Phosphorylated intermediate of (Ca2+ + K+)-stimulated Mg2+-dependent transport ATPase in endoplasmic reticulum from rat pancreatic acinar cells

Calcium transport and calcium activated ATPase activity in microsomal vesicles of rat gastric mucosa

1. Microsomal and plasma membrane vesicles, isolated from rat gastric mucosa, were found to exhibit Ca(2+)-dependent ATPase activities of 14.1 +/- 1.4 and 7.8 +/- 1.1 mumol/mg/hr, respectively. The optimum conditions for the microsomal Ca(2+)-ATPase was pH 6-7, and required Mg2+, while divalent cation such as Cu2+, Zn2+, Fe2+, Ba2+ and Cd2+ had no significant effect. 2. As in the case of Ca2+, Mg(2+)-ATPase, the Ca2+ uptake activity of the microsomal membrane required Mg2+. Both processes were stimulated by submicro molar concentrations of Ca2+ and the apparent Km for Ca2+, Mg2+ ATPase and Ca2+ uptake activities were 0.06 microM and 0.02 microM, respectively. 3. Divalent cations Ba2+ and Fe2+, inhibited both microsomal activities, while Zn2+ and Cd2+ showed no effect on them. However, the monovalent cation K+ did not stimulate Ca2+, Mg(2+)-ATPase and Ca2+ uptake activities. 4. The Ca2+ pumping ATPase of rat gastric mucosal microsome cross-reacted with a monoclonal antibody (mAb-5F10) against the human erythrocyte Ca2+ pump. The apparent molecular weight of mucosal Ca2+ pump was 98 kDa. 5. Close relationship between the kinetic parameters of Ca2+, Mg(2+)-ATPase and Ca2+ uptake activities, and the cross reaction of 98 kDa protein of mucosal microsome with erythrocyte Ca2+ pump antibody, strongly suggest the expression of Ca2+ pump in rat gastric mucosa.

Demonstration of two isoforms of the SERCA-2b type Ca2+,Mg(2+)-ATPase in pancreatic endoplasmic reticulum

An antibody raised against a 12 amino acid peptide corresponding to the C-terminal sequence of the SERCA-2b Ca2+,Mg(2+)-ATPase precipitated Ca2+,Mg(2+)-ATPase activity from pancreatic rough ER. Thapsigargin and vanadate inhibited the activity with the same concentration-dependence as for native ER membranes. Partial purification of Ca2+,Mg(2+)-ATPase using Reactive Dye-agarose affinity chromatography resulted in activation of the enzyme, suggesting the presence of an endogenous inhibitor which was detached by binding to the Reactive Dye. Immunoblots and analysis of immunoprecipitated protein revealed two bands of molecular masses approx. 111 kDa and 97 kDa. It is concluded that pancreatic ER Ca2+,Mg(2+)-ATPase is of the SERCA-2b type and consists of two isoforms.

Ca(2+)-storage organelles

Intracellular Ca(2+)-storage organelles are found in virtually all eukaryotic cells. They play an important role in the regulation of the cytosolic free Ca2+ concentration and, thereby, in the regulation of cellular activity. Ca(2+)-storage organelles consist, in the simplest model of a Ca2+ pump, of a Ca(2+)-storage protein and a Ca(2+)-release channel. The primary structure of these functionally important proteins of Ca(2+)-storage organelles is similar in different cell types and conserved through evolution. In contrast, their spatial arrangement and, thus, the architecture of Ca(2+)-storage organelles may vary dramatically from one cell type to another.

Effect of GTP and Ca2+ on inositol 1,4,5-trisphosphate induced Ca2+ release from permeabilized rat exocrine pancreatic acinar cells

The effects of Ca2+ and GTP on the release of Ca2+ from the inositol 1,4,5-trisphosphate (IP3) sensitive Ca2+ compartment were investigated with digitonin permeabilized rat pancreatic acinar cells. The amount of Ca2+ released due to IP3 directly correlated with the amount of stored Ca2+ and was found to be inversely proportional to the medium free Ca2+ concentration. Ca2+ release induced by 0.18 microM IP3 was half maximally inhibited at 0.5 microM free Ca2+, i.e. at concentrations observed in the cytosol of pancreatic acinar cells. GTP did not cause Ca2+ release on its own, but a single addition of GTP (20 microM) abolished the apparent desensitization of the Ca2+ release which was observed during repeated IP3 applications. This effect of GTP was reversible. GTP gamma S could not replace GTP. Desensitization still occurred when GTP gamma S was added prior to GTP. The reported data indicate that GTP, stored Ca2+ and cytosolic free Ca2+ modulate the IP3 induced Ca2+ release.

Cytosolic free calcium spiking affected by intracellular pH change

The characteristics underlying cytosolic free calcium oscillation were evaluated by superfused dual wave-length microspectrofluorometry of fura-2-loaded single acinar cells from rat pancreas. Application of a physiological concentration of cholecystokinin octapeptide (CCK) (20 pM) induced a small basal increase in cytosolic free calcium concentration ([Ca2+]i) averaging 34 nM above the prestimulation level (69 nM) with superimposed repetitive Ca2+ spike oscillation. The oscillation amplitude averaged 121 nM above the basal increase in [Ca2+]i and occurred at a frequency of one pulse every 49 s. Although extracellular Ca2+ was required for maintenance of high frequency and amplitude of the spikes with increase in basal [Ca2+]i, the primary source utilized for oscillation was intracellular. The threshold of the peak [Ca2+]i amplitude for causing synchronized and same-sized oscillations was less than 300 nM. The [Ca2+]i oscillation was sensitive to intracellular pH (pHi) change. This is shown by the fact that the large pHi shift toward acidification (delta pHi decrease, 0.95) led to a basal increase in [Ca2+]i to the spike peak level with inhibiting Ca2+ oscillation. The pHi shift toward alkalinization (delta pHi increase, 0.33) led to a basal decrease in [Ca2+]i to the prestimulation level, possibly due to reuptake of Ca2+ into the Ca2+ stores, with inhibiting Ca2+ oscillation. Whereas extracellular pH (pHo) change had only minimal effects on Ca2+ oscillation (and/or Ca2+ release from intracellular stores), the extra-Ca2+ entry process, which was induced by higher concentrations of CCK, was totally inhibited by decreasing pHo from 7.4 to 6.5. Thus the major regulatory sites by which H+ affects Ca2+ oscillation are accessible from the intracellular space.

The role of dietary electrolytes in hypertension

The possible contribution of dietary electrolyte intake as a cause of or contributor to the development of hypertension has been intensively investigated for over 50 years. Evidence from various sources suggests a role for sodium-salt, chloride, calcium, and magnesium. In this article, we will review the evidence supporting a role for each of these electrolytes in human hypertension.

H+/Ca2+ exchange in rabbit renal cortical endosomes

We have examined the effect of second messengers on ATP-driven H+ transport in an H+ ATPase-bearing endosomal fraction isolated from rabbit renal cortex. cAMP (0.1 mM) had no effect on H+ transport. Acridine orange fluorescence in the presence of 0.5 mM Ca2+ (+1 mM EGTA) was 19 +/- 6% of control. Inhibition of ATP-driven H+ transport by Ca2+ was concentration dependent; 0.25 and 0.5 mM Ca2+ (+1 mM EGTA) inhibited acridine orange fluorescence by approximately 50 and approximately 80%, respectively. Ca2+ also produced a concentration-dependent increase in the rate of pH-gradient dissipation. Ca2+ did not affect ATP hydrolysis. ATP-dependent Br- uptake was virtually unchanged in the presence of 0.5 mM Ca2+ (+1 mM EGTA). These vesicles were also shown to transport Ca2+ in an ATP-dependent mode. Inositol 1,4,5-trisphosphate had no effect on ATP-dependent Ca2+ uptake. These results are consistent with the co-existence of an H+ ATPase and an H+/Ca2+ exchanger on these endosomes, the latter transport system using the H+ gradient to energize Ca2+ uptake. Attempts to demonstrate an H+/Ca2+ antiporter in the absence of ATP have been unsuccessful. Yet, when a pH gradient was established by preincubation with ATP and residual ATP was subsequently removed by hexokinase + glucose, stimulation of Ca2+ uptake could be demonstrated. A Ca2(+)-dependent increase in H+ permeability and an ATP-dependent Ca2+ uptake might have important implications for the regulation of vacuolar H+ ATPase activity as well as the homeostasis of cytosolic Ca2+ concentration.

Mechanism of the calcium pump in the endoplasmic reticulum of liver: phosphoproteins as reaction intermediates

Microsomal fractions, highly enriched with endoplasmic reticulum of rat and human liver exhibit Ca2+ uptake catalyzed by a Ca2+-pumping ATPase. The mechanism of Ca2+-translocation involves: (i) reversible Ca2+-dependent formation of an acyl-phosphoenzyme intermediate (Mr 116,000 to 118,000) with bound Ca2+, which in the reversed reaction can transphosphorylate its Pi to ADP to re-synthesize ATP; (ii) reversible transition of the ADP-reactive phosphoenzyme into an isomer without bound Ca2+, not further reactive to ADP; (iii) hydrolytic cleavage, stimulated by Mg2+, K+, and ATP of the ADP-unreactive phosphoenzyme with liberation of Pi. By analogy to a mechanism proposed for the Ca2+ pump of sarcoplasmic reticulum, the translocation of Ca2+ to and dissociation from the inner side of the membrane is suggested to occur by a conformational change, coupled with a decrease in Ca2+-affinity of the phosphoenzyme during its transition into the ADP-unreactive isomer. With CaATP as the effective substrate the reactions proceed normally but at a considerably slower rate.

Characterization of inositol 1,4,5-trisphosphate-sensitive (IsCaP) and -insensitive (IisCaP) nonmitochondrial Ca2+ pools in rat pancreatic acinar cells

We have measured Ca2+ uptake and Ca2+ release in isolated permeabilized pancreatic acinar cells and in isolated membrane vesicles of endoplasmic reticulum prepared from these cells. Ca2+ uptake into cells was monitored with a Ca2+ electrode, whereas Ca2+ uptake into membrane vesicles was measured with 45Ca2+. Using inhibitors of known action, such as the H+ ATPase inhibitors NBD-Cl and NEM, the Ca2+ ATPase inhibitor vanadate as well as the second messenger inositol 1,4,5-trisphosphate (IP3) and its analog inositol 1,4,5-trisphosphorothioate (IPS3), we could functionally differentiate two nonmitochondrial Ca2+ pools. Ca2+ uptake into the IP3-sensitive Ca2+ pool (IsCaP) occurs by a MgATP-dependent Ca2+ uptake mechanism that exchanges Ca2+ for H+ ions. In the absence of ATP Ca2+ uptake can occur to some extent at the expense of an H+ gradient that is established by a vacuolar-type MgATP-dependent H+ pump present in the same organelle. The other Ca2+ pool takes up Ca2+ by a vanadate-sensitive Ca2+ ATPase and is insensitive to IP3 (IisCaP). The IsCaP is filled at "higher" Ca2+ concentrations (approximately 10(-6) mol/liter) which may occur during stimulation. The low steady-state [Ca2+] of approximately 10(-7) mol/liter is adjusted by the IisCaP. It is speculated that both Ca2+ pools can communicate with each other, the possible mechanism of which, however, is at present unknown.

Characterization of MgATP-driven H+ uptake into a microsomal vesicle fraction from rat pancreatic acinar cells

In microsomal vesicles, as isolated from exocrine pancreas cells, MgATP-driven H+ transport was evaluated by measuring H+-dependent accumulation of acridine orange (AO). Active H+ uptake showed an absolute requirement for ATP with simple Michaelis-Menten kinetics (Km for ATP 0.43 mmol/liter) with a Hill coefficient of 0.99. H+ transport was maximal at an external pH of 6.7, generating an intravesicular pH of 4.8. MgATP-dependent H+ accumulation was abolished by protonophores, such as nigericin (10(-6) mol/liter) or CCCP (10(-5) mol/liter), and by inhibitors of nonmitochondrial H+ ATPases, such as NEM or NBD-Cl, at a concentration of 10(-5) mol/liter. Inhibitors of both mitochondrial and nonmitochondrial H+ pumps, such as DCCD (10(-5) mol/liter) or Dio 9 (0.25 mg/ml), reduced microsomal H+ transport by about 90%. Vanadate (2 x 10(-3) mol/liter), a blocker of those ATPases, which form a phosphorylated intermediate, did not inhibit H+ transport. The stilbene derivative DIDS (10(-4) mol/liter), which inhibits anion transport systems, abolished H+ transport completely. MgATP-dependent H+ transport was found to be anion dependent in the sequence Cl- greater than Br- greater than gluconate-; in the presence of SO2-4, CH3COO- or No-3, no H+ transport was observed. MgATP-dependent H+ accumulation was also cation dependent in the sequence K+ greater than Li+ greater than Na+ = choline+. As shown by dissipation experiments in the presence of different ion gradients and ionophores, both a Cl- and a K+ conductance, as well as a small H+ conductance, were found in the microsomal membranes. When membranes containing the H+ pump were further purified by Percoll gradient centrifugation (ninefold enrichment compared to homogenate), no correlation with markers for endoplasmic reticulum, mitochondria, plasma membranes, zymogen granules or Golgi membranes was found. The present data indicate that the H+ pump located in microsomes from rat exocrine pancreas is a vacuolar- or "V" -type H+ ATPase and has most similarities to that described in endoplasmic reticulum, Golgi apparatus or endosomes.

Anion channels in giant liposomes made of endoplasmic reticulum vesicles from rat exocrine pancreas

Using the method of dehydration and rehydration, rough endoplasmic reticulum (RER) vesicles, isolated by differential centrifugation, can be enlarged to giant liposomes with diameters ranging from 5 to 200 micron. Patch-clamp studies on these giant RER liposomes revealed the existence of a channel with a mean conductance of 260 +/- 7 pS (n = 23; 140 mmol/liter KCl on both sides). The channel is about four times more permeable for Cl- than for K+. Its activity is strongly voltage regulated. At low potentials (+/- 20 mV) the channel is predominantly in its open state with an open probability near 1.0, whereas it closes permanently at high positive and negative voltages (+/- 70 mV). The channel activity is not influenced by changing the free Ca2+ concentration from 1 mmol/liter to less than 10(-9) mol/liter on either side, and is also not affected by typical Cl- -channel blockers like diphenylamine-2-carboxylate (DPC, 1 mmol/liter) or 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS, 1 mmol/liter). Another chloride channel with a single-channel conductance of 79 +/- 6 pS (n = 4) was less frequently observed. In the potential range of -80 to +40 mV this channel displayed no voltage-dependent gating. We assume that these anion channels are involved in the maintenance of electroneutrality during Ca2+ uptake in the RER.

Ionic and substrate requirements of the high affinity calcium pumping ATPase in endoplasmic reticulum of pancreas

1. Calcium transport and ATPase activities were determined in microsomal vesicles from pancreatic tissue enriched in endoplasmic reticulum membranes. 2. Calcium transport and ATPase share the following properties: (i) magnesium was required with a K0.5 of 0.7 mM and maximal pumping ATPase activity at 5 mM Mg-ATP; (ii) at saturating magnesium concentrations, calcium increased ATP splitting activity up to three times with an apparent K0.5 close to 0.3 microM calcium; (iii) potassium stimulated the high calcium affinity Mg2+-dependent ATPase and calcium transport. 3. The properties of the calcium pumping system fulfil the cationic and substrate requirements from a physiological point of view.

Calcium concentration and amylase secretion in guinea pig pancreatic acini: interactions between carbachol, cholecystokinin octapeptide and the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate

The effects of the phorbol ester, 12-O-tetradecanoylphorbol 13-acetate (TPA) on amylase secretion and cytoplasmic free calcium concentration ([Ca2+]i) were investigated in dispersed guinea pig pancreatic acini. Carbachol evoked dose-dependent increases in amylase secretion and [Ca2+]i with half-maximal responses at 2.5 and 5 microM, respectively. Carbachol-induced calcium transients could be blocked by atropine. In the presence of a maximal effective dose of carbachol, cholecystokinin octapeptide caused no further increase in [Ca2+]i, suggesting that both agonists act on the same pool of trigger calcium. TPA (10(-9)-10(-6) M) stimulated amylase secretion with no change in [Ca2+]i. Maximum amylase secretion occurred at 0.5 microM TPA. Preincubation of acini in the presence of TPA resulted in a time- and dose-dependent inhibition (IC50 = 30 nM) of the carbachol-induced rise in [Ca2+]i, the maximal effect being observed within 3 min. The inactive phorbol ester, 4 alpha-phorbol 12,13-didecanoate was ineffective in inhibiting the carbachol-stimulated rise in [Ca2+]i. These findings suggest that, in addition to stimulating amylase secretion, probably through protein kinase C, TPA may also exert a negative feedback control over secretagogue-induced calcium transients.

Histochemical localization of adenosine triphosphatase activity in thymus: a light microscopical and ultrastructural study

Ultrastructural localization of ATPase at high pH in the presence of Ca2+ showed that activity in thymocyte precursors was stronger than in mature thymocytes. The activity was localized in the nuclear envelope, rough endoplasmic reticulum, Golgi apparatus and mitochondria. The difference in activity was attributed to a marked decrease in ATPase-containing organelles, mainly the endoplasmic reticulum in the mature thymocytes. This appears to be related to the proliferative activity of the cells rather than to the immunological maturity of the thymocytes. A very strong activity, also localized in the same organelles, was present in the macrophages and interdigitating cells which might have a secretory function and possibly contribute to thymocyte maturation. The Ca2+-ATPase activity in the nuclear envelope-endoplasmic reticulum system suggests that these may be the sites for storage and regulation of cytoplasmic calcium.

Ca2+-dependent protein phosphorylation associated with microsomal fraction of rat pancreas

Microsomes isolated from cat pancreas were incubated with [gamma-32P]ATP in the presence or absence of Ca2+. Following fractionation of phosphoproteins by sodium dodecyl sulfate-polyacrylamide gel electrophoresis a single microsomal protein with an apparent molecular mass of 77,000 dalton (77K) was found to be phosphorylated in a Ca2+-dependent mechanism. Maximal phosphate incorporation into the 77K protein was observed at 10(-6) mol/l [Ca2+] and was 4-fold higher than in the absence of Ca2+. The 77K phosphoprotein showed characteristic of a stable phosphoester rather than an acyl phosphate. Measurable phosphate incorporation into the 77K protein was noted 5 s following addition of [gamma-32P]ATP and reached maximum at 9-10th min. The lack of effect of exogenous cyclic AMP, cyclic AMP-dependent protein kinase, calmodulin, the calmodulin antagonist trifluoperazine, leupeptin and the suppression of phosphorylation by some phospholipid-interacting drugs suggested that the 77K protein is a substrate for cyclic AMP- and calmodulin-independent, Ca2+-activated phospholipid-sensitive kinase activity. Centrifugation of the pancreatic homogenate in a ficoll-sucrose density gradient indicated that both the 77K protein and enzyme were associated in a fraction enriched in rough endoplasmic reticulum.