Intact vesicles of canine cardiac sarcolemma: evidence from vectorial properties of Na+, K+-ATPase

Nucleotidase activities in membrane preparations of nervous ganglia and digestive gland of the snail Helix aspersa

Nucleotide-metabolizing enzymes play an important role in the regulation of nucleotide levels. In the present report, we demonstrated an enzyme activity with different kinetic properties in membrane preparations of the nervous ganglia and digestive gland from Helix aspersa. ATPase and ADPase activities were dependent on Ca2+ and Mg2+ with pH optima approximately 7.2 and between 6.0 and 8.0 in digestive gland and nervous ganglia, respectively. The enzyme activities present in membrane preparations of these tissues preferentially hydrolyzed triphosphate nucleotides. In nervous ganglia, the enzyme was insensitive to the classical ATPases inhibitors. In contrast, in digestive gland, N-ethylmaleimide (NEM) produced 45% inhibition of Ca(2+)-ATP hydrolysis. Sodium azide, at 100 microM and 20 mM, inhibited Mg(2+)-ATP hydrolysis by 36% and 55% in digestive gland, respectively. The presence of nucleotide-metabolizing enzymes in these tissues may be important for the modulation of nucleotide and nucleoside levels, controlling their actions on specific purinoceptors in these species.

Chronic aspartame affects T-maze performance, brain cholinergic receptors and Na+,K+-ATPase in rats

This study demonstrated that chronic aspartame consumption in rats can lead to altered T-maze performance and increased muscarinic cholinergic receptor densities in certain brain regions. Control and treated rats were trained in a T-maze to a particular side and then periodically tested to see how well they retained the learned response. Rats that had received aspartame (250 mg/kg/day) in the drinking water for 3 or 4 months showed a significant increase in time to reach the reward in the T-maze, suggesting a possible effect on memory due to the artificial sweetener. Using [(3)H]quinuclidinyl benzilate (QNB) (1 nM) to label muscarinic cholinergic receptors and atropine (10(-6) M) to determine nonspecific binding in whole-brain preparations, aspartame-treated rats showed a 31% increase in receptor numbers when compared to controls. In aspartame-treated rats, there was a significant increase in muscarinic receptor densities in the frontal cortex, midcortex, posterior cortex, hippocampus, hypothalamus and cerebellum of 80%, 60%, 61%, 65%, 66% and 60%, respectively. The midbrain was the only area where preparations from aspartame-treated rats showed a significant increase in Na(+),K(+)-ATPase activity. It can be concluded from these data that long-term consumption of aspartame can affect T-maze performance in rats and alter receptor densities or enzymes in brain.

Characterization of Na/Ca exchange in plasmalemmal vesicles from zona fasciculata cells of the bovine adrenal gland

The presence of an Na/Ca exchange system in fasciculata cells of the bovine adrenal gland was tested using isolated plasmalemmal vesicles. In the presence of an outwardly Na(+) gradient, Ca(2+) uptake was about 2-fold higher than in K(+) condition. Li(+) did not substitute for Na(+) and 5 mM Ni(2+) inhibited Ca(2+) uptake. Ca(2+) efflux from Ca(2+)-loaded vesicles was Na(+)-stimulated and Ni(2+)-inhibited. The saturable part of Na(+)-dependent Ca(2+) uptake displayed Michaelis-Menten kinetics. The relationship of Na(+)-dependent Ca(2+) uptake versus intravesicular Na(+) concentration was sigmoid (apparent K(0.5) approximately 24 mM; Hill number approximately 3) and Na(+) acted on V(max) without significant effect on K(m). Na(+)-stimulated Ca(2+) uptake was temperature-dependent (apparent Q(10) approximately 2.2). The inhibition properties of several divalent cations (Cd(2+), Sr(2+), Ni(2+), Ba(2+), Mn(2+), Mg(2+)) were tested and were similar to those observed in kidney basolateral membrane. The above results indicate the presence of an Na/Ca exchanger located on plasma membrane of zona fasciculata cells of bovine adrenal gland. This exchanger displays similarities with that of renal basolateral cell membrane.

Cardiac sarcoplasmic reticulum and sarcolemmal proteins separated by two-dimensional electrophoresis: surfactant effects on membrane solubilization

We evaluated the use of the alkyaryl amidosulfobetaine zwitterionic detergent, designated as C8psi, to facilitate the solubilization of cardiac subcellular, membrane-associated proteins. Hearts from 7-week-old male Sprague-Dawley rats were isolated, and the left ventricles dissected and subsequently homogenized. The sarcolemma (SL) and the sarcoplasmic reticulum (SR) were isolated from different homogenate preparations originating from rat hearts by ultracentrifugation methods. The isolated membrane preparations were solubilized and the proteins precipitated. After resuspension, protein separation was achieved in first-dimensional IEF using an immobilized (pH 4-7) gradient and in the second dimension using 12% sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Gels were then stained, images analyzed, and protein spots excised for subsequent identification. Protein identification from both SR and SL samples did not identify any of the known major membrane-associated proteins. Solubilization of whole tissue lysates with C8psi resulted in no increase in the total number of proteins detected relative to samples solubilized in the presence of 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulforate (CHAPS). The data suggest the utility of newer surfactants such as C8psi to improve both the resolution of (2-D) protein profiles and increase the number of proteins extracted from subcellular organelle fractions.

Acute and chronic regulation of Na+/K+-ATPase transport activity in the RN22 Schwann cell line in response to stimulation of cyclic AMP production

Na+/K+-ATPase-dependent Rb+ uptake of RN22 Schwann cells was stimulated by cholera toxin (0.25 microg/ml), forskolin (2 mM), or 8-bromo cAMP (1 mM). At 2 h Rb+ uptake was increased by 162+/-6% (cholera toxin), 151+/-14% (forskolin), and 207+/-15% (8-bromo cAMP). Cholera toxin or 8-bromo cAMP treatment for 12-24 h resulted in a second peak of Na+/K+-ATPase-dependent Rb+ transport activity of 186+/-12 and 265+/-9% of control, respectively. Cholera toxin also transiently stimulated the activity of the Na+, K+, 2Cl- -cotransporter with a peak at 2 h (179+/-9%), returning to basal levels by 24 h. Inhibition of the Na+,K+,2Cl- -cotransporter by bumetanide (0.1 mM) or by reduction of the Na+ gradient (10 mM veratridine treatment) prevented the early peak in ATPase activity but not the second peak. These results indicated that the early transient stimulation of Na+/K+ ATPase activity by cholera toxin was due to an increase in cellular Na+, secondary to stimulation of Na+,K+,2Cl -cotransport activity. Western blot analysis of cellular homogenates and purified membrane fractions showed that the second peak of Rb+ uptake activity was a result of translocation of transport protein from an intracellular microsomal pool to the plasma membrane. Rb+ uptake by dominant negative protein kinase A mutants of the RN22 cell was not stimulated by cholera toxin treatment (acute or chronic) confirming the cAMP/protein kinase A dependency of both acute and long-term regulation of transport activity. In the absence of a change in Michaelis constants or of an increase in total transport protein of cellular homogenates, neither a change in enzyme kinetics nor an increase in de novo synthesis of transport protein could account for the increase in transport activity.

Characterization and localization of an ATP diphosphohydrolase activity (EC 3.6.1.5) in sarcolemmal membrane from rat heart

In the present report we describe an ATP diphosphohydrolase (apyrase EC 3.6.1.5) in rat cardiac sarcolemma. It is Ca2+ dependent and is insensitive to ouabain, orthovanadate, N-ethylmaleimide (NEM), lanthanum, and oligomycin that are classical ATPase inhibitors. Sodium azide that is a mitochondrial inhibitor at low concentrations, did not affect the enzyme activity at 5.0 mM or below. In contrast, at high concentrations (> 10 mM) sodium azide inhibited the enzyme. Levamisole, a specific inhibitor of alkaline phosphatase and P1, P5-di(adenosine 5'-)pentaphosphate (Ap5A), a specific inhibitor of adenylate kinase did not inhibit the enzyme. Mercury chloride showed a parallel inhibition of the hydrolysis of both substrates of apyrase. Similar inhibition profiles are powerful evidence for a common catalytic site for the hydrolysis of both substrates. The enzyme has an optimum pH range of 7.5-8.0 and catalyzes the hydrolysis of triphospho- and diphosphonucleosides other than ATP or ADP. The apparent Km (Michaelis constant) and Vmax (maximal velocity) are 62.1 +/- 5.2 microM and 1255.7 +/- 178 micromol inorganic phosphate liberated/min/mg with ATP and 59.4 +/- 4.3 microM and 269.2 +/- 39 micromol inorganic phosphate liberated/min/mg with ADP. Enzyme markers indicated that this apyrase is associated with the plasma membrane. A deposition of lead phosphate granules on the outer surface of the sarcolemmal vesicles was observed by electron microscopy in the presence of either ATP or ADP as substrate. It is suggested that the ATP diphosphohydrolase could regulate the concentration of extracellular adenosine, and thus is important in the control of vascular tone and coronary flow.

Regulation of cardiac sarcolemmal Ca2+ channels and Ca2+ transporters by thyroid hormone

In order to examine the regulatory role of thyroid hormone on sarcolemmal Ca(2+)-channels, Na(+)-Ca2+ exchange and Ca(2+)-pump as well as heart function, the effects of hypothyroidism and hyperthyroidism on rat heart performance and sarcolemmal Ca(2+)-handling were studied. Hyperthyroid rats showed higher values for heart rate (HR), maximal rates of ventricular pressure development +(dP/dt)max and pressure fall -(dP/dt)max, but shorter time to peak ventricular pressure (TPVP) and contraction time (CT) when compared with euthyroid rats. The left ventricular systolic pressure (LVSP) and left ventricular end-diastolic pressure (LVEDP), as well as aortic systolic and diastolic pressures (ASP and ADP, respectively) were not significantly altered. Hypothyroid rats exhibited decreased values of LVSP, HR, ASP, ADP, +(dP/dt)max and -(dP/dt)max but higher CT when compared with euthyroid rats; the values of LVEDP and TPVP were not changed. Studies with isolated-perfused hearts showed that while hypothyroidism did not modulate the inotropic response to extracellular Ca2+ and Ca2+ channel blocker verapamil, hyperthyroidism increased sensitivity to Ca2+ and decreased sensitivity to verapamil in comparison to euthyroid hearts. Studies of [3H]-nitrendipine binding with purified cardiac sarcolemmal membrane revealed decreased number of high affinity binding sites (Bmax) without any change in the dissociation constant for receptor-ligand complex (Kd) in the hyperthyroid group when compared with euthyroid sarcolemma; hypothyroidism had no effect on these parameters. The activities of sarcolemmal Ca(2+)-stimulated ATPase, ATP-dependent Ca2+ uptake and ouabain-sensitive Na(+)-K+ ATPase were decreased whereas the Mg(2+)-ATPase activity was increased in hypothyroid hearts. On the other hand, sarcolemmal membranes from hyperthyroid samples exhibited increased ouabain-sensitive Na(+)-K+ ATPase activity, whereas Ca(2+)-stimulated ATPase, ATP-dependent Ca2+ uptake, and Mg(2+)-ATPase activities were unchanged. The Vmax and Ka for Ca2+ of cardiac sarcolemmal Na(+)-Ca2+ exchange were not altered in both hyperthyroid and hypothyroid states. These results indicate that the status of sarcolemmal Ca(2+)-transport processes is regulated by thyroid hormones and the modification of Ca(2+)-fluxes across the sarcolemmal membrane may play a crucial role in the development of thyroid state-dependent contractile changes in the heart.

Seasonal variations in the rate and capacity of cardiac SR calcium accumulation in a hibernating species

The rate of calcium uptake and the level of calcium accumulation was measured in cardiac muscle SR from hibernating and nonhibernating Richardson's ground squirrels. In whole heart homogenates, the rate of calcium uptake was higher (P less than 0.05) in hibernating animals than it was in active animals. Further purification of homogenates into sacroplasmic reticulum (SR) preparations showed that the hibernating animals had the highest rate of calcium uptake and the greatest level of calcium accumulation. These results could not be explained by variations in non-SR membrane contaminants nor by changes in the maximal activity or total amount of a SR marker enzyme, the Ca(2+)-ATPase. The addition of ryanodine to the calcium uptake medium increased the level of calcium accumulation in all groups by a similar amount. It is concluded that the high rate of calcium uptake by isolated cardiac SR vesicles from hibernating ground squirrels reflects the activity of the organelle in vivo, and that the ability of the ryanodine-insensitive population of SR vesicles to accumulate calcium is affected by hibernation.

Effect of hyper- and hypothyroidism on phospholipid fatty acid composition and phospholipases activity in sarcolemma of rabbit cardiac muscle

Lipid content and composition of fatty acids esterified to phospholipids of cardiac sarcolemma isolated from hyperthyroid, hypothyroid and control rabbits were analysed. Hyperthyroidism resulted in a significant reduction of the cholesterol to phospholipid molar ratio as compared to control animals, while hypothyroidism exerted the opposite effect. Complex changes in composition of phospholipid fatty acids observed in hyperthyroid state led to an elevation of the fatty acid unsaturation index over the control value. The unsaturation index value was, however, not affected in the hypothyroid state. Thyroxine hormone administration increased phospholipase A1 and decreased phospholipase A2 activity. The opposite effect was observed in thyreodectomized animals. The effect of changes in sarcolemmal bulk phospholipids upon thyroxine administration or deficiency on regulation of activity of membrane-bound enzymes is discussed.

Long-term consequences of prenatal exposure to cocaine or related drugs: effects on rat brain monoaminergic receptors

Reports from both this laboratory and others indicate that prenatal exposure of rats to cocaine can produce alterations in development, activity and responses to environmental stimuli. In order to determine a biochemical basis for these effects, radioligand receptor-binding assays for different monoaminergic receptors were performed on rat brain tissues obtained from offspring of dams treated SC with saline, cocaine (15 mg/kg b.i.d.), amitriptyline (10 mg/kg) or amfonelic acid (AFA, 1.5 mg/kg). Male rat pups were fostered by surrogate dams and one rat per litter taken at 30, 60 or 180 days postnatal for determination of striatal and prefrontal cortical D2 receptors, prefrontal cortical 5HT2 receptors, cortical alpha 1-, alpha 2-, beta 1- and beta 2-adrenoceptors. Across all drug treatments and times, the only significant change was at 30 days of age when beta 1-adrenoceptors were increased 68% in the cocaine exposed pups--a time when these rats show hyperactivity--and at 180 days postnatal when a 20% decrease in DA2 receptor Bmax was observed. Also, cortical membrane Mg(2+)-dependent Na+, K(+)-ATPase activities and basal ATPase activities were unaltered by any of the treatments at any of the times. These results suggest that few changes have occurred in monoaminergic receptor sensitivity as a result of the exposure to these drugs during gestation. The behavioral changes that are known to occur following prenatal exposure to cocaine may be due to presynaptic alterations in neurotransmitter function rather than changes in postsynaptic receptors.

Effect of hypothermic ischemia and reperfusion on calcium transport by myocardial sarcolemma and sarcoplasmic reticulum

The effects of hypothermic ischemia and reperfusion on sarcolemma and sarcoplasmic reticulum Ca2+ transport were studied in vesicles isolated from rabbit hearts. Hypothermic global ischemia was produced by immersing hearts in saline at 4 degrees C for 3 h. Following hypothermic ischemia, reperfusion was carried out for 40 min using a Langendorff perfusion system for the working heart. Na+,K(+)-ATPase activity of sarcolemmal vesicles (SL), was not depressed by hypothermic ischemia nor by ischemia and reperfusion. The initial rate of Na(+)-Ca2+ exchange in SL vesicles was not depressed, but the maximum amount of Ca2+ uptake was increased both after hypothermic ischemia and after reperfusion. Ca2+ uptake activity of sarcoplasmic reticulum vesicles (SR) isolated from hearts subjected to hypothermic ischemia was slightly lower than that of control, and was further reduced following reperfusion. Ca(2+)-ATPase activity of SR was unaffected by hypothermic ischemia, while it was markedly lowered after reperfusion. Although the phosphoenzyme level in SR vesicles was slightly decreased, the turnover rate was reduced after reperfusion. Reperfusion injury thus took place mainly in SR while SL appeared to be tolerant to ischemia and reperfusion.

Unsaturated aminophospholipids are preferentially retained by the fast skeletal muscle CaATPase during detergent solubilization. Evidence for a specific association between aminophospholipids and the calcium pump protein

When fast twitch skeletal muscle vesicles (SR) and purified calcium pump protein are stripped with the nonionic detergent C12E8 (octaethylene glycol dodecyl ether), not all the membrane phospholipids are removed from the calcium pump protein. Maximal extraction produces a remnant of 6-8 mol of phospholipid/mole of calcium ATPase (CaATPase). In contrast to native SR and the prestripped purified CaATPase, the remaining phospholipid is markedly enriched in phosphatidylethanolamine (PE) and phosphatidylserine (PS) in both preparations; the remaining lipid is also enriched in phospholipid that is predominantly unsaturated. In addition, virtually all of the associated PE is plasmalogenic (96% as opposed to 63% in the native SR). The amino-specific cross-linking reagent DFDNB (1,5-difluoro-2,4-dinitrobenzene sulfonic acid) and the amino binding reagent TNBS (2,4,6-trinitrobenzene sulfonic acid) were utilized to identify the monolayer of the native preparation where these phospholipids reside, and to determine which phospholipids are closely associated with the calcium pump protein following detergent treatment. These studies demonstrate that PE and PS are closely associated with the pump protein, PE residing almost exclusively in the outer monolayer of SR, while PS resides in the inner monolayer. Nonspecific phospholipid exchange protein was shown to be capable of exchanging phospholipids from donor vesicles into those phospholipids associated with the CaATPase; stripping of lipid-exchanged vesicles with C12E8 exhibited the same specificity with regard to head-group species (i.e., PE is markedly enriched in the extracted protein associated fraction). The results suggest that specific protein-lipid interactions exist, favoring the association of plasmalogenic aminophospholipids with the calcium pump protein.

Lactate transport is mediated by a membrane-bound carrier in rat skeletal muscle sarcolemmal vesicles

To study the kinetics of lactate transport in an isolated, nonmetabolizing system, skeletal muscle sarcolemmal membrane vesicles were purified from 22 female Sprague-Dawley rats. L(+)-[U-14C] Lactate at 10 concentrations demonstrated saturation kinetics with a Vmax of 139.4 nmol/mg/min, and an apparent Km of 40.1 mM. Threefold higher initial rates of L(+)-lactate uptake were seen at 37 degrees C than at 25 degrees C, indicating temperature sensitivity. Transport was stereospecific for the L(+) isomer: isotopic D(-) uptake rates remained linear at concentrations from 1 to 200 mM, and 1 mM D(-) remained 6-fold lower in net uptake after 60 min than the L(+) isomer. Furthermore, unlabeled 10 mM D(-)-lactate in the external medium could only inhibit 1 mM isotopic (L(+) uptake by 12%, whereas unlabeled 10 mM L(+)-lactate and pyruvate inhibited 82 and 71%, respectively. Additionally, 10 mM beta-hydroxybutyrate and acetoacetate could moderately inhibit (27 and 32%, respectively) 1 mM L(+)-lactate transport, but the unsubstituted aliphatic monocarboxylates (formate, acetate, propionate), tricarboxylic acid cycle intermediates (malate, succinate, oxaloacetate, alpha-ketoglutyrate, citrate), amino acids (alanine, aspartate, glutamate), and palmitate or adenosine in 10-fold excess could not effectively inhibit 1 mM L(+)lactate uptake under cis-transport conditions. 4,4'-Diisothiocyanostilbene-2,2'-disulfonic acid could inhibit L(+)-lactate transport by only 13%, so that lactate transport does not appear to be affected directly by Cl- or HCO3- fluxes. It was demonstrated that KCl could not evoke a membrane potential-induced overshoot of lactate uptake in the presence or absence of valinomycin. Moreover, gluconate could substitute for Cl-, indicating that Cl- flux does not contribute to a membrane potential-dependent component of the transport mechanism, suggesting an electroneutral translocation process. Protein-modifying reagents significantly inhibited 1 mM L(+)-lactate transport during pH-stimulated conditions (p-chloromercuriphenyl-sulfonic acid, 83%; N-ethylmaleimide, 86%; HgCl2, 56%; mersalyl, 63% inhibition). We conclude that the skeletal muscle lactate transporter is a membrane-bound protein, specifically associated with the sarcolemma, that demonstrates saturation kinetics, competition, stereospecificity, and sensitivity to temperature as well as various ionic cis-inhibitors. The lactate transporter is a potentially important regulator of lactate flux across skeletal muscle, and may help to regulate intracellular pH and intermediary metabolism during lactic acidosis.

Beta-adrenergic inhibition of AGEPC-stimulated Na+/Ca2+ exchange and AGEPC-induced platelet activation

Recently, AGEPC (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine) was found to initiate contraction of ileal smooth muscle strips and to enhance Na+/Ca2+ exchange in ileal plasmalemmal vesicles. In the present study, the effects of the smooth muscle relaxant, isoproterenol, on Na+/Ca2+ exchange in rat ileal plasmalemmal vesicles was examined. In this preparation, Na+/Ca2+ exchange was stimulated 131 +/- 8% and 264 +/- 19% by addition of 50 nM and 100 nM AGEPC, respectively. Isoproterenol, a beta-adrenergic agonist, inhibited AGEPC stimulation of Na+/Ca2+ exchange in a dose- and time-dependent manner but had no effect on basal rates of Na+/Ca2+ antiport. At 1 microM, isoproterenol inhibited 86% of the Na+/Ca2+ exchange stimulated by 50 nM AGEPC. Vesicular cAMP levels were increased over 100% following the addition of 1 microM isoproterenol for 30 s. Inhibition of AGEPC-stimulated vesicular Na+/Ca2+ exchange and elevation of vesicular cAMP levels by isoproterenol was prevented by the beta-receptor antagonist propranolol (5 microM), demonstrating that these effects of isoproterenol were mediated by interaction with vesicular beta-adrenergic receptors. Additional studies with washed rabbit platelets demonstrated that isoproterenol inhibited AGEPC-induced aggregation and serotonin release. These effects of isoproterenol were dose- and time-dependent and were antagonized by propranolol. Isoproterenol had no effect on thrombin-induced aggregation and did not change appreciably platelet cAMP levels. Moreover, dibutyryl cAMP could not mimic the effect of isoproterenol to inhibit an AGEPC-induced aggregation. On a molar basis, the inhibitory effects of isoproterenol toward AGEPC action were greater in the ileal preparation than in the platelets. It is suggested that beta-adrenergic agonists may modulate AGEPC-induced ileal Na+/Ca2+ exchange and AGEPC-induced platelet aggregation through cAMP-dependent and-independent mechanisms, respectively.

Effects of microgravity on brain neurotransmitter receptors

Neurotransmitter receptor binding and Na+, K+-ATPase activity were examined in the brains of six rats exposed to 7 days of microgravity during the flight of Spacelab 3. The same variables were examined in a group of six ground control rats. 5-HT1 receptor number in the hippocampus was significantly elevated by exposure to the microgravity environment, and cortical sodium-potassium pump activity was significantly depressed. A marginal depression in dopamine D-2 binding in the striatum was noted. Dopamine and 5-HT binding in a wide variety of other central regions, in addition to GABAA, muscarinic acetylcholine, adenosine A1, and opiate receptor binding, and adrenoceptor binding, was unaffected by microgravity exposure.

Effect of dietary trans fatty acids on some membrane-associated enzymes and receptors in rat heart

Three groups of male weanling Sprague-Dawley rats were fed diets containing 20% corn oil, 20% partially hydrogenated soybean oil (PHSBO) or 18% PHSBO + 2% corn oil. PHSBO contained about 48% of its total fatty acids as trans-octadecenoate. Rats were killed after 16-18 weeks of feeding the various diets, hearts were dissected and crude sarcolemma was prepared by differential centrifugation. The activities of ouabain-sensitive (Na+ + K+)ATPase were significantly lower in membranes of rats fed 20% PHSBO than the control rats fed 20% corn oil. The feeding of 2% corn oil with 18% PHSBO resulted in partial restoration of the enzyme activity. The maximum number of [3H]ouabain-binding sites (Bmax) was also lower in cardiac membranes of rats fed 20% PHSBO than those fed 20% corn oil. Similar to (Na+ + K+)-ATPase activity, some restoration of the number of [3H]ouabain-binding sites was observed when 2% corn oil was fed with 18% PHSBO-containing diet. There was no difference in the binding affinity of the radioligand for the receptor among the 3 dietary groups. Adenylate cyclase activities (fluoride-, isoproterenol- and forskolin-stimulated) were lower in membranes of rats fed 20% PHSBO or 18% PHSBO + 2% corn oil than in the control group fed 20% corn oil. Density of the beta-adrenergic receptor was the lowest in cardiac membranes of rats fed 20% PHSBO. The feeding of 2% corn oil with 18% PHSBO resulted in partial restoration of the maximum number of [3H]dihydroalprenolol (DHA)-binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of [3H]ouabain binding sites in intact cells and cell homogenates: apparent lack of glycoside receptors unrelated to sarcolemmal Na+, K+-ATPase in guinea-pig heart

In intact heart muscle cells incubated in a physiological solution, i.e. under the condition in which the cardiac glycosides produce pharmacological and toxicological effects, receptors for these actions of the glycosides should be available to ouabain. In cell homogenates, [3H]ouabain binding observed in the presence of Mg2+ and inorganic phosphate represents binding of the glycoside to Na+, K+-ATPase. Therefore, numbers of these two types of [3H]ouabain binding sites were compared using viable myocyte preparations obtained from ventricular muscle of guinea-pig heart. The number of ouabain binding sites observed in viable myocytes in the absence of Ca2+ and K+ was not different from the number of ouabain binding sites on Na+, K+-ATPase observed with sodium dodecylsulfate-treated homogenates prepared from isolated myocytes. These results do not support the hypothesis that there are receptors for the pharmacological or toxic actions of ouabain other than those that are associated with sarcolemmal Na+, K+-ATPase.

Pressure-induced cardiac hypertrophy: changes in Na+,K+-ATPase and glycoside actions in cats

Effects of myocardial hypertrophy caused by pressure overload on sarcolemmal Na+,K+-ATPase and positive inotropic action of strophanthidin were examined in cats. Partial ligation of the main pulmonary artery for four weeks resulted in right ventricular hypertrophy with no significant changes in left ventricular muscle. Hypertrophy was associated with a reduction in the number of active Na+,K+-ATPase units. Affinity of the remaining enzyme for [3H]ouabain was unchanged. No apparent right or left shift in dose-response curve for the positive inotropic effect of strophanthidin was observed and toxic concentrations of strophanthidin were unchanged; however, the degree of the positive inotropic effect produced by high concentrations of strophanthidin was significantly smaller in hypertrophied muscle. Moreover, decreases in developed tension rather than tachyarrhythmias was the predominant form of toxicity observed in hypertrophied muscle. These results indicate that myocardial hypertrophy reduces the number of active Na+,K+-ATPase units per milligram protein, decreases maximal positive inotropic effect of strophanthidin, and alters the prevailing form of strophanthidin toxicity.

Skeletal muscle sarcolemma in malignant hyperthermia: evidence for a defect in calcium regulation

Sarcolemmal properties implicated in the skeletal muscle disorder, malignant hyperthermia (MH), were examined using sarcolemma-membrane vesicles isolated from normal and MH-susceptible (MHS) porcine skeletal muscle. MHS and normal sarcolemma did not differ in the distribution of the major proteins, cholesterol or phospholipid content, vesicle size and sidedness, (Na+ + K+)-ATPase activity, ouabain binding, or adenylate cyclase activity (total and isoproterenol sensitivity). The regulation of the initial rates of MHS and normal sarcolemmal ATP-dependent calcium transport (calcium uptake after 1 min) by Ca2+ (K1/2 = 0.64-0.81 microM), calmodulin, and cAMP-dependent protein kinase were similar. However, when sarcolemmal calcium content was measured at either 2 or 20 min after the initiation of active calcium transport, a significant difference between MHS and normal sarcolemmal calcium uptake became apparent, with MHS sarcolemma accumulating approximately 25% less calcium than normal sarcolemma. Calcium transport by MHS and normal sarcolemma, at 2 or 20 min, had a similar calmodulin dependence (C1/2 = 150 nM), and was stimulated to a similar extent by cAMP-dependent protein kinase or calmodulin. Halothane inhibited MHS and normal sarcolemmal active calcium uptake in a similar fashion (half-maximal inhibition at 10 mM halothane), while dantrolene (30 microM) and nitrendipine (1 microM) had little effect on either MHS or normal sarcolemmal calcium transport. After 20 min of ATP-supported calcium uptake, 2 mM EGTA plus 10 microM sodium orthovanadate were added to initiate sarcolemmal calcium efflux. Following an initial rapid phase of calcium release, an extended slow phase of calcium efflux (k = 0.012 min-1) was similar for both MHS and normal sarcolemma vesicles. We conclude that although a number of sarcolemmal properties, including passive calcium permeability, are normal in MH, a small but significant defect in MHS sarcolemmal ATP-dependent calcium transport may contribute to the abnormal calcium homeostasis and altered contractile properties of MHS skeletal muscle.

Alkylacetylglycerophosphocholine stimulates Na+-Ca2+ exchange, protein phosphorylation and polyphosphoinositide turnover in rat ileal plasmalemmal vesicles

The novel ether phospholipid, 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (AGEPC), isometrically contracted helically cut rat ileal smooth muscle strips in a dose- and time-dependent manner. Utilizing an enriched plasma membrane vesicular preparation from rat ileal longitudinal smooth muscle, AGEPC specifically stimulated Na+-Ca2+ exchange in a dose- and time-dependent manner. Concomitant with the AGEPC stimulation of Na+-dependent Ca2+ influx in plasma membrane vesicles is an enhanced turnover of the polyphosphoinositides, an elevated concentration of phosphatidic acid and also an enhanced phosphorylation of an Mr 40,000 plasmalemmal protein. The mechanisms by which AGEPC may regulate ileal plasmalemmal Ca2+ flux and contractility are considered.

Effect of mononuclear cell factors on calcium transport in sarcoplasmic reticulum vesicles

The effect of supernatants from cultures of mitogen-stimulated human mononuclear cells on calcium transport by sarcoplasmic reticulum was examined. Calcium transport was assayed by measuring the time course of calcium accumulation by sarcoplasmic reticulum incubated with supernatants from stimulated mononuclear cells was 20% less than that by vesicles exposed to control supernatants (P less than 0.001). In contrast, no difference in calcium-dependent ATPase activity was noted between vesicles incubated with either active or control supernatants. The results suggest that mononuclear cell factors disturb calcium transport in sarcoplasmic reticulum membrane.

Contamination of a cardiac sarcolemmal preparation with endothelial plasma membrane

Preparation of sarcolemma from whole rabbit heart using the method of Jones et al. (Jones,L.R., Besch, H.R., Fleming, J.W., McConnaughey, M.M. and Watanabe, A.M. (1979) J. Biol. Chem. 254, 530-539) results in a 46-fold purification of the endothelial plasmalemma-specific marker angiotensin converting enzyme. This implies contamination of the sarcolemma with vascular endothelial plasmalemma. During preparation of sarcolemma from sheep heart, using the same method, angiotensin converting enzyme copurified with the general plasma membrane marker (Na+ + K+)-ATPase. The ratio of myocyte to endothelial plasma membrane in the final preparation is therefore similar to that in the whole heart homogenate. Ultrastructural analysis has shown that the myocyte/endothelial surface area is 70:30 in whole cardiac muscle. Comparison of angiotensin converting enzyme activity of an endothelial plasma membrane fraction with that of whole heart sarcolemma suggests an upper limit of 42% for endothelial contamination. Contamination by endothelial plasmalemma was dramatically reduced by preparing sarcolemma from myocytes produced by proteolytic disruption of whole hearts. Following disruption, myocytes were separated from non-muscle cells by sedimentation through 0.5 M sucrose. Sarcolemma prepared from sheep cardiac myocytes had approximately 15-fold less angiotensin converting enzyme activity than whole sheep heart sarcolemma but comparable ouabain-inhibitable (Na+ + K+)-ATPase activity.

Insulin binding and glucose transport in rat skeletal muscle sarcolemmal vesicles

A new method is described for isolation of sarcolemma (SL) from skeletal muscle of rats that produces vesicles of high purity and yield. There was a mean 59-fold purification (n = 22) of the SL marker enzyme K+-p-nitrophenylphosphatase. Specific activities of marker enzymes for sarcoplasmic reticulum and mitochondria were low, indicating minimal contamination. Despite the high purity and low contamination, a relatively high protein yield was achieved (0.43 +/- 0.03 mg/g wet wt, n = 25). Electron microscopy showed that the membranes were primarily vesicles. Specific 125I-insulin binding association constants derived from the high- and low-affinity portion of the Scatchard plots were 0.764 +/- 0.154 and 0.0096 +/- 0.0012 X 10(9) M-1, whereas the apparent number of receptors were 15.0 +/- 4.1 and 925 +/- 80 X 10(9) per mg of SL protein. Equilibrium exchange glucose transport studies at 37 degrees C indicated that the SL vesicles exhibited specific D-glucose transport which was responsive to in vivo insulin stimulation. We conclude that this isolation procedure, especially in light of the high purity and yield, provides a good and practical experimental model for studying insulin binding and glucose transport in skeletal muscle.

Myometrial (Na+ + K+)-activated ATPase and its Ca2+ sensitivity

Ouabain-sensitive (Na+ + K+)-ATPase activity in the rat myometrial microsome fraction could only be determined following detergent treatment. The (Na+ + K+)-ATPase activity manifested by detergent treatment proved very stable even to high concentrations of NaN3, in contrast Mg+-ATPase activity was reduced to about 30 percent of the control. The major part of the Mg2+-ATPase in the myometrial membrane preparation was found to be identical with the NaN3-sensitive ATP diphosphohydrolase capable of ATP and ADP hydrolysis. This monovalent-cation-insensitive ATP hydrolysis could be extensively reduced by DMSO. Furthermore DMSO prevented the inactivation of the (Na+ + K+)-ATPase activity. 10-100 microM Ca2+ inhibited the (Na+ + K+)-ATPase activity obtained in the presence of SDS by 15-50 percent. The Ca2+ sensitivity of the enzyme was considerably decreased if the proteins solubilized by the detergent had been separated from the membrane fragments by ultracentrifugation. The inhibitory effect could be regained by combining the supernatant with the pellet. Ca2+ sensitivity of the (Na+ + K+)-ATPase activity was preserved even after removal of the solubilized proteins provided that DMSO had been applied. It appears that a factor in the plasma membrane solubilized by SDS may be responsible for the loss of Ca2+ sensitivity of the (Na+ + K+)-ATPase activity, the solubilization of which can be prevented by DMSO.

A possible action of nicardipine on the cardiac sarcolemmal Na+-Ca2+ exchange

The effects of nicardipine on sodium-calcium exchange activity of cardiac sarcolemma-enriched vesicles isolated from the rat heart were examined. Sodium-loaded, sarcolemma-enriched vesicles, when exposed to a medium containing 40 microM CaCl2, exhibited about 5 nmoles Ca2+/mg protein of the maximal calcium uptake; the initial rate was 21 nmoles Ca2+/mg protein/min. The calcium uptake was dependent on the extravesicular concentration of calcium ion. Nicardipine at concentrations of 0.1 to 10 microM depressed the rate of calcium uptake activity by 60-90%. The isolated membrane vesicles preloaded with Ca2+ showed a calcium efflux activity, when exposed to a medium containing sodium ion. The rate of calcium efflux was 2.5 nmoles Ca2+/mg protein/min, when measured in a medium containing 6.5 mM NaCl. The efflux rate was facilitated with increased concentrations of sodium ion in the medium. About 75% of the preloaded calcium in the vesicles was released within 3 min of incubation. The rate of calcium efflux was stimulated in the presence of 0.1 to 10 microM nicardipine (2.5- to 4-fold increase). The present results suggest a possible action of nicardipine on the sodium-calcium exchange mechanism at cardiac sarcolemmal sites.

The isolation and characterization of Ca++-accumulating subcellular membrane fractions from cerebral arteries

A study was undertaken using differential centrifugation methods to isolate from rabbit cerebral arteries the subcellular microsomal protein fractions capable of actively sequestering Ca++. One isolated protein fraction displayed a relatively large adenosine triphosphate (ATP)-dependent Ca++-accumulating capacity that was completely inhibited by NaN3, and was therefore designated the "mitochondrial fraction." Electron microscopy confirmed that this fraction consisted of numerous mitochondrial elements. Another isolated membrane fraction possessed a Ca++-accumulating capacity dependent on ATP and oxalate and only partially sensitive to NaN3. In the presence of mersalyl acid or the Ca++ ionophore, A23187, Ca++ uptake by this fraction was inhibited 98.0% and 87.4%, respectively. Electron microscopy revealed that this fraction consisted of numerous membrane vesicles, and measurements of Na+-K+-ATPase (adenosine triphosphatase) activity indicated minimal plasma membrane contamination. It was concluded that this microsomal fraction consisted primarily of sarcoplasmic reticulum. At physiological free [Ca++] levels, Ca++ uptake by this fraction was inhibited by norepinephrine through a process sensitive to tolazoline but not propranolol. The effects on Ca++ uptake of added cyclic adenosine monophosphate (cAMP) alone or with rabbit or bovine protein kinase were inconclusive. The organic Ca++ channel blockers, nifedipine and verapamil, significantly inhibited Ca++ uptake by sarcoplasmic reticulum.

Dysfunction of the sarcoplasmic reticulum in polymyositis

The transport of calcium in vesicles of sarcoplasmic reticulum isolated from muscle specimens from 6 patients with early, active polymyositis and from 11 controls was examined. The time courses of calcium uptake and calcium-dependent ATPase activity were measured simultaneously. Calcium uptake by sarcoplasmic reticulum vesicles from patients with polymyositis was 50% less than that by vesicles from controls (P less than 0.001). In contrast, no difference in calcium-dependent ATPase activity was noted between vesicles from patients with polymyositis and controls. The demonstrated defect may be important in the pathogenesis of muscle weakness in polymyositis.

Effect of gentamicin treatment on adenylate cyclase and Na+, K+-ATPase activities in renal tissues of rats

Gentamicin (20 mg/kg) treatment of male rats reduced Na+,K+-ATPase activity by 32% in renal cortical plasma membranes. In contrast, adenylate cyclase stimulation by isoproterenol or a guanyl nucleotide or both was enhanced by as much as twofold in glomeruli and in plasma membranes of gentamicin-treated rats. These effects of gentamicin are suggested to be related to the changes in renal phospholipid metabolism produced by the drug.

Changes in renal phospholipid fatty acids in diabetes mellitus: correlation with changes in adenylate cyclase activity

Male Sprague-Dawley rats made diabetic with alloxan (37.5 mg/kg) or streptozotocin (65 mg/kg) were killed after 3-6 weeks of disease; renal tissues were studied for phospholipid content and for fatty acid composition of the phospholipids. No consistent change was noted in total phospholipid content nor in the proportion of various phospholipids in diabetics. However, diabetic animals showed a consistent reduction of arachidonic acid content in phosphatidylcholine (PC) and phosphatidylethanolamine in whole renal cortex, plasma membranes purified from renal cortex, and in isolated glomeruli. Associated with the fall in arachidonic acid was a rise in linoleic acid in the samples studied. Insulin therapy returned the fatty acid profiles to normal. These results are similar to patterns observed in other diabetic tissues and suggest that diabetes is associated with generalized changes in cell membranes. That these structural changes may have functional significance is suggested by demonstrated alterations in the temperature-dependence of adenylate cyclase in renal plasma membranes of diabetic animals. Adenylate cyclase is thought to be intimately associated with PC in plasma membranes, a phospholipid showing significant changes in fatty acid content in diabetes (unsaturation index 165 +/- 2 for normals, 147 +/- 5 for diabetics). Na+,K+-ATPase which is thought to be primarily associated in vivo with phosphatidylinositol (PI), shows no change in apparent energy of activation in diabetes. The fatty acid content of PI is minimally altered in diabetes, and the unsaturation index is unchanged.

Inhibition of Ca2+-dependent protein kinase and Ca2+/Mg2+-ATPase in cardiac sarcolemma by the anti-calmodulin drug calmidazolium

Sarcolemma (SL) vesicles, isolated from pig heart, contain both a Ca2+-calmodulin-dependent protein kinase (CaM-PK) and a Ca2+-dependent Mg2+-ATPase (Ca2+/Mg2+)-ATPase). Some of their properties have been compared: their affinity for Ca2+ ions, dependence on exogenous calmodulin (CaM) and sensitivity to the anti-CaM drug calmidazolium (R24571). The properties of Ca2+-CaM-dependent brain phosphodiesterase (PDE) have also been examined. R24571 appeared to be the most potent inhibitor from brain PDE. For the three enzymes studied, exogenously added CaM was able to antagonize the R24571 inhibition, although the efficiency to counteract was rather low in the case of the SL Ca2+/Mg2+-ATPase. R24571 decreased the affinity of the Ca2+/Mg2+-ATPase for Ca2+ ions (KCa 0.35 versus 0.75 microM) and exerted an inhibition non-competitive with Ca2+ ions on the other CaM-dependent enzymes. Membrane-bound CaM, which is involved in controlling the Ca2+/Mg2+-ATPase, appeared to be present in a stoichiometry varying from 1:1 to 1:4 compared to the 32P-intermediate of the ATPase. R24571 treatment of SL vesicles selectively solubilized a number of proteins in the molecular range of 13-20 kD, which may include CaM. The results suggest that different mechanisms are involved in the CaM control of the two SL enzymes studied.

Phosphorylation of purified bovine cardiac sarcolemma and potassium-stimulated calcium uptake

Sarcolemmal vesicles were prepared from bovine cardiac muscle by differential and discontinuous sucrose density gradient centrifugation. Na+/K+-ATPase was purified 33-fold to a specific activity of 53 +/- 0.5 (12) mumol Pi X mg-1 X h-1, binding sites for strophantin 20-fold to a density of 56.3 +/- 5.3 (14) pmol/mg and that for the calcium antagonist nitrendipine 5.5-fold to a density of 0.72 +/- 0.07 (6) pmol/mg. The specific activity of the Na+/Ca2+ exchanger was 61.1 +/- 3.7 (6) nmol/mg. The vesicles had an intravesicular volume of 20 +/- 4 (4) microliter/mg and 56.9 +/- 6 (4)% of the vesicles were right-side-out oriented. Several peptides of the purified membranes were phosphorylated in the presence of Mg . ATP and EGTA. Most of the radioactive phosphate was incorporated into a peptide with an apparent molecular mass of 22 kDa. Denaturation of the membranes at 100 degrees C changed the mobility of this peptide to 15 kDa and 11 kDa. This peptide could not be distinguished from a sarcoplasmic reticulum peptide of similar molecular mass. The phosphorylation of the sarcolemmal peptide was stimulated by Ca2+/calmodulin, cAMP and the catalytic subunit of cAMP-dependent protein kinase. A comparison of the phosphorylation of sarcolemmal membranes with that of sarcoplasmic reticulum showed that Ca2+/calmodulin stimulated in each membrane, the phosphorylation of the 22-kDa peptide and a 44-kDa peptide, and in the sarcoplasmic reticulum the phosphorylation of an additional peptide of 55-kDa. Ca2+/calmodulin-dependent phosphorylation of a 55-kDa peptide could not be demonstrated in sarcolemma, regardless if sarcolemmal membranes were incubated together with sarcoplasmic reticulum or if the phosphorylation was carried out in the presence of purified cardiac myosin light chain kinase or phosphorylase kinase. 'Depolarization' induced Ca2+ uptake which was measured according to Bartschat, D.K., Cyr, D.L. and Lindenmayer, G.E. [(1980) J. Biol. Chem. 255, 10044-10047] was 5 nmol/mg protein. This uptake was not enhanced after preincubation of the vesicles with Mg . ATP or Mg . ATP and cAMP-dependent protein kinase. The value of 5 nmol/mg protein is in agreement with the theoretical amount of Ca2+ which can be accumulated by the bovine cardiac sarcolemma in the absence of a driving force other than the Ca2+ gradient. The potassium-stimulated Ca2+ uptake was not blocked by the organic Ca2+ channel blockers. Prolonged incubation of Mg . ATP with sarcolemmal vesicles in the presence of various ATPase inhibitors led to the hydrolysis of ATP. The liberated phosphate precipitated with Ca2+ in the presence of LaCl3. These precipitates amounted to an apparent Ca2+ uptake ranging from 50 to over 1000 nmol/mg. The results suggest that potassium-stimulated Ca2+ uptake of bovine cardiac sarcolemmal vesicles is not enhanced in the presence of ATP or by phosphorylation of a 22-kDa peptide.

Differences in sarcolemmal preparations: cell surface material and membrane sidedness

Two different procedures were employed for the isolation of sarcolemma from the rat heart and the membranes were studied with respect to the presence of cell surface material as well as their functional characteristics. Both hypotonic shock-LiBr treatment method (fraction HL) and sucrose density gradient method (fraction S) yielded membranes enriched 8 to 13 fold with respect to Na+-K+ ATPase and adenylate cyclase activities in comparison to heart homogenate. Cell surface material was demonstrated on the outer surface of the vesicles only in fraction HL with cationic dyes, lanthanum and ferritin, applied either to the isolated fractions or perfused in the heart through coronaries. Fraction HL also had high sialic acid content. ATP independent Ca2+ binding in fraction HL was about 6 times more than that in fraction S which had little sialic acid and showed no cell surface staining with cationic dyes. On the other hand, ATP-dependent Ca2+ binding and Ca2+-stimulated Mg2+ dependent ATPase activities in fraction S were 4 to 6 times higher than those in fraction HL. Epinephrine stimulated adenylate cyclase in fractions HL and S by 24 and 3% whereas ouabain was found to inhibit Na+-K+ ATPase in these fractions by 80 and 10% respectively. A mild treatment of the membranes with deoxycholate to eliminate the semipermeable characteristics or effects of sidedness of the vesicles resulted in an almost complete ouabain inhibition of Na+-K+ ATPase in both fractions. These data suggest that presence of cell surface material as well as membrane sidedness has an important role in in vitro expression of functional characteristics of sarcolemma. It is emphasized that sarcolemmal preparations containing cell surface material will provide information more realistic to the native conditions in situ.

A high-affinity (Ca2+ + Mg2+)-ATPase in plasma membranes of rat ascites hepatoma AH109A cells

The activity of calcium-stimulated and magnesium-dependent adenosinetriphosphatase which possesses a high affinity for free calcium (high-affinity (Ca2+ + Mg2+)-ATPase, EC 3.6.1.3) has been detected in rat ascites hepatoma AH109A cell plasma membranes. The high-affinity (Ca2+ + Mg2+)-ATPase had an apparent half saturation constant of 77 +/- 31 nM for free calcium, a maximum reaction velocity of 9.9 +/- 3.5 nmol ATP hydrolyzed/mg protein per min, and a Hill number of 0.8. Maximum activity was obtained at 0.2 microM free calcium. The high-affinity (Ca2+ + Mg2+)-ATPase was absolutely dependent on 3-10 mM magnesium and the pH optimum was within physiological range (pH 7.2-7.5). Among the nucleoside trisphosphates tested, ATP was the best substrate, with an apparent Km of 30 microM. The distribution pattern of this enzyme in the subcellular fractions of the ascites hepatoma cell homogenate (as shown by the linear sucrose density gradient ultracentrifugation method) was similar to that of the known plasma membrane marker enzyme alkaline phosphatase (EC 3.1.3.1), indicating that the ATPase was located in the plasma membrane. Various agents, such as K+, Na+, ouabain, KCN, dicyclohexylcarbodiimide and NaN3, had no significant effect on the activity of high-affinity (Ca2+ + Mg2+)-ATPase. Orthovanadate inhibited this enzyme activity with an apparent half-maximal inhibition constant of 40 microM. The high-affinity (Ca2+ + Mg2+)-ATPase was neither inhibited by trifluoperazine, a calmodulin-antagonist, nor stimulated by bovine brain calmodulin, whether the plasma membranes were prepared with or without ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetra-acetic acid. Since the kinetic properties of the high-affinity (Ca2+ + Mg2+)-ATPase showed a close resemblance to those of erythrocyte plasma membrane (Ca2+ + Mg2+)-ATPase, the high-affinity (Ca2+ + Mg2+)-ATPase of rat ascites hepatoma cell plasma membrane is proposed to be a calcium-pumping ATPase of these cells.

Alteration of sarcoplasmic reticulum after denervation of chicken pectoralis muscle

To determine the neural influence on the function of the sarcoplasmic reticulum (SR) of fast-twitch skeletal muscle, the superior pectoralis muscle of adult chicken was denervated, and the SR was isolated at 20 days post-denervation. The isolated SR was probably derived from the longitudinal SR and was relatively free of contaminants. The protein profile of the SR was quantitatively changed after denervation with an increase in the M55 and 30000-mol.wt. proteins relative to the Ca2+-ATPase. Ca2+-dependent ATPase activity and phosphoenzyme formation were lower in the denervated-muscle SR; however, the enzyme catalytic-centre activity was similar to the control value. The decrease in Ca2+-ATPase activity in denervated-muscle SR was accompanied by a lower Ca2+ accumulation so that the relationship between Ca2+ accumulation and Ca2+-dependent ATPase activity was well maintained in the SR from denervated muscle. The data imply that denervation may result in a diminution of functional Ca2+ pump sites. Evidence is presented, though, which suggests that denervation affects a single class of Ca2+-binding sites of the Ca2+-ATPase, resulting in a lower affinity for Ca2+.