺ uptake. The effect of CCK was maximal after 10 min stimulation and at a not. The effect of CCK was maximal after 10 min stimulation and at a concentration of 1 nM; these conditions are comparable to the effects of CCK on ⁴⁵Ca²⁺ fluxes in intact acini. The increased microsomal ⁴⁵Ca²⁺ uptake induced by CCK was due to an increase in the maximal rate of ⁴⁵Ca²⁺ uptake as there was no effect on the K_m for Ca²⁺ (1 μM). It is concluded that secretagogues increase the ATP-dependent uptake of ⁴⁵Ca²⁺ by an isolated pancreatic microsomal component under the same conditions that also stimulate both digestive enzyme secretion and bi-directional Ca²⁺ movements.     

Phosphate and calcium uptake in beech (Fagus sylvatica) in the presence of aluminium and natural fulvic acids

In the present study we examine the effects of Al on the uptake of Ca²⁺ and H₂PO^-₄ in beech (Fagus sylvatica L.) grown in inorganic nutrient solutions and nutrient solutions supplied with natural fulvic acids (FA). All the solutions used were chemically well characterized. The uptake of Al by roots of intact plants exposed to solutions containing 0, 0.15 or 0.3 mM AlCl₃ for 24 h, was significantly less if FA (300 mg l⁻¹) were also present in the solutions. The Ca²⁺(⁴⁵Ca²⁺) uptake was less affected by Al in solutions supplied with FA than in solutions without FA. There was a strong negative correlation between the Al and Ca²⁺ uptake (r²=0.98). When the Al and Ca²⁺ (⁴⁵Ca²⁺) uptake were plotted as a function of the Al³⁺ activity (or concentration of inorganic mononuclear Al), almost the same response curves were obtained for the -FA and +FA treatments. We conclude that FA-complexed Al was not available for root uptake and therefore could not affect the Ca²⁺ uptake. The competitive effect of Al on the Ca²⁺ uptake was also shown in a 5-week cultivation experiment, where the Ca concentration in shoots decreased at an AlCl₃ concentration of 0.3 mM. The effect of Al on H₂PO⁻₄ uptake was more complex. The P content in roots and shoots was not significantly affected, compared with the control, by cultivation for 5 weeks in a solution supplied with 0.3 mM AlCl₃, despite a reduction of the H₂PO⁻₄ concentration in the nutrient solution to about one-tenth. At this concentration Al obviously had a positive effect on H₂PO⁻₄ uptake. The presence of FA decreased ³²P-phosphate uptake by more than 60% during 24 h, and the addition of 0.15 or 0.3 mM AlCl₃ to these solutions did not alter the uptake of ³²P-phosphate.     

Further Characteristics of the ATP-Stimulated Uptake of Calcium into Chromaffin Granules

Abstract: The ATP-stimulated uptake of ⁴⁵Ca²⁺ [and [³H](-)-noradrenaline ([³H]NA)] into chromaffin granules and that into mitochondria are driven by a protonic gradient ΔμH⁺, composed of the components ΔpH (concentration gradient of protons) and ΔΨ(electrical potential difference). The granular ATPase pumps protons into the matrix (ΔpH inside acid, ΔΨ positive), but the mitochondrial ATPase ejects protons from the matrix (ΔpH alkaline, ΔΨ negative inside). To show different driving forces of uptake, the rate of the ATP-stimulated uptake of ⁴⁵Ca²⁺ (and [³H]NA) into chromaffin granules was compared with the rate of the ATP-stimulated uptake of ⁴⁵Ca²⁺ into mitochondria (adrenomedullary or rat liver). In the presence of nitrate, the rate of the ATP-stimulated uptake of ⁴⁵Ca²⁺ into chromaffin granules is higher than in the presence of acetate, because the lyotropic anion nitrate stimulates the granular ATPase and increases ΔpH (acid inside). Compared with nitrate, the rate of the ATP-stimulated uptake of ⁴⁵Ca²⁺ into mitochondria is higher in the presence of the proton-carrying anion acetate, which, after permeation, provides protons for ejection by the ATPase. In the absence of ATP, a valinomycin-mediated potassium influx (ΔΨ inside positive) stimulates the granular uptake of [³H]NA, which has an electrogenic component, but not the granular uptake of ⁴⁵Ca²⁺, which is electroneutral. The electrogenic uptake of ⁴⁵Ca²⁺ into mitochondria is stimulated by a valinomycin-mediated potassium efflux (ΔΨ negative inside). The ATP-stimulated uptake of ⁴⁵Ca²⁺ into chromaffin granules is sensitive to ruthenium red, suggesting a carrier-mediated mechanism of uptake, and it is sensitive to atractyloside, indicating the simultaneous uptake of ATP. After collapse of ΔpH by ammonia, the ATP-stimulated uptake of ⁴⁵Ca²⁺ into chromaffin granules is abolished, but not that into mitochondria. In the presence of ammonia, the rate of the ATP-stimulated uptake of [³H]NA is very low, and an ATP-independent uptake of ⁴⁵Ca²⁺ into chromaffin granules is observed which is similar to the ATP-independent Ca²⁺/Na⁺ exchange at the granular membrane.     

Calcium channels and cation transport ATPases in cardiac hypertrophy induced by aortic constriction in newborn rats

In order to evaluate the contribution of pinocytosis to basal (no agonist) and lanthanide-insensitive store-activated Ca²⁺ inflow in freshly-isolated rat hepatocytes, the uptake of extracellular fluid by pinocytosis was measured at 20°C and used to predict the amount of extracellular Ca²⁺ taken up by pinocytosis. This was compared with the measured rate of Ca²⁺ uptake in the basal state, and with the measured lanthanide-insensitive component of divalent cation uptake stimulated by 2,5-di-tert-butylhydroquinone (DBHQ), an inhibitor of the smooth endoplasmic reticulum (Ca²⁺ + Mg²⁺)ATP-ase. Fluid uptake by pinocytosis was measured using [¹⁴C]sucrose. In hepatocytes incubated at 20°C, DBHQ increased the initial rate of sucrose uptake by about 35%. The data for sucrose uptake were used to calculate the volume of extracellular fluid taken up by pinocytosis which, in turn, was used to predict the amount of extracellular Ca ²⁺ taken up through pinocytosis in the basal and DBHQ-stimulated states. Rates of divalent cation inflow in the basal state were determined at 20°C by measuring the uptake of ⁴⁵Ca²⁺. The degree of stimulation of Ca²⁺ inflow by DBHQ and the lanthanide-insensitive component of DBHQ-stimulated divalent cation inflow were determined by measuring the rate of Mn²⁺-induced quenching of intracellular quin-2 in the absence of an agonist, and in the presence of DBHQ or DBHQ plus Gd³⁺. It was calculated that the process of pinocytosis accounts for at least 15% of Ca²⁺ uptake in the basal (no agonist) state, and for about 10% of DBHQ-stimulated lanthanide-insensitive Ca²⁺ uptake. It is concluded that in isolated hepatocytes (i) the release of Ca²⁺ from intracellular stores stimulates pinocytosis and (ii) the process of pinocytosis can account for a substantial proportion of basal Ca²⁺ inflow and a small proportion of DBHQ-stimulated lanthanide-insensitive Ca²⁺ inflow.Abbreviations RACC receptor-activated Ca²⁺ channel - DBHQ 2,5-di-tert-butylhydroquinone - [Ca²⁺] intracellular free Ca²⁺ concentration     

An ion exchange chromatographic method for the determination of synaptosomal calcium uptake

A simple, rapid method for determining depolarization-induced⁴⁵Ca influx into synaptosomes is described. Synaptosomes which had been depolarized in the presence of⁴⁵Ca were applied to a small column of Chelex-100 resin to separate free Ca²⁺ from that taken up by the tissue. Approximately 70% of the synaptosomal protein applied to the column was recovered in the initial eluate. The magnitude of⁴⁵Ca uptake was dependent on the amount of Ca²⁺ in the incubation medium and on the KCl concentration. Calcium influx reached a plateau after 90 sec of incubation at 24°C. The Na⁺ channel activator veratridine also produced a substantial influx of⁴⁵Ca, and this effect was blocked by tetrodotoxin. Thus, this ion exchange procedure makes it possible to measure depolarization-induced Ca²⁺ influx in synaptosomes without subjecting them to high vacuum or centrifugation pressures or to EGTA-containing solutions.     

Ca2+-independent form of protein kinase C may regulate Na+ transport across frog skin

Summary Activators of protein kinase C (PKC) stimulate Na^– transport (J _Na) across frog skin. We have examined the effect of Ca²⁺ on PKC stimulation ofJ _Na. Both the phorbol ester 12-O-tetradecanoylglycerol (DiC₈) were used as PKC activators. Blocking Ca²⁺ entry into the cytosol (either from external or internal stores) reduced the subsequent natriferic effect of the PKC activators. This negative interaction did not simply reflect saturation of activation of the apical Na⁺ channels, since the stimulations produced by blocking Ca²⁺ entry and adding cyclic AMP were simply additive.The Ca²⁺ dependence of the natriferic effect could have reflected either a direct action of cytosolic Ca²⁺ on PKC or an indirect action on the final receptor site (the Na⁺ channel). To distinguish between these possibilities, the TPA- and phospholipid-dependent kinase activity of broken-cell preparations was assayed. The kinase activity was not stimulated by physiological levels of Ca²⁺, and in fact was inhibited at millimolar concentrations of Ca²⁺.We conclude that the effects of Ca²⁺ on the natriferic response to PKC activators are indirect. Reducing cytosolic uptake of Ca²⁺ may have stimulated Na⁺ transport by a chemical modification of the apical channels observed in other tight epithelia. The usual stimulation of Na⁺ transport produced by PKC activators in frog skin may reflect the operation of a nonconventional form of PKC. This enzyme is Ca²⁺ independent and seems related to thenPKC or PKC observed in other systems.     

Role of calcium and calmodulin in the regulation of the rabbit ileal brush-border membrane Na+/H+ antiporter

Summary In rabbit ileum, Ca²⁺/calmodulin (CaM) appears to be involved in physiologically inhibiting the linked NaCl absorptive process, since inhibitors of Ca²⁺/CaM stimulate linked Na⁺ and Cl^– absorption. The role of Ca²⁺/CaM-dependent phosphorylation in regulation of the brush-border Na⁺/H⁺ antiporter, which is believed to be part of the neutral linked NaCl absorptive process, was studied using purified brush-border membrane vesicles, which contain both the Na⁺/H⁺ antiporter and Ca²⁺/CaM-dependent protein kinase(s) and its phosphoprotein substrates. Rabbit ileal villus cell brush-border membrane vesicles were prepared by Mg precipitation and depleted of ATP. Using a freezethaw technique, the ATP-depleted vesicles were loaded with Ca²⁺, CaM, ATP and an ATP-regenerating system consisting of creatine kinase and creatine phosphate. The combination of Ca²⁺/CaM and ATP inhibited Na⁺/H⁺ exchange by 45±13%. This effect was specific since Ca²⁺/CaM and ATP did not alter diffusive Na⁺ uptake, Na⁺-dependent glucose entry, or Na⁺ or glucose equilibrium volumes. The inhibition of the Na⁺/H⁺ exchanger by Ca²⁺/CaM/ATP was due to an effect on theV _max and not on theK _m for Na⁺. In the presence of CaM and ATP, Ca²⁺ caused a concentration-dependent inhibition of Na⁺ uptake, with an effect 50% of maximum occurring at 120nm. This Ca²⁺ concentration dependence was similar to the Ca²⁺ concentration dependence of Ca²⁺/CaM-dependent phosphorylation of specific proteins in the vesicles. The Ca²⁺/CaM/ATP-inhibition of Na⁺/H⁺ exchange was reversed by W₁₃, a Ca²⁺/CaM antagonist, but not by a hydrophobic control, W₁₂, or by H-7, a protein kinase C antagonist. we conclude that Ca²⁺, acting through CaM, regulates ileal brush-border Na⁺/H⁺ exchange, and that this may be involved in the regulation of neutral linked NaCl absorption.     

Calcium Uptake and Catecholamine Secretion by Cultured Bovine Adrenal Medulla Cells

Abstract: The uptake of ⁴⁵Ca²⁺ and secretion of catecholamines by primary cultures of adrenal medulla cells were studied. Nicotine, veratridine, potassium, and Ionomycin stimulate both the accumulation of ⁴⁵Ca²⁺ and the secretion of catecholamines. Nicotinic antagonists block ⁴⁵Ca²⁺ uptake induced by nicotine, tetrodotoxin blocks ⁴⁵Ca²⁺ uptake induced by veratridine, and D600 blocks uptake induced by K⁺, nicotine, and veratridine, but not ⁴⁵Ca²⁺ uptake or secretion induced by Ionomycin. The EC₅₀ for nicotine is 3 μm for catecholamine secretion and 10 μm for ⁴⁵Ca²⁺ uptake, while the EC₅₀S for veratridinestimulated uptake and secretion are approximately the same (75 μm ). Kinetic studies show that the uptake of Ca²⁺ is rapid and appears to precede the secretion of catecholamines, and that the rate of uptake declines rapidly. The uptake of ⁴⁵Ca²⁺ and secretion of catecholamines stimulated by veratridine and 50 mm -K⁺ show saturation kinetics with respect to external calcium concentrations at about 2 mm . On the other hand, the uptake of ⁴⁵ Ca²⁺ stimulated by nicotine does not become saturated at external calcium concentrations of 10 mm although the secretion of catecholamines reaches a maximum at external calcium concentrations of 2 mm . The data suggest that depolarizing agents such as veratridine and 50 mm -K⁺ stimulate ⁴⁵Ca²⁺ entry through voltage-sensitive calcium channels, while nicotinic agonists stimulate calcium entry through the acetylcholine receptor ion channels as well as through voltage-sensitive calcium channels.