Calmodulin-regulated, ATP-driven calcium transport by basolateral membranes of rat small intestine

Mechanisms and regulation of epithelial Ca2+ absorption in health and disease

Ca2+ is essential for numerous physiological functions in our bodies. Therefore, its homeostasis is finely maintained through the coordination of intestinal absorption, renal reabsorption, and bone resorption. The Ca2+-selective epithelial channels TRPV5 and TRPV6 have been identified, and their physiological roles have been revealed: TRPV5 is important in final renal Ca2+ reabsorption, and TRPV6 has a key role in intestinal Ca2+ absorption. The TRPV5 knockout mice exhibit renal leak hypercalciuria and accordingly upregulate their intestinal TRPV6 expression to compensate for their negative Ca2+ balance. In contrast, despite their severe negative Ca2+ balance, TRPV6-null mice do not display any compensatory mechanism, thus resulting in secondary hyperparathyroidism. These results indicate that the genes for TRPV5 and TRPV6 are differentially regulated in human diseases associated with disturbed Ca2+ balance such as hypercalciuria, osteoporosis, and vitamin D-resistant rickets.

Sperm phenotype of mice carrying a gene deletion for the plasma membrane calcium/calmodulin dependent ATPase 4

The sarcolemmal calcium pumps (PMCA for plasma membrane calcium/calmodulin dependent ATPase) are a family of 10 transmembrane domain proteins ejecting calcium from the cytosol. They are encoded by four independent genes and at least 21 splice variants have been described. Isoforms 1 and 4 are ubiquitous, whereas isoforms 2 and 3 are confined to neurons and few other cells (e.g. isoform 2 in the myocardium). In non-excitable cells they are thought to be the only calcium ejection systems and their function as governors of calcium balance is hence intuitive since cells cannot survive in a state of calcium overload. Differences in the affinity of the various isoforms for calcium, ATP and calmodulin have been described, but it is unclear whether the pumps have specialized functions over and above their 'housekeeping' role. In particular, in excitable cells, most calcium is ejected by the sodium/calcium exchanger suggesting that the PMCAs may have evolved into a specialized role. Recently, our group has identified a number of specialized functions of the PMCAs, notably a prominent regulatory role of PMCA4 (splice variant b) for neuronal NO synthase as well as for the Ras pathway. In addition, mice carrying a genetic deletion of the PMCA4 gene showed normal female, but completely infertile male animals. This is due to a highly specific defect in sperm motility, which is reduced to zero, with normal fertilization capacity. Overall, a scenario emerges where the plasma membrane calcium pumps fulfil roles far beyond the traditional housekeeping function, notably in cell signaling, sperm motility, and potentially in cell division. Consequently, we are currently exploring their potential as future drug targets for a variety of conditions, as well as their potential use in the development of a male contraception.

Mechanisms of intestinal calcium absorption

Calcium is absorbed in the mammalian small intestine by two general mechanisms: a transcellular active transport process, located largely in the duodenum and upper jejunum; and a paracellular, passive process that functions throughout the length of the intestine. The transcellular process involves three major steps: entry across the brush border, mediated by a molecular structure termed CaT1, intracellular diffusion, mediated largely by the cytosolic calcium-binding protein (calbindinD(9k) or CaBP); and extrusion, mediated largely by the CaATPase. Chyme travels down the intestinal lumen in approximately 3 h, spending only minutes in the duodenum, but over 2 h in the distal half of the small intestine. When calcium intake is low, transcellular calcium transport accounts for a substantial fraction of the absorbed calcium. When calcium intake is high, transcellular transport accounts for only a minor portion of the absorbed calcium, because of the short sojourn time and because CaT1 and CaBP, both rate-limiting, are downregulated when calcium intake is high. Biosynthesis of CaBP is fully and CaT1 function is approximately 90% vitamin D-dependent. At high calcium intakes CaT1 and CaBP are downregulated because 1,25(OH)(2)D(3), the active vitamin D metabolite, is downregulated.

Ca-ATPase of human syncytiotrophoblast basal plasma membranes

In the present work, a Mg(2+)-dependent, Ca(2+)-stimulated ATPase activity was determined and characterized in purified preparations of syncytiotrophoblast basal (fetal facing) plasma membranes, and its characteristics were compared to those of the active Ca(2+)-transport already demonstrated in this tissue. Similar to the active Ca(2+)transport, the Ca-ATPase is Mg(2+)-dependent, is stimulated by calmodulin, and is inhibited by vanadate. The K(m) for Ca(2+)activation is 0.25+/- 0.02microM, a value near to that described for calcium active transport in this tissue. Consequently, the Ca-ATPase activity of human syncytiotrophoblast basal plasma membrane described in this paper could be responsible for the active extrusion of calcium from the syncytiotrophoblast toward the fetal circulation.

ATP-dependent calcium uptake into basolateral vesicles from transporting epithelia of intermolt crayfish

ATP-dependent Ca2+ uptake was determined into inside-out basolateral membrane vesicles (BLMV) from intermolt crayfish (Procambarus clarkii) Ca2+-transporting epithelia: gill, hepatopancreas (liver), and antennal gland (kidney). Extravesicular (EV) ATP (5 mM) increased 45Ca2+ uptake (free Ca2+ 5 microM) by fivefold but was abolished by pretreatment with either vanadate or the ionophore A-23187. Addition of A-23187 to Ca2+-loaded vesicles produced 70% efflux. The saturable carrier exhibited a Km for Ca2+ of 0.11-0.27 microM and maximal influx of 20-123 pmol. mg-1. min-1. The Km for ATP was 0.01-0.04 mM. The temperature coefficient ranged from 1.43 to 2.06. EGTA treatment of hepatopancreas and antennal gland vesicles decreased 45Ca2+ uptake by 50-90%; uptake was restorable by calmodulin. However, in gill, 45Ca2+ uptake was unaffected by EGTA treatment and calmodulin decreased uptake in both EGTA-treated and untreated vesicles. Addition of EV Na+ (5 mM) increased ATP-dependent Ca2+ uptake into hepatopancreas and antennal gland BLMV by 60%; in hepatopancreas BLMV, this increase was inhibitable by ouabain. However, ATP-dependent Ca2+ uptake in gill vesicles was Na+ independent. The relative role of each epithelium in whole animal Ca2+ homeostasis has been interpreted based on in vitro characteristics.

Mechanism of the enhancing effect of sorbitol on ileal Ca uptake in rat enterocytes

The effect of sorbitol on Ca uptake by isolated ileal epithelial cells was investigated. Intestinal cells were isolated from rat ileum by mechanical vibration. 45Ca uptake was approximately 2 times higher in cells exposed to 200 mM sorbitol of D-alanine than in control cells. This enhancing effect of sorbitol on percentage Ca uptake decreased with increasing Ca concentrations in the incubation medium suggesting an effect on Ca entry velocity. The addition of 10 microM nifedipine or 200 microM verapamil to the incubation medium was devoid of any effect on Ca uptake in ileal cells, whereas 100 microM trifluoperazine or chlorpromazine abolished the stimulatory effect of sorbitol. Finally, the effect of sorbitol on isolated cells was independent of a measurable change of cellular ATP content. In conclusion, the stimulatory effect of sorbitol on ileal Ca uptake is probably exerted through mechanisms other than an increase in intracellular ATP concentration. Sorbitol may enhance enterocyte Ca transport via a direct interaction with calmodulin and/or the Ca pump. It may also exert its effect through an inhibition of the basolateral Na Ca exchanger.

Demonstration of calmodulin-sensitive calcium translocation by isolated osteoclast plasma membrane vesicles

Plasma membrane vesicles were prepared from chicken osteoclasts, and active calcium transport was demonstrated in a spectrofluorimetric assay using the fluorescent calcium concentration indicator, fura-2. Transport activity was inhibited by quercetin (10 microM), sodium vanadate (10 microM), and the anticalmodulin agents, compound 48/80 (20 and 200 micrograms/ml) and calmidazolium (10 and 20 microM). The transport rate (Vmax, 1.3 nmol/mg protein/min) was not altered in the presence of the protonophore, nigericin (1 microM), indicating that proton transport was not driving calcium transport. Release of accumulated calcium in the vesicles occurred with the addition of bromo-A23187 (5 microM) or ionomycin (5 microM). Increasing calcium transport occurred with increasing calcium concentration. Finally, the calmodulin content of the vesicles was demonstrated to be 54-134 U/mg protein. These results demonstrate that a calmodulin-sensitive, ATP-dependent calcium transporter is present in the osteoclast plasma membrane.

Ca2+/calmodulin regulation of putrescine uptake in cultured gastrointestinal epithelial cells

Regulation of putrescine uptake in a small intestinal crypt cell line, IEC-6 cells, was examined. Uptake of [14C]putrescine was measured throughout a normal growth curve and was found to be inversely related to growth. Kinetic analysis at low and high cell density revealed the inhibition of uptake in confluent cells was due to a five-fold reduction in Vmax of uptake, 199.5 vs. 43.1 pmol.10(5) cells-1.h-1, respectively. Three gastrointestinal hormones, gastrin, secretin, and cholecystokinin, produced partial inhibition of [14C]putrescine uptake. Conversely, treatment of quiescent cells with 5% fetal bovine serum to stimulate growth did not affect uptake. Influence of putrescine uptake on free ionized intracellular Ca2+ ([Ca2+]i) was measured by microspectrofluorometry using the Ca(2+)-sensitive fluoroprobe fura-2. Basal [Ca2+]i was calculated to be 112 nM and increased rapidly to 313 nM upon addition of 10 microM putrescine. Preventing the rise in [Ca2+]i using an intracellular Ca2+ buffer, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, decreased [14C]putrescine uptake to 29.5 +/- 5.3% of control values. 45Ca2+ flux experiments and measurement of transport in 0 Ca2+ and 0.5 mM EDTA suggested an intracellular source of calcium was mobilized during putrescine uptake. Finally, use of the putative calmodulin antagonist N-(6-aminohexyl)-5-chloro-l-naphthalenesulfonamide caused a dose-dependent inhibition of [14C]putrescine uptake with 50% inhibitory concentration of approximately 7 microM. These data suggest that putrescine uptake in IEC-6 cells may be regulated by a Ca2+/calmodulin-dependent mechanism.

Plasma membrane calcium pump expression in human placenta and small intestine

To identify the forms of the plasma membrane calcium pump present in tissues that transport calcium, cDNA from human placenta and proximal small intestine was amplified by the polymerase chain reaction using a pair of mixed primers based on all the known human and rat plasma membrane calcium pump sequences. Clones were identified from the two human forms HPMCA1 and HPMCA4, but no new sequences were found in either tissue. RNA blots probed with HPMCA1 showed two bands in both tissues; probing with HPMCA4 gave a single, larger species. In placenta, HPMCA4 was the more abundant form and similar expression was found in full-term and second-trimester placentas. In contrast, in the small intestine, HPMCA1 was more abundant, suggesting that calcium absorption is not associated with any one specific isoform in calcium transporting cells.

Vitamin D and mineral deficiencies increase the plasma membrane calcium pump of chicken intestine

The basolateral membrane of the enterocyte was previously shown to contain an adenosine triphosphate-dependent calcium pump. Using immunological procedures, the localization of the Ca2+ pump in chick intestine, and the effect of dietary variables on the concentration of the pump, were studied. A monoclonal antibody produced against the human erythrocyte calcium pump was shown to cross-react with a chick intestinal Ca2+ pump epitope. The most intense staining of intestinal tissue, as determined immunohistochemically, occurred at the basolateral membrane of the duodenum, jejunum, ileum, and colon, with minor staining elsewhere. By the Western blotting procedure, vitamin D repletion of vitamin D-deficient chicks was shown to significantly increase the concentration of the Ca2+ pump epitope of duodenal, jejunal, and ileal mucosa by a factor of 2-3. Chicks were also fed diets deficient in calcium or phosphorus, a situation known to result in the stimulation of the synthesis of calbindin-D28k and an enhancement of the efficiency of Ca2+ absorption. Adaptation of the chicks to these deficient diets was verified by an increase in intestinal levels of calbindin-D28k, and is now shown to increase the Ca2+ pump epitope. From these immunological studies, it seems apparent that dietary variables that enhance intestinal Ca2+ absorption also increase the amount of the intestinal basolateral Ca2+ pump.

Calmodulin content and activity in normal and coeliac duodenum

Calmodulin is an important modulator of intracellular calcium processes and may be implicated in the calcium malabsorption of coeliac disease. The calmodulin content in extracts of duodenal biopsy specimens from 48 normal control subjects and 28 patients with coeliac disease was determined. Radioimmunoassay was used to measure immunoreactive calmodulin while a cyclic adenosine 3',5'-monophosphate phosphodiesterase activity assay was used to measure biologically active calmodulin. Calmodulin values measured by both assays were similar for control and disease groups. Mean (SEM) immunoreactive calmodulin values were 1.68 (0.09) micrograms/mg protein for controls and 1.67 (0.15) and 1.45 (0.15) micrograms/mg protein for partial and total villous atrophy respectively. These values were not significantly different. Biologically active calmodulin values were 2.77 (0.21), 1.82 (0.34), and 3.24 (0.33) micrograms/mg protein for control, partial, and total villous atrophy subjects respectively. The biologically active calmodulin values in the partial villous atrophy group were significantly lower than in controls and total villous atrophy subjects. In the phosphodiesterase assay, the calmodulin antagonist trifluoperazine inhibited the activity stimulated by purified calmodulin and by the extracts to the same extent. These results show that calmodulin values are normal in coeliac disease and provide no evidence that changes in calmodulin account for the abnormal calcium absorption in these patients.

Role of facilitated diffusion of calcium by calbindin in intestinal calcium absorption

Computer simulations of transcellular Ca2+ transport in enterocytes were carried out using the simulation program SPICE. The program incorporated a negative-feedback entry of Ca2+ at the brush-border membrane that was characterized by an inhibitor constant of 0.5 microM cytosolic Ca2+ concentration ([Ca2+]). The basolateral Ca(2+)-ATPase was simulated by a four-step mechanism that resulted in Michaelis-Menten kinetics with a Michaelis constant of 0.24 microM [Ca2+]. The cytosolic diffusion of Ca2+ was simulated by dividing the cytosol into 10 slabs of equal width. Ca2+ binding to calbindin-D9K was simulated in each slab, and diffusion of free Ca2+, free calbindin, and Ca(2+)-laden calbindin was simulated between each slab. The cytosolic [Ca2+] of the simulated cells was regulated within the physiological range. Calbindin-D9K reduced the cytosolic [Ca2+] gradient, increased Ca2+ entry into the cell by removing the negative-feedback inhibition of Ca2+ entry, increased cytosolic Ca2+ flow, and increased the efflux of Ca2+ across the basolateral membrane by increasing the free [Ca2+] immediately adjacent to the pump. The enhancement of transcellular Ca2+ transport was nearly linearly dependent on calbindin-D9K concentration. The values of the dissociation constant (Kd) for calbindin-D9K were previously obtained experimentally in the presence and absence of KCl. Calbindin with the Kd obtained in the presence of KCl enhanced the simulated Ca2+ transport more than with the Kd obtained in the absence of KCl. This result suggests that the physiological Kd of calbindin is optimal for the enhancement of transcellular Ca2+ transport. The simulated Ca2+ flow was less than that predicted from the "near-equilibrium" analytic solution of the reaction-diffusion problem.

Quantification of Ca(2+)-ATPases in porcine duodenum. Effects of 1,25(OH)2D3 deficiency

Previous studies have identified a calmodulin-stimulated ATP-dependent Ca2+ pump as the major Ca2+ efflux pathway in enterocytes. Here, we developed methods to quantify the number of Ca2+ pumps in basolateral and intracellular membranes from porcine duodenum. By the use of a pig strain with a genetic defect in renal 1 alpha-hydroxylase, we were able to investigate the influence of 1,25(OH)2D3-deficiency on the number of Ca(2+)-ATPases in porcine duodenum. The amount of Ca(2+)-ATPase in isolated basolateral membranes was 5.5 +/- 0.7 micrograms/mg protein, while the Vmax of ATP-dependent Ca2+ transport into inside-out resealed basolateral membrane vesicles was 2.6 +/- 0.4 nmol/mg protein per min. From these data we estimated roughly about 95 x 10(3) plasma membrane Ca2+ pump sites per enterocyte. In addition, the amount of intracellular Ca(2+)-ATPase in microsomal fractions was 0.41 +/- 0.02 microgram/mg protein. Comparison of these parameters between control and rachitic animals showed that Ca2+ pump capacities in both basolateral membranes and microsomal fractions of porcine duodenum are not influenced by 1,25(OH)2D3-deficiency. In conclusion, stimulatory effects of 1,25(OH)2D3 on intestinal Ca2+ transport most likely result from specific effects on apical influx and facilitation of cytosolic Ca2+ diffusion by Ca(2+)-binding proteins and not from an increase in Ca2+ pumping capacity in basolateral membranes.

The effect of diets adequate and deficient in calcium on blood pressures and the activities of intestinal and kidney plasma membrane enzymes in normotensive and spontaneously hypertensive rats

Basolateral and brush-border membranes were prepared from the intestines and kidneys of spontaneously hypertensive (SHR) and normotensive (WKY) rats fed on a calcium-adequate diet and assayed for their enzyme activities. In intestinal basolateral membranes the activities of Na+ K(+)-ATPase (EC 3.6.1.37) Ca2(+)-ATPase (EC 3.6.1.38) and alkaline phosphatase (EC 3.1.3.1) were lower in SHR rats when compared with WKY rats, whilst 5'-nucleotidase (EC 3.1.3.5) (a marker for basolateral membranes) was unaffected. In kidney basolateral membranes all enzymes were similar in activity in SHR and WKY rats. In intestinal brush-border membranes the activities of Ca2(+)-ATPase and alkaline phosphatase were lower in SHR rats when compared with WKY rats, whilst microvillus aminopeptidase (EC 3.4.11.2) (a marker for brush-border membranes) was unaffected. In kidney brush-border membranes all enzymes were similar in activity in SHR and WKY rats. The blood pressures of the SHR rats were considerably higher than those of the WKY rats. When SHR rats were fed on a Ca-deficient diet the activities of Na+K(+)-ATPase, Ca2(+)-ATPase and alkaline phosphatase in basolateral membranes and Ca2(+)-ATPase and alkaline phosphatase in brush-border membranes were all increased in the intestine when compared with SHR rats fed on a Ca-adequate diet. The equivalent enzymes in the kidneys of SHR rats, and the intestines and kidneys of WKY rats, were not affected by altering the Ca in the diet. The blood pressures of SHR rats fed on a Ca-deficient diet were higher than in those fed on a Ca-adequate diet. Blood pressures of WKY rats were not affected by altering the diet in this way. The results indicate that the absorption of Ca by active mechanisms may be reduced in SHR rats compared with WKY rats. Changing the level of Ca in the diet modified both blood pressure and the activities of enzymes which catalyse active Ca transport. The implications of these results to the aetiology, and possible nutritional treatment, of essential hypertension are discussed.

Relationship between the uptake of calcium or phosphorus and alkaline phosphatase activity induced by certain modulators in rat organs

The effects of phorbol ester or calmodulin on the calcium and phosphorus uptakes by rat tissues and their relationship to the alkaline phosphatase activity (ALP) were investigated in vivo. In rat tissues, ALP activity and calcium uptake in the duodenum and liver were clearly induced by phorbol ester treatment, whereas in the calvarium and ileum they were decreased. Phosphorus uptake was increased by the administration of phorbol ester only in the calvarium. In rats pretreated with an injection of indomethacin as an inhibitor of prostaglandin-synthesizing enzyme, the selective uptake of calcium by phorbol ester was eliminated in the duodenum and liver, as was the ALP activity. In contrast, rats showed a marked increase in ALP activity in the ileum after calmodulin treatment. Moreover, the increased uptake of calcium after calmodulin treatment was clearly seen in the ileum, calvarium, and kidney, and an increased uptake in phosphorus was seen in the duodenum, ileum, and calvarium, but not in kidney. Furthermore, prior injection of W-7 or calmidazolium as an antagonist of calmodulin, reduced the increased ALP activities and the uptake of calcium in all organs tested, but did not reduce the increased phosphorus uptake by the calvarium. Consequently, it is suggested that calcium uptake under the above conditions correlated well with changes in the ALP activity; however, phosphorus uptake seemed to be less in accord with ALP activity. The amount of tested other mineral metabolic markers suggests that the Ca uptake and ALP activity induced by certain effectors regulate 1,25(OH)2D3 level by the modulation of 25(OH)D3-1 alpha-hydroxylase activity.

Comparison of the enhanced steady-state diffusion of calcium by calbindin-D9K and calmodulin: possible importance in intestinal calcium absorption

The diffusion of calcium was measured using the unidirectional flux of 45Ca across an aqueous layer. The aqueous layer was bounded by two dialysis membranes and convection was eliminated by gelling the aqueous layer with agarose. The apparent self-diffusion coefficient was determined by the dependence of the tracer flux on the diffusion distance. The apparent self-diffusion coefficient increased linearly with the concentration of calbindin-D9K and calmodulin, but the effect of calmodulin was markedly less than that of calbindin-D9K. This difference is attributed to the lower association constant for calmodulin. The ion-exchange resin Chelex-100 also increased the steady-state of 45Ca, but the effect of Chelex-100 was much less efficient than the effect of calbindin-D9K. The mechanism of enhanced diffusion was attributed to an enhanced gradient of total 45Ca. These results indicate that the steady-state unidirectional calcium flux is a superposition of free calcium diffusion and bound calcium diffusion, with only a small contribution due to a 'bucket brigade' mechanism. We suggest that this phenomenon may be important in calcium absorption across the intestine.

Rat ileal alkaline phosphatase activity and secretion is stimulated by alterations in calcium metabolism

The mechanism of calmodulin-stimulated alkaline phosphatase activity was studied in the rat. In calmodulin-treated rats (2.5 micrograms/animal, intraperitoneally) alkaline phosphatase (ALP) activity was elevated 11-fold in the ileum, 1.5-fold in the duodenum and calvarium, 3-fold in serum, and not at all in liver. The elevated ALP activity was prevented by prior treatment with flunarizine, a calcium channel blocker, and by W-7, a calmodulin antagonist. cAMP content in ileum paralleled the timing and changes in ALP activity, but was not elevated in the duodenum or calvarium. Calcium ionophore A23187 and calcitonin treatment also increased ileal, duodenal, and calvarial ALP activity, but by less than the response to calmodulin. All of these treatments caused a 2-fold elevation in serum 1,25-dihydroxyvitamin D-3 (1,25(OH)2D3) levels. Pretreatment of the animals with parathyroid hormone prevented the rise of both ALP activity and of 1,25(OH)2D3. Administration of 1,25(OH)2D3 alone stimulated a different pattern of increased ALP activity, greater in duodenum than ileum. The uptake of 45Ca by calmodulin was also elevated in ileum and calvarium. These data suggest that shifts in calcium movement, perhaps mediated by vitamin D, can alter ALP activity, and may provide a mechanism for rapid control of the secretion of this enzyme.

On the molecular mechanism of intestinal calcium transport

The intestinal absorption of calcium is certainly a complex process, dependent on several factors of which vitamin D, via 1,25(OH)2D3, is the major controlling hormone. The efficiency of calcium absorption is a function of calcium status and calcium need. As the body's demand for calcium increases, the process commonly termed, adaptation, is activated in which the synthesis of 1,25(OH)2D3 from precursor is increased, resulting in the stimulation of the rate of calcium absorption. The increased demand for calcium might result from the ingestion of a diet deficient in calcium, from growth, pregnancy, lactation and egg shell formation in the laying hen. Accomapanying the change in calcium absorptive efficiency are molecular modifications of the transporting enterocytes, some mentioned herein and elsewhere (Wasserman & Chandler, 1985; Wasserman, 1980; Wasserman et al., 1984). Highly correlated with the rate of calcium absorption under a wide variety of conditions is the concentration of the vitamin D-induced calcium-binding protein, calbindin-D28K (avian type) and calbindin-D9K (mammalian intestinal type). The role of calbindin-D in this transport process is not precisely known but is considered to act at the present time as a cytosolic facilitator of Ca2+ diffusion from the brush border membrane to the basolateral membrane. In addition to the induction of calbindin-D synthesis, 1,25(OH)2D3 exerts other effects on the intestinal epithelium that can have consequences on the calcium absorptive process. Some of these effects are summarized in Figure 14. Vitamin D-dependent reactions might be either direct effects of 1,25(OH)2D3 or indirect effects due to elevated intracellular Ca2+ concentrations. These include changes in the fluidity of the brush border membrane, an increase in microvillar alkaline phosphatase-low affinity Ca-activated ATPase activity, an association of calmodulin with the 105 kD brush border cytoskeletal protein and, following calbindin D synthesis, the binding of calbindin D to a 60 kD brush border protein and to microtubules. The latter has been suggested to be related to the proposed transfer of Ca2+ by an endocytotic-exocytotic mechanism. In addition, a vitamin D-dependent intestinal membrane calcium-binding protein has been identified (Kowarski & Schachter, 1980). Playing into this multi-component system is a stimulation of cyclic nucleotide synthesis by 1,25(OH)2D3 which, through activation of cyclic nucleotide-dependent protein kinases, might modify membrane Ca2+ "channels" by phosphorylation reactions.4+ Intracellular organelles, i.e., the endoplasmic reticulum, mitochondria, the Golgi apparatus, are potent sequesters of Ca2+ and could contribute to the protection of the cell from excessively high Ca2+ concentrations by transiently storing absorbed Ca2+.

Effects of lanthanum in cellular systems. A review

Lanthanum belongs to the group of elements known as "lanthanons," which also includes cerium, europium, promethium, and thulium. It is the most electropositive element of the rare earth group, is uniformly trivalent, and is similar in its chemical properties to the alkaline earth elements. The effects of this element and its compounds on cellular systems are of considerable interest because of their increasing use in industry and as a substitute or antagonist for calcium in a variety of cellular reactions. Lanthanum is also being employed extensively in studying anatomical barriers, membrane structure, and subcellular transport systems, particularly the calcium pathway.

Calcium transport by basal lateral membrane vesicles from rat small intestine decreases with age

There is a marked decrease in active Ca2+ transport by the rat small intestine with age, particularly between 2 and 12 months. Much evidence suggests that the active component of Ca2+ transport resides in the energy-dependent pumping of Ca2+ across the intestinal basal lateral membrane. Therefore, we have characterized Ca2+ uptake by basal lateral membrane vesicles isolated from young (2-3 month old) and adult (12-14 month old) rats. In vesicles from the proximal duodenum, ATP-dependent Ca2+ uptake was about 4-times greater in the young animal than in the adult. There were no age differences in Ca2+ uptake in the absence of ATP. In vesicles from the ileum, Ca2+ uptake was much less than in the duodenum. The age differences in the ileum were smaller, and ATP-dependent Ca2+ uptake in the young was only twice that seen in the adult. Osmotic lysis of duodenal vesicles reduced Ca2+ uptake to low levels in both age groups, indicating that most of the Ca2+ was being taken up into an osmotically active space. Kinetic studies of Ca2+ uptake showed that there was no change in the apparent affinity but a 5-fold decrease in the Vmax of the adult Ca2+ transport system compared to that of the young animal. This marked decrease in the capacity of basal lateral membrane vesicles to actively transport Ca2+ may contribute to the decline in intestinal Ca2+ absorption with age.

Na+/H+ exchange mechanism in the basolateral membrane of the rat enterocyte

Basolateral membrane vesicles from rat jejunal enterocytes, especially purified of brush-border contamination, were used for Na+ uptake. The basolateral membrane vesicles are osmotically active and under our experimental conditions Na+ binding is much lower than transport. An outwardly directed proton gradient stimulates Na+ uptake at both 5 microM and 5 mM concentrations. The proton gradient effect can be inhibited completely by 2 mM amiloride and partially by either FCCP or NH4Cl (NH3 diffusion). Membrane potential effects can be excluded by having valinomycin plus K+ on both sides of the vesicles. These results suggest that there is an Na+/H+ exchanger in the basolateral membrane of rat enterocytes.

Localisation of calcium-activated adenosine-triphosphatase (Ca2+-ATPase) in intracerebral arterioles in acute hypertension

The plasma membrane calcium-activated adenosine triphosphatase (Ca2+-ATPase) is known to regulate intracellular calcium levels. This enzyme was localised in intracerebral cortical vessels of normotensive and acutely hypertensive rats. Of interest was whether the arterioles that develop increased permeability to horseradish peroxidase (HRP) in acute hypertension demonstrate any alteration in localisation of Ca2+-ATPase as compared to normotensive controls. Rats were injected with HRP intravenously and acute hypertension was induced by a 2-min infusion of angiotensin amide. Following perfusion of fixative, brains were sliced and reacted for demonstration of HRP reaction product and Ca2+-ATPase. Normotensive rats showed discontinuous distribution of Ca2+-ATPase on the outer plasma membranes of endothelial, smooth muscle and adventitial cells of arterioles. The localisation of Ca2+-ATPase in pinocytotic vesicles present in endothelial and smooth muscle cells was quite striking. Focal cortical areas of hypertensive rats showed increased arteriolar permeability to HRP. Permeable arterioles showed marked reduction of Ca2+-ATPase on the outer plasma membranes of endothelium and smooth muscle cells as compared to nonpermeable arterioles of the same animals and arterioles of normotensive controls. The latter finding suggests that calcium may be involved in increased cerebrovascular permeability mechanisms in acute hypertension.

Histochemical, ultrastructural and X-ray microprobe analytical studies of localization of calcium in the mucous lining of the rat duodenum

Localization of calcium in a rapid frozen and freeze substituted duodenum of normal, starved or calcium-repleted rat was examined using either of the glyoxal bis(2-hydroxyanil) (GBHA) staining method, a sensitive histochemical calcium stain or electron microscopy. In normally-fed rats, a majority of absorptive cells of the duodenum showed numerous discrete red granular GBHA reactions, approximately 1 micron or less in diameter, located primarily along their lateral plasma membranes and within intercellular spaces. Electron microscopy also revealed electron-dense granules, 30-100 nm in diameter, showing a similar distribution as the GBHA granules in the respective absorptive cells, and confirmed their absence in mitochondria and other intracellular compartments. Some of the absorptive cells located exclusively at the tip of each villus contained highly GBHA-reactive tubulo-vesicular structures extending throughout the cytoplasm. However, they displayed virtually no granular GBHA reaction. In these cells, electron microscopy revealed numerous electron-dense granules in the nucleus, mitochondria and in other unidentified organelles. X-ray microprobe analyses of ultrathin sections confirmed the presence of calcium within electron-dense granules associated with both types of absorptive cells. The number and intensity of all GBHA reactions fluctuated according to luminal calcium concentration. In calcium-repleted rats, strong GBHA reactions appeared in a narrow zone of lamina propria at the tip of the villus, overlaid, predominantly, with absorptive cells showing tubulo-vesicular GBHA reactions. These results suggest the existence of distinct types of absorptive epithelial cells in the rat duodenum, with respect to patterns of calcium localization which they display.

Electron microscopic cytochemical localization of a basolateral calcium adenosine triphosphatase in vitamin D replete chick enterocytes

A cytochemical technique for the electron microscopic localization of calcium adenosine triphosphatase (Ca-ATPase) was utilized to localize this enzyme in the enterocytes of rachitic and vitamin D-replete chicks. In animals treated with cholecalciferol (CC, vitamin D3), an electron-dense reaction product was located along the basolateral membranes of the absorptive cells within 72 hr after injection. Similarly, a reaction product was identified in association with the basolateral membranes within 24 hr after injection of 1,25-dihydroxycholecalciferol, the active metabolite of vitamin D. A microvillar reaction product was not seen in either of these two groups. Electron-dense reaction products were also seen in association with mitochondria and scattered throughout the cytoplasm of these enterocytes. The Ca-ATPase reaction product was dependent upon the presence of medium calcium and substrate (ATP), was inhibited by vanadate, and was heat labile. In the rachitic animals, a reaction product indicative of Ca-ATPase activity was not seen in association with either the basolateral membranes or the mitochondria. These data appear to indicate that an energy-requiring calcium-activated membrane pump plays a role in the flux of calcium across the enterocytes of the small intestine.

Effect of two potent calmodulin antagonists on calcium transport of brush border and basolateral vesicles from human duodenum

In the present in-vitro study we investigated the possible role of the calmodulin-antagonistic drugs loperamide and calmidazolium in the regulation of transepithelial Ca2+ transport of human duodenum. Brush border membrane vesicles and basolateral membrane vesicles were simultaneously prepared from surgically resected pieces of morphologically intact human duodenum with a modified Percoll-gradient centrifugation method. Brush border and basolateral membrane vesicles were characterized using enzyme marker analysis and electron microscopy: alkaline phosphatase was enriched 20-fold in brush border membrane vesicles, whereas [Na+ + K+]-stimulated adenosine triphosphatase was enriched 15-fold in basolateral membrane vesicles. Calmodulin activity was determined by a specific radioimmunoassay after solubilizing brush border and basolateral membrane vesicles in 1% Triton X-100. In basolateral membrane vesicles, we found no calmodulin activity. In brush border membrane vesicles calmodulin activity was impaired by 50% after pre-incubation with loperamide or calmidazolium. We measured calcium, sodium, D-glucose and D-mannitol uptake with a rapid filtration technique. Before the transport experiments, brush border and basolateral membrane vesicles were pre-incubated with 5 microM loperamide or 5 microM calmidazolium for 60 min at 5 degrees C. In drug-pretreated, brush border membrane vesicles calcium uptake was significantly reduced after 1 min incubation (-25% +/- 5%, P less than 0.05); this effect was completely reversed in the presence of 5 microM calmodulin. In basolateral membrane vesicles, we found two Ca2+ transport systems: (1) Na+/Ca2+ exchange and (2) ATP-dependent Ca2+ transport. In basolateral membrane vesicles loperamide had no effect. Calmidazolium had no effect on Na+/Ca2+ exchange, but significantly inhibited ATP-dependent Ca2+ transport. This effect could not be reversed by calmodulin.

ATP-dependent Ca2+ uptake in brush border membranes of human term placenta

In this communication, an ATP-dependent Ca2+ uptake was characterized in term human placental brush border membranes. Multiple freeze-thaw cycles greatly diminished uptake activity by 90 per cent, indicating an intravesicular accumulation of Ca2+. Kinetic studies indicate an apparent Km for Ca2+ of 0.22 +/- 0.04 microM and a Vmax of 441 +/- 137 pmoles/min/mg protein at 37 degrees C. The uptake was shown to have an optimum pH between 7.0 and 7.2, and was unaffected by the addition of oxalate, characteristics which are consistent with a plasma membrane origin of uptake. The process was temperature-dependent with a Q10 of 1.11 and was significantly inhibited (50 per cent) by 100 microM concentrations of trifluoperazine and vanadate. The characteristics of placental brush border Ca2+ uptake are similar to those of other Ca2+ uptake systems known to regulate intracellular calcium concentrations. By analogy, we suggest a similar role for this process in the maintenance of the low intracellular Ca2+ levels necessary for placental syncytiotrophoblast viability.

Exogenous ATP-stimulated calcium uptake in isolated rat intestinal epithelial cells

ATP in the extracellular medium is known to stimulate Ca uptake into avian intestinal epithelial cells. We have now demonstrated a similar effect of ATP in mammalian intestinal epithelial cells and have further characterized this effect. Exogenous ATP increased 45Ca uptake 2-6 fold in isolated rat small intestinal epithelial cells, with a maximal effect at 1 mM and an ED50 of 290 microM. A strict structural requirement for nucleotide-stimulated 45Ca uptake was observed. ADP was much less effective than ATP and gamma-thio-ATP, and 5'-AMP, cyclic AMP, adenosine, non-adenine nucleotides, non-hydrolyzable ATP analogs and ATP analogs with ring substitutions at the 8 position were inactive. Prenylamine (100 microM) completely inhibited ATP-stimulated 45Ca uptake, while verapamil (100 microM) had only a small effect. In the intact intestine, ATP increased short-circuit current (Isc) when added to the mucosal side of the tissue. This effect was reduced by 10 microM and abolished by 100 microM prenylamine. The effect of ATP on Isc was markedly reduced in Cl-free solutions and in reduced-Ca solutions. Serosal and mucosal addition of the nonhydrolyzable ATP analog, beta, gamma-methylene-ATP, and serosal addition of ATP had little or no effect on Isc. The similarities between the effects of ATP in isolated cells and in the intact intestine suggest that the effect of ATP on Isc may be at least partially mediated through stimulation of Ca uptake into the epithelial cells.

Calcium uptake into rat small intestinal brush border membrane vesicles: characterization of transmembrane calcium transport at short initial incubation times

Calcium transport into brush border vesicles from rat small intestine was investigated by determining uptake rates at very short incubation periods. At incubation times up to 1 second a linear relationship between calcium uptake and time was observed at free calcium concentrations ranging from 1 microM to 5 mM. At time points above 1 second calcium uptake deviates progressively from linearity. Several lines of evidences (EGTA-wash, dependency on membrane potential, temperature sensitivity and effect of the calcium ionophore A23187) suggest transmembrane transport rather than extravesicular binding of calcium as being responsible for calcium uptake. Saturation experiments performed under initial linear and curvilinear uptake conditions show a saturable transport component in the mu molar and only a tendency to saturate in the molar concentration range. It is concluded that uptake values far from equilibrium are characteristic for transmembrane flux of calcium. Transmembrane flux of calcium is mediated by multiple and potential-sensitive mechanisms.

Intestinal basolateral membrane Ca-ATPase activity with properties distinct from those of the Ca-pump

The Ca-pump in rat intestinal basolateral membranes had been studied previously as vesicular ATP-dependent Ca-uptake. In the present studies, Ca-stimulated ATP hydrolysis (Ca-ATPase activity) was measured and found to differ from the Ca-pump in having higher activity and being insensitive to vanadate. Whereas the pump was specific for ATP, hydrolytic activity was found with ATP, GTP or ADP but not with AMP or p-nitro-phenyl-phosphate. In contrast to Ca-pump activity, Ca-ATPase activities were similar for different intestinal segments, for duodenal villus/crypt cell-fractions and for vitamin D-deficient animals. Thus, as usually measured, intestinal basolateral membrane Ca-ATPase activity is not equivalent to the Ca-pump.

Permeability properties of isolated enterocytes from rat small intestine

Metabolic and permeability properties of enterocytes isolated by treatment of rat small intestine with hyaluronidase or EDTA were compared. No significant difference was observed in the ability of the two types of cell to produce lactate from glucose. However, while cells obtained with hyaluronidase accumulate alpha-methylglucoside, cells obtained with EDTA were unable to accumulate the sugar above the medium concentrations. When resuspended in a medium designed to resemble the intracellular medium, potentiometric measurements showed that cells obtained with hyaluronidase released Ca2+ to the medium while cells obtained with EDTA accumulated it. Using 45Ca transport assays, this was shown to be an ATP-dependent process, the accumulated 45Ca being totally released by the addition of the ionophore A23187. When cells obtained with EDTA were resuspended in a medium containing concentrations of free Ca2+ higher that 10 microM, the uptake was partially inhibited by sodium orthovanadate and also by oligomycin and antimycin. At free Ca2+ concentrations lower than 1 microM, the accumulation was inhibited up to 87% by sodium orthovanadate while mitochondrial inhibitors inhibited only 5%. Thus, it appears that during their preparation cells obtained with hyaluronidase retain their integrity while cells obtained with EDTA become permeable to Ca2+ and other ions. The usefulness of both types of preparation in metabolic and transport studies is discussed.

Subcellular distribution of ATP-dependent calcium transport in rat duodenal epithelium

Subcellular fractionation studies were performed to delineate plasma membrane and intracellular membrane populations which might be involved in intracellular Ca2+-homeostasis of rat small intestinal epithelial cells. After a low-speed supernatant fraction had been suspended in 5% sorbitol and subjected to equilibrium centrifugation in a zonal rotor, the Golgi and endoplasmic reticulum markers, galactosyltransferase and NADPH-cytochrome -c reductase, were concentrated in a density region designated Window II. The basal-lateral membrane marker (Na+-K+)-ATPase was concentrated in a higher-density region designated Window III. ATP-dependent Ca2+ transport was equally distributed between the two windows. Several membrane populations could be resolved from each window with good recovery of Ca2+-transport activity by a second density gradient centrifugation step. Second density gradient fractions were subjected to counter-current partitioning in an aqueous polymer two-phase system. Basal-lateral membranes, characterized by an 11-fold enrichment of (Na+-K+)-ATPase, contained ATP-dependent Ca2+-transport activity with Vmax = 3.7 nmol/mg per min and Km = 0.5 microM. A major Golgi-derived population exhibited Ca2+-transport activity with Vmax and Km values similar to those of the basal-lateral membranes. One membrane population, presumed to have been derived from the endoplasmic reticulum, contained Ca2+-transport activity with Vmax = 4 nmol/mg per min and Km = 0.5 microM. In addition to demonstrating that ATP-dependent Ca2+-transport activity has a complex distribution within enterocytes, this study raises the possibility that the basolateral plasma membranes might account for a relatively minor portion of the cell's Ca2+-pumping ability.

Preparation of subcellular membranes from rat intestinal scrapings or isolated cells. Different Ca2+ binding, nonesterified fatty acid levels, and lipolytic activity

Basolateral, brush-border, and Golgi-enriched subcellular membrane fractions, prepared from homogenates of rat small intestinal mucosa obtained by scraping, had unusually high concentrations of nonesterified fatty acids. These fatty acids appear to be responsible for the large amount of calcium binding, an effect that previously was shown to be reduced in vitamin D deficiency. In contrast, basolateral and Golgi membranes prepared from isolated cells had low levels of nonesterified fatty acids and calcium binding. Intermediate levels were found with isolated cells that were not put through the usual washing procedures. Addition to homogenates of scrapings of a lipase inhibitor, diethyl-p-nitrophenyl phosphate, reduced calcium binding and nonesterified fatty acids to levels similar to those in membranes prepared from isolated cells. Phospholipase A activity was low in homogenates of isolated cells and high in scrapings; this was reduced in intestinal scrapings of vitamin D-deficient rats. Ileal membranes had more calcium binding than duodenal membranes, and ileal homogenates also had greater phospholipase A activity. Preparation of subcellular membranes from rat intestinal scrapings can result in altered lipid composition, probably due to lipolytic enzyme activity; in addition to increasing cation-binding, these high levels of fatty acids may affect other membrane properties and enzyme function.

An endogenous inhibitor protein of synaptic plasma membrane (Ca2+ + Mg2+)-ATPase

An inhibitor protein of synaptic plasma membrane (Ca2+ + Mg2+)-ATPase was purified to apparent homogeneity from rat cerebrum by a molecular weight cut followed by chromatography of cytosol proteins with molecular weights between 10 000 and 3500 on DEAE-Sephadex at pH 5.2. The inhibitor could be partially inactivated by proteinases and dithiothreitol, but was heat-stable. Gel filtration gave a molecular weight of about 6000. Like the (Ca2+ + Mg2+)-ATPase inhibitor protein isolated from erythrocytes, the inhibitor from brain contains a characteristic high proportion of glutamic acid (36%) and glycine (37%) residues. Synaptic plasma membrane Mg2+-ATPase and microsomal membrane (Ca2+ + Mg2+)-ATPase did not respond to the inhibitor. Synaptic plasma membrane and erythrocyte membrane (Ca2+ + Mg2+)-ATPases, however, were affected. Inhibitory influence on synaptic membrane (Ca2+ + Mg2+)-ATPase was reversible, since inhibition could be relieved upon removal of inhibitor from saturable sites on the membrane. The inhibitor is not a calmodulin-binding protein, since the concentration of calmodulin for half-maximal activation of the ATPase was unaffected by its presence. Mode of inhibition of the (Ca2+ + Mg2+)-ATPase by the inhibitor was non-competitive.

Effects of trifluoperazine and verapamil on the hydro-osmotic response to antidiuretic hormone in the urinary bladder of the toad

To investigate the role of the intracellular calcium-calmodulin complex in the hydro-osmotic response to antidiuretic hormone (ADH), the effects of trifluoperazine (TFP), a well-established inhibitor of calmodulin-mediated functions, and of verapamil (V), a calcium entry blocker, were examined in the urinary bladder of the toad, a model for the late distal tubule and the collecting duct of the mammalian nephron. Preincubation of the hemibladders with TFP at serosal concentrations of 10(-5) and 10(-4) M was without effect on basal water flow but markedly reduced the maximal hydroosmotic response to ADH (50 mU/ml) in a dose-dependent manner as compared to control hemibladders (23.60 +/- 1.23 vs. 42.17 +/- 4.18 mg/min per hemibladder (10(-5) M TFP) and 5.43 +/- 0.59 vs. 52.50 +/- 4.67 mg/min per hemibladder (10(-4) M TFP). This inhibitory effect of TFP on the ADH-stimulated osmotic water flow persisted in the presence of naproxen (10(-5) M), a known inhibitor of prostaglandin synthesis. The hydro-osmotic response to cyclic adenosine 3',5' monophosphate (cAMP, 10(-3) M) was also significantly reduced in TFP-pretreated tissues (11.68 +/- 1.84 vs. 32.83 +/- 3.14 mg/min per hemibladder), suggesting a post-cAMP inhibitory effect of TFP. V (10(-4) M) had no effect on basal water flow but significantly reduced the hydro-osmotic effect of 50 mU/ml ADH (15.17 +/- 1.05 vs. 38.00 +/- 3.39 mg/min per hemibladder). In contrast, cAMP-stimulated osmotic water flow was significantly stimulated in V-treated tissues (48.07 +/- 1.95 vs. 27.13 +/- 1.50 mg/min per hemibladder).(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2-adrenergic receptor-mediated regulation of intestinal calcium transport

The role of alpha 2-adrenergic receptors in the regulation of intestinal Ca2+ transport was studied in vitro in rabbit proximal colon and distal ileum. Unidirectional mucosal-to-serosal and serosal-to-mucosal fluxes (Jm----s and Js----m) of 45Ca2+ were measured across isolated mucosal sheets bathed on both sides with identical Ringer solutions (1.25 mM Ca2+) and short-circuited. In the proximal colon, mucosal-to-epithelial influx (Jm----e) of Ca2+ was also determined. Proximal colonic mucosa secreted Ca2+ (in nmol X h-1 X cm-2, Jm----s = 12.1 and Js----m = 42.2). Epinephrine (10 microM) abolished Ca2+ secretion (Jm----s = 24.1 and Js----m = 24.9) but did not affect Jm----e (21.4 in controls and 23.6 after epinephrine). These effects were mediated by alpha 2-adrenergic receptors: yohimbine abolished the effect of epinephrine, but prazosin and propranolol did not; furthermore, clonidine mimicked the effects of epinephrine, whereas methoxamine and isoproterenol had no effect. Ileal mucosa did not transport Ca2+ spontaneously; epinephrine (10 microM) increased Jm----s in the ileum without affecting Js----m, thereby stimulating net Ca2+ absorption. The mechanism for these alpha 2-receptor-mediated effects was examined in proximal colon. The effect of epinephrine on Jm----s was dependent on serosal Na+ but was unaffected by ouabain or the Na+-Ca2+ exchange inhibitor, benzamil. The effect of epinephrine on Js----m was dependent on serosal Na+ and was blocked by both ouabain and benzamil. In the absence of epinephrine benzamil did not alter Jm----s or Js----m.(ABSTRACT TRUNCATED AT 250 WORDS)

Substrate requirements and subcellular distribution of calcium transport activities in brain membranes

Ca2+ transport activity in synaptosomal membranes has been identified as having two major components: Ca2+-stimulated ATP hydrolysis and ATP-dependent CA2+ uptake. Both processes exhibit similar affinities for Ca2+ and operate maximally under identical buffer conditions. Subcellular fractionation studies revealed the Ca2+/Mg2+ ATPase and ATP-dependent CA2+ uptake activities to be highest in synaptic plasma membrane fractions 1 and 2, with lesser activity in synaptic vesicles and mitochondria. Progressive treatment with Triton X-100 activated, then decreased Ca2+/Mg2+ ATPase, Mg2+ ATPase and Ca2+ ATPase. ATP-dependent Ca2+ uptake was progressively decreased by similar treatment with Triton X-100. These studies illustrate that Ca2+ ATPase and ATP-dependent Ca2+ uptake may provide two important mechanisms for buffering of cytosolic Ca2+ at the nerve terminal. These systems may function to rapidly sequester cytosolic Ca2+ following a rise during depolarization and then extrude Ca2+ from the terminal against a concentration gradient. This regulation of cytosolic Ca2+, represented by two processes of the type seen in other plasma membranes, may play critical roles in calcium homeostasis in nerve cells.

Complex subcellular distributions of enzymatic markers in intestinal epithelial cells

Current procedures for isolating intestinal epithelial cell surface and intracellular membranes are based on the assumption that each organelle is marked by some unique constituent. This assumption seemed inconsistent with the dynamic picture of subcellular organization emerging from studies of membrane turnover and recycling. Therefore, we have designed an alternative fractionation which is independent of a priori marker assignments. We subjected mucosal homogenates to a sequence of separations based on sedimentation coefficient, equilibrium density, and partitioning in aqueous polymer two-phase systems. The resulting distributions of protein and enzymatic markers define a total of 17 physically and biochemically distinct membrane populations. Among these are: basal-lateral membranes, with Na,K-ATPase enriched 21-fold; brush-border membranes, with alkaline phosphatase enriched as much as 38-fold; two populations apparently derived from the endoplasmic reticulum; a series of five populations believed to have been derived from the Golgi complex; and a series of five acid phosphatase-rich populations which we cannot identify unequivocally. Each of the five enzymatic markers we have followed is associated with a multiplicity of membrane populations. Basallateral, endoplasmic reticulum, and Golgi membranes contain alkaline phosphatase at the same specific activity as the initial homogenate. Similarly, Na,K-ATPase appears to be associated with Golgi, endoplasmic reticulum, and brush-border membranes at specific activities two- to seven-fold that of the initial homogenate.

Synthesis of ATP catalyzed by the (Ca2+ + Mg2+)-ATPase from erythrocyte ghosts. Energy conservation in plasma membranes

The (Ca2+ + Mg2+)-ATPase from erythrocyte ghosts catalyzed the hydrolysis of ATP together with the synthesis of ATP or ATP in equilibrium 'Pi exchange. The modulation of the ATPase reaction cycle was controlled by high- and low-affinity calcium-binding sites asymmetrically located on the enzyme. Calmodulin accelerated the reaction cycle in both directions, stimulating the overall turnover of the enzyme. Calcium transport was achieved utilizing optimal conditions for the expression of the ATP in equilibrium Pi exchange system.

The effects of vanadate on 45Ca exchange and enzyme secretion in the rat exocrine pancreas

The effects of vanadate on calcium homeostasis and enzyme secretion have been assessed in the incubated pancreas of young rats. Vanadate causes an acceleration of 45Ca efflux from pre-loaded uncinate glands; amylase release is reversibly increased for the duration of exposure to vanadate. Alkaline orthovanadate is most effective in eliciting these responses; its effects are greatly reduced at pH 7.4. However, changes in pH alone do not mimic these effects. Other vanadium oxides (meta-vanadate, vanadium pentoxide and vanadyl sulphate) are poor secretagogues. Alkaline ortho-, or meta-vanadate also causes an increased calcium uptake although this does not seem to be responsible for the observed secretory response. Vanadate is thought to stimulate pancreatic secretion by an effect on intracellular calcium store(s).