Electron probe analysis of calcium compartments in cryo sections of smooth and striated muscles

Compartmentalized signalling: Ca2+ compartments, microdomains and the many facets of Ca2+ signalling

Ca(2+) regulates a multitude of cellular processes and does so by partitioning its actions in space and time. In this review, we discuss how Ca(2+) responses are constructed from small quantal (elementary) events that have the potential to propagate to produce large pan-cellular responses. We review how Ca(2+) is compartmentalized both physically and functionally, and describe how each organelle has its own distinct Ca(2+)-handling properties. We explain how coordination of the movement of Ca(2+) between organelles is used to shape and hone Ca(2+) signals. Finally, we provide a number of specific examples of where compartmentation and localization of Ca(2+) are crucial to cell function.

Dynamics and consequences of potassium shifts in skeletal muscle and heart during exercise

Since it became clear that K(+) shifts with exercise are extensive and can cause more than a doubling of the extracellular [K(+)] ([K(+)](s)) as reviewed here, it has been suggested that these shifts may cause fatigue through the effect on muscle excitability and action potentials (AP). The cause of the K(+) shifts is a transient or long-lasting mismatch between outward repolarizing K(+) currents and K(+) influx carried by the Na(+)-K(+) pump. Several factors modify the effect of raised [K(+)](s) during exercise on membrane potential (E(m)) and force production. 1) Membrane conductance to K(+) is variable and controlled by various K(+) channels. Low relative K(+) conductance will reduce the contribution of [K(+)](s) to the E(m). In addition, high Cl(-) conductance may stabilize the E(m) during brief periods of large K(+) shifts. 2) The Na(+)-K(+) pump contributes with a hyperpolarizing current. 3) Cell swelling accompanies muscle contractions especially in fast-twitch muscle, although little in the heart. This will contribute considerably to the lowering of intracellular [K(+)] ([K(+)](c)) and will attenuate the exercise-induced rise of intracellular [Na(+)] ([Na(+)](c)). 4) The rise of [Na(+)](c) is sufficient to activate the Na(+)-K(+) pump to completely compensate increased K(+) release in the heart, yet not in skeletal muscle. In skeletal muscle there is strong evidence for control of pump activity not only through hormones, but through a hitherto unidentified mechanism. 5) Ionic shifts within the skeletal muscle t tubules and in the heart in extracellular clefts may markedly affect excitation-contraction coupling. 6) Age and state of training together with nutritional state modify muscle K(+) content and the abundance of Na(+)-K(+) pumps. We conclude that despite modifying factors coming into play during muscle activity, the K(+) shifts with high-intensity exercise may contribute substantially to fatigue in skeletal muscle, whereas in the heart, except during ischemia, the K(+) balance is controlled much more effectively.

Spontaneous sarcoplasmic reticulum calcium release and extrusion from bovine, not porcine, coronary artery smooth muscle

1. We tested the hypothesis that the Ca(2+)-loaded sarcoplasmic reticulum (SR) of coronary artery smooth muscle spontaneously releases Ca2+ preferentially toward the sarcolemma to be extruded from the cell without increasing the average free myoplasmic [Ca2+] (Ca(im)) concentration. 2. The SR of bovine cells was Ca(2+)-loaded by depolarization-induced Ca2+ influx. Release (unloading) of Ca2+ from the SR during recovery from depolarization was determined by Fura-2 microfluorometry of Ca(im). The SR Ca2+ unloading was maximal following a long (14 min) recovery from depolarization, as shown by the 66% decrease in the peak caffeine-induced Ca(im) transient compared to the Ca(im) transient after a short (2 min) recovery. No increase in Ca(im) occurred during the long recovery. No unloading of the SR Ca2+ store was noted in porcine cells. 3. Approximately 80% of the outward K+ current in bovine and porcine cells was sensitive to subsarcolemmal Ca2+ (Ca(is)) concentrations. Whole-cell voltage clamp using pipette solutions with Ca2+ concentrations clamped between 0 and 1000 nM with Ca(2+)-EGTA or Ca(2+)-BAPTA buffers showed increasing K+ currents (normalized for cell membrane surface area) as a function of both membrane potential and Ca(is). Clamping of Ca(im) and Ca(is) was verified by the lack of changes in K+ current and Fura-2 ratio in response to Ca2+ influx, Ca(2+)-free external solution, or caffeine-induced Ca2+ release. At +30 to +50 mV the K+ current amplitude showed a similar sensitivity to Ca2+ as Fura-2. These data indicate that in this experimental preparation Ca(2+)-activated K+ current is a valid estimate of Ca(is). 4. Simultaneous Ca(im) and Ca(is) measurements in bovine cells which were not Ca(2+)-clamped (2 x 10(-4) M-EGTA pipette solution) showed that during the long recovery period the K+ current (reflecting Ca(is)) increased 55%, while Ca(im) did not change. 5. In quiescent bovine cells the Ca(is) was higher than Ca(im), while the higher resting Ca(is) gradient was not apparent in porcine cells. 6. The Ca(is) concentration was directly related to the amount of Ca2+ in the SR in bovine, but not porcine cells. Depletion of the SR in bovine cells by caffeine resulted in a 58% decrease in K+ current compared to the resting K+ current. 7. Caffeine-induced Ca2+ release caused an increase in Ca(is) which preceded the increase in Ca(im) by approximately 2 s.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium and magnesium transport by in situ mitochondria: electron probe analysis of vascular smooth muscle

The extent, time course, and reversibility of mitochondrial Ca2+ uptake secondary to cellular Ca2+ influx stimulated by massive Na+ efflux were evaluated by electron probe microanalysis of rabbit portal vein smooth muscle. Strips of portal vein were Na+ loaded for 3 hours at 37 degrees C in a K+-free 1 mM ouabain solution, after which rapid Na+ efflux was induced by washing with a Na+-free K+-Li+ solution (1 mM ouabain). Li+ washing Na+-loaded portal vein produced a large transient contraction accompanied by an increase (over 100-fold) in mitochondrial Ca2+ and also significant (p less than 0.05) increases in phosphorus and Mg2+. The Ca2+ loading of the mitochondria was reversed during prolonged Li+ wash, and by 2 hours, mitochondrial Ca2+, Mg2+, and phosphorus had returned to control levels. The maximal contractile response to stimulation remained normal, demonstrating that pathologic Ca2+ loading of mitochondria is reversible in situ and compatible with normal maximal force developed by the smooth muscle. Mitochondrial Ca2+ and phosphorus uptake were reduced but still significant when the Li+ wash contained 0.2 mM Ca2+ or when ouabain was omitted. The fact that mitochondrial Ca2+ loading accompanied submaximal contractions during 0.2 mM Ca2+-Li wash suggests "supranormal" affinity of mitochondria for Ca2+ and may be due, in part, to reverse operation of the mitochondrial Na+-Ca2+ exchanger. Mitochondrial Ca2+, Mg2+, and phosphorus uptake were eliminated when the Li+ wash was performed at 2 degrees C or when the wash contained no Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Resolution of intracellular calcium metabolism in intact segments of rabbit aorta

A new method, based on computer-assisted kinetic analysis of 45Ca efflux data, was used to measure calcium contents and fluxes for extracellular and intracellular compartments in intact segments of rabbit aorta. After a 1-hour loading period, efflux data were collected for 8 hours using a flow-through tissue chamber. These long-term effluxes were necessary because information on intracellular calcium metabolism was concentrated in the slow components of the efflux curves while earlier components appeared to be dominated by washout of extracellular calcium. Intracellular compartments were identified as those whose calcium contents were altered by 10 microM phenylephrine. This method complements previous approaches by providing simultaneous estimates of compartmental calcium contents and fluxes without requiring the assumption of isotopic equilibrium and without recourse to standard wash techniques for removal of extracellular calcium. In normal, calcium-containing, bicarbonate-buffered physiological salt solution these compartments contained a total of approximately 300 nmol Ca/g wet aorta. Of this total, 55 nmol/g were associated with the slowest resolvable compartment whose turnover time was 170 minutes and whose exchange flux was 0.32 nmol min-1g-1. Two other intracellular compartments had turnover times of 30 minutes. One of these was phenylephrine releasable and contained 145 nmol/g; it exchanged calcium at 4.9 nmol min-1g-1. In normal physiological salt solution the plasma membrane was, surprisingly, not rate limiting for Ca efflux; and in 10 microM phenylephrine the membrane Ca flux was even greater, increasing 3.5-fold compared to control.

Kinetic identification of an intracellular calcium compartment sensitive to phosphate and dinitrophenol in intact isolated rabbit aorta

Previous work from this laboratory revealed the presence of at least three distinct intracellular calcium compartments in intact segments of rabbit aorta. In this study one of these intracellular compartments is shown to be sensitive to dinitrophenol and to increased extracellular phosphate. Intact aortic segments were loaded with 45Ca in bicarbonate-buffered physiologic salt solution for 1 hour, and then transferred to a flow-through chamber perfused with physiologic salt solution. Effluent from the chamber was collected for 8 hours, and 45Ca efflux curves were analyzed using compartmental analysis. When aortic segments were loaded and washed out in dinitrophenol, the slowest component of the efflux curve was less prominent; in high phosphate it was more prominent. The rate constant changes required to account for these data were primarily in the exchange between the cytosolic and slowest intracellular calcium compartment, suggesting that the slowest calcium compartment resolved during the 8-hour washout was mitochondrial. This compartment contained 5.4 +/- 3.2 nmol calcium/g wet wt. tissue. The calcium flux across its membranes was 0.32 +/- 0.04 nmol min-1g-1. Because this flux is much smaller than the plasma-membrane calcium flux, we suggest that, in normal physiological circumstances, plasma-membrane extrusion is more important for the removal of Ca from the smooth muscle cytosol than is uptake into this slow intracellular compartment.

Heterogeneity of calcium compartmentation: electron probe analysis of renal tubules

The objective of this study has been to determine the intracellular localization of calcium in cryofixed, cryosectioned suspensions of kidney proximal tubules using quantitative electron probe X-ray microanalysis. Two populations of cells have been identified: 1) "Viable" cells, representing the majority of cells probed, are defined by their relatively normal K/Na concentration ratio of approximately 4:1. Their measured Ca content is 4.1 +/- 1.4 (SEM) mmol/kg dry wt in the cytoplasm and 3.1 +/- 1.1 mmol/kg dry wt in the mitochondria, or an average cell calcium content of approximately 3.8 mmol/kg dry wt. 2) "Nonviable" cells, defined by the presence of dense inclusions in their mitochondria and a K/Na concentration ratio of approximately 1. The Ca content is 15 +/- 2 mmol/kg dry wt in the cytoplasm and 685 +/- 139 mmol/kg dry wt in the mitochondria of such cells. Assuming 25 to 30% of the cell volume is mitochondrial, the overall calcium content of such nonviable cells is approximately 210 mmol/kg dry wt. The presence of these inclusions in 4 to 5% of the cells would account for the average total Ca content measured in perchloric acid extracts of isolated proximal tubule suspensions (approximately equal to 18 nmol/mg protein or 12.6 mmol/kg dry wt). Whole kidney tissues display a large variability in total Ca content (4.5 to 18 nmol/mg protein, or 3.4 to 13.5 mmol/kg dry wt), which could be accounted for by inclusions in 0 to 4% of the cells. The electron probe X-ray microanalysis (EPXMA) data conclusively demonstrate that the in situ mitochondrial Ca content of viable cells from the kidney proximal tubule is low and support the idea that mitochondrial Ca may regulate dehydrogenase activity but probably does not normally control cytosolic free Ca.

The relevance of in vitro smooth muscle experiments to cerebral vasospasm

An overview of the possible factors that might contribute to the development of cerebral vasospasm is presented, with particular emphasis on the possibility that spasm arises from a malfunction of the regulatory or contractile processes in smooth muscle cells. This possibility is emphasized because the evidence for cellular damage and the delayed occurrence of vasospasm are suggestive of pathological alteration. Data regarding the development of spasm in vivo has been reviewed and, to the extent possible, correlated with in vitro studies of cerebrovascular smooth muscle contractility. Short-term in vitro studies of normal cerebral arteries may be of little relevance to the prolonged and severe cerebral vasoconstriction that occurs only after a delay of several days from the initial insult.

An assessment of the calcium content of rat liver mitochondria in vivo

The effect of systematically altering the isolation conditions on the total calcium content of mitochondria isolated from perfused rat liver was examined. We showed that, under most isolation conditions, significant redistributions of mitochondrial calcium occurred resulting in up to 5-fold changes of the total calcium content. Mitochondrial Ca2+ flux inhibitors such as Ruthenium Red and nupercaine were only partially effective in inhibiting such redistributions. We present evidence indicating that the total calcium content of rat liver mitochondria in situ may approximate 2 nmol X (mg of protein)-1.

Ultrastructure, function and composition of smooth muscle

Filamentous myosin is present in both relaxed (myosin light chains unphosphorylated) and contracted (light chains phosphorylated) vascular smooth muscle. The organization of myosin and actin filaments and the insertion of the latter on cytoplasmic and plasma membrane bound dense bodies is consistent with a mini sarcomere-like organization and a sliding filament mechanism of contraction in smooth muscle. Mitochondria are high capacity, low affinity Ca stores in smooth muscle. They do not play a role in the regulation of cytoplasmic Ca2+ at physiological levels. The localization and Ca content of the junctional sarcoplasmatic reticulum (SR) is consistent with this organelle being the major intracellular source of activator Ca released by excitatory transmitters. Repeated contractions in the absence of extracellular Ca2+ (thought to represent recycling of intracellular activator Ca2+) can be demonstrated if the excitatory agent is not allowed to remain in contact with the smooth muscle throughout relaxation.; the demonstration of "recycling" is facilitated if the efflux of cellular Ca2+ is blocked. The rise in total cytoplasmic calcium measured with electron probe analysis during a maintained (30 min) contracture in rabbit portal-anterior mesenteric vein smooth muscle (approximately 0.9 mol/kg dry cytoplasm) is greater than the amount of Ca that could be bound to calmodulin.

Cell calcium in human peripheral blood lymphocytes and the effect of mitogen

The apparent cell concentration of calcium in human peripheral blood lymphocytes is 143.3 +/- 17.7 microM, as measured by two different techniques using 45Ca. The steady-state level of accumulation, and possibly the rate of uptake, are increased in the presence of succinyl-concanavalin A. Initiation of the mitogen-induced alteration of cell calcium occurs within 1-2 min and the change is complete within 5-10 min. Determinations of cell calcium in cells suspended in low Na media indicate that (1) there is no difference in cell calcium between cells incubated in 142 mM extracellular Na and cells incubated in 63 mM extracellular Na, and (2) the mitogen-induced increase in cell calcium is unaffected by a decrease in extracellular sodium concentration (to 63 mM).

Electron probe and electron energy loss analysis in biology

Methods, applications and limitations of quantitative electron probe analysis, X-ray mapping, electron energy loss analysis and energy filtered imaging are described, with emphasis on the analysis of thin (less than 200nm) cryosections. Energy dispersion electron probe analysis can measure reliably 5 to 10mM/Kg of biologically prevalent elements in 50nm diameter areas of 100 to 150 nm thick cryo sections during 100-300 sec counts. The minimal detectable mass (MDM) with a conventional thermionic electron source is approximately 10(-19)g Fe (100 sec count) and can be reduced to 10(-20)g through the use of a field emission gun (FEG). A spatial resolution of 8.7nm is demonstrated in two-dimensional Fourier transforms of Mo X-ray maps of stained catalase crystals. Significant biological results of quantitative electron probe analysis include the measurement of total Ca released from the Mg and K taken up by the sarcoplasmic reticulum during muscle contraction, and the demonstration that mitochondria do not contribute to the physiological regulation of cytoplasmic free Ca levels in cardiac, vascular smooth and striated muscle. Electron energy loss analysis (EELS) promises a significant improvement in sensitivity for the measurement of Ca; based on statistical errors of the measurement, 250 microM/Kg Ca should be measureable with EELS in 250 sec. through the Ca L-edge loss. The use of a doubly corrected magnetic sector spectrometer as a transmission electron microscope imaging filter outside the microscope vacuum is illustrated, and the resolution of the iron core (7.5nm) and surrounding organic shell of single ferritin molecules is demonstrated in, respectively, iron M and carbon K loss images.

A 45Ca autoradiographic and stereological study of freeze-dried smooth muscle of the guinea pig vas deferens

In an effort to more clearly elucidate the role of cellular structures as calcium sinks and sources in smooth muscle cells, the intracellular distribution of radioactive calcium was evaluated by a new method based on freeze-drying. The guinea pig vas deferens was exposed to a physiological salt solution that contained 45Ca. The muscle was then freeze-dried and prepared for electron microscope autoradiography. The grain density over the plasma membrane, mitochondria, and sarcoplasmic reticulum (SR) was significantly greater than that of the matrix. These results suggest that the plasma membrane, mitochondria and SR have the capacity to accumulate calcium. Which of these structures serve as a source of calcium for contraction remains to be determined. A stereological comparison between freeze-dried and conventionally prepared smooth muscles revealed several differences. The cross-sectional area of freeze-dried cells was about twice that of conventionally prepared cells. Moreover, mitochondria and sub-surface vesicles occupied a significantly smaller percentage of the cell in the freeze-dried tissue than they did in the conventionally prepared tissue.

The mechanism of voltage-sensitive dye responses on sarcoplasmic reticulum

The mechanism of voltage-sensitive dye responses was analyzed on sarcoplasmic reticulum vesicles to assess the changes in membrane potential related to Ca2+ transport. The absorbance and fluorescence responses of 3,3'-diethyl-2,2'-indodicarbocyanine and oxonol VI during ATP-dependent Ca2+ transport are influenced by the effect of accumulated Ca2+ upon the surface potential of the vesicle membrane. These observations place definite limitations on the use of these probes as indicators of ion-diffusion potential in processes which involve large fluctuations in free Ca2+ concentrations. Nile Blue A appeared to produce the cleanest optical signal to negative transmembrane potential, with least direct interference from Ca2+, encouraging the use of Nile Blue A for measurement of the membrane potential of sarcoplasmic reticulum in vivo and in vitro. 1,3-dibutylbarbituric acid (5)-1-(p-sulfophenyl)-3 methyl, 5-pyrazolone pentamethinoxonol (WW 781) gave no optical response during ATP-induced Ca2+ transport and responded primarily to changes in surface potential on the same side of the membrane where the dye was applied. Binding of these probes to the membrane plays a major role in the optical response to potential, and changes in surface potential influence the optical response by regulating the amount of membrane-bound dye. The observations are consistent with the electrogenic nature of ATP-dependent Ca2+ transport and indicate the generation of about 10 mV inside-positive membrane potential during the initial phase of Ca2+ translocation. The potential generated during Ca2+ transport is rapidly dissipated by passive ion fluxes across the membrane.

Localization of calcium in an annelid visceral muscle by pyroantimonate deposition and by x-ray microprobe analysis

The loci of calcium distribution in Nereis pharyngeal visceral muscle have been examined by cytochemical precipitation using potassium pyroantimonate. In Na-, Ca- and Mg-free media, pyroantimonate incubation was used to pinpoint loci of intracellularly bound calcium. This method also revealed heavy deposition on the inner face of the plasma membrane, in the sarcoplasmic reticulum and nucleus. X-ray microprobe analysis of the precipitate confirmed the presence of calcium and antimony peaks. It is concluded that the plasma membrane may constitute a major calcium pool for the activation of contraction in this muscle.

Parallel recording of electron energy loss spectra

Two silicon photo diode array devices were tested as parallel recording detectors for electron energy loss spectrometry (EELS). The direct bombardment of a Reticon photodiode array detector with high energy electrons (80 keV) causes an irreversible increase in diode dark current. The dark current saturates the detector amplifier after a dose of 10(-6) C/diode making it unsuitable for EELS. A scintillator coupled SIT vidicon is sensitive enough to count two high energy electrons with a spatial resolution of 100 micrometers, corresponding to 5 eV energy resolution with the electron optical system described. The large pixel-to-pixel gain variation inherent in the scintillator and vidicon can be reduced by averaging the spectrum over a large area of the target perpendicular to the dispersion direction. The L-edge of calcium for a 4 X 10(-3) weight fraction concentration biological specimen is observable in a 40 s parallel recorded spectrum. The minimum detectable concentration of calcium is estimated to be ten times better for ELLS than EDS X-ray analysis.

Localization of Ca++-containing antimonate precipitates during mitosis

Intracellular bound Ca++ has been localized throughout mitosis and cytokinesis in two plant species by means of in situ precipitation with potassium antimonate and electron microscope visualization. Identification of Ca++ as the major cation precipitated was made by comparing solubility properties in water, EDTA, and EGTA of the intracellular deposits with respect to those of K+-, Mg++-, and Ca++-antimonate standards. In spermatogenous cells of the water fern, Marsilea vestita, and stomatal complex cells of barley, Hordeum vulgare, antimonate deposits have been found associated with the endoplasmic reticulum (ER), vacuoles, euchromatin/nucleoplasm, and mitochondria. The last contain a much higher density of precipitates in Marsilea than in Hordeum. Dictyosomes and the nuclear envelope of Marsilea also contain antimonate deposits, as do the plasmalemma, cell wall, and phragmoplast vesicles of Hordeum. Microtubule-organizing centers such as kinetochores and the blepharoplast of Marsilea do not stain. In spite of differences in associated antimonate between certain organelles of the two species, the presence of antimonate aong the ER throughout the cell cycle is common to both. Of particular interest are those precipitates seen along the tubules and cisternae of the extensive smooth ER that surrounds and invades the mitotic spindle in both species. The ability to bind divalent cations makes the mitotic apparatus (MA)-associated ER a likely candidate for regulation of free Ca++ levels in the immediate vicinity of structural components and processes that are Ca++-sensitive and proposed to be Ca++-regulated.

Sulphur and phosphorus content in relation to fibre composition and atrophy of skeletal muscle in patients with Parkinson's disease

Seventeen patients with Parkinson's disease have been compared with 8 normal individuals by biopsy of either the biceps brachii or quadriceps femoris muscles. All biopsies were investigated by enzyme histochemistry. With 13 patients, as well as all controls, scanning electron microscopy with X-ray microanalysis was employed on cryo-sections adjacent to those prepared for light microscopy. Thus, the elemental composition of single muscle fibres was obtained and could be related to histochemical fibre types. Fibre type analysis on the diseased material, based on differential stainability for alkali- and acid-stable ATPase, showed a normal type I and type IIA fibre frequency. A mild type IIB dominance at the expense of type IIA fibres was regarded as a significant deviation from normal. A slight to moderate muscle atrophy affected type IIB fibres almost exclusively. Normal content of sulphur and phosphorus was detected in type I and type IIA Fibres but a lowered sulphur content was obvious in type IIB fibres, especially in the atrophic ones, which also exhibited an increase in phosphorus content. The shift in fibre composition from IIA to IIB, the type IIB fibre atrophy and the change in sulphur and phosphorus content of type IIB fibres are interpreted as signs of a disuse which preferentially affects fast twitch type IIB motor units. These presumably have the highest threshold for activation under pathological conditions characterized by increased muscular tone and difficulties in the performance of rapid and strong voluntary movements.