The localization and assay of chemical elements by microprobe methods

Calcium content of mitochondria and endoplasmic reticulum in liver frozen rapidly in vivo

The recognition that the endoplasmic reticulum (ER), rather than the mitochondria, is the main organelle regulating the cytoplasmic Ca2+ concentration in non-muscle cells supports the notion that an alternative physiological role of mitochondrial Ca transport is the modulation of Ca-sensitive mitochondrial enzymes through small (micromolar) fluctuations in the concentration of mitochondrial matrix Ca2+. The latter mechanism could operate only if the mitochondrial Ca concentration were low, as it is in muscle and retinal rods, below the levels saturating the regulated enzymes. In contrast, if the ER serves as an intracellular Ca store, its Ca content would be expected to be high. In view of the major metabolic function of the liver, the question of whether hepatic mitochondrial matrix Ca2+ regulates metabolism is particularly important, but the range of Ca concentrations reported for isolated liver mitochondria is too wide to provide a conclusive answer. Therefore, we have used electron probe X-ray microanalysis (EPMA) to measure the subcellular distribution of Ca in liver snap-frozen in vivo, and report here that the endoplasmic reticulum is a major intracellular store of Ca, while the concentration of Ca in mitochondria is low and compatible with the regulation of mitochondrial enzymes.

A comparison of subcellular element concentrations in frozen-dried, plastic-embedded, dry-cut sections and frozen-dried cryosections

Biological X-ray microanalysis of diffusible elements within cellular and subcellular compartments requires preparation methods to retain electrolytes in the compartments they occupied in vivo. X-ray microanalysis of frozen-dried, plastic-embedded samples has been used to quantitate electrolytes at the cellular level. We have compared the subcellular elemental distribution in dry cut sections from such samples with that in ultrathin frozen-dried cryosections. Rat pancreases were quench-frozen onto a helium-vapor-cooled copper block. Cryosections were cut at 130-150 K, transferred using a Gatan cold stage, frozen-dried in the column and analysed at 190 K. Tissue fragments were frozen-dried at 190 K, and cut on a dry knife at 293 K. Both samples provided images permitting unambiguous identification of all major compartments except the Golgi complex. Intracellular potassium-to-sodium ratios obtained on frozen-dried plastic-embedded sections were lower than for cryosections (e.g. 1.77 in basal cytoplasm in plastic sections as compared to 4.34 for cryosections) and varied with the pre-embedding procedure (e.g. 1.77 in formaldehyde-fixed as compared to 2.87 in osmium-fixed plastic sections). Potassium gradients between adjacent organelles were large in cryosections and insignificant in plastic-embedded material. Higher cytoplasmic phosphorus, potassium and sulfur concentrations were observed in cryosections. Therefore, a redistribution of electrolytes and covalently bound elements occurred subcellularly in the plastic sections. Calcium was quantifiable in most organelles in cryosections but the plastic lowered sensitivity too much to permit routine calcium quantification. We conclude that in our hands frozen-dried, plastic-embedded samples were compromised and provided lower sensitivity than cryosections.(ABSTRACT TRUNCATED AT 250 WORDS)

Elemental distribution in Rana pipiens retinal rods: quantitative electron probe analysis

The composition of dark-adapted and illuminated retinal rod outer and inner segments and mitochondria was determined with electron probe X-ray micro-analysis of cryosections. The concentration of Ca in the outer segment was 0.4 mmol/kg dry wt. (0.1 Ca/rhodopsin) and did not measurably change upon illumination with saturating light for 5 min. The non-mitochondrial regions of the inner segment contained the highest concentrations (up to 13 mmol/kg dry wt.) of Ca in rods; these regions probably represent the endoplasmic reticulum. The equilibrium potentials estimated from the measured elemental concentrations and the known water content of dark-adapted outer segments were (mV): ENa = +17, EK = -83, ECl = -27. The respective values in the inner segment were: ENa = +20, EK = -89, ECl = -26. The above values were obtained in frog rods bathed in 0.18 mM-Ca Ringer solution. In the outer segment of toad rods bathed in 1.8 mM-Ca Ringer, ENa = +33 mV. The Mg content of the rods was high. The (computed) concentration in the dark-adapted retinae was 11 mM in the outer segment and 24 mM in the inner segment. Illumination caused a reduction in Mg to 9 mM (outer segment) and 16 mM (inner segment). Illumination caused a highly significant reduction in Na and Cl concentrations, and an increase in K concentration in both outer and inner segments. Exposure to Na-free (choline Ringer) solution resulted in reduction in Na to just-detectable levels (3 +/- 1 mmol/kg dry wt.) in the outer segment and to 5 +/- 1 mM in the inner segment. This was associated with a significant loss of Cl and decrease in ECl to -50 mV. The low Na content of the outer segment in the Na-depleted rods is not compatible with an extracellular concentration (105 mM) of inexchangeable Na in the intradiskal space. Mitochondrial Na and Mg paralleled the changes in the cytoplasmic concentrations: both mitochondrial Na and Mg were significantly decreased in illuminated, compared to dark-adapted rods. There was no detectable Ca (0 +/- 0.2 mmol/kg dry wt.) in mitochondria of dark-adapted rods containing high concentrations of Na; mitochondrial Ca was slightly higher (0.5 +/- 0.2 mmol/kg dry wt.) in the mitochondria that contained low Na following illumination.

Total cytoplasmic calcium in relaxed and maximally contracted rabbit portal vein smooth muscle

The concentration of total cytoplasmic Ca in vascular smooth muscle was measured by electron probe microanalysis of strips of rabbit portal anterior mesenteric vein that were rapidly frozen either when relaxed or during a maintained (30 min) maximal contraction stimulated with high K and noradrenaline. Strips were also frozen and analysed after incubation in Ca-free, high-Mg2+ solution. Probe diameters of 0.1-0.2 micron and 1.0-1.5 micron were used to measure, respectively, cytoplasmic and cellular (including stored) Ca. There was a highly significant increase (P less than 0.0005) in cytoplasmic Ca of 1.0 +/- 0.2 (S.D.) mmol Ca/kg dry wt. from 0.8 +/- 0.2 (S.E. of mean) mmol/kg dry wt. (n = 262 spectra, six animals) to 1.8 +/- 0.2 (S.E. of mean) mmol Ca/kg dry wt. (n = 296 spectra, six animals), during maximal contraction. This increase is greater than can be accounted for by Ca binding to calmodulin and to myosin, suggesting the presence of other Ca-binding proteins in smooth muscle. A small amount (0.4-0.6 mmol/kg dry wt.) of cytoplasmic Ca remained after incubation in Ca-free, high-Mg2+ EGTA solution. This tightly bound, cytoplasmic Ca is insufficient to account for the total amount of divalent cation known to be bound to F-actin. We conclude that Mg is the major inexchangeably bound cation in F-actin in smooth as in striated muscle. In the contracted muscles, the cellular Ca concentration, measured with the large probes that include Ca stored in the sarcoplasmic reticulum (s.r.), was 3.2 +/- 0.3 (S.E. of mean) mmol Ca/kg dry wt. (n = 93), significantly higher than the cytoplasmic Ca concentration measured with small probes. This value of cellular Ca is probably an underestimate, as the large-diameter probes did not cover all of the peripheral s.r. The cellular Ca (measured with large probes) was highest in the contracted and lower in the relaxed tissue, and was significantly reduced in the muscles incubated in Ca-free solution. In contracted muscle, cytoplasmic Mg significantly decreased and mitochondrial Mg increased. In 0 Ca, high-Mg2+ solution, the cytoplasmic Mg increased significantly. Mitochondrial Ca did not significantly change during a maintained contraction, but was significantly lower (0.0 +/- 0.2 (S.E. of mean) mmol Ca/kg dry wt.) after incubation in Ca-free, high-Mg2+ solution than in the relaxed tissue (1.6 +/- 0.2 mmol Ca/kg dry wt.) in normal Ca-containing solution.

The application of EDXS to the biological sciences

The distribution of chemical elements in soft tissues may be faithfully preserved by very rapid freezing. Most often the material is then cryosectioned and the sections frozen-dried prior to analysis, but direct analysis in the hydrated state is an established alternative. For bulk specimens, the shape of the analysed volume is uncertain. But whichever current model is accepted, analytical spatial resolution must generally be limited to the order of 1 micron. Such specimens can be suitable for the specific analysis of cytoplasm, cell nuclei and large extracellular spaces but not for study on a finer scale. Analytical spatial resolution in the range 200-500 nm is obtainable with sections cut approximately 1 micron thick. In the frozen-hydrated state, small extracellular spaces can be analysed but multiple scattering obscures intracellular detail in the STEM image. The irradiation required for an EDXS analysis, approximately 50 nanoCoulomb (50 nanoAmpere seconds), need not produce intolerable radiation damage when spread over an area 200 nm or more in diameter. Finer structure, for example mitochondria and regions of rough or smooth endoplasmic reticulum, can be identified and analysed in frozen-dried cryosections cut approximately 100 nm thick. Recently such features have been visualized in 100 nm frozen-hydrated sections where the water is vitreous. This opens the prospect of analysing material where elemental distributions have been preserved on a very fine scale, since one might avoid even the ionic shifts from aqueous solution to supramolecular structures which must occur on freeze-drying. But radiation damage may be prohibitive when an irradiation of 50 nanoCoulomb is concentrated into a hydrated area less than 200 nm in diameter.

Release and recycling of calcium by the sarcoplasmic reticulum in guinea-pig portal vein smooth muscle

The amplitude of interrupted contractions evoked by noradrenaline or caffeine in Ca2+-free, high-K+ solutions containing EGTA or La3+ was determined in small (40-60 micron thick) bundles of guinea-pig portal anterior mesenteric vein. Interrupted contractions were produced by removing the stimulating agent as soon as the amplitude of the tension record reached its peak. The distribution of intracellular Ca2+ was determined, with electron probe X-ray microanalysis, in cryosections of preparations frozen in the relaxed state and at the peak of noradrenaline-induced contractions. Interrupted contractions of maximal or near-maximal amplitudes could be evoked every 2 min for up to 15 min in the virtual absence of extracellular Ca2+. If noradrenaline was allowed to remain in the solution throughout the period of spontaneous relaxation, a subsequent contraction could no longer be evoked in the absence of extracellular Ca2+. Interrupted contractions, similar to those evoked by noradrenaline, could also be stimulated by caffeine. The amplitude of reproducible interrupted contractions in Ca2+-free, high-K+ solution was graded with noradrenaline concentration. The ability of these smooth muscles to contract repeatedly and maximally in Ca2+-free solutions indicates the recycling of Ca2+ released from an intracellular store. The occurrence of these contractions in high-K+ (depolarizing) solutions supports the conclusion (Devine, Somlyo & Somlyo, 1972) that the release of intracellular Ca2+ is one of the mechanisms of pharmacomechanical coupling. The number of subplasmalemmal regions in which high Ca concentrations (greater than 10 mmol/kg dry wt.) were detected, with approximately 75 nm diameter electron probes, was reduced in muscles frozen at the peak of contraction, from 4.7/cell periphery in the relaxed to 1.4/cell periphery in the contracted preparations. In freeze-substituted smooth muscles, in which the membranes of the junctional sarcoplasmic reticulum could be visualized, the regions containing high Ca were identified as part of the sarcoplasmic reticulum (s.r.), indicating that the s.r. is the store from which noradrenaline and caffeine release Ca2+.

Morphology and X-ray microprobe analysis of spermatozoa from fertile and infertile men in in vitro fertilization

This paper reports differences observed in the elemental content of fertile and infertile human spermatozoa used in an in vitro fertilization (IVF) program. "Fertile" and "infertile" were designated by the successful penetration or failure to penetrate an oocyte in vitro. We report morphological and morphometric differences which, together with elemental changes, may be causes of infertility in apparently normal spermatozoa. There were significant differences (P less than 0.05) in sodium and chlorine concentrations between fertile and infertile samples and there was more chlorine than could be accounted for as sodium chloride. Many spermatozoa showed particles adhering to tails, with a higher incidence of "contamination" in the infertile spermatozoa. There were significant differences in both shapes of heads and lengths of tails between fertile and infertile spermatozoa.