Movement of endoplasmic reticulum in the living axon is distinct from other membranous vesicles in its rate, form, and sensitivity to microtubule inhibitors

The endoplasmic reticulum (ER) is the major membranous component present throughout the axon. Although other membranous structures such as synaptic vesicles are known to move via fast axonal transport, the dynamics of ER in the axon still remains unknown. To study the dynamics of ER in the axon, we have directly visualized the movement of two ER-specific membrane proteins, the sarcoplasmic/endoplasmic reticulum calcium-ATPase and the inositol 1,4,5-trisphosphate receptor, both of which were tagged with green fluorescence protein (GFP) and expressed in cultured chick dorsal root ganglion neurons. In contrast to GFP-tagged synaptophysin that moved as vesicles at 1 microm/sec predominantly in the anterograde direction in the typical style of fast axonal transport, the two ER proteins did not move in a discrete vesicular form. Their movement determined by the fluorescence recovery after photobleaching technique was bi-directional, 10-fold slower (approximately 0.1 microm/sec), and temperature-sensitive. The rate of movement of ER was also sensitive to low doses of vinblastine and nocodazole that did not affect the rate of synaptophysin-GFP, further suggesting that it is also distinct from the well-documented movement of membranous vesicles in its relation with microtubules.

Time course of reverse remodeling of the left ventricle during support with a left ventricular assist device

BACKGROUND

Support with a left ventricular assist device leads to normalization of left ventricular chamber geometry, regression of myocyte hypertrophy, alterations in left ventricular collagen content, and normalized expression of genes involved with excitation-contraction coupling in patients with heart failure. The objective of this study was to investigate the time course of these processes.

METHODS

Passive left ventricular pressure-volume relationships were obtained from explanted hearts of 19 patients with heart failure undergoing transplantation without left ventricular assist device support, 25 patients with heart failure supported before transplantation (duration of support ranging between 8 and 155 days), and 5 normal human hearts not suitable for transplantation. Left ventricular size was indexed by the volume at which left ventricular pressure reached 30 mm Hg. Left ventricular tissue samples were probed for sarcoplasmic endoreticular calcium adenosine triphosphatase 2a expression and processed for analysis of myocyte diameter and relative myocardial collagen content.

RESULTS

The volume at which left ventricular pressure reached 30 mm Hg was not significantly different between hearts without and with assist device support for less than 40 days. However, the volume at which left ventricular pressure reached 30 mm Hg in patients with assist devices supported for more than 40 days was significantly smaller than that of the hearts without assist devices but was larger than that of normal hearts. A similar pattern was observed for myocyte diameter. Sarcoplasmic endoreticular calcium adenosine triphosphatase 2a expression increased to normal levels by about 20 days of support with an assist device. Relative collagen content was significantly increased in hearts supported for more than 40 days.

CONCLUSION

Maximum structural reverse remodeling by left ventricular assist devices is complete by about 40 days. Molecular reverse remodeling of sarcoplasmic endoreticular calcium adenosine triphosphatase 2a expression is quicker, being complete by about 20 days.

Expression and functional characterization of a Plasmodium falciparum Ca2+-ATPase (PfATP4) belonging to a subclass unique to apicomplexan organisms

We have obtained a full-length P type ATPase sequence (PfATP4) encoded by Plasmodium falciparum and expressed PfATP4 in Xenopus laevis oocytes to study its function. Comparison of the hitherto incomplete open reading frame with other Ca(2+)-ATPase sequences reveals that PfATP4 differs significantly from previously defined categories. The Ca(2+)-dependent ATPase activity of PfATP4 is stimulated by a much broader range of [Ca(2+)](free) (3.2-320 micrometer) than are an avian SERCA1 pump or rabbit SERCA 1a (maximal activity < 10 micrometer). The activity of PfATP4 is resistant to inhibition by ouabain (200 micrometer) or thapsigargin (0.8 micrometer) but is inhibited by vanadate (1 mM) or cyclopiazonic acid (1 microM). We used a quantitative polymerase chain reaction to assay expression of mRNA encoding PfATP4 relative to that for beta-tubulin in synchronized asexual stages and found variable expression throughout the life cycle with a maximal 5-fold increase in meronts compared with ring stages. This analysis suggests that PfATP4 defines a novel subclass of Ca(2+)-ATPases unique to apicomplexan organisms and therefore offers potential as a drug target.

Effects of a 21-day expedition to 6,194 m on human skeletal muscle SR Ca2+-ATPase

We investigated the effects of a 21-day expedition to the summit of Mount Denali, Alaska (6,194 m) on selected Ca2+ sequestration properties of sarcoplasmic reticulum (SR) calcium pump in vastus lateralis muscle. Muscle samples were obtained by biopsy from 5 male climbers (peak oxygen consumption, VO2peak = 52.3 +/- 2.1 mL.kg(-1).min(-1)) approximately 7 days prior to (PRE) and 4 days following (POST) the expedition. A comparison of PRE versus POST measures of maximal Ca2+-ATPase activities (117 +/- 8.5 vs. 97.6 +/- 5.6 nmol.mg protein(-1).min(-1)) and Ca2+-uptake (204 +/- 15 vs. 161 +/- 11 nmol.mg protein(-1).min(-1)) measured in crude homogenates obtained from pre-exercised muscle, indicated only an effect (p < 0.05) of the expedition on Ca2+-uptake. The reduction in Ca2+-ATPase activity, representing 16.6%, was not significant (p = 0.089). The sarco endoplasmic reticulum calcium (SERCA)-ATPase isoforms, measured using Western blotting techniques, revealed a small reduction (p < 0.05) in SERCA 1 (-4.6 +/- 1.9%), but not in SERCA 2a (+2.0 +/- 1.4%). Prior to the expedition, both Ca2+-ATPase activity and Ca2+-uptake were reduced (p < 0.05) by approximately 34 and 18%, respectively, following 40 min of a two-step continuous cycling task (20 min at 59% VO2peak and 20 min at 74% VO2peak). The exercise-induced reduction in Ca2+-ATPase activity was independent of fiber type. Only in the case of Ca2+-uptake was a lower exercise response (p < 0.05) observed following the expedition, an effect that was due to the lower resting value. It is concluded that acclimatization as experienced during a mountaineering expedition induces changes in the properties of the SR Ca2+-pump, and particularly to Ca2+-sequestering function.

Ca-ATPase of human myometrium plasma membranes

We determined and characterized the Mg2+-dependent, Ca2+-stimulated ATPase (Ca-ATPase) activity in cell plasma membranes from the myometrium of pregnant women, and compared these characteristics to those of the active Ca2+-transport already demonstrated in this tissue. Similarly to the Ca2+-transport system, the Ca2+-ATPase is Mg2+-dependent, stimulated by calmodulin, and inhibited by vanadate. The Km for Ca2+ activation is 0.40 microM, very similar to that found for active calcium transport, i.e. 0.25 microM. Consequently, this Ca2+-ATPase can be responsible for the active calcium transport across the plasma membranes of smooth muscle cells.

Novel subcellular and molecular tools to study Ca(2+) transport mechanisms during the elusive moulting stages of crustaceans: flow cytometry and polyclonal antibodies

Our understanding of calcium homeostasis during the crustacean moulting cycle derives from research on intermoult animals that has been extrapolated to other stages. In terms of transepithelial Ca(2+) flux, the more interesting stages are those surrounding ecdysis since crustaceans experience a sizeable negative calcium balance in immediate premoult and a significant positive calcium balance in immediate postmoult. These stages are elusive in the sense that larger species such as lobsters are rarely captured at this time, and smaller species such as blue crabs and crayfish are seldom synchronized in their moulting cycle. The reductionist approaches employed in cellular physiology, such as vesicle techniques, employ pooling of fresh tissues from many organisms. Examination of the elusive moulting stages requires more sensitive approaches that can utilize tissue from an individual crustacean to characterize Ca(2+) pumps (Sarco/Endoplasmic Reticulum Ca(2+)-ATPase, SERCA; Plasma Membrane Ca(2+)-ATPase, PMCA) and the Na(+)/Ca(2+) eXchanger (NCX). An emerging subcellular approach described in this paper is to use flow cytometry as a technique to monitor Ca(2+) uptake into Fluo-3-loaded membrane vesicles. This paper illustrates the utility of this technique for measuring ATP-dependent Ca(2+) uptake into hepatopancreatic basolateral membrane vesicles. Obstacles to progress in molecular studies have not been limited by synchronization of moulting since tissue can be snap-frozen and collected from many animals over time. Here, the problem has been the lack of specific antibodies that hybridize with the Ca(2+) transporters of interest so that they can be localized within epithelia. In this paper, we introduce polyclonal antibodies raised in rabbits against crayfish SERCA, PMCA and NCX. Immunocytochemistry of SERCA in muscle, PMCA in antennal gland and NCX in heart confirms the specificity of the antibodies.

Adenoviral SERCA1a gene transfer to adult rat ventricular myocytes induces physiological changes in calcium handling

OBJECTIVE

We examined the functional consequences of expressing adult rabbit fast skeletal sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA1a) in isolated adult rat ventricular myocytes.

METHODS

Myocytes were infected with a recombinant adenovirus harboring SERCA1a. Then 2 days after myocyte infection, protein expression was estimated using Western blot and SDS-PAGE analysis. We also measured the ATP-dependent oxalate-facilitated Ca(2+) uptake of myocyte homogenates and monitored Ca(2+) transient in myocytes loaded with the Ca(2+) dye, indo-1.

RESULTS

SERCA1a gene expression resulted in a 36% increase in the total SERCA protein level in infected myocytes compared to controls (P<0.01), while SERCA2 and phospholamban levels did not change. This increase was associated with a 42% rise in SR Ca(2+) uptake (P<0.01), while tau (the time constant of Ca(2+) transient decay), and the time to peak fell by 32% (P<0.01) and 38% (P<0.001), respectively. Increasing the frequency of stimulation from 0.2 to 2 Hz decreased tau in both cell types (P<0.01). However, the decrease was much smaller in infected (P<0.01) than in uninfected cells (P<0.001). Isoproterenol (1 microM) further decreased tau in infected myocytes by 23% (P<0.05). In these cells, the diastolic [Ca(2+)](i) decreased by 50% (P<0.05) while the systolic [Ca(2+)](i) increased by 19% (P<0.05). No difference was found in the speed of SR Ca(2+) reloading after caffeine washout between the two cell types.

CONCLUSION

Adenovirus-mediated SERCA1a gene transfer to adult rat ventricular myocytes enhances SR Ca(2+) handling to a degree similar to that observed following physiological stimulation.

Detection and localization of a Ca2+-ATPase activity in Toxoplasma gondii

Toxoplasma gondii, the agent causing toxoplasmosis, is an obligate intracellular protozoan parasite. A calcium signal appears to be essential for intracellular transduction during the active process of host cell invasion. We have looked for a Ca2+-transport ATPase in tachyzoites and found Ca2+-ATPase activity (11-22 nmol Pi liberated/mg protein/min) in the tachyzoite membrane fraction. This ATP-dependent activity was stimulated by Ca2+ and Mg2+ ions and by calmodulin, and was inhibited by pump inhibitors (sodium orthovanadate or thapsigargin). We used cytochemistry and X-ray microanalysis of cerium phosphate precipitates and immunolabelling to find the Ca2+, Mg2+-ATPase. It was located mainly in the membrane complex, the conoid, nucleus, secretory organelles (rhoptries, dense granules) and in vesicles with a high calcium concentration. Thus, Toxoplasma gondii possesses Ca2+-pump ATPase (Ca2+, Mg2+-ATPase) as do eukaryotic cells.

Caveolae from canine airway smooth muscle contain the necessary components for a role in Ca(2+) handling

To explain that bronchial smooth muscle undergoes sustained agonist-induced contractions in a Ca(2+)-free medium, we hypothesized that caveolae in the plasma membrane (PM) contain protected Ca(2+). We isolated caveolae from canine tracheal smooth muscle by detergent treatment of PM-derived microsomes. Detergent-resistant membranes were enriched in caveolin-1, a specific marker for caveolae as well as for L-type Ca(2+) channels and Ca(2+) binding proteins (calsequestrin and calreticulin) as determined by Western blotting. Also, the PM Ca(2+) pump was present but not connexin 43 (a noncaveolae PM protein), the sarcoplasmic reticulum (SR) Ca(2+) pump, or the type 1 inositol 1,4, 5-trisphosphate receptor, supporting the idea that SR-derived membranes were not present. Antibodies to caveolin coimmunoprecipitated caveolin with calsequestrin or calreticulin. Thus some of the cellular calsequestrin and calreticulin associated with caveolin on the cytoplasmic face of each caveola. Immunohistochemistry of tracheal smooth muscle crysosections confirmed the localization of caveolin and the PM Ca(2+) pump to the cell periphery, whereas the SR Ca(2+) pump was located deeper in the cell. The presence of L-type Ca(2+) channels, the PM Ca(2+) pump, and the Ca(2+) bindng proteins calsequestrin and calreticulin in caveolin-enriched membranes supports caveola involvement in airway smooth muscle Ca(2+) handling.

Age-related appearance of tubular aggregates in the skeletal muscle of almost all male inbred mice

Tubular aggregates (TAs) which have been recently observed in a few mouse myopathies are identical to those described in human diseases. In this study we show that TAs are also found in the skeletal muscle of almost all normal inbred mice strains. In these inbred strains of mice the presence of TAs is shown to be related to both age and sex. Nine different muscles were stained with the modified Gomori trichrome method to reveal the general morphology of the muscles. Anti-SERCA1 ATPase was used to confirm that the TAs were in fact accumulations of sarcoplasmic reticulum and anti-MyHC IIB to demonstrate that these accumulations were found exclusively in the type IIB muscle fibers. An ultrastructural study confirmed the observations revealed by light microscopy that the TAs were derived from the sarcoplasmic reticulum. TAs were never observed in female inbred mice and were only found in type IIB glycolytic muscle fibers of male inbred mice. Therefore when analyzing the effect of genetic knock out and knock in experiments on the muscle phenotype of transgenic mice one should be aware that the presence of these aggregates is a non-specific phenomenon induced by inbreeding.

Differential lusitropic responsiveness to beta-adrenergic stimulation in rat atrial and ventricular cardiac myocytes

Cardiac myocyte relaxation is brought about mainly through Ca2+ uptake into the sarcoplasmic reticulum (SR) by a Ca2+-ATPase isoform, SERCA2a. Its activity is modulated by another protein, phospholamban (PLB). The levels of both proteins differ in some mammals between atrial and ventricular myocardium and this may lead to differences in relaxation, especially under stimulatory conditions. At a concentration of 100 nM, the beta-adrenergic agonist isoprenaline (ISO) accelerates the relaxation of rat papillary muscle more than that of the left atria (16.4 versus 4.0% hastening of time to 50% relaxation, respectively). Ventricular myocytes were 24.7% quicker in reaching 50% of their diastolic length after contraction when treated with ISO compared to atrial myocytes, which were only 3.6% faster. Ca2+ fluorescence transients were also abbreviated in ventricular compared to atrial myocytes exposed to ISO (41.9 versus 25.2% hastening of time to 50% peak Ca2+ respectively). Ca2+ uptake into ventricular SR vesicles was increased by 13% in the presence of protein kinase A while that into atrial SR vesicles remained unaffected. Western blotting analysis revealed 23% less SERCA2a protein, but 76% more PLB in ventricular compared to atrial tissue. We conclude that the distinct levels of SERCA2a and PLB in ventricular and atrial myocardium are responsible for the differential modulation of the relaxation process arising from beta-adrenergic stimulation in single rat atrial and ventricular myocytes.

Coordinate expression of Ca2+-ATPase slow-twitch isoform and of beta calmodulin-dependent protein kinase in phospholamban-deficient sarcoplasmic reticulum of rabbit masseter muscle

Modulation of sarcoplasmic reticulum (SR) Ca(2+) transport by endogenous calmodulin-dependent protein kinase II (CaM K II) involves covalent changes of regulatory protein phospholamban (PLB), as a common, but not the only mechanism, in limb slow-twitch muscles of certain mammalian species, such as the rabbit. Here, using immunofluorescent techniques in situ, and biochemical and immunological methods on the isolated SR, we have demonstrated that rabbit masseter, a muscle with a distinct embryological origin, lacks PLB. Accommodating embryological heterogeneity in the paradigm of neural-dependent expression of specific isogenes in skeletal muscle fibers, our results provide novel evidence for the differential expression in the SR of 72 kDa beta components of CaM K II, together with the expression of a slow-twitch sarcoendoplasmic reticulum Ca(2+)-ATPase isoform, both in limb muscle and in the masseter.

Ca++, Mg++-ATPase activity in insulin-dependent and non-insulin dependent diabetic Nigerians

A study of Ca++, Mg++-ATPase activity was carried out in normal (HHm) and diabetic Nigerians of both sexes with insulin-dependent diabetes mellitus (IDDM) and non-insulin dependent diabetes mellitus (NIDDM). The results showed that protein concentration of erythrocyte ghost membranes of healthy humans (HHm) was the highest when compared with protein concentrations of IDDM and NIDDM patients. The protein concentration was lowest in IDDM, while the value in NIDDM was between those of HHm and IDDM. The basal activities of erythrocyte Ca++-ATPase from IDDM and NIDDM were determined and were found to be significantly lower than that of HHm. The addition of calmodulin (CaM) 2 microg/ml stimulated the activity of the calcium pump in all the groups (IDDM, NIDDM and HHm). The effects of calcium (Ca++) and adenosine triphosphate (ATP) on the activity of the pump from each group were determined. Enzyme kinetics (Km and Vmax) revealed that the activity of Ca++, Mg++-ATPase was initiated by ATP in the presence of Ca++ in a dose-dependent manner. Calmodulin also enhanced the activity of the enzyme in the presence of Ca++ in all the groups, though activities in IDDM and NIDDM were significantly lower than in HHm. There was no significant difference in the activities between IDDM and NIDDM. These results suggest a defective calcium translocating mechanism in diabetic Nigerians.

Overexpression of SERCA2b in the heart leads to an increase in sarcoplasmic reticulum calcium transport function and increased cardiac contractility

The sarcoplasmic reticulum calcium ATPase SERCA2b is an alternate isoform encoded by the SERCA2 gene. SERCA2b is expressed ubiquitously and has a higher Ca(2+) affinity compared with SERCA2a. We made transgenic mice that overexpress the rat SERCA2b cDNA in the heart. SERCA2b mRNA level was approximately approximately 20-fold higher than endogenous SERCA2b mRNA in transgenic hearts. SERCA2b protein was increased 8-10-fold in the heart, whereas SERCA2a mRNA/protein level remained unchanged. Confocal microscopy showed that SERCA2b is localized preferentially around the T-tubules of the SR, whereas SERCA2a isoform is distributed both transversely and longitudinally in the SR membrane. Calcium-dependent calcium uptake measurements showed that the maximal velocity of Ca(2+) uptake was not changed, but the apparent pump affinity for Ca(2+) (K(0.5)) was increased in SERCA2b transgenic mice (0.199 +/- 0.011 micrometer) compared with wild-type control mice (0.269 +/- 0.012 micrometer, p < 0.01). Work-performing heart preparations showed that SERCA2b transgenic hearts had a higher rates of contraction and relaxation, shorter time to peak pressure and half-time for relaxation than wild-type hearts. These data show that SERCA2b is associated in a subcompartment within the sarcoplasmic reticulum of cardiac myocytes. Overexpression of SERCA2b leads to an increase in SR calcium transport function and increased cardiac contractility, suggesting that SERCA2b plays a highly specialized role in regulating the beat-to-beat contraction of the heart.

The sarco/endoplasmic reticulum calcium-ATPase 2b is an endoplasmic reticulum stress-inducible protein

The sarco/endoplasmic reticulum calcium-ATPase (SERCA) translocates Ca(2+) from the cytosol to the lumen of the endoplasmic reticulum. This Ca(2+) storage is important for cellular processes such as calcium signaling and endoplasmic reticulum (ER)-associated posttranslational protein modifications. We investigated the expression of the SERCA2 and SERCA3 isozymes in PC12 cells exposed to agents interfering with different aspects of the posttranslational protein processing within the ER, thereby activating the ER stress-induced unfolded protein response (UPR). All agents increased the SERCA2b mRNA level 3-4-fold, in parallel with increasing mRNA levels for the ER stress marker proteins BiP/GRP78 and CHOP/GADD153. In contrast, SERCA3 mRNA levels did not change. SERCA2b mRNA stability was not changed, indicating that the mechanism of its up-regulation was transcriptional, in accordance with the presence of ER stress response elements in the promoter region of the SERCA2 gene. SERCA2b was also increased at the protein level upon ER stress treatments. Induction of ER stress by tunicamycin, dithiothreitol, or l-azetidine 2-carboxylic acid did not result in depletion of ER calcium, showing that such depletion was not necessary for up-regulation of SERCA2b expression or UPR activation in general. We conclude that the SERCA2b expression can be controlled by the UPR pathway independently of ER Ca(2+) depletion.

Presynaptic proteins of ribbon synapses in the retina

The synapses of photoreceptors and bipolar cells in the retina are characterized ultrastructurally by the presence of an electron-dense bar, the synaptic ribbon, lying perpendicular to the plasma membrane at the active zone and extending about 0.5 microm into the cytoplasm. Hence, these synapses are known as ribbon synapses. All neurons that make ribbon synapses release neurotransmitter tonically. That is, neurotransmitter is released continuously from these neurons and the rate of release is modulated in response to graded changes in the membrane potential. This contrasts with action potential-driven, phasic release from other neurons. Similar to other synapses, neurotransmitter is released at ribbon synapses by the calcium-dependent exocytosis of synaptic vesicles. Most components of the molecular machinery governing transmitter release are conserved between ribbon and conventional synapses, but several differences that may be important determinants of tonic transmitter release have been identified in the retina by immunohistochemistry. For example, the presynaptic calcium channels of bipolar cells and photoreceptors are different from those elsewhere in the brain. Differences have also been found in the proteins involved in synaptic vesicle recruitment to the active zone and in synaptic vesicle fusion. These differences and others are discussed in terms of their implications for neurotransmitter release from photoreceptors and bipolar cells in the retina.

Thyroid hormone regulation of phospholamban phosphorylation in the rat heart

Thyroid hormone exerts predictable effects on the contractile performance of the heart in part by regulating the transcription of genes encoding specific calcium transporter proteins. In a rat model of hypothyroidism, left ventricular (LV) contractile function as measured by ejection fraction was decreased by 22% (P < 0.05), and this was returned to control values with T3 treatment. In confirmation of prior studies, LV phospholamban (PLB) protein content was significantly decreased by 25% and 40% compared with hypothyroid LV when the animals were treated with T3 at two doses, 2.5 and 7.0 microg/day, respectively. The ratio of sarcoplasmic reticulum calcium adenosine triphosphatase (SERCA2) to PLB protein content was thus increased by 171% and 207%, respectively (P < 0.01). Resolution of the phosphorylated PLB pentamers by SDS-PAGE showed that T3 infusion at 2.5 and 7.0 microg/day decreased (P < 0.001) the amount nonphosphorylated pentamers by 82% and 95%, respectively, in a dose-dependent manner. T3 treatment produced an increase in the proportion of highly phosphorylated PLB pentamers (more than five phosphates) when expressed as a fraction of total pentameric molecules (P < 0.05). Site-specific antibodies showed that the T3-induced increase in phosphorylated PLB pentamers was the result of an increase in both serine 16 and threonine 17 phosphorylation. We conclude that thyroid hormone, in addition to regulating the expression of cardiac PLB, is able to alter the degree of PLB phosphorylation, which correlates with enhancement of LV contractile function. These studies suggest that T3 may augment myocyte calcium cycling via changes in both cAMP- and calcium/calmodulin-dependent protein kinase activities.

Calcium-dependent ATPase unlike ecto-ATPase is located primarily on the luminal surface of brain endothelial cells

Numerous cytochemical studies have reported that calcium-activated adenosine triphosphatase (Ca2+-ATPase) is localized on the abluminal plasma membrane of mature brain endothelial cells. Since the effects of fixation and co-localization of ecto-ATPase have never been properly addressed, we investigated the influence of these parameters on Ca2+-ATPase localization in rat cerebral microvessel endothelium. Formaldehyde at 2% resulted in only abluminal staining while both luminal and abluminal surfaces were equally stained following 4% formaldehyde. Fixation with 2% formaldehyde plus 0.25% glutaraldehyde revealed more abluminal staining than luminal while 2% formaldehyde plus 0.5% glutaraldehyde produced vessels with staining similar to 4% and 2% formaldehyde plus 0.25% glutaraldehyde. The abluminal reaction appeared unaltered when ATP was replaced by GTP, CTP, UTP, ADP or when Ca2+ was replaced by Mg2+ or Mn2+ or p-chloromercuribenzoate included as inhibitor. But the luminal reaction was diminished. Contrary to previous reports, our results showed that Ca2+-specific ATPase is located more on the luminal surface while the abluminal reaction is primarily due to ecto-ATPase. The strong Ca2+-specific-ATPase luminal localization explains the stable Ca2+ gradient between blood and brain, and is not necessarily indicative of immature or pathological vessels as interpreted in the past.

Sarcoplasmic reticulum Ca(2+)/Calmodulin-dependent protein kinase is altered in heart failure

Although Ca(2+)/calmodulin-dependent protein kinase-II (CaMK) is known to phosphorylate different Ca(2+) cycling proteins in the cardiac sarcoplasmic reticulum (SR) and regulate its function, the status of CaMK in heart failure has not been investigated previously. In this study, we examined the hypothesis that changes in the CaMK-mediated phosphorylation of the SR Ca(2+) cycling proteins are associated with heart failure. For this purpose, heart failure in rats was induced by occluding the coronary artery for 8 weeks, and animals with >30% infarct of the left ventricle wall plus septum mass were used. Noninfarcted left ventricle was used for biochemical assessment; sham-operated animals served as control. A significant depression in SR Ca(2+) uptake and release activities was associated with a decrease in SR CaMK phosphorylation of the SR proteins, ryanodine receptor (RyR), Ca(2+) pump ATPase (SR/endoplasmic reticulum Ca(2+) ATPase [SERCA2a]), and phospholamban (PLB) in the failing heart. The SR protein contents for RyR, SERCA2a, and PLB were decreased in the failing hearts. Although the SR Ca(2+)/calmodulin-dependent CaMK activity, CaMK content, and CaMK autophosphorylation were depressed, the SR phosphatase activity was enhanced in the failing heart. On the other hand, the cAMP-dependent protein kinase-mediated phosphorylation of RyR and PLB was not affected in the failing heart. On the basis of these results, we conclude that alterations in SR CaMK-mediated phosphorylation may be partly responsible for impaired SR function in heart failure.

Distribution of plasma-membrane Ca2+ pump in mandibular condyles from growing and adult rabbits

Chondrocytes may control the mineralization of the extracellular matrix of condylar cartilage by several mechanisms including the release of microvesicles involved in the initial nucleation, the creation or modification of the local matrix to help propagate or restrict mineralization, and the regulation of the ionic environment at the calcifying foci within the matrix. The plasma membrane Ca2+-Mg2+ ATPase (Ca2+ pump) is known to play a part in the vectorial efflux of calcium in a variety of cells including chondrocytes. The purpose here was to study the distribution of Ca2+-pump protein in mandibular condyles from growing and adult rabbits, and compare the expression of that protein in progressively differentiating chondrocytes whose final stage is associated with a mineralized extracellular matrix. Ca2+-pump antigen was identified immunohistochemically in six growing and six adult rabbit mandibular condyles with a Ca2+ pump-specific monoclonal antibody. The presence of Ca2+-pump antigen was established in hypertrophic chondrocytes, and in osteoblasts and osteoclasts of subchondral bone. Slot-blot analysis of nitrocellulose-immobilized chondrocyte homogenates showed that the amount of Ca2+ pump in growing cartilage was more than twice that in adult cartilage (p < 0.05). The demonstration of Ca2+-pump antigen in the hypertrophic chondrocytes of growing rabbit condyles is consistent with a role for the plasma-membrane Ca2+ pump in the calcification of mandibular condylar cartilage.

Calcium regulation by skeletal muscle membranes of horses with recurrent exertional rhabdomyolysis

OBJECTIVE

To determine whether an alteration in calcium regulation by skeletal muscle sarcoplasmic reticulum, similar to known defects that cause malignant hyperthermia (MH), could be identified in membrane vesicles isolated from the muscles of Thoroughbreds with recurrent exertional rhabdomyolysis (RER).

SAMPLE POPULATION

Muscle biopsy specimens from 6 Thoroughbreds with RER and 6 healthy (control) horses.

PROCEDURES

RER was diagnosed on the basis of a history of > 3 episodes of exertional rhabdomyolysis confirmed by increases in serum creatine kinase (CK) activity. Skeletal muscle membrane vesicles, prepared by differential centrifugation of muscle tissue homogenates obtained from the horses, were characterized for sarcoplasmic reticulum (SR) activities, including the Ca2+ release rate for the ryanodine receptor-Ca2+ release channel, [3H]ryanodine binding activities, and rate of SR Ca2+-ATPase activity and its activation by Ca2+.

RESULTS

Time course of SR Ca2+-induced Ca2+ release and [3H]ryanodine binding to the ryanodine receptor after incubation with varying concentrations of ryanodine, caffeine, and ionized calcium did not differ between muscle membranes obtained from control and RER horses. Furthermore, the maximal rate of SR Ca2+-ATPase activity and its affinity for Ca2+ did not differ between muscle membranes from control horses and horses with RER.

CONCLUSIONS AND CLINICAL RELEVANCE

Despite clinical and physiologic similarities between RER and MH, we concluded that RER in Thoroughbreds does not resemble the SR ryanodine receptor defect responsible for MH and may represent a novel defect in muscle excitation-contraction coupling, calcium regulation, or contractility.

Endoplasmic reticulum Ca(2+)-ATPase inhibitors stimulate membrane guanylate cyclase in pancreatic acinar cells

In this study, we show that particulate guanylate cyclase (GC) is present in rat pancreatic acinar cells and is located both on plasma membrane and membranes of endoplasmic reticulum (ER). Western blot analysis indicates that the enzyme isoform GC-A is present in the acinar cell membranes. The specific inhibitors of ER Ca(2+)-ATPase thapsigargin, 2,5-di-(t-butyl)-1,4-hydroquinone (BHQ), and cyclopiazonic acid all activated particulate GC in pancreatic acini, both in membrane fractions and intact cells. These inhibitors also induced dephosphorylation of GC. Dose dependencies of Ca(2+)-ATPase inhibition and GC activation by BHQ are very similar, and those for thapsigargin partially overlap. ER Ca(2+)-ATPase and GC are coimmunoprecipitated both by antisera against membrane GC and by antisera against ER Ca(2+)-ATPase, suggesting a physical association between the two enzymes. The results suggest that thapsigargin and the other inhibitors act through ER Ca(2+)-ATPase to activate membrane GC in pancreatic acinar cells, although their direct effect on GC cannot be excluded.