Ca2+-induced redistribution of Ca2+/calmodulin-dependent protein kinase II associated with an endoplasmic reticulum stress response in vascular smooth muscle

The relation between CaM kinase II activity and high Ca²⁺-mediated stress responses was studied in cultured vascular smooth muscle cells. Treatment with ionomycin (1 M) for 5 min caused a significant loss of CaM kinase II activity in whole cell homegenates and prominent vesiculation of the endoplasmic reticulum (ER). Similar losses of CaM kinase II activity were observed in the soluble lysate as assessed by activity measurements and Western blotting. Examination of the post-lysate particulate fraction showed that the loss of CaM kinase II from the soluble lysate was accompanied by a redistribution of CaM kinase II to this fraction. The ionomycin-mediated response was limited to this concentration (1 M); lower concentrations of ionomycin as well as stimulation with angiotensin II (1 M) or ATP (100 M) did not cause a shift in CaM kinase II distribution. Treatment with neither the CaM kinase II inhibitor KN-93 nor the phosphatase inhibitor okadaic acid altered the ionomycin-induced redistribution indicating that CaM kinase II activation and/or phosphorylation was not part of the mechanism. The response, however, was eliminated when the cells were treated in Ca²⁺-free medium. Washout of ionomycin led to only a partial restoration of the kinase activity in the soluble fraction after 10 min. Immunofluorescence microscopy of resting cells indicated colocalization of antibodies to CaM kinase II and an ER protein marker. ER vesiculation induced by ionomycin coincided with a parallel redistribution of CaM kinase II and ER marker proteins. These data link ionomycin-induced ER restructuring to a progressive redistribution of CaM kinase II protein to an insoluble particulate fraction and loss of cellular CaM kinase II activity. We propose that redistribution of CaM kinase II and loss of cellular activity are components of a common Ca²⁺-overload induced cellular stress response in cells.     

Ca2+-overload inhibits the cardiac SR Ca2+-calmodulin protein kinase activity

There is increasing evidence to suggest that Ca2+-calmodulin dependent protein kinase (CaMK) regulates the sarcoplasmic reticulum (SR) function and thus plays an important role in modulating the cardiac performance. Because intracellular Ca2+-overload is an important factor underlying cardiac dysfunction in a heart disease, its effect on SR CaMK was examined in the isolated rat heart preparations. Ca2+-depletion for 5 min followed by Ca2+-repletion for 30 min, which is known to produce intracellular Ca2+-overload, was observed to attenuate cardiac function as well as SR Ca2+-uptake and Ca2+-release activities. Attenuated SR function in the heart was associated with reduced CaMK phosphorylation of the SR Ca2+-cycling proteins such as Ca2+-release channel, Ca2+-pump ATPase, and phospholamban, decreased CaMK activity, and depressed levels of SR Ca2+-cycling proteins. These results indicate that alterations in cardiac performance and SR function following the occurrence of intracellular Ca2+-overload may partly be due to changes in the SR CaMK activity.     

Ca2+ oscillations stimulate an ATP increase during fertilization of mouse eggs

Mammalian eggs and embryos rely upon mitochondrial ATP production to survive and proceed through preimplantation development. Ca(2+) oscillations at fertilization have been shown to cause a reduction of mitochondrial NAD+ and flavoproteins, suggesting they might also cause changes in cytosolic ATP levels. Here, we have monitored intracellular Ca(2+) and ATP levels in fertilizing mouse eggs by imaging the fluorescence of a Ca(2+) dye and luminescence of firefly luciferase. At fertilization an initial increase in ATP levels occurs with the first Ca(2+) transient, with a second increase occurring about 1 h later. The increase in cytosolic ATP was estimated to be from a prefertilization concentration of 1.9 mM to a peak value of 3 mM. ATP levels returned to prefertilization values as the Ca(2+) oscillations terminated. An increase in ATP also occurred with other stimuli that increase Ca(2+) and it was blocked when Ca(2+) oscillations were inhibited by BAPTA injection. Additionally, an ATP increase was not seen when eggs were activated by cycloheximide, which does not cause a Ca(2+) increase. These data suggest that mammalian fertilization is associated with a sudden but transient increase in cytosolic ATP and that Ca(2+) oscillations are both necessary and sufficient to cause this increase in ATP levels.     

Heme-induced Trypanosoma cruzi proliferation is mediated by CaM kinase II

Trypanosoma cruzi, the etiologic agent of Chagas disease, is transmitted through triatomine vectors during their blood-meal on vertebrate hosts. These hematophagous insects usually ingest approximately 10 mM of heme bound to hemoglobin in a single meal. Blood forms of the parasite are transformed into epimastigotes in the crop which initiates a few hours after parasite ingestion. In a previous work, we investigated the role of heme in parasite cell proliferation and showed that the addition of heme significantly increased parasite proliferation in a dose-dependent manner [1]. To investigate whether the heme effect is mediated by protein kinase signalling pathways, parasite proliferation was evaluated in the presence of several protein kinase (PK) inhibitors. We found that only KN-93, a classical inhibitor of calcium-calmodulin-dependent kinases (CaMKs), blocked heme-induced cell proliferation. KN-92, an inactive analogue of KN-93, was not able to block this effect. A T. cruzi CaMKII homologue is most likely the main enzyme involved in this process since parasite proliferation was also blocked when Myr-AIP, an inhibitory peptide for mammalian CaMKII, was included in the cell proliferation assay. Moreover, CaMK activity increased in parasite cells with the addition of heme as shown by immunological and biochemical assays. In conclusion, the present results are the first strong indications that CaMKII is involved in the heme-induced cell signalling pathway that mediates parasite proliferation.     

Specificity of ligand binding to transport sites: Ca2+ binding to the Ca2+ transport ATPase and its dependence on H+ and Mg2+

Ligand binding to transport sites constitutes the initial step in the catalytic cycle of transport ATPases. Here, we consider the well characterized Ca²⁺ ATPase of sarcoplasmic reticulum (SERCA) and describe a series of Ca²⁺ binding isotherms obtained by equilibrium measurements in the presence of various H⁺ and Mg²⁺ concentrations. We subject the isotherms to statistical mechanics analysis, using a model based on a minimal number of mechanistic steps. The analysis allows satisfactory fits and yields information on occupancy of the specific Ca²⁺ sites under various conditions. It also provides a fundamental method for analysis of binding specificity to transport sites under equilibrium conditions that lead to tightly coupled catalytic activation.     

The involvement of Ca2+ in the Ca2+-effect on Photosystem-II oxygen evolution

A number of recent reports have concluded that Ca²⁺ is not released by treatments which are usually thought to induce the depletion of Ca²⁺. Consequently, it was proposed that the Ca²⁺ demand was not related to a specific rôle for Ca²⁺ in Photosystem-II oxygen evolution. In this letter, we scrutinize the data behind these conclusions and argue that, based on these data, it is premature to question the view that intrinsic Ca²⁺ is actually being released.     

Involvement of CaM kinase II in gonadotropin-releasing hormone-induced activation of MAP kinase in cultured hypothalamic neurons

Gonadotropin-releasing hormone (GnRH) is secreted from hypothalamic GnRH neurons. There is accumulating evidence that GnRH neurons have GnRH receptors and that the autocrine action of GnRH activates MAP kinase. In this study, we found that KN93, an inhibitor of Ca(2+)/calmodulin-dependent protein kinases (CaM kinases), inhibited the GnRH-induced activation of MAP kinase in immortalized GnRH neurons (GT1-7 cells). Immunoblot analysis indicated that the CaM kinase IIdelta2 isoform (CaM kinase IIdelta2) and synapsin I were expressed in GT1-7 cells. GnRH treatment rapidly increased phosphorylation of synapsin I at serine 603, a specific phosphorylation site for CaM kinase II, suggesting that GnRH treatment rapidly activated CaM kinase IIdelta2. In addition, when we stably overexpressed CaM kinase IIdelta2 in GT1-7 cells, the activation of MAP kinase was strongly enhanced. These results suggest that CaM kinase IIdelta2 was involved in the GnRH-induced activation of MAP kinase in GT1-7 cells.     

Sarcolemmal Na+-Ca2+ exchange and Ca2+-pump activities in cardiomyopathies due to intracellular Ca2+-overload

In order to identify defects in Na⁺-Ca²⁺ exchange and Ca²⁺-pump systems in cardiomyopathic hearts, the activities of sarcolemmal Na⁺-dependent Ca²⁺ uptake, Na⁺-induced Ca²⁺ release, ATP-dependent Ca²⁺ uptake and Ca²⁺-stimulated ATPase were examined by employing cardiomyopathic hamsters (UM-X7.1) and catecholamine-induced cardiomyopathy produced by injecting isoproterenol into rats. The rates of Na⁺-dependent Ca²⁺ uptake, ATP-dependent Ca²⁺ uptake and Ca²⁺-stimulated ATPase activities of sarcolemmal vesicles from genetically-linked cardiomyopathic as well as catecholamine-induced cardiomyopathic hearts were decreased without any changes in Na⁺-induced Ca²⁺-release. Similar results were obtained in Ca²⁺-paradox when isolated rat hearts were perfused for 5 min with a medium containing 1.25 mM Ca²⁺ following a 5 min perfusion with Ca²⁺-free medium. Although a 2 min reperfusion of the Ca²⁺-free perfused hearts depressed sarcolemmal Ca²⁺-pump activities without any changes in Na⁺-induced Ca²⁺-release, Na⁺-dependent Ca²⁺ uptake was increased. These results indicate that alterations in the sarcolemmal Ca²⁺-efflux mechanisms may play an important role in cardiomyopathies associated with the development of intracellular Ca²⁺ overload.     

Tyrosine kinase activity of a Ca2+/calmodulin-dependent protein kinase II catalytic fragment

A 30-kDa fragment of Ca²⁺/calmodulin-dependent protein kinase II (30K-CaMKII) is a constitutively active protein Ser/Thr kinase devoid of autophosphorylation activity. We have produced a chimeric enzyme of 30K-CaMKII (designated CX₄₀-30K-CaMKII), in which the N-terminal 40 amino acids of Xenopus Ca²⁺/calmodulin-dependent protein kinase I (CX₄₀) were fused to the N-terminal end of 30K-CaMKII. Although CX₄₀-30K-CaMKII exhibited essentially the same substrate specificity as 30K-CaMKII, it underwent significant autophosphorylation. Surprisingly, its autophosphorylation site was found to be Tyr-18 within the N-terminal CX₄₀ region of the fusion protein, although it did not show any Tyr kinase activity toward exogenous substrates. Several lines of evidence suggested that the autophosphorylation occurred via an intramolecular mechanism. These data suggest that even typical Ser/Thr kinases such as 30K-CaMKII can phosphorylate Tyr residues under certain conditions. The possible mechanism of the Tyr residue autophosphorylation is discussed.     

Comparison of the effects of the membraneassociated Ca2+/calmodulin-dependent protein kinase on Ca2+-ATPase function in cardiac and slow-twitch skeletal muscle sarcoplasmic reticulum

In both cardiac and slow-twitch skeletal muscle sarcoplasmic reticulum (SR) there are several systems involved in the regulation of Ca²⁺-ATPase function. These include substrate level regulation, covalent modification via phosphorylation-dephosphorylation of phospholamban by both cAMP-dependent protein kinase (PKA) and Ca²⁺/calmodulin-dependent protein kinase (CaM kinase) as well as direct CaM kinase phosphorylation of the Ca²⁺-ATPase. Studies comparing, the effects of PKA and CaM kinase on cardiac Ca²⁺-ATPase function have yielded differing results; similar studies have not been performed in slow-twitch skeletal muscle. It has been suggested recently, however, that phospholamban is not tightly coupled to the Ca²⁺-ATPase in SR vesicles from slow-twitch skeletal muscle. Our results indicate that assay conditions strongly influence the extent of CaM kinase-dependent Ca²⁺-ATPase stimulation seen in both cardiac and slow-twitch skeletal muscle. Addition of calmodulin (0.2 M) directly to the Ca²⁺ transport assay medium results in minimal ( 112–130% of control) stimulation of Ca²⁺ uptake activity when the Ca²⁺ uptake reaction is initiated by the addition of either ATP or Ca²⁺/EGTA. On the other hand, prephosphorylation of the SR by the endogenous CaM kinase and subsequent transfer of the membranes to the Ca²⁺ transport assay medium results in stimulation of Ca²⁺ uptake activity (202% of control). These effects are observable in both cardiac and slow-twitch skeletal muscle SR. PKA stimulates Ca²⁺ uptake markedly (215% of control) when the Ca²⁺ uptake reaction is initiated by the addition of prephosphorylated SR membranes or by Ca²⁺/EGTA but minimally (130% of control) when the Ca²⁺ uptake reaction is initiated by the addition of ATP. These findings imply that (a) phospholamban is coupled to the Ca²⁺-ATPase in slow-twitch skeletal muscle SR (as in cardiac SR), and (b) the amount of Ca²⁺ uptake stimulation seen upon the addition of calmodulin or PKA depends strongly on the assay conditions employed. Our observations help to explain the wide range of effects of calmodulin or PKA addition reported in previous studies. It should be noted that, since CaM kinase is now known to phosphorylate the Ca²⁺-ATPase in addition to phospholamban, further studies are required to determine the relative contributions of phospholambanversus Ca²⁺-ATPase phosphorylation in the stimulation of Ca²⁺-ATPase function by CaM kinase. Also, earlier studies attributing all of the effects of CaM kinase stimulation of Ca²⁺ uptake and Ca²⁺-ATPase activity to phospholamban phosphorylation need to be re-examined.     

Caffeine-induced Ca2+ oscillations in type I horizontal cell of carp retina: A mathematical model

Oscillations in intracellular free Ca²⁺ concentration ([Ca²⁺]_i) have been observed in a variety of cell types. In the present study, we constructed a mathematical model to simulate the caffeine-induced [Ca²⁺]_i oscillations based on experimental data obtained from isolated type I horizontal cell of carp retina. The results of model analysis confirm the notion that the caffeine-induced [Ca²⁺]_i oscillations involve a number of cytoplasmic and endoplasmic Ca²⁺ processes that interact with each other. Using this model, we evaluated the importance of store-operated channel (SOC) in caffeine-induced [Ca²⁺]_i oscillations. The model suggests that store-operated Ca²⁺ entry (SOCE) is elicited upon depletion of the endoplasmic reticulum (ER). When the SOC conductance is set to 0, caffeine-induced [Ca²⁺]_i oscillations are abolished, which agrees with the experimental observation that [Ca²⁺]_i oscillations were abolished when SOC was blocked pharmacologically, verifying that SOC is necessary for sustained [Ca²⁺]_i oscillations.     

Caffeine-induced Ca2+ oscillations in type I horizontal cell of carp retina: A mathematical model

Oscillations in intracellular free Ca²⁺ concentration ([Ca²⁺]_i) have been observed in a variety of cell types. In the present study, we constructed a mathematical model to simulate the caffeine-induced [Ca²⁺]_i oscillations based on experimental data obtained from isolated type I horizontal cell of carp retina. The results of model analysis confirm the notion that the caffeine-induced [Ca²⁺]_i oscillations involve a number of cytoplasmic and endoplasmic Ca²⁺ processes that interact with each other. Using this model, we evaluated the importance of store-operated channel (SOC) in caffeine-induced [Ca²⁺]_i oscillations. The model suggests that store-operated Ca²⁺ entry (SOCE) is elicited upon depletion of the endoplasmic reticulum (ER). When the SOC conductance is set to 0, caffeine-induced [Ca²⁺]_i oscillations are abolished, which agrees with the experimental observation that [Ca²⁺]_i oscillations were abolished when SOC was blocked pharmacologically, verifying that SOC is necessary for sustained [Ca²⁺]_i oscillations.     

Comparison of the Effects Exerted by Luminal Ca2+ on the Sensitivity of the Cardiac Ryanodine Receptor to Caffeine and Cytosolic Ca2+

Ca²⁺ released from the sarcoplasmic reticulum (SR) via ryanodine receptor type 2 (RYR2) is the key determinant of cardiac contractility. Although activity of RYR2 channels is primary controlled by Ca²⁺ entry through the plasma membrane, there is growing evidence that Ca²⁺ in the lumen of the SR can also be effectively involved in the regulation of RYR2 channel function. In the present study, we investigated the effect of luminal Ca²⁺ on the response of RYR2 channels reconstituted into a planar lipid membrane to caffeine and Ca²⁺ added to the cytosolic side of the channel. We performed two sets of experiments when the channel was exposed to either luminal Ba²⁺ or Ca²⁺. The given ion served also as a charge carrier. Luminal Ca²⁺ effectively shifted the EC₅₀ for caffeine sensitivity to a lower concentration but did not modify the response of RYR2 channels to cytosolic Ca²⁺. Importantly, luminal Ca²⁺ exerted an effect on channel gating kinetics. Both the open and closed dwell times were considerably prolonged over the whole range (response to caffeine) or the partial range (response to cytosolic Ca²⁺) of open probability. Our results provide strong evidence that an alteration of the gating kinetics is the result of the interaction of luminal Ca²⁺ with the luminally located Ca²⁺ regulatory sites on the RYR2 channel complex.     

Role of Na+/Ca2+ exchange and the plasma membrane Ca2+ pump in hormone-mediated Ca2+ efflux from pancreatic acini

Summary The relative contributions of the Na⁺/Ca²⁺ exchange and the plasma membrane Ca²⁺ pump to active Ca²⁺ efflux from stimulated rat pancreatic acini were studied. Na⁺ gradients across the plasma membrane were manipulated by loading the cells with Na⁺ or suspending the cells in Na⁺-free media. The rates of Ca²⁺ efflux were estimated from measurements of [Ca²⁺] _i using the Ca²⁺-sensitive fluorescent dye Fura 2 and⁴⁵Ca efflux. During the first 3 min of cell stimulation, the pattern of Ca²⁺ efflux is described by a single exponential function under control, Na⁺-loaded, and Na⁺-depleted conditions. Manipulation of Na⁺ gradients had no effect on the hormone-induced increase in [Ca²⁺] _i . The results indicate that Ca²⁺ efflux from stimulated pancreatic acinar cells is mediated by the plasma membrane Ca²⁺ pump. The effects of several cations, which were used to substitute for Na⁺, on cellular activity were also studied. Choline⁺ and tetramethylammonium⁺ (TMA⁺) released Ca²⁺ from intracellular stores of pancreatic acinar, gastric parietal and peptic cells. These cations also stimulated enzyme and acid secretion from the cells. All effects of these cations were blocked by atropine. Measurements of cholecystokinin-octapeptide (CCK-OP)-stimulated amylase release from pancreatic acini, suspended in Na⁺, TMA⁺, choline⁺, or N-methyl-d-glucamine⁺ (NMG⁺) media containing atropine, were used to evaluate the effect of the cations on cellular function. NMG⁺, choline⁺, and TMA⁺ inhibited amylase release by 55, 40 and 14%, respectively. NMG⁺ also increased the Ca²⁺ permeability of the plasma membrane. Thus, to study Na⁺ dependency of cellular function, TMA⁺ is the preferred cation to substitute for Na⁺. The stimulatory effect of TMA⁺ can be blocked by atropine.     

Mg2+ release coupled to Ca2+ uptake: A novel Ca2+ accumulation mechanism in rat liver

Isolated hepatocytes release 2–3 nmol Mg²⁺/mg protein or ~10% of the total cellular Mg²⁺ content within 2 minutes from the addition of agonists that increase cellular cAMP, for example, isoproterenol (ISO). During Mg²⁺ release, a quantitatively similar amount of Ca²⁺ enters the hepatocyte, thus suggesting a stoichiometric exchange ratio of 1 Mg²⁺:1Ca²⁺. Calcium induced Mg²⁺ extrusion is also observed in apical liver plasma membranes (aLPM), in which the process presents the same 1 Mg²⁺:1Ca²⁺ exchange ratio. The uptake of Ca²⁺ for the release of Mg²⁺ occurs in the absence of significant changes in Δψ as evidenced by electroneutral exchange measurements with a tetraphenylphosphonium (TPP⁺) electrode or ³H-TPP⁺. Collapsing the Δψ by high concentrations of TPP⁺ or protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) does not inhibit the Ca²⁺-induced Mg²⁺ extrusion in cells or aLPM. Further, the process is strictly unidirectional, serving only in Ca²⁺ uptake and Mg²⁺ release. These data demonstrate the operation of an electroneutral Ca²⁺/Mg²⁺ exchanger which represents a novel pathway for Ca²⁺ accumulation in liver cells following adrenergic receptor stimulation. This work was supported by National Institutes of Health Grant HL 18708.     

Regulation of Ca2+ mobilization by prolactin in mammary gland cells: possible role of secretory pathway Ca2+-ATPase type 2

Regulatory role of prolactin (PRL) on Ca2+ mobilization in human mammary gland cell line MCF-7 was examined. Direct addition of PRL did not affect cytoplasmic Ca2+ concentration ([Ca2+]i); however, treatment with PRL for 24h significantly decreased the peak level and duration time of [Ca2+]i elevation evoked by ATP or thapsigargin (TG). Intracellular Ca2+ release by IP3 or TG in permeablized cells was not decreased after PRL-treatment, indicating that the Ca2+ release was not impaired by PRL treatment. Extracellular Ca2+ entry evoked by ATP or TG was likely to be intact, because entry of extracellular Ba2+ was not affected by PRL treatment. Among Ca2+-ATPases expressed in MCF-7 cells, we found significant increase of secretory pathway Ca2+-ATPase type 2 (SPCA2) mRNA in PRL-treated cells by RT-PCR experiments including quantitative RT-PCR. Knockdown of SPCA2 by siRNA in PRL-treated cells showed similar Ca2+ mobilization to that in PRL-untreated cells. The present results suggest that PRL facilitates Ca2+ transport into Golgi apparatus and may contribute the supply of Ca2+ to milk.     

Ca2+/H+ exchange,lumenal Ca2+ release and Ca2+/ATP coupling ratios in the sarcoplasmic reticulum ATPase

The Ca²⁺ transport ATPase (SERCA) of sarcoplasmic reticulum (SR) plays an important role in muscle cytosolic signaling, as it stores Ca²⁺ in intracellular membrane bound compartments, thereby lowering cytosolic Ca²⁺ to induce relaxation. The stored Ca²⁺ is in turn released upon membrane excitation to trigger muscle contraction. SERCA is activated by high affinity binding of cytosolic Ca²⁺, whereupon ATP is utilized by formation of a phosphoenzyme intermediate, which undergoes protein conformational transitions yielding reduced affinity and vectorial translocation of bound Ca²⁺. We review here biochemical and biophysical evidence demonstrating that release of bound Ca²⁺ into the lumen of SR requires Ca²⁺/H⁺ exchange at the low affinity Ca²⁺ sites. Rise of lumenal Ca²⁺ above its dissociation constant from low affinity sites, or reduction of the H⁺ concentration by high pH, prevent Ca²⁺/H⁺ exchange. Under these conditions Ca²⁺ release into the lumen of SR is bypassed, and hydrolytic cleavage of phosphoenzyme may yield uncoupled ATPase cycles. We clarify how such Ca²⁺pump slippage does not occur within the time length of muscle twitches, but under special conditions and in special cells may contribute to thermogenesis.     

Functional Approach to the Catalytic Site of the Sarcoplasmic Reticulum Ca2+-ATPase: Binding and Hydrolysis of ATP in the Absence of Ca2+

Isolated sarcoplasmic reticulum vesicles in the presence of Mg(2+) and absence of Ca(2+) retain significant ATP hydrolytic activity that can be attributed to the Ca(2+)-ATPase protein. At neutral pH and the presence of 5 mM Mg(2+), the dependence of the hydrolysis rate on a linear ATP concentration scale can be fitted by a single hyperbolic function. MgATP hydrolysis is inhibited by either free Mg(2+) or free ATP. The rate of ATP hydrolysis is not perturbed by vanadate, whereas the rate of p-nitrophenyl phosphate hydrolysis is not altered by a nonhydrolyzable ATP analog. ATP binding affinity at neutral pH and in a Ca(2+)-free medium is increased by Mg(2+) but decreased by vanadate when Mg(2+) is present. It is suggested that MgATP hydrolysis in the absence of Ca(2+) requires some optimal adjustment of the enzyme cytoplasmic domains. The Ca(2+)-independent activity is operative at basal levels of cytoplasmic Ca(2+) or when the Ca(2+) binding transition is impeded.     

Heart sarcolemmal Ca2+ transport in endotoxin shock: I. Impairment of ATP-dependent Ca2+ transport

Effects of endotoxin administration on the ATP-dependent Ca²⁺ transport in canine cardiac sarcolemma were investigated. The results show that the sidedness of the sarcolemmal vesicles was not affected but the ATP-dependent Ca²⁺ transport in cardiac sarcolemma was decreased by 22 to 46% (p < 0.05) at 4 h following endotoxin administration. The kinetic analysis indicates that the V_max for ATP and for Ca²⁺ were decreased by 50% (p < 0.01) and 32% (p < 0.01), respectively, while the Km values for ATP and Ca²⁺ were not significantly affected after endotoxin administration. Magnesium (1–5 mM) stimulated while vanadate (0.25–3.0 M) inhibited the ATP-dependent Ca²⁺ transport, but the Mg²⁺-stimulated and the vanadate-inhibitable activities remained significantly lower in the endotoxin-treated animals. These data demonstrate that endotoxin administration impairs the ATP-dependent Ca²⁺ transport in canine cardiac sarcolemma and that the impairment is associated with a mechanism not affecting the affinity towards ATP and Ca²⁺. Additional experiments show that the Ca²⁺ sensitivity of the Ca²⁺-ATPase activity was indifferent between the control and endotoxic groups suggesting that endotoxic injury impairs Ca²⁺ pumping without affecting Ca²⁺-ATPase activity. Since sarcolemmal ATP-dependent Ca²⁺ transport plays an important role in the regulation of cytosolic Ca²⁺ homeostasis, an impairment in the sarcolemmal ATP-dependent Ca²⁺ transport induced by endotoxin administration may have a pathophysiological significance in contributing to the development of myocardial dysfunction in endotoxin shock.