Ultrastructural localization of the plasmalemmal calcium pump in cerebellar neurons

In a previous study, fluorescence labeling of a plasmalemmal ATPase protein with the 5F10 monoclonal antibody revealed prominent antigen in the cerebellar molecular layer surrounding the somata and dendrites of Purkinje cells. In the present study, this antibody labeled with silver enhanced nano-sized gold particles on semithin plastic sections revealed a clearly demarcated plasma membrane outlining the somata and entire dendritic arbors of Purkinje cells including their spines. Ultrastructural analysis of horseradish peroxidase preparations showed reaction product along the plasmalemma and extending on to the sub-plasmalemmal endoplasmic reticulum. In the granular layer, somata of granule cells were reactive, as were their dendritic extensions into glomeruli where reactive claws surrounded voids formed by mossy fiber rosettes. Somata and dendrites of cerebellar nuclear cells also had reactive zones that were limited to the plasma membrane and a narrow zone of the sub-plasmalemmal endoplasmic reticulum. Comparative labeling of this protein and P channel protein revealed similar plasmalemmal locations. This study shows that a specific calcium ATPase pump protein is located on the plasmalemma of certain types of cerebellar neurons. The ultrastructural distribution of calcium pump and P channel antibodies occurred in punctate sites along the plasma membrane of dendrites and spines of Purkinje cells. The close association between P-type calcium channels and the plasma membrane calcium pump is consistent with rapid extrusion of intracellular calcium from neurons endowed with large numbers of voltage-gated calcium channels.

Plasma membrane calcium pump isoform 4a has a longer calmodulin-binding domain than 4b

Alternate splicing of human plasma membrane calcium pump isoform 4 (hPMCA4) transcripts causes the expression of two variants, hPMCA4a and hPMCA4b, which have different downstream regulatory regions. Of the two, hPMCA4a has a lower affinity for calmodulin and a lower effective affinity for Ca2+ (Enyedi, A., Verma, A. K., Heim, R., Adamo, H. P., Filoteo, A. G., Strehler, E. E., and Penniston, J. T. (1994) J. Biol. Chem. 269, 41-43). Additional consequences of the alternate splice were studied by analyzing the characteristics of constructs (expressed in COS-1 cells) containing different portions of the carboxyl terminus of hPMCA4a. Our results show striking differences in the structure of the calmodulin-binding and autoinhibitory domains of the two variants. The calmodulin-binding region of hPMCA4b is a region of about 28 residues, whereas that of hPMCA4a is about 49 residues long and is probably interrupted by a region not involved in the binding. The autoinhibitory region of hPMCA4b (a part of the downstream region that keeps the molecule inactive in the absence of Ca2+-calmodulin) is divided between the 28-residue calmodulin-binding region and a downstream region, whereas in hPMCA4a, all of it is contained within the 49-residue calmodulin-binding region.

Mutants in the putative nucleotide-binding region of the plasma membrane Ca(2+)-pump. A reduction in activity due to slow dephosphorylation

Mutants of individual residues of the plasma membrane Ca(2+)-pump were made in the highly conserved region that (in related P-type ATPases) has been associated with nucleotide binding. Alteration of the strictly conserved Asp672 to Glu nearly eliminated the ability of the pump to transport Ca2+, while alteration at Val674, Arg675, and Lys686 reduced the activity. High levels of ATP (25 mM) did not overcome the reduced activity, indicating that it could not be due to a reduction in the affinity for ATP. Effects not directly related to ATP binding seemed to result from mutations in this area. For instance, the amount of phosphorylated intermediate in the most severely inhibited mutant, Asp672-->Glu, was nearly as high as that in the wild type, a much larger amount of phosphorylated intermediate than was expected from its low activity. However, the rate of decomposition of this intermediate was much slower than that of the wild type, indicating that the inhibition of this mutant resulted from an inhibition of the E approximately P-->E step in the enzyme cycle.

Two residues that may ligate Ca2+ in transmembrane domain six of the plasma membrane Ca(2+)-ATPase

In order to identify Ca2+ ligands in the putative transmembrane domain 6 of the plasma membrane Ca2+ pump, amino acids Asn879, Met882, Asp883, and Ser887 were singly altered. Asn879, Met882, and Asp883 were chosen because the corresponding amino acids have been proposed as Ca2+ ligands in the sarcoplasmic reticulum Ca2+ pump (Clarke, D. M., Loo, T. W., and MacLennan, D. H. (1990) J. Biol. Chem. 265, 6262-6267). For the alterations, a fully active truncated version of the pump was used, because the interaction of Ca2+ with the pump could be studied without interference from calmodulin binding. The mutants at Asn and Asp did not carry out ATP-supported Ca2+ uptake and formed no acylphosphate from [gamma-32P]ATP, suggesting that, like the corresponding amino acids in the sarcoplasmic reticulum Ca2+ pump, these two are Ca2+ ligands. However, all the mutants at the position of Met882 showed some activity. Indeed, the Met882--> Ile mutant was fully active at a saturating Ca2+ concentration and only the K1/2 for Ca2+ activation was shifted slightly upward. Converting the Met to Thr (which is the corresponding residue in the sarcoplasmic reticulum Ca2+ pump) reduced the activity to 20% of the wild type, further emphasizing the differences between the two Ca2+ pumps. The mutant Ser887--> Ala was expressed in greater amounts than, and had a specific activity about 50% higher than, the wild type, indicating that this serine also could not be a Ca2+ ligand and could not replace the missing Thr at position Met882.

Unique localization of mRNA encoding plasma membrane Ca2+ pump isoform 3 in rat thin descending loop of Henle

In our previous study of plasma membrane Ca2+ pump (PMCA) isozymes in the rat kidney, we found that the mRNA coding isoform PMCA3 was detected primarily in the outer medullary zone of rat kidney tissue. We now investigated the location of the mRNAs coding for isoforms PMCA3 and PMCA4 of Ca2+ pump in the nephron segments that are present in outer medullary parenchyma using the method of reverse transcription and polymerase chain reaction. In mRNA extracted from whole dissected outer medulla we detected mRNA encoding three splice variants (a, b, and c) of isoform PMCA3; isoform PMCA4 was found in the outer medulla almost exclusively as variant b. Analysis of mRNA from microdissected tubule segments show that proximal straight tubules (PST), medullary thick ascending limbs, outer medullary collecting ducts, and descending thin limb of Henle's loop (DTL) all contained mRNA for isoform 4b. In contrast, the mRNA encoding isoform 3 was detected exclusively in DTL and not in other nephron segments. The unique presence of isoform 3 in DTL is rather surprising, since the specific role of this nephron segment in vectorial Ca2+ transport or in intracellular Ca2+ signaling is not known. The data suggest that isoform 3 in cells of the DTL may have a hitherto unrecognized specific role in Ca2+ signaling or transport of Ca2+, which is distinct from the role of the isoforms of the PMCA in all other nephron segments.

Regulatory region of plasma membrane Ca2+ pump. 28 residues suffice to bind calmodulin but more are needed for full auto-inhibition of the activity

The Ca(2+)-pumping activity of constructs containing various portions of the putative 28-residue calmodulin-binding domain (C domain) of hPMCA4b were compared. As the length of the C domain in the pump increased, the pump's activity decreased and the ability of calmodulin to stimulate the activity increased. Study of the calmodulin dependence of activity showed that the construct containing all 28 residues of the C domain had a K1/2 for calmodulin equal to that of the complete molecule; the constructs containing less of the C domain interacted less strongly with calmodulin. On the other hand, incorporation of all 28 residues of the C domain did not decrease the activity of the pump (in the absence of calmodulin) as low as the activity of the complete molecule. This indicates that other segments of the molecule, further toward the COOH terminus, are also required for the degree of inhibition seen in the complete molecule.

The Ca2+ affinity of the plasma membrane Ca2+ pump is controlled by alternative splicing

The plasma membrane Ca2+ pump is a calmodulin-regulated P-type ATPase that is an essential element in controlling intracellular Ca2+ concentration. Studies on the gene structure of this pump have revealed an alternate splice option that changes the structure of the calmodulin-binding domain. This change in the structure of the enzyme results in a reduced calmodulin affinity. Tests of the enzyme's activity in the presence of a high calmodulin concentration, approximating that found inside living cells, show that this reduced calmodulin affinity causes a reduced apparent affinity of the enzyme for Ca2+. This shift in the Ca2+ activation occurs in a Ca2+ concentration range crucial to cellular function and is probably the physiologically important consequence of the alternate splice.

Immunocytochemical localization of the plasma membrane calcium pump, calbindin-D28k, and parvalbumin in Purkinje cells of avian and mammalian cerebellum

A monoclonal antibody produced against the human erythrocyte plasma membrane calcium pump (PMCA) was shown to react immunohistochemically with an epitope of the PMCA in avian and mammalian cerebellum. Western blot analysis of purified synaptosomes and homogenates from avian cerebellum revealed major immunoreactive proteins with molecular masses (130 kDa and 138 kDa) similar to those of purified erythrocyte PMCA. Dual-imaging confocal immunofluorescence microscopy of avian cerebellum showed that the PMCA antibody stained the periphery of the soma whereas calbindin-D28k was located in the cytosol. PMCA heavily stained the more distal dendrites of the Purkinje cells and, within the resolution of the fluorescence procedure, colocalized with calbindin-D28k. By using alkaline phosphatase-conjugated second antibody, PMCA was again localized to the peripheral soma, to a segmental pattern in dendrites, and to presumed spiny elements. The soma periphery and dendrites of Purkinje cells of the rat cerebellum were also prominently stained with anti-PMCA antibody and compared to parvalbumin localization. Dendritic depolarization and dendritic spiking behavior are significant Ca(2+)-dependent events of Purkinje cells. The rapid decline of intracellular free Ca2+ after the rapid rise time of Ca2+ transients is considered to be due to sequestration by Ca2+ buffers, uptake by intracellular stores, and Ca2+ extrusion mechanisms, the latter a function of PMCA now shown immunohistochemically to be a prominent feature of Purkinje cell dendrites.

Autoinhibitory domains of various Ca2+ transporters cross-react

Synthetic peptides having the sequence of the calmodulin-binding autoinhibitory domain of the Na+/Ca2+ exchanger (XIP) and of the plasma membrane Ca2+ pump (C28R2) inhibited the sarcoplasmic reticulum (SERCA) and plasma membrane (PMCA) Ca2+ pumps. XIP was nearly as effective in inhibiting both systems as C28R2, which had previously been considered to be the most powerful peptide inhibitor of PMCA. In contrast, calmodulin-binding peptides from non-Ca2+ transport proteins (calmodulin-dependent protein kinase II and myosin light chain kinase) showed only weak inhibition. The relative specificity and effectiveness of the peptides from the Ca2+ transport proteins, as well as the characteristics of their actions were similar in both SERCA and PMCA, suggesting a high degree of functional relatedness between these autoinhibitory regions. Secondary structure predictions show that the calmodulin-binding autoinhibitory domains of the Na+/Ca2+ exchanger and PMCA are predicted to form a beta structure which might be necessary for the observed cross-inhibition. The results also indicate that other domains of the Ca2+ transporters have common structural elements which interact with the autoinhibitory domains.

A highly active 120-kDa truncated mutant of the plasma membrane Ca2+ pump

A truncated mutant (hPMCA4b(ct120)) of the plasma membrane Ca2+ pump was expressed in COS-1 cells. The full-length pump (hPMCA4b) consisted of 1205 residues, and the mutant lacked 120 residues (including the 28-residue calmodulin-binding inhibitory domain) at the COOH terminus. To characterize this construct, Ca2+ transport was determined in a microsomal fraction. Phosphate was added to increase Ca2+ uptake, and specificity was enhanced by adding thapsigargin to inhibit the endoplasmic reticulum Ca2+ pump. The mutant showed a similar level of expression as hPMCA4b and a Ca2+ affinity and Ca2+ transport activity about equal to that of hPMCA4b when hPMCA4b was activated. Addition of the synthetic peptide C28R2, corresponding to the calmodulin binding region of the pump, inhibited the mutant and restored the non-activated state of the enzyme. In these respects, the truncated mutant acted like hPMCA4b, when hPMCA4b had been proteolyzed to cleave a bond between the calmodulin binding region and the NH2-terminal portion of the molecule. This indicates that the effects of proteolysis are due to removal of the COOH terminus and not to rearrangement of the two fragments. Since the truncated mutant was fully active and its tryptic digestion resulted in the appearance of the expected 81- and 76-kDa active fragments, we concluded that the COOH-terminal portion which is missing cannot be important in synthesis or proper folding of the enzyme.

Molecular cloning of a plasma membrane calcium pump from human osteoblasts

The osteoblast plays a critical role in bone formation, bone remodelling, bone matrix formation, and matrix calcification. To better understand the process of osteoblast-controlled bone formation, we determined the structure and isoform types of the plasma membrane calcium pump from normal human osteoblasts. A complementary DNA library from normal human osteoblasts was screened for plasma membrane calcium pump clones. Sequencing and analysis of cDNA clones revealed the presence of a 3986 base pair cDNA that encoded a 1220 amino acid protein that was similar to the human plasma membrane calcium pump isoform 1. Polyadenylated RNA from human osteoblast cells contains bands of RNA approximately 5050 and 6750 bases long. Reverse transcription of polyadenylated RNA from human osteoblasts followed by amplification of the RNA-DNA duplex with calcium pump isoform-specific primers revealed the presence of isoforms 1 and 2 of the calcium pump. Isoform 4 was not detected. We conclude that normal adult human osteoblasts contain a plasma membrane calcium pump that is similar to the human plasma membrane calcium pump isoform 1. It is likely that this pump plays an important role in the cell biology of the human osteoblast.

Vitamin D and adaptation to dietary calcium and phosphate deficiencies increase intestinal plasma membrane calcium pump gene expression

The effect of vitamin D and other variables on the synthesis of the chicken intestinal plasma membrane calcium pump (PMCA) mRNA was assessed. The DNA probe for Northern analysis was obtained by reverse transcription and PCR with intestinal poly(A)+ RNA, using two 20-mer oligonucleotide primers homologous to the 3' coding region of the human teratoma PMCA. An EcoRI restriction fragment of the PCR product was cloned into the pBluescript II KS(-) phagemid vector, and the chimeric plasmid was used to transform Escherichia coli. The amino acid sequence deduced from the nucleotide DNA sequence of the PCR product and the cloned DNA were 96% homologous with the teratoma sequence. Northern blots of intestinal poly(A)+ RNA with 32P-labeled DNA showed the presence of three major species of chicken PMCA mRNAs at about 6.6, 5.4, and 4.5 kb. Northern analysis with the chicken PMCA DNA indicated that repletion of vitamin D-deficient chickens with vitamin D increased PMCA mRNAs in the duodenum, jejunum, ileum, and colon. After injection of 1,25-dihydroxyvitamin D3 intravenously into vitamin D-deficient chickens, duodenal PMCA mRNA tended to increase by 2 hr, reached a maximum at about 16 hr, and returned to baseline levels at 48 hr. Adaptation of chickens to either a calcium- or phosphorus-deficient diet resulted in a 2- to 3-fold increase in duodenal PMCA mRNA. These results indicate that vitamin D and specific variables that affect calcium absorption through the vitamin D-endocrine system increase intestinal PMCA gene expression.

Expression of plasma membrane Ca++ pump epitopes parallels the progression of enamel and dentin mineralization in rat incisor

We investigated the expression of Ca++ pump epitopes during enamel and dentin mineralization in the rat incisor. Secretory and maturation ameloblasts were studied as well as odontoblasts, using a monoclonal antibody (5F10) against human erythrocyte plasma membrane Ca++, Mg(++)-ATPase. A progressive increase in staining intensity in ameloblasts and the odontoblasts was observed beginning with the onset of mineralization. The mainly membrane-related labeling of ameloblasts showed variable intensity depending on the stage of enamel formation, whereas that of the odontoblasts showed even intensity during continued dentinogenesis. Staining of papillary cells was evident only during enamel maturation. Western blot analysis of freeze-dried ameloblasts was also used to determine the molecular weight of the Ca++ pump epitopes as well as the distribution and relative concentration of epitopes at each stage. An immunoreactive band of MW 140 KD and lower molecular weight bands that are more intense in late than in early maturation were demonstrated. Our studies suggest that the expression of plasma membrane Ca++ pump parallels the progression of mineralization in rat incisor enamel and dentin.

Genetic linkage analysis in familial benign hypercalcemia using a candidate gene strategy. I. Studies in four families

Despite extensive study since the first report of familial benign hypercalcemia (FBH, or hypocalciuric hypercalcemia) in 1972, there is no evidence of the specific abnormal gene product. FBH is highly suitable for either a candidate gene or a reverse genetics approach to localizing the genetic abnormality, because it is inherited in an autosomal dominant pattern, is highly penetrant, does not affect survival, and can be diagnosed in families with readily available measurements. Importantly, several candidate genes have been cloned and mapped. Therefore, we collected blood samples and extracted leukocyte DNA from 94 members of 4 families with well documented FBH (44 affected, 45 unaffected, and 5 unclassifiable). We digested the DNA samples with various restriction endonucleases, conducted standard Southern blotting, and searched for restriction fragment length polymorphisms for the following candidate genes (probe names in parentheses): multiple endocrine neoplasia (MEN) type 1 (pMCMP.1, pHBI59, p3C7, and pTHH26), MEN 2a (MCK2 and cTB14.34), basic fibroblast growth factor (pHFL1-7), (Ca2+,Mg2+)ATPase isoform 4 (hPMCA4), membrane Na/Ca exchanger (cNC28 M-A), PTH (pPTH-LF), and calbindin-D28K (pSKCalb). In addition, we used the anonymous variable number tandem repeat marker pYNH24 to verify pedigree structures by excluding misinheritances. Data were analyzed using the Linkage program. For none of the genes was there significant linkage with the FBH trait; logarithm of odds scores ranged from -1.3 to -26.0 at a recombination fraction of 0.001, and from 0.6 to -5.6 at a recombination fraction of 0.10. We conclude that FBH is unrelated to the MEN syndromes and is not caused by mutations in any of the calcium-regulating or -binding proteins or growth factors studied thus far.

Overexpression of the erythrocyte plasma membrane Ca2+ pump in COS-1 cells

A full-length cDNA corresponding to the hPMCA4 plasma membrane Ca2+ pump was assembled and expressed in COS-1 cells. The original sequence of hPMCA4 gave a very low expression. The mutation of the initiation translation site of this sequence to the consensus A/G-X-X-AUG-G increased the production of the protein. The Ca2+ pump activity in transfected cells was 1.5-3.5-fold higher than in controls. The Ca(2+)-dependence and the calmodulin stimulation of hPMCA4 expressed in COS-1 cells were comparable with those of the erythrocyte Ca2+ pump. Immunohistochemistry experiments showed that most of the expressed protein remained in intracellular membranes. Possible explanations for this targeting of the pump are discussed.

Use of expression mutants and monoclonal antibodies to map the erythrocyte Ca2+ pump

Deletion and truncation mutants of the human erythrocyte Ca2+ pump (hPMCA4b) were expressed in COS-1 cells. The reactivity patterns of these mutants with seven monoclonal antibodies were examined. Of the seven, six (JA9, JA3, 1G4, 4A4, 3E10 and 5F10) react from the cytoplasmic side. JA9 and JA3 reacted near the NH2 terminus and the COOH terminus of the molecule, respectively. 5F10 and 3E10 recognized portions of the large hydrophilic region in the middle of the protein. The epitopes of 1G4 and 4A4 were discontinuous and included residues from the long hydrophilic domain and residues between the proposed transmembrane domains M2 and M3. Antibody 1B10, which reacts from the extracellular side, recognized the COOH-terminal half of the molecule. These results show that the NH2 terminus, the COOH terminus, the region between M2 and M3, and the large hydrophilic region are all on the cytoplasmic side. This means that there are an even number of membrane crossings in both the NH2-terminal and the COOH-terminal halves. Between residues 75 and 300 there must be at least two membrane crossings, and there are at least two membrane crossings in the COOH-terminal half of the molecule.

Ca2+ accumulation and loss by aberrant endocytic vesicles in sickle erythrocytes

Sickle cells contain internal vesicles which accumulate Ca2+. As shown here, the membrane enclosing the vesicles contains the plasma membrane Ca(2+)-ATPase, or Ca2+ pump, as judged by staining with an antibody directed against the protein. Moreover, the number of cells containing such vesicles increases upon deoxygenation. These findings argue strongly that the vesicles arise by endocytosis from the plasma membrane, and explain how they accumulate Ca2+. When sickle cells are depleted of ATP, Ca2+ is lost from the vesicles, as judged by the disappearance of staining with the Ca2+/membrane probe chlortetracycline (CTC), without a corresponding loss of antibody staining. This loss of Ca2+ can be inhibited by nitrendipine, a Ca2+ channel blocker. These results suggest that the vesicle membrane allows outward passage of Ca2+ by a nitrendipine-sensitive pathway, which can be overcome by the inward-directed activity of the Ca2+ pump of the vesicle membrane. If so, the Ca2+ which vesicles contain is in dynamic equilibrium with the cytoplasm of the sickle erythrocyte.

Localization of mRNAs coding for isozymes of plasma membrane Ca(2+)-ATPase pump in rat kidney

We have studied localization of mRNAs coding isozymes of rat plasma membrane Ca(2+)-adenosinetriphosphatase pump (rPMCA) in the rat kidney, with use of reverse transcription (RT) with subsequent amplification by polymerase chain reaction (PCR). When zones of the kidney were separated by macrodissection, a large amount of mRNA coding isozyme rPMCA1 was found in all zones; mRNA for isozyme rPMCA2 was abundant in cortex and in outer medulla, and mRNA for isozyme rPMCA3 was prominent in outer medulla. The mRNAs were analyzed in microdissected cortical nephron segments by use of RT-PCR approach described previously [T. Moriyama, H. R. Murphy, B. M. Martin, and A. Garcia-Perez. Am. J. Physiol. 258 (Renal Fluid Electrolyte Physiol. 27): F1470-F1474, 1990]. We detected mRNA for isozyme rPMCA2 in microdissected distal convoluted tubules (DCT) and in cortical thick ascending limbs (CTAL) and, less consistently, also in proximal convoluted tubule and in glomeruli. The mRNA for isozyme rPMCA1 was abundant in glomeruli but was absent in all examined cortical tubular segments. Our results document that mRNAs for all three major isozymes of rPMCA are present and show a unique distribution in the three major zones of rat renal parenchyma. Specific mRNA coding for rPMCA2 was detected in cortical tubules, namely in CTAL and DCT, whereas mRNA coding isozyme rPMCA1 was found in glomeruli. We suggest that isozyme rPMCA2 might be specifically related to epithelial cells and their function, whereas rPMCA1 is probably a component of nonepithelial cells including these in glomeruli.

The lipid-binding peptide from the plasma membrane Ca2+ pump binds calmodulin, and the primary calmodulin-binding domain interacts with lipid

Peptide G25 (KKAVKVPKKEKSVLQGKLTRLAVQI) representing the putative lipid-binding region (G region) of the erythrocyte Ca2+ pump was synthesized. This peptide interacted with acidic lipids, as shown by the increase in size of phosphatidylserine liposomes in its presence. This lipid interaction is consistent with the previous evidence suggesting that the portion of the pump from which this peptide was taken is responsible for the activation of the pump by acidic lipid. G25 also bound to calmodulin, as was shown by its cause of a shift in the fluorescence of 5-dimethylamino naphthalene-1-sulfonyl- (dansyl)-calmodulin, and by its competition with Ca2+ pump for calmodulin. Its Kd for dansyl-calmodulin was much higher (0.8 microM) than that of the peptides representing the primary calmodulin-binding region (C region) of the Ca2+ pump. Although the presence of the G region provided the possibility of a second calmodulin-binding site, activation of the pump by calmodulin always could be fitted by simple saturation kinetics. The calmodulin-binding peptide from the C region of the pump, C28R2, also interacted with lipid with even greater effectiveness than G25. When the C region of the pump was saturated with calmodulin, acidic lipid activation of the pump followed simple saturation kinetics. However, when calmodulin was omitted, a higher concentration of lipid was needed for saturation and the kinetics became complex. The data are consistent with the idea that calmodulin activates the pump only by interaction at the C region, but that acidic lipid activates by interaction at both of the C and G regions.

New Ca2+ pump isoforms generated by alternative splicing of rPMCA2 mRNA

Alternative splices capable of generating proteins with altered functions were found (by PCR) in isoform 2 of the rat plasma membrane Ca2+ pump. These splices were concentrated in two hypervariable regions. One of these regions, near the N-terminus and the lipid-binding region, could be altered by the insertion of either or both of inserts x and y. Insertion of both x and y would add 45 amino acids to the molecule. The y insert causes the appearance of a rather hydrophobic stretch of amino acids in the middle of a highly polar region. The second variable region begins in the middle of the calmodulin-binding domain. Insertion of 229 nucleotides at this point of the message converts the b form to the a form, which has an altered (and shorter) C-terminus. The calmodulin-binding domain of this shortened form has a less basic character, which would decrease the affinity for calmodulin. The b form of isoenzyme 2 contains relatively weak protein kinase A substrate sequences, such as KQNSS and KNNS. These sequences are eliminated in form a, and a strongly activated kinase substrate sequence, RRQSS, appears in a different place. Different tissues use different combinations of alternative splices, with heart and brain showing the greatest diversity.

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.

Intestinal calcium transport and calcium extrusion processes at the basolateral membrane

The intestinal absorption of calcium has been proposed to occur by the transcellular transfer of Ca2+ through the enterocyte proper and between the cells of the intestinal epithelium, i.e., the paracellular path. Attention in this report is given to the transcellular models of Ca2+ absorption and, more specifically, the Ca2+ extrusion events occurring at the basolateral membrane. These extrusion processes include the operation of an ATP-dependent Ca2+ pump and a Na+/Ca2+ exchanger, as well as exocytosis as the terminal event in a proposed vesicular transport mechanism. Evidence for the presence of an ATP-dependent Ca2+ pump at the basolateral membrane is documented and illustrated with biochemical and immunological data from studies on the avian intestinal basolateral membrane. As shown immunohistochemically, the Ca2+ pump was primarily localized on the enterocyte basolateral membrane. The ATP-dependency and vitamin D enhancement of Ca2+ uptake by isolated basolateral membrane vesicles are shown. Western blot analysis of intestinal mucosa, by using a monoclonal antibody produced against the erythrocyte Ca2+ pump, indicated that the number of pump units is increased by 1,25-dihydroxycholecalciferol. The possible involvement of calbindin-D28K as a direct stimulator of the Ca2+ pump is discussed, and the quantitative relationship between Ca2+ transport rates and Ca2+ pumping activity has been estimated. Information related to the basolateral membrane Na+/Ca2+ exchanger and the vesicular transport model of Ca2+ absorption is also briefly reviewed.

Ca2+ or Mg2+ nucleotide phosphohydrolases in myometrium: two ecto-enzymes

A high level of Ca2+ or Mg2+ nucleotide phosphohydrolase activity is present on the outside surface of intact myometrial cells and is also observed in the isolated plasma membranes. About half of this activity is labile while the remainder is stable. The characteristics of the activities suggest the presence of at least two different ecto-enzymes. The stable component (Km for Ca2+ about 0.1 mM) accepts XTP or XDP as substrate, is not inhibited by p-chloromercuriphenylsulfonate or inorganic phosphate, but is inhibited by 20 mM NaN3. The labile component (Km for Ca2+ nearly 1 mM) cleaves XTP but not XDP, and is inhibited by p-chloromercuriphenyl-sulfonate and inorganic phosphate, but not by NaN3. The activity of the labile component can be restored by removing the cells from the incubation medium and resuspending them in fresh medium. This suggests that the 'lability' is due to product inhibition, probably by inorganic orthophosphate. While the Ca2+ pump of myometrial plasma membranes was inhibited by 0.1 microM oxytocin, these ecto-enzymes were unaffected by oxytocin concentrations up to 10 microM. Because of its high activity and rapid inactivation by product inhibition, the labile enzyme may be involved in the regulation of purinergic receptors.