Calcium-binding sites in rat myocardial sarcolemma

Positive inotropic, negative chronotropic, and coronary vasoconstrictor effects of acetylcholine in isolated rat hearts: role of muscarinic receptors, prostaglandins, protein kinase C, influx of extracellular ca2+, intracellular Ca2+ release, and endothelium

The involvement of nitric oxide (NO), muscarinic receptors, prostaglandins, calcium influx via slow calcium channels, Ca2+ release from intracellular stores, protein kinase C, and endothelium in the positive inotropic, negative chronotropic, and coronary vasoconstrictor effects of acetylcholine (ACh) has been investigated in isolated rat hearts. The perfusion of hearts with ACh (10(-7), 5 x 10(-7), and 10(-6) M) produced marked decreases in heart rate and coronary flow and a marked increase in contractile force. Similar effects have been observed during the perfusion of hearts with ACh in the presence of Nomega-nitro-L-arginine methyl ester (L-NAME), which is an inhibitor of NO synthesis. The positive inotropic, negative chronotropic, and coronary vasoconstrictor effects of ACh were abolished by muscarinic receptor blocker atropine. In hearts pretreated with cyclooxygenase inhibitor indomethacin, ACh significantly decreased heart rate but did not significantly affect coronary flow and contractile force. In the presence of calcium channel antagonist verapamil or protein kinase C inhibitor staurosporine, ACh produced a significant drop in heart rate but did not significantly affect coronary perfusion pressure and force of contraction. In the presence of the inhibitor of the release of Ca2+ from intracellular stores dantrolene sodium, ACh produced a significant increase in coronary perfusion pressure and a marked decline in heart rate, but did not significantly affect force of contraction. Furthermore, the disruption of endothelium by perfusing the hearts with saponin abolished the vasoconstrictor effect of ACh but did not alter negative chronotropic and positive inotropic effect. Our results suggest that ACh causes vasoconstrictor, negative chronotropic, and positive inotropic effects in isolated rat hearts. Cardiac effects of ACh are related to muscarinic receptor activation, and prostaglandins modulate ACh-induced vasoconstriction and positive inotropy. Our data also suggest that protein kinase C and calcium influx from extracellular source may be responsible for the vasoconstrictor and positive inotropic effect of ACh. The calcium release from intracellular stores may mediate the positive inotropic effect, and the vasoconstrictor effect of ACh depends on an intact endothelium.

Sarcolemmal calcium binding sites in heart: I. Molecular origin in "gas-dissected" sarcolemma

Calcium in the myocardial cell is highly compartmentalized and a fast, an intermediate, a slow and a nonexchangeable calcium pool have been described. The fast pool contains 66% of the total cell exchangeable calcium in cultured neonatal rat heart cells with a t1/2 of less than 1.5 sec. Though the cellular origin of this fast pool is unknown, its rapidity and its displacement by La3+ most likely places it at the sarcolemma or at least in rapid equilibrium with the sarcolemma. We isolated the sarcolemma of cultured neonatal rat heart cells using the gas-dissection technique, which yields a pure sarcolemmal preparation in less than a second, thereby precluding membrane changes which might occur during conventional plasma membrane isolation. We determined the calcium binding characteristics of these membranes, using an on-line technique to monitor 45Ca, which allows measurement of 45Ca binding characteristics in the presence of unbound 45Ca. Two classes of calcium binding sites were determined: (i) Kd of 13 microM, capacity 7 nmol/mg and (ii) Kd of 1.1 mM, capacity of 84 nmol/mg. To assess the molecular origin of the sarcolemmal calcium binding we treated the membranes with a variety of enzymes. Protease or neuraminidase treatment did not cause large changes in these parameters. Simultaneous treatment with two different phospholipases C or the extraction of the lipids with isopropanol resulted in a dramatic loss of the low-affinity binding sites. These results, in association with previously defined sarcolemmal phospholipid distribution, places the low-affinity binding sites at the cytoplasmic leaflet. The physiological implication of this localization as it pertains to cellular calcium exchange is discussed.

Two major antigens of heart sarcolemma are Ca2(+)-binding glycoproteins that copurify with the dihydropyridine receptor

Ca2+ binding has been studied in isolated heart sarcolemmal membranes using the 45Ca overlay technique. 45Ca bound to two sarcolemmal polypeptides of 125 kDa and 97 kDa in preparations from dog, rabbit, cow and pig. During fractionation on DEAE ion-exchange and wheat-germ lectin affinity columns, the two Ca2(+)-binding polypeptides copurified with the dihydropyridine receptor associated with the voltage gated Ca2+ channel. These polypeptides were the major proteins in the isolated fraction as judged by silver staining in SDS-PAGE. Antisera raised against purified dog heart, sarcolemma indicated that the 125 and 97 kDa polypeptides were highly antigenic components of this membrane. The antisera cross-reacted with similar polypeptides in cardiac sarcolemmal preparations from rabbit, cow and pig, but not sarcoplasmic reticulum membranes. Purified antibodies against the 125 kDa polypeptide did not cross-react with the 97 kDa polypeptide, while antibodies against the 97 kDa polypeptide did not cross-react with the 125 kDa polypeptide. Both the 125 kDa and 97 kDa polypeptides bound wheat-germ lectin, suggesting both were glycoproteins. It is unlikely that these Ca2+ binding glycoproteins represent subunits of the dihydropyridine receptor-Ca2+ channel in this membrane.

Roles of proteins in cation/membrane interactions of isolated rat cardiac sarcolemmal vesicles

To ascertain the roles of the membrane proteins in cation/sarcolemmal membrane binding, isolated rat cardiac sarcolemmal vesicles were extensively treated with Protease (S. aureus strain V.8). SDS-gel electrophoresis, protein and phosphate analysis confirmed that at least 20-22% of the protein, but none of the phospholipid, was solubilized by this procedure, and that the remaining membrane proteins were extensively hydrolyzed into small fragments. The cation binding properties of the treated vesicles were then examined by analyzing their aggregation behavior. The results demonstrate that this procedure had no effect on the selectivity series for di- and trivalent cation binding, or the divalent cation-induced aggregation behavior of the sarcolemmal vesicles at different pHs, indicating that proteins are probably not involved in these interactions and cannot be the low affinity cation binding sites previously observed. It did, however, change the pH at which protons induced sarcolemmal vesicle aggregation, suggesting a possible role for proteins in these processes. Protease treatment also modified the effects of fluorescamine labelling on divalent cation-induced vesicle aggregation, indicating that the NH2 groups being labelled with fluorescamine are located on the sarcolemmal proteins. Together, these results support the hypothesis that di- and trivalent cation binding to the sarcolemmal membrane is largely determined by lipid/lipid and/or lipid/carbohydrate interactions within the plane of the sarcolemmal membrane, and that membrane proteins may exert an influence on these interactions, but only under very specialized conditions.

Changes in the surface charge properties of isolated cardiac sarcolemmal vesicles measured by light scattering. I. Characteristics of rat and canine preparations

The cation-binding characteristics of isolated sarcolemmal vesicles from rat and canine cardiac muscle cells were investigated. To help elucidate the molecular properties involved in these interactions the cation-induced aggregation behavior of rat and canine cardiac sarcolemmal vesicles, sonicated unilamellar vesicles (SUVs) made from sarcolemmal lipid extracts, and SUVs generated from combinations of synthetic lipids similar to those found in the sarcolemmal membrane, as well as mitochondrial and sarcoplasmic reticulum enriched membrane fractions were examined. Our results indicate that cations, such as Ca2+, to indeed bind to the sarcolemmal membrane surface. They also suggest that two (or more) interacting sites are involved in the Ca2+-induced aggregation of the isolated sarcolemmal vesicles, and that sarcolemmal lipid components could be the primary binding sites. The modulating (secondary) sites on the other hand may be protein or carbohydrate in nature, or require specific lipid organizational properties. Finally, the results indicate that the interactions of cations, such as Ca2+, with the sarcolemmal surface are species specific, with the sarcolemmal membranes of both rat and canine preparations having different physico-chemical properties.

Comparative biochemical study of sarcolemma and sarcoplasmic reticulum fractions isolated from mouse skeletal and cardiac muscles

1. Ca-ATPase activity, calcium-binding proteins and Concanavalin-A-bound glycoproteins of sarcolemma and sarcoplasmic reticulum were compared in mouse cardiac and skeletal muscles. 2. Ca-ATPase activity and calsequestrin were quite reduced in cardiac muscle, and the quantity of calcium bound to these two proteins was practically negligible, contrary to what was observed with skeletal muscle. In addition, the quantity of lipid bound calcium was not greater in cardiac muscle than in skeletal muscle. 3. Certain proteins seemed exclusively specific for skeletal muscle, including a 30,000 mol. wt glycoprotein which was totally absent in cardiac muscle sarcolemma.

Sodium-sensitive calcium binding to sarcolemma-enriched preparations from canine ventricle

Calcium binding to sarcolemma-enriched preparations from canine ventricle was evaluated. The preparation was exposed to calcium and 45Ca at physiological ionic strength, pH 7.4, for 15-18 hours at 5 degrees C. Bound calcium was separated from free by filtration and washing of the filter with solutions containing calcium and LaCl3. After equilibration at 5 degrees C, exposure to 37 degrees C caused an irreversible loss of binding. Monovalent cations (157 mM) reduced calcium binding: Na+ much greater than Li+ greater than Cs+ greater than K+ greater than Rb+ approximately equal to choline. In 1 microM calcium, divalent cations (3 mM) reduced binding: Sr++ greater than Ba++ greater than Mg++ approximately equal to Mn++. At 1-300 microM calcium, inhibition of the sodium-sensitive component of binding was characterized by I50's of 3.2-9.5 mM sodium. Comparison of binding by centrifugation versus filtration suggested that the sodium-sensitive component resided on constituents within the membrane vesicles. Calcium binding in 1 mM ethyleneglycol-bis-(beta-aminoethylether)N,N'-tetraacetic acid at pH 7.1 and 5 degrees C, revealed a single species of sodium-sensitive calcium-binding sites: Kd = 0.052 microM and Bmax = 6.73 nmol/mg. In 3 mM magnesium, the Kd was 0.205 microM and the Bmax was 9.03 nmol/mg. Nearly complete inhibition of binding was observed as sodium was raised from 1 to 10 mM. Thus, a substantial number of calcium-binding sites were detected at 5 degrees C in 3 mM magnesium at physiological ionic strength and pH 7.1. The affinity of these sites was in the range necessary to modulate intracellular free calcium. The sensitivity to sodium was at the lower end of the range estimated for intracellular sodium.

Sarcomere dynamics in single myocardial cells as revealed by high-resolution light diffractometry

A specially designed diffractometer with a high spatial and temporal resolution recorded the diffraction of a laser beam by single enzymatically isolated myocardial cells. The fine structures within the first-order diffraction were resolved and each structure was interpreted as the diffraction from a group of sarcomeres of nearly equal length. During activation of the cell dynamics of each discrete group of sarcomeres was uniform and independent of the other groups. However, a small nonuniform component in the sarcomere dynamics was observed and attributed to the coupling between the shortening tension and the radial stress resulting from the expansion of the myofibrillar cross-section. The time-course of the diffraction fine structures during contractile activity revealed (1) the period of the contraction-relaxation cycle, (2) the latent period, (3) the shortening and relengthening speeds and (4) the variation in the line width and intensity of the fine structure. Measurements showed that the latent period was dependent on the free Ca2+ of the cell's bathing solution while the initial shortening speed was not. The diffraction line width and intensity of the shortening cell were explained by the grating model.

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.

The effect of a calcium channel antagonist, Nisoldipine, on the ischemia-induced change of canine sarcolemmal membrane

Ischemic injury was produced in the dog heart by occluding the left anterior descending coronary artery just below the second diagonal branch for a duration of 1.5 h followed by the release of occlusion. Nisoldipine, 3.5 micrograms/kg was injected intravenously 10 min before the occlusion and again 10 min before the commencement of reperfusion. The activity of serum creatine phosphokinase greatly increased after the reperfusion, and this increase was significantly suppressed by Nisoldipine. This drug, in addition, prevented ischemia-induced myocardial hemorrhage and premature ventricular contraction. Sarcolemmal membrane vesicles were prepared from an ischemic and non-ischemic portions of the myocardium 3 h after the commencement of reflow. The fraction was purified approximately 12-fold with respect to ouabain-sensitive (Na+K+)-ATPase as an indicator; contamination of mitochondria was minimum with cytochrome c oxidase as an indicator. Without treatment of Nisoldipine, the total amount of sarcolemmal phospholipid obtained from the ischemic area, as well as the amounts of phosphatidyl-choline and phosphatidyl-ethanolamine, were significantly decreased as compared with those obtained from the non-ischemic area. Nisoldipine treatment abolished the decrease in the sarcolemmal phospholipids, total as well as phosphatidyl-choline and -ethanolamine, induced by ischemia plus reperfusion. Therefore, our work indicates that the Ca++ channel antagonist, Nisoldipine, suppresses the ischemia-induced increase in phospholipid breakdown of cardiac sarcolemma probably through its inhibitory effect on the Ca++-mediated activation of membrane phospholipase, through its vasodilatory action, or both.

Ca2+ displacement by Polymyxin B from sarcolemma isolated by 'gas dissection' from cultured neonatal rat myocardial cells

Amphiphilic, cationic Polymyxin B is shown to displace Ca2+ from 'gas dissected' cardiac sarcolemma in a dose-dependent, saturable fashion. The Ca2+ displacement is only partially reversible, 57% and 63%, in the presence of 1 mM or 10 mM Ca2+, respectively. Total Ca2+ displaced by a non-specific cationic probe, lanthanum (La3+), at maximal displacing concentration (1 mM) was 0.172 +/- 0.02 nmol/microgram membrane protein. At 0.1 mM, Polymyxin B displaced 42% of the total La3+-displaceable Ca2+ or 0.072 +/- 0.01 nmol/microgram protein. 5 mM Polymyxin displaced Ca2+ in amounts equal to those displaced by 1 mM La3+. Pretreatment of the membranes with neuraminidase (removal of sialic acid) and protease leads to a decrease in La3+-displaceable Ca2+ but to an increase in the fraction displaced by 0.1 mM Polymyxin from 42% to 54%. Phospholipase D (cabbage) treatment significantly increased the La3+-displaceable Ca2+ to 0.227 +/- 0.02 nmol/microgram protein (P less than 0.05), a gain of 0.055 nmol. All of this phospholipid specific increment in bound Ca2+ was displaced by 0.1 mM Polymyxin B. The results suggest that Polymyxin B will be useful as a probe for phospholipid Ca2+-binding sites in natural membranes.

Ca2+- and Mg2+-dependent ATPase activities in the deoxycholate-treated rat heart sarcolemma

1. Isolated rat heart sarcolemma was treated with different concentrations of an ionic detergent, deoxycholate (DOC) and ATP hydrolysis in the presence of Ca2+ or Mg2+ was determined. 2. Both Ca2+-dependent ATPase and Mg2+-dependent ATPase activities were decreased in the DOC-treated membranes; however, the depression of Mg2+-dependent ATPase activity was greater than that of Ca2+-dependent ATPase. 3. The differential changes in Ca2+-dependent ATPase and Mg2+-dependent ATPase activities were apparent when incubations with DOC were carried out for different time intervals and at different temperatures. 4. In DOC-treated preparations, the Km value for Ca2+-dependent ATPase was decreased whereas that for Mg2+-dependent ATPase was increased. The half maximal velocities of the Ca2+-dependent ATPase and Mg2+-dependent ATPase enzyme reactions in the treated preparations were obtained at a DOC: membrane protein ratio of 3.0 and 0.6, respectively. 5. In the DOC-treated membranes exhibiting the half maximal velocities of enzyme reactions, the Ka value for Ca2+-dependent ATPase was drastically reduced but remained unchanged for Mg2+-dependent ATPase. 6. The DOC treatment was associated with a loss of protein as well phospholipids and resulted in changes in the ultrastructural integrity of the membrane. 7. Varying degrees of decreases in the activities of sarcolemmal adenylate cyclase. (Na+-K+)-ATPase, 5'-nucleotidase and calcium binding were seen upon DOC treatment. 8. The extent of reduction in Ca2+-dependent ATPase and Mg2+-dependent ATPase activities were also different when the membrane was treated with a non-ionic detergent, Lubrol PX. 9. These data suggest that Ca2+-dependent ATPase in heart sarcolemma is more resistant than Mg2+-dependent ATPase to detergent treatments and further indicate some differences in the properties of these enzymes.

The effect of temperature on the contractile response of cardiac muscle from two frog species, Rana temporaria and Xenopus laevis, at two different calcium concentrations-II. At pH 7.0 and pH 6.6

1. Mechanical parameters were recorded from paced ventricular cardiac muscle strips of two amphibian species. Rana temporaria and xenopus laevis. 2. The strips contracted at pH 6.6 and 7.0 with either 1.6 or 3.0 mM Ca2+ while the temperature was changed from 5 to 30 degree C. 3. In the hearts of R. temporaria, but not in those of X. laevis, in increased Ca2+ concentration at low pH could compensate for the effect of lowered pH. 4. It is concluded that during high CO2 tension and/or low pH the myocardium of X. laevis was more temperature dependent, less Ca2+ dependent in the Ca2+ range examined and less sensitive to the pH and/or the CO2 tension during physiological temperature and Ca2+ concentration, than the cardiac muscle of R. temporaria.

Control of intracellular calcium in heart muscle

Regulation of the intracellular availability of Ca2+ in heart muscle cells is a necessary prerequisite for survival. Various subcellular organelles are involved, including the sarcoplasmic reticulum, the mitochondria, and the sarcolemmal complex. When isolated, the sarcolemmal complex forms either "inside-out" or "right-side-out" vesicles, and these have been used to study the regulatory role of the sarcolemma. Right-side-out vesicles exhibit passive Ca2+ binding that is inhibited by certain divalent cations including Ni2+, Co2+. By contrast, the binding that is exhibited by the inside-out vesicles is highly sensitive to H+ and cAMP. These findings will be discussed in accordance with the hypothesis that both the surface coat and the cytosolic surface of the plasmalemma play important roles in regulating the intracellular availability of Ca2+.

Negatively charged sites and calcium binding in the isolated rat heart sarcolemma

Colloidal iron staining, calcium binding and enzyme activities were studied in the isolated rat heart sarcolemma. Colloidal iron staining of the sarcolemma revealed a high density of negatively charged sites associated with the cell surface. This membrane fraction was found to have calcium binding activity at both low (0.1 mM) and high (1.25 mM) concentrations of calcium. Pretreatment of the sarcolemma with either trypsin, phospholipase C or neuraminidase, was associated with a reduction in colloidal iron staining as well as decreased calcium-binding activity at high concentrations of calcium. Calcium binding at low concentrations was decreased by both trypsin and neuraminidase. Mg2+ ATPase, Ca2+ ATPase, and Na+-K+ ATPase activities were altered by neuraminidase and trypsin treatments, whereas phospholipase C treatment altered Na+-K+ ATPase only. It is concluded that both surface negative charge and calcium-binding sites associated with the isolated rat heart sarcolemma are contributed by a mosaic of biomolecules including proteins, phospholipids and glycoproteins, and alterations in the surface charge may influence the activities of membrane-bound enzymes.

Sialic acid: regulation of electrogenesis in cultured heart cells

Removal of up to 50% of the sialic acid from aggregates of 7-day chick embryo heart cells during incubation in a highly purified preparation of neuraminidase resulted in hyperpolarization of the maximum diastolic potential, reduction in the slope of diastolic depolarization leading to slowing of beating, negative shifts of threshold potential and the voltage at which upstroke velocity was maximal, and an initial increase in the action potential overshoot. These effects were opposite to those induced by phospholipase C, a possible contaminant. The modification of electrical properties by neuraminidase is suggested to result from an enhanced influx of calcium ions that might "screen" or bind specifically to internal negative fixed charges and thereby shift the voltage dependence of conductance and kinetic parameters to more negative potentials. This hypothesis is supported by the results above and by greater uptake of 45Ca following release of sialic acid, augmentation of enzyme-induced changes in elevated Ca2+, and the similarity of effects produced by A23187, and inophore which also increased 45Ca uptake. However, other mechanisms cannot be ruled out at the present time.

Effect of phospholipid methylation on beta-adrenergic receptors in the normal and hypertrophied rat myocardium

Abdominal aortic constriction in rats results in mild cardiac hypertrophy (20% increase in left ventricular weight compared to sham-operated controls) which is associated with increased numbers of beta-adrenergic receptors (123 +/- 7.3 fmol/mg protein (mean +/- SE) vs. 87 +/- 4.3 fmol/mg in controls, P < 0.01) without changes in their affinities for dihydroalprenolol. In vitro synthesis of phosphatidylcholine through successive methylation of phosphatidylethanolamine by S-adenosyl-L-methionine is enhanced in the hypertrophied myocardium) 0.38 +/- 0.03 nmol/mg per 30 minutes vs. 0.23 +/- 0.03 nmol/mg per 30 minutes in controls, P < 0.01). In both experimental groups, methyltransferase activity has a high affinity for S-adenosyl-L-methionine (Km = 6.8 microM), depends on Mg2+, is optimal around pH 9.0, and is inhibited by S-adenosyl-L-homocysteine (ki = 8.3 microM). The possible relationship between phospholipid methylation and changes in myocardial beta-adrenergic receptors was studied in both normal and hypertrophied hearts. Preincubation of cardiac membranes with S-adenosyl-L-methionine increased the numbers of b eta-adrenergic receptors in proportion to the duration of incubation and the concentration of S-adenosyl-L-methionine. In both groups, S-adenosyl-homocysteine, but not 5'-AMP or L-methionine, attenuated the increase in beta-adrenoreceptors. These results indicate that phospholipid methylation may be an important mechanism for regulation of beta-adrenergic mechanisms in both normal and hypertrophied myocardium.

Effect of inotropic agents on the calcium binding to isolated cardiac sarcolemma

Ca2+ binding to fragmented sarcolemma isolated from canine heart was measured by an ultracentrifugation technique. Two classes of binding site with dissociation constants of 2.0 . 10(-5) and 1.2 . 10(-3) M were identified. The capacities of the high- and low-affinity sites were 15 and 452 nmol/mg, respectively. These sites were not affected by treatment with neuraminidase. The effects of various cations and drugs on Ca2+ binding were studied. All cations tested inhibited Ca2+ binding with the following order of potency: trivalent greater than divalent greater than monovalent cations. The order of potency for the monovalent ions was: Na greater than K+ greater than Li+ greater than or equal to Cs+ and for the divalent and trivalent ions: La3+ greater than or equal to Mn2+ greater than Sr2+ greater than or equal to Ba2+ greater than Mg2+. 1 . 10(-3) M caffeine and 1 . 10(-8) M ouabain increased the capacity of the low-affinity sites to 1531 and 837 nmol/mg, respectively. 1 . 10(-7) M verapamil, acidosis (pH 6.4), 1 -10(-5) M Mn2+ and 1 . 10(-4) M ouabain depressed the capacity of the low-affinity sites to a range of 154--291 nmol/mg. The dissociation constants of the high- and low-affinity sites and the capacity of the high-affinity sites were not affected by these agents.

Isolation and characterization of cardiac sarcolemma

A procedure was developed for the isolation of cardiac sarcolemmal vesicles. These vesicles are enriched about ten-fold (with respect to the tissue homogenate) in K+-stimulated p-nitrophenylphosphatase, (Na+ + K+)-ATPase, 5'-nucleotidase activities and sialic acid content, all of which are believed to be components of the sarcolemma. The sarcolemma of tissue culture cardiac cells were radioiodinated and the distribution of this radioiodine paralleled the distribution of the other membrane markers above. There was very little contamination of the sarcolemmal fraction by sarcoplasmic reticulum (as judged by Ca2+-ATPase and glucose-6-phosphatase activities) or inner mitochondrial membranes (as judged by succinate dehydrogenase activity). There may, however, be some contamination by outer mitochondrial membranes (as judged by monoamine oxidase and rotenone-insensitive NADH cytochrome c reductase activities) which have rarely been monitored in cardiac sarcolemmal preparations. The purity of this preparation is good when compared with other cardiac sarcolemmal preparations. This preparation should be very useful in studying the roles of the cardiac sarcolemma (e.g. in excitation contraction coupling and Ca2+ binding).

Taurine enhancement of calcium binding to rat heart sarcolemma

The effect of taurine on calcium binding to isolated rat heart sarcolemmal membrane was examined. Taurine was observed to increase calcium binding to the low affinity sites in both high sodium-low potassium and low sodium-high potassium buffers. Taurine was also seen to antagonize the inhibition of calcium binding to the sarcolemma caused by both verapamil and lanthanum. Nevertheless, membrane structural changes due to taurine could not be detected using the spin label ESR probe 2N14. A possible regulatory role of taurine is discussed.

Spin label studies on rat liver and heart plasma membranes: effects of temperature, calcium, and lanthanum on membrane fluidity

The structures of rat liver and heart plasma membranes were studied with the 5-nitroxide stearic acid spin probe, I(12,3). The polarity-corrected order parameters (S) of liver and heart plasma membranes were independent of probe concentration only if experimentally determined low I(12,3)/lipid ratios were employed. At higher probe/lipid ratios, the order parameters of both membrane systems decreased with increasing probe concentration, and these effects were attributed to enhanced nitroxide radical interactions. Examination of the temperature dependence of approximate and polarity-corrected order parameters indicated that lipid phase separations occur in liver (between 19 degrees and 28 degrees C) and heart (between 21 degrees and 32 degrees C) plasma membranes. The possibility that a wide variety of membrane-associated functions may be influenced by these thermotropic phase separations is considered. Addition of 3.9 mM CaCl2 to I(12,3)-labeled liver plasma membrane decreased the fluidity as indicated by a 5% increase in S at 37 degrees C. Similarly, titrating I(12,3)-labeled heart plasma membranes with either CaCl2 or LaCl3 decreased the lipid fluidity at 37 degrees C, although the magnitude of the La3+ effect was larger and occurred at lower concentrations than that induced by Ca2+; addition of 0.2 mM La3+ or 3.2 mM Ca2+ increased S by approximately 7% and 5%, respectively. The above cation effects reflected only alterations in the membrane fluidity and were not due to changes in probe--probe interactions. Ca2+ and La3+ at these concentrations decrease the activities of such plasma membrane enzymes as Na+, K+-ATPase and adenylyl cyclase, and it is suggested that the inhibition of these enzymes may be due in part to cation-mediated decreases in the lipid fluidity.