Smooth muscle cell sarcolemma. Purification and properties of plasma membranes from the rat uterus.

A membrane fraction with sarcolemmal properties was purified from the smooth muscle layers (myometrium) of rat uterus by successive differential and equilibrium centrifugation in sucrose. The putative sarcolemmal fraction was identified by iodination with [125I]iodosulfanilic acid, had an equilibrium density of 1.15, and was enriched in enzyme activities usually associated with the plasma membrane including 5'-nucleotidase (EC 3.1.3.5) and (Na+ + K+) ATPase (EC 3.6.1.3). These membranes were free of mitochondrial or nuclear membrane contamination, suggesting the relative enrichment of sarcolemmal membranes in the fraction. Proteins of the membranes were heterogeneous with respect to molecular weight, but only a few were labelled when intact muscle was radioiodinated. Uniform resistance of sarcolemmal proteins to trypsin digestion and salt extraction suggested many are tightly bound or intrinsic membrane proteins and was a further indication of the homogeneity of membranes in this fraction.     

Na+−Ca2+ exchange and Ca2+ channel characteristics in bovine aorta and coronary artery smooth muscle sarcolemmal membranes

Tension generation and Ca²⁺ flux in smooth muscle varies depending upon the diameter of a vessel and its location. The purpose of the present investigation was to determine if the biochemical characteristics of the Na⁺–Ca²⁺ exchanger and the Ca²⁺ channel differ in sarcolemmal membrane preparations isolated from a large conduit vessel (thoracic aorta) or from large and small coronary arteries. We also investigated the possibility of differences between sarcolemmal membranes isolated from coronary arteries dissected from the right and left ventricles. The purification of the sarcolemmal membranes was of a similar magnitude amongst the different groups. Contamination of the sarcolemmal membranes with other membranous organelles was negligible and similar amongst the groups. The K_m and V_max of Na⁺-dependent Ca²⁺ uptake in sarcolemmal vesicles was similar amongst the groups. Calcium channel characteristics were examined by measuring [³H]PN200-110 binding to sarcolemmal vesicles. The right coronary artery membranes from both large and small caliber vessels exhibited a higher K_d and the small right coronary artery sarcolemmal preparation had a lower maximal binding density for [³H] PN200-110. The results suggest that the right coronary artery, and in particular the small diameter right coronary artery, possesses altered Ca²⁺ channel characteristics in isolated sarcolemmal membranes.     

Activation of Ca2+/Mg2+ ATPase in heart sarcolemma upon electrical stimulation

Electrical stimulation of the rat heart sarcolemmal membranes with a square wave current was found to increase Ca²⁺-ATPase activity. This activation of the enzyme was dependent upon the voltage of the electric current, frequency of stimulation and duration of stimulation of the sarcolemmal membranes. The increase in ca²⁺-ATPase was reversible upon terminating the electrical stimulation. The activation of sarcolemmal Ca²⁺-ATPase due to electrical stimulation was markedly depressed when the reaction was carried out at high pH (7.8 to 8.2), low pH (6.6 to 7.0), high temperatures (45 to 50°C) and low temperatures (17 to 25°C) of the incubation medium. Ca²⁺-antagonists, verapamil and D-600, unlike other types of inhibitors such as propranolol and ouabain, were found to reduce the activation of sarcolemmal Ca²⁺-ATPase by electrical stimulation. These results support the view that Ca²⁺/Mg²⁺ ATPase may be involved in the gating mechanism for opening Ca²⁺-channels in the sarcolemmal membrane upon excitation of the cardiac muscle.     

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).     

Subcellular distribution and isolation of the Ca2+ antagonist receptor associated with the voltage regulated Ca2+ channel from rabbit heart muscle

The Ca²⁺ antagonist binding sites associated with the voltage dependent calcium channel in rabbit myocardium were found to distribute with the sarcolemmal Na^– + K⁺ ATPase and adenylate cyclase activities during subcellular fractionation on sucrose-density gradients. The equilibrium dissociation constants (K_D) for the binding of [³H]nitrendipine and [³H]verapamil were 0.31 ± 0.04 nM and 4.1 ± 0.5 nM respectively, and displayed an average density of 0.55 ± 0.05 pmol/mg and 0.4 ± 0.03 pmol/mg protein respectively for the most enriched membrane fraction. The Ca²⁺2 antagonist binding sites were solubilized from the membranes with the detergent 3-[(3-cholamidopropyl)dimethylammonio]propanesulfonate, and specific binding sites for [³H]PN200-110, [³H]verapamil and [³H]diltiazem were isolated on a wheat-germ lectin column. The binding sites for [³H]PN200-110 were enriched about 2500 fold as compared with the original homogenate and displayed a density of 28.5 ± 8 pmole/mg protein in the isolated fraction. Sodium dodecyl sulfate gel electrophoresis of the isolated drug binding proteins indicated enrichment of proteins of Mr 170000, 140000, 130000, 100 000 and 53000. The isolated receptor contained an intrinsic kinase activity that phosphorylated glycoproteins of Mr 170 000 and 53000. Exogenously added cAMP-kinase stimulated phosphorylation of the 170000, 100000, 53 000 and 28000 Mr glycoproteins in the receptor fraction. The results of this study indicate that the binding sites for [³H]nitrendipine, [³H]PN200-110, [³H]verapamil and [³H]diltiazem residue on glycoprotein(s) which are of sarcolemmal origin, and co-purify together on wheat germ lectin columns. The polypeptide composition of the Ca²⁺ antagonist binding sites from cardiac muscle appears to be very similar to that of the dihydropyridine receptor in skeletal muscle.Abbreviations CHAPS 3-[-(3-cholamidopropyl) dimethylammonio]-propanesulphonate - SDS sodium dodecyl sulphate Scholar of the Ontario Heart and Stroke Foundation.     

Isolation of plasma membrane and binding of the Ca2+ antagonist nimodipine in Chlamydomonas reinhardtii

Chlorophyll-free plasma membranes of the unicellular green alga Chlamydomonas reinhardtii Dangeard were purified from a microsomal fraction using an aqueous polymer two-phase system of 6.5% (w/w) dextran T500, 6·5% (w/w) polyethylene glycol 3350, 60 mM NaCI, 0 33 M sucrose and 5 mM potassium phosphate (pH 7·8). The plasma membrane fraction contained only 2·4% of the microsomal membrane protein. Specific activity of the plasma membrane marker enzyme, K*, Mg²⁺-ATPase (EC 3.6.1.3). was enriched 9-fold over the microsomal fraction, and 22% of total activity was recovered in the upper, polyethylene glycol-rich phase. Contamination from intracellular membranes was minimal. K*, Mg²⁺-ATPase showed a pH optimum at about 6·5, and addition of 0·05% (w/v) Triton X-100 stimulated the activity 3-fold. [³H]-Nimodipinc was employed to characterize 1,4-dihydropyridine-specific membrane receptors. Two apparent binding sites with different affinities to nimodipine were found in the crude microsomal fraction. The separation of plasma membranes from intracellular membranes revealed that one binding site with higher affinity (K_D= 9 nM) was located on the plasma membrane and a second binding site with lower affinity (K_D= 36 nM) on an intracellular membrane The apparent dissociation constants determined from the association and dissociation rate constants in kinetic experiments were comparable to those determined by equilibrium experiments. The maximum number of binding sites of the plasma membrane fraction and the intracellular membrane fraction was B_max= 440 and 470 fmol (mg protein)^-1, respectively. [³H]-Nimodipinc binding was inhibited by (±) verapamil and stimulated by D-cis-diltiazem in both fractions. Moreover, ethyle-neglycol-bis(2-aminoethylcther)-N, N'-tetraacctic acid (EGTA) inhibited [³H]-nimo-dipinc binding in the plasma membrane fraction but not in the intracellular membrane fraction This effect was cancelled by the addition of CaCl₂.     

Sodium-calcium ion exchange in skeletal muscle sarcolemmal vesicles

Summary The Ca²⁺ permeability of rabbit skeletal muscle sarcolemmal vesicles was investigated by means of radioisotope flux measurements. A membrane vesicle fraction highly enriched in sarcolemma, as revealed by enzymatic markers, was obtained from the 22–27% region of sucrose gradients after isopycnic centrifugation. The ability of sarcolemmal vesicles to exchange Na⁺ for Ca²⁺ was investigated by measuring Ca²⁺ influx into and efflux from sarcolemmal vesicles in the presence and absence of a Na⁺ gradient. It was found that Ca²⁺ movements were enhanced in the direction of the higher Na⁺ concentration. When intra- and extravesicular Na⁺ concentrations were high, Na⁺–Na⁺ exchange predominated and Na⁺–Ca²⁺ exchange was low or absent. The presence of the Ca²⁺ ionophore A23187 in the dilution medium resulted in the rapid release of Ca²⁺ and the elimination of the Na⁺-enhanced efflux of Ca²⁺, suggesting that internal rather than bound external Ca²⁺ was exchanged with Na⁺. La³⁺ abolished Na⁺–Ca²⁺ exchange and decreased overall membrane permeability. Na⁺–Ca²⁺ exchange was not due to sarcoplasmic reticulum or mitochondrial contaminants. This investigation suggests that skeletal muscle, like cardiac muscle and neurons, is capable of a transmembranous Na⁺–Ca²⁺ exchange.     

The isolation of plasma membrane from frog cardiac muscle cells

A vesicular preparation consisting largely of the plasma membrane of frog cardiac cells was isolated and its enzymatic activities and lipid content were investigated.The enriched plasma membrane preparation was obtained by (1) mildly homogenizing washed ventricles, (2) separating away the cellular debris by low speed differential centrifugation, (3) separating the plasma membrane fraction from other membranous components by centrifugation to equilibrium in various sucrose gradients. The frog cardiac plasma membranes were found to be concentrated between specific gravities of 1.07 and 1.11. In the membrane fraction the specific activities of membrane marker enzymes: 5′-nucleotides (EC 3.1.3.5), alklaline phosphatase (EC 3.1.3.1) and Na⁺---:K⁺)-activated ATPase were, respectively, 19, 15, and 14 times greater than in the homogenate. Activities of mitochondrial marker enzymes were either very low or absent. No unusual lipid types were found. Cardiolipin was less than 0.1% (by wt) in the membrane fraction. The molar ratio of phospholipid to cholesterol was approximatley 3 : 2.     

Molecular architecture of Ca2+ signaling control in muscle and heart cells

Ca²⁺ signaling in skeletal and cardiac muscles is a bi-directional process that involves cross-talk between signaling molecules in the sarcolemmal membrane and Ca²⁺ release machinery in the intracellular organelles. Maintenance of a junctional membrane structure between the sarcolemmal membrane and the sarcoplasmic reticulum (SR) provides a framework for the conversion of action potential arrived at the sarcolemma into release of Ca²⁺ from the SR, leading to activation of a variety of physiological processes. Activity-dependent changes in Ca²⁺ storage inside the SR provides a retrograde signal for the activation of store-operated Ca²⁺ channel (SOC) on the sarcolemmal membrane, which plays important roles in the maintenance of Ca²⁺ homeostasis in physiology and pathophysiology. Research progress during the last 30 years had advanced our understanding of the cellular and molecular mechanisms for the control of Ca²⁺ signaling in muscle and cardiovascular physiology. Here we summarize the functions of three key molecules that are located in the junctional membrane complex of skeletal and cardiac muscle cells: junctophilin as a “glue” that physiologically links the SR membrane to the sarcolemmal membrane for formation of the junctional membrane framework, mitsugumin29 as a muscle-specific synaptophysin family protein that contributes to maintain the coordinated Ca²⁺ signaling in skeletal muscle, and TRIC as a novel cation-selective channel located on the SR membrane that provides counter-ion current during the rapid process of Ca²⁺ release from the SR.     

Distribution of calmodulin and calmodulin-binding proteins in bovine pituitary: association of myosin light chain kinase with pituitary secretory granule membranes

Calcium is necessary for secretion of pituitary hormones. Many of the biological effects of Ca²⁺ are mediated by the Ca²⁺-binding protein calmodulin (CaM), which interacts specifically with proteins regulated by the Ca²⁺-CaM complex. One of these proteins is myosin light chain kinase (MLCK), a Ca²⁺-calmodulin dependent enzyme that phosphorylates the regulatory light chains of myosin, and has been implicated in motile processes in both muscle and non-muscle tissues. We determined the content and distribution of CaM and CaM-binding proteins in bovine pituitary homogenates, and subcellular fractions including secretory granules and secretory granule membranes. CaM measured by radioimmunoassay was found in each fraction; although approximately one-half was in the cytosolic fraction, CaM was also associated with the plasma membrane and secretory granule fractions. CaM-binding proteins were identified by an ²⁵¹-CaM gel overlay technique and quantitated by densitometric analysis of the autoradiograms. Pituitary homogenates contained nine major CaM-binding proteins of 146, 131, 90, 64, 58, 56, 52, 31 and 22 kilodaltons (kDa). Binding to all the bands was specific, Cat+-sensitive, and displaceable with excess unlabeled CaM. Severe heat treatment (100°C, 15 min), which results in a 75% reduction in phosphodiesterase activation by CaM, markedly decreased ²⁵¹I-CaM binding to all protein bands. Secretory granule membranes showed enhancement for CaM-binding proteins with molecular weights of 184, 146, 131, 90, and 52000. A specific, affinity purified antibody to chicken gizzard MLCK bound to the 146 kDa band in homogenates, centrifugal subcellular fractions, and secretory granule membranes. No such binding was associated with the granule contents. The enrichment of MLCK and other CaM-binding proteins in pituitary secretory granule membranes suggests a possible role for CaM and/or CaM-binding proteins in granule membrane function and possibly exocytosis.     

Components of purified sarcolemma from porcine skeletal muscle

Sarcolemmal membranes were isolated from porcine skeletal muscle by modifications of a LiBr-extraction technique. Latency determinations of acetylcholinesterase, ouabain-sensitive p-nitrophenylphosphatase, [3H]ouabain binding, and (Na+ + K+)-ATPase activities indicated that 65-76% of the membranes were sealed inside-out vesicles. The preparations were enriched in cholesterol and phospholipid, and demonstrated adenylate cyclase activity and both cAMP and cGMP phosphodiesterase activities. An indication of the purity of this fraction was that the Ca2+-ATPase activity (0.13 mumol Pi mg-1 min-1 at 37 degrees C) was 3.8% of that of porcine skeletal muscle sarcoplasmic reticulum preparations. Pertussis toxin specifically catalyzed the ADP-ribosylation of a Mr 41,000 sarcolemmal protein, indicating the presence of the inhibitory guanine nucleotide regulatory protein of adenylate cyclase, Ni. An endogenous ADP-ribosyltransferase activity, with several membrane protein substrates, was also demonstrated. The addition of exogenous cAMP-dependent protein kinase or calmodulin promoted the phosphorylation of a number of sarcolemmal proteins. The calmodulin-dependent phosphorylation exhibited an approximate K 1/2 for Ca2+ of 0.5 microM, and an approximate K 1/2 for calmodulin of 0.1 microM. 125I-Calmodulin affinity labeling of the sarcolemma, using dithiobis(succinimidyl propionate), demonstrated the presence of Mr 160,000 and 280,000 calmodulin-binding components in these membranes. These results demonstrate that this porcine preparation will be valuable in the study of skeletal muscle sarcolemmal ion transport, protein and hormonal receptors, and protein kinase-catalyzed phosphorylation.     

Immunolocalization of the sarcolemmal Ca2+/Mg2+ ecto-ATPase (myoglein) in rat myocardium

Cardiac plasma membrane Ca²⁺/Mg²⁺ ecto-ATPase (myoglein) requires millimolar concentrations of either Ca²⁺ or Mg²⁺ for maximal activity. In this paper, we report its localization by employing an antiserum raised against the purified rat cardiac Ca²⁺/Mg²⁺ ATPase. As assessed by Western blot analysis, the antiserum and the purified immunoglobulin were specific for Ca²⁺/Mg²⁺ ecto-ATPase; no cross reaction was observed towards other membrane bound enzymes such as cardiac sarcoplasmic reticulum Ca²⁺-pump ATPase or sarcolemmal Ca²⁺-pump ATPase. On the other hand, the cardiac Ca²⁺/Mg²⁺ ecto-ATPase was not recognized by antibodies specific for either cardiac sarcoplasmic reticulum Ca²⁺-pump ATPase or plasma membrane Ca²⁺-pump ATPase. Furthermore, the immune serum inhibited the Ca²⁺/Mg²⁺ ecto-ATPase activity of the purified enzyme preparation. Immunofluorescence of cardiac tissue sections and neonatal cultured cardiomyocytes with the Ca²⁺/Mg²⁺ ecto-ATPase antibodies indicated the localization of Ca²⁺/Mg²⁺ ecto-ATPase in association with the plasma membrane of myocytes, in areas of cell-matrix or cell-cell contact. Staining for the Ca²⁺/Mg²⁺ ecto-ATPase was not cardiac specific since the antibodies detected the presence of membrane proteins in sections from skeletal muscle, brain, liver and kidney. The results indicate that Ca²⁺/Mg²⁺ ecto-ATPase is localized to the plasma membranes of cardiomyocytes as well as other tissues such as brain, liver, kidney and skeletal muscle.     

Plasma membrane associated membranes (PAM) from Jurkat cells contain STIM1 protein: Is PAM involved in the capacitative calcium entry?

A proper cooperation between the plasma membrane, the endoplasmic reticulum and the mitochondria seems to be essential for numerous cellular processes involved in Ca²⁺ signalling and maintenance of Ca²⁺ homeostasis. A presence of microsomal and mitochondrial proteins together with those characteristic for the plasma membrane in the fraction of the plasma membrane associated membranes (PAM) indicates a formation of stabile interactions between these three structures. We isolated the plasma membrane associated membranes from Jurkat cells and found its significant enrichment in the plasma membrane markers including plasma membrane Ca²⁺-ATPase, Na⁺, K⁺-ATPase and CD3 as well as sarco/endoplasmic reticulum Ca²⁺ ATPase as a marker of the endoplasmic reticulum membranes. In addition, two proteins involved in the store-operated Ca²⁺ entry, Orai1 located in the plasma membrane and an endoplasmic reticulum protein STIM1 were found in this fraction. Furthermore, we observed a rearrangement of STIM1-containing protein complexes isolated from Jurkat cells undergoing stimulation by thapsigargin. We suggest that the inter-membrane compartment composed of the plasma membrane and the endoplasmic reticulum, and isolated as a stabile plasma membrane associated membranes fraction, might be involved in the store-operated Ca²⁺ entry, and their formation and rebuilding have an important regulatory role in cellular Ca²⁺ homeostasis.     

Isolation of plasma membrane vesicles,derived from transverse tubules,by selective homogenization of subcellular fractions of frog skeletal muscle in isotonic media

A new technique for isolating fragmented plasma membranes from skeletal muscle has been developed that is based on gentle mechanical disruption of selected homogenate fractions. $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-stimulated}$, Mg²⁺-dependent ATPase was used as an enzymatic marker for the plasma membrane, Ca²⁺-stimulated, Mg²⁺-dependent ATPase as a marker for sarcoplasmic reticulum, and succinate dehydrogenase for mitochondria. Cell Cell segments in an amber low-speed ($\text{800 × g}$) pellet of a frog muscle homogenate were disrupted by repeated gentle shearing with a Polytron homogenizer. Sarcoplasmic reticulum was released into the low-speed supernatant, whereas most of the plasma membrane marker remained in a white, fluffy layer of the sediment, which contained sarcolemma and myofibrils. Additional gentle shearing of the white low-speed sediment extracted plasma membranes in a form that required centrifugation at $\text{100 000 × g}$ for pelleting. This pellet, the fragmented plasma membrane fraction, had a relatively high specific activity of $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-stimulated}$ ATPase compared with the other fractions, but it had essentially no Ca²⁺-stimulated ATPase activity and only a small percentage of the succinate dehydrogenase activity of the homogenate.Experimental evidence suggests that the fragmented plasma membrane fraction is derived from delicate transverse tubules rather than from the thicker, basement membrane-coated sarcolemmal sheath of muscle cells. Electron microscopy showed small vesicles lined by a single thin membrane. Hydroxyproline, a characteristic constituent of collagen and basement membrane, could not be detected in this fraction.     

Membrane proteins involved in potassium shifts during muscle activity and fatigue

Muscle activity is associated with potassium displacements, which may cause fatigue. It was reported previously that the density of the large-conductance Ca2+-dependent K+ (BK(Ca)) channel is higher in the T tubule membrane than in the sarcolemmal membrane and that the opposite is the case for the ATP-sensitive K+ (K(ATP)) channel. In the present experiments, we investigated the subcellular localizations of the strong inward rectifier 2.1 K+ (Kir2.1) channel and the Na+-K+-2Cl- (NKCC)1 cotransporter with Western blot analysis of different muscle fractions. Furthermore, muscle function was studied while trying to manipulate the opening probability or transport capacity of these proteins during electrical stimulation of isolated soleus muscles. All experiments were made with excised muscle from male Wistar rats. Kir2.1 channels were almost undetectable in the sarcolemmal membrane but present in the T tubule membrane, whereas NKCC1 cotransporters were present in the sarcolemmal membrane. For muscles incubated in a buffer containing pinacidil, NS1619, Ba2+, or bumetanide, there was a faster reduction in peak force (P < 0.05). Furthermore, bumetanide incubation reduced the peak force at the onset of electrical stimulation (P < 0.05). Thus the effects on muscle force indicate that these drugs can affect K+-transporting proteins and thereby influence K+ accumulation, especially in the T tubules, suggesting that K(ATP) and BK(Ca) channels are responsible for K+ release and decrease in force during repeated muscle contractions, whereas Kir2.1 and NKCC1 may have a role in K+ reuptake.     

The mammalian skeletal muscle DHPR has larger Ca2+ conductance and is phylogenetically ancient to the early ray-finned fish sterlet (Acipenser ruthenus)

The L-type Ca²⁺ channel or dihydropyridine receptor (DHPR) in vertebrate skeletal muscle is responsible for sensing sarcolemmal depolarizations and transducing this signal to the sarcoplasmic Ca²⁺ release channel RyR1 via conformational coupling to initiate muscle contraction. During this excitation-contraction (EC) coupling process there is a slow Ca²⁺ current through the mammalian DHPR which is fully missing in euteleost fishes. In contrast to ancestral evolutionary stages where skeletal muscle EC coupling is still depended on Ca²⁺-induced Ca²⁺-release (CICR), it is possible that the DHPR Ca²⁺ conductivity during mammalian (conformational) EC coupling was retained as an evolutionary remnant (vestigiality). Here, we wanted to test the hypothesis that due to the lack of evolutionary pressure in post-CICR species skeletal muscle DHPR Ca²⁺ conductivity gradually reduced as evolution progressed. Interestingly, we identified that the DHPR of the early ray-finned fish sterlet (Acipenser ruthenus) is phylogenetically positioned above the mammalian rabbit DHPR which retained robust Ca²⁺ conductivity, but below the euteleost zebrafish DHPR which completely lost Ca²⁺ conductivity. Remarkably, our results revealed that sterlet DHPR still retained the Ca²⁺ conductivity but currents are significantly reduced compared to rabbit. This decrease is due to lower DHPR membrane expression similar to zebrafish, as well as due to reduced channel open probability (P_o). In both these fish species the lower DHPR expression density is partially compensated by higher efficacy of DHPR-RyR1 coupling. The complete loss of P_o in zebrafish and other euteleost species was presumably based on the teleost specific 3rd round of genome duplication (Ts3R). Ts3R headed into the appearance of two skeletal muscle DHPR isoforms which finally, together with the radiation of the euteleost clade, fully lost the P_o.     

Transient Receptor Potential Canonical Type 1 (TRPC1) Operates as a Sarcoplasmic Reticulum Calcium Leak Channel in Skeletal Muscle

Extensive studies performed in nonexcitable cells and expression systems have shown that type 1 transient receptor potential canonical (TRPC1) channels operate mainly in plasma membranes and open through phospholipase C-dependent processes, membrane stretch, or depletion of Ca²⁺ stores. In skeletal muscle, it is proposed that TRPC1 channels are involved in plasmalemmal Ca²⁺ influx and stimulated by store depletion or membrane stretch, but direct evidence for TRPC1 sarcolemmal channel activity is not available. We investigated here the functional role of TRPC1 using an overexpressing strategy in adult mouse muscle fibers. Immunostaining for endogenous TRPC1 revealed a striated expression pattern that matched sarcoplasmic reticulum (SR) Ca²⁺ pump immunolabeling. In cells expressing TRPC1-yellow fluorescent protein (YFP), the same pattern of expression was observed, compatible with a longitudinal SR localization. Resting electric properties, action potentials, and resting divalent cation influx were not altered in TRPC1-YFP-positive cells. Poisoning with the SR Ca²⁺ pump blocker cyclopiazonic acid elicited a contracture of the fiber at the level of the overexpression site in presence and absence of external Ca²⁺ which was not observed in control cells. Ca²⁺ measurements indicated that resting Ca²⁺ and the rate of Ca²⁺ increase induced by cyclopiazonic acid were higher in the TRPC1-YFP-positive zone than in the TRPC1-YFP-negative zone and control cells. Ca²⁺ transients evoked by 200-ms voltage clamp pulses decayed slower in TRPC1-YFP-positive cells. In contrast to previous hypotheses, these data demonstrate that TRPC1 operates as a SR Ca²⁺ leak channel in skeletal muscle.     

Actomyosin-like ATPase extracted from plain synaptic vesicle fractions

Ca²⁺-sensitive Mg²⁺-dependent ATP phosphohydrolase (EC 3.6.1.3, ATPase) was extracted from the plain synaptic vesicle fractions that were virtually devoid of contamination. The protein pattern of the ATPase preparation on SDS polyacrylamide gel electrophoresis closely resembled that of actomyosin from skeletal muscle. The finding suggests that the main components of the ATPase are actin- and myosin-like proteins of the brain (stenin and neurin). Microsome and synaptosomal plasmalemma fractions were extracted under the same conditions to examine the possibility that the ATPase extracted derived from contaminating particulates. An entirely different ATPase was extracted from microsomes, and no protein from plasma membranes. Although Ca²⁺-sensitive Mg²⁺-dependent ATPase was extracted from coated vesicle fraction, the electrophoretic pattern was dissimilar to that of the ATPase from plain synaptic vesicle fractions. It may be inferred that the whole complex of neurostenin is located in plain synaptic vesicles from the brain.     

Studies on plasma membranes: XIX. Isolation and characterization of a plasma membrane fraction from calf thymocytes

A plasma membrane fraction was isolated from calf thymocytes by a modification of the method of Wallach and Kamat (Wallach, D. F. H. and Kamat, V. B. (1966) in Methods in Enzymology) (Colowick, S. P. and Kaplan, N. O., eds), Vol. 8, pp. 164–172, Academic Press, New York). Fractions were examined electron microscopically and subjected to chemical and enzymic assays.With respect to the cell homogenate and the final microsomal fraction, respectively, the plasma membrane fraction was enriched by a factor 23 and 5.1 in cholesterol, 11 and 2.4 in phospholipid, 5.1 and 4.2 in sialic acid, 20 and 5.2 in Mg²⁺-ATPase (EC 3.6.1.3), and 8 and 2.6 in 5′-nucleotidase (EC 3.1.3.5). Succinate:cytochrome $\text{c}$ oxidoreductase (EC 1.3.99.1) was lacking, and DNA was hardly if at all present in the plasma membrane fraction. The major part of the RNA found in this fraction (30 μg · mg^?1 protein) was concluded to be an authentic component of the plasma membrane.The concept of membrane “markers” was briefly considered and the conclusion was reached that by current criteria and electron microscopic evidence the plasma membrane fraction obtained from calf thymocytes consisted of reasonably clean plasma membranes.     

Interactions of Cations with Membrane Fractions of Smooth and Rough Strains of Salmonella typhimurium and Other Gram-Negative Bacteria

Addition of cations (20 to 50 mM for Mg²⁺ or Ca²⁺ or 100 to 500 mM for Na⁺) to N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid buffer during preparation of membranes from smooth and rough strains of Salmonella typhimurium LT2, Salmonella minnesota, and Escherichia coli O8 had two effects on the composition of the membranes isolated. First, in rough strains of chemotypes Ra to Re the “total membranes” (pellets from high-speed centrifugation) were deficient in the proteins of the outer membrane. The missing proteins were found to have been sedimented in a prior low-speed centrifugation in a fraction we call “cation-aggregated membranes.” Since these membranes were enriched for lipopolysaccharide and for outer membrane proteins, deficient in succinic dehydrogenase, and contained primarily the dense peak after sucrose gradient centrifugation, it appears to be relatively pure outer membrane. About 10% of the membrane protein of smooth strains and up to 50% that of rough strains were cation-aggregated membranes, appearing to contain most of the outer membrane of rough strains. Thus, cation aggregation may be a useful means of preparation of outer membrane samples. The second effect was that with cation addition, several high-molecular-weight proteins not seen when membranes were prepared without cation addition were found in the total membranes of both smooth and rough strains after high-speed centrifugation. These proteins were bound by cations to the inner membranes, since they were soluble in Triton X-100 and separated into the less dense peak upon sucrose gradient centrifugation. They originated from the cytoplasm or the periplasm, since they corresponded to soluble proteins found in the supernatant after high-speed centrifugation and were depleted from this supernatant when preparation was done in the presence of cations.