Mitochondria and other calcium buffers of squid axon studied in situ

Arsenazo III and antipyrylazo III calcium transients in single skeletal muscle fibers

The metallochrome calcium indicators arsenazo III and antipyrylazo III have been introduced individually into cut single frog skeletal muscle fibers from which calcium transients have been elicited either by action potential stimulation or by voltage-clamp pulses of up to 50 ms in duration. Calcium transients recorded with both dyes at selected wavelengths have similar characteristics when elicited by action potentials. Longer voltage-clamp pulse stimulation reveals differences in the late phases of the optical signals obtained with the two dyes. The effects of different tension blocking methods on Ca transients were compared experimentally. Internal application of EGTA at concentrations up to 3 mM was demonstrated to be efficient in blocking movement artifacts without affecting Ca transients. Higher EGTA concentrations affect the Ca signals' characteristics. Differential effects of internally applied EGTA on tension development as opposed to calcium transients suggest that diffusion with binding from Ca++ release sites to filament overlap sites may be significant. The spectral characteristics of the absorbance transients recorded with arsenazo III suggest that in situ recorded signals cannot be easily interpreted in terms of Ca concentration changes. A more exhaustic knowledge of the dye chemistry and/or in situ complications in the use of the dye will be necessary.

Cytoplasmic alkalization reduces calcium buffering in molluscan central neurons

The effect of raised cytoplasmic pH (pHi) on intracellular concentration ([Ca2+]i) transients following calcium influx during membrane depolarization was studied in identified neurons in the abdominal ganglion of Aplysia californica. The pHi was monitored with pH-sensitive microelectrodes. Sea water containing 15 mM NH4Cl at pH 7.7 elevated pHi about 0.35 pH units from the normal level of 7.17. These cells have an estimated buffering power of about 60 mM/pH unit. Calcium influx was elicited by depolarizing pulses under voltage clamp and [Ca2+]i transients were monitored with the photoprotein aequorin or the metallochromic dye arsenazo III. Aequorin photo-emissions increased by 21--131% (mean, 48%) and arsenazo III absorbance changes accompanying depolarization increased by 9--33% (mean, 20%) after 30 min in NH4+, corresponding roughly to a 14% increase in [Ca2+]i transients. Calcium-dependent potassium tail currents following a depolarizing pulse were somewhat slower and 4--91% (mean, 38%) large in NH4+. The magnitude and time- and voltage-dependence of the membrane calcium conductance was studied using calcium tail currents following depolarizing pulses. The calcium current was unaffected by NH4+, so the enhanced [Ca2+]i transients must reflect reduced calcium buffering at high pHi. Either reduced cytoplasmic calcium binding or slowed active extrusion of calcium may be responsible for this effect.

Studies on the in vivo sensitivity of spindle microtubules to calcium ions and evidence for a vesicular calcium-sequestering system

I microinjected calcium ions into echinoderm eggs during mitosis to determine the calcium sensitivity of microtubules (Mts) in vivo. Spindle birefringence (BR), a measure of the number of aligned Mts in the spindle, is locally, rapidly, and reversibly abolished by small volumes of microinjected CaCl2 (1 mM). Rapid return of BR is followed by anaphase, and subsequent divisions are normal. Similar doses of MgCl2, BaCl2, KCl, NaCl, pH buffers, distilled water, or vegetable oil have no effect on spindle BR, whereas large doses of such agents sometimes cause slow, uniform loss in BR over the course of a minute or more. Of the ions tested, only Sr++ causes effects comparable to Ca++. Ca-EGTA buffers, containing greater than micromolar free Ca++, abolishes BR in a manner similar to millimolar concentrations of injected CaCl2. Caffeine, a potent uncoupler of the Ca++-pump/ATPase of sarcoplasmic reticulum, causes a local, transient depression in spindle BR in the injected region. Finally, injection of potassium oxalate results in the formation of small, highly BR crystals, presumably CA-oxalate, in Triton-sensitive compartments in the cytoplasm. Taken together, these findings demonstrate that spindle Mts are sensitive to levels of free Ca++ in the physiological range, provide evidence for the existence of a strong cytoplasmic Ca++-sequestering system, and support the notion that Mt assembly and disassembly in local regions of the spindle may be orchestrated by local changes in the cytoplasmic free Ca++ concentration during mitosis. An appendix offers the design of a new chamber for immobilizing echinoderm eggs for injection, a new method for determining the volume of the injected solution, and a description of the microinjection technique, which was designed, but never fully described, by Hiramoto (Y. Hiramoto, Exp. Cell. Res., 1962, 27:416-426.).

Calcium-ion transport by intact synaptosomes. Intrasynaptosomal compartmentation and the role of the mitochondrial membrane potential

The association of Ca2+ with isolated nerve endings (synaptosomes) is investigated and resolved into two components, that bound to the outer surface of the plasma membrane and that transported across the plasma membrane. When synaptosomes are added directly to a Ca2+-containing medium, there is an initial rapid uptake of Ca2+ across the plasma membrane, followed by a slow uptake that proceeds for 20 min. The rapid phase is not observed if the synaptosomes are initially pre-incubated in a Ca2+-free medium. Rapid disruption of synaptosomes reveals that less than 3 nmol of transported Ca2+ per mg of synaptosomal protein can be ascribed to non-mitochondrial components, whereas the remainder, up to 79% of the total, is further transported into the mitochondrial matrix. Abolition of oxidative phosphorylation while the mitochondrial membrane potential is retained leads to a time-dependent increase in transported Ca2+, whereas abolition of the mitochondrial membrane potential decreases both plasma-membrane transport and accumulation of Ca2+ in the mitochondrial matrix. It is concluded that intrasynaptosomal mitochondria are major regulators of synaptosomal Ca2+.

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.

Aequorin response facilitation and intracellular calcium accumulation in molluscan neurones

1. When molluscan neural somata are filled with the calcium-indicating photo-protein aequorin and subjected to a 1 Hz train of depolarizing pulses (0.3 sec duration to + 15 mV) under voltage clamp, the successive photo-emissions due to calcium influx facilitate. The origin of this phenomenon was investigated in identified neurones from the abdominal ganglion of Aplysia californica.2. Since outward currents inactivate cumulatively in successive pulses, the effective depolarization increases due to a series resistance error. Elimination of this error by electronic compensation or pharmacological block of outward current reduced aequorin response facilitation by only about 30%, on the average.3. When voltage-dependent sodium and potassium currents are blocked in tetraethylammonium (TEA)-substituted zero-sodium sea water, the remaining inward calcium currents display no facilitation. On the contrary, a slow decline during a pulse and a slight progressive depression in successive pulses are observed. Barium-substitution for calcium in the same medium eliminates a small residual potassium current insensitive to external TEA. The remaining inward barium currents also display depression instead of facilitation.4. A non-pharmacological separation of calcium current was accomplished by measuring tail currents at the potassium equilibrium potential following depolarizing pulses. Calcium tail currents activate rapidly and then decline gradually and incompletely as depolarizing pulse duration is lengthened. Tail currents also show no evidence of facilitation; there is instead a slight depression of currents after successive pulses.5. Increments of optical absorbance in neurones filled with the calcium-sensitive dye arsenazo III show a depression rather than facilitation to successive depolarizations in a train. The time course of these absorbance signals is consistent with the time-dependent depression of calcium current.6. Calibration of arsenazo III response amplitude indicates that the dye reports only about 1% of the calcium concentration increment expected from knowledge of cell volume and the charge carried by calcium current during a depolarizing pulse. This suggests that cytoplasmic buffering of free calcium must occur rapidly, on a time scale comparable to the response time of arsenazo III (about 1 msec) or more rapidly.7. The slow potassium tail current following a depolarizing pulse is calcium-dependent and probably provides an approximate index of the internal sub-membrane calcium concentration. Increments in this current after repetitive pulses display a slight progressive depression rather than facilitation.8. Since neither calcium currents nor the concentration transients show facilitation, we conclude that aequorin response facilitation is due to the non-linear dependence of aequorin photo-emissions on calcium concentration. This conclusion is supported by a finding that the very different kinetics of arsenazo III responses and aequorin responses can be reconciled by a simple model representing calcium accumulation and known response properties of the two indicator substances.9. In a train of impulses evoked by injecting depolarizing current into a neurone, the successive action potentials grow in duration. Nevertheless, a nearly constant calcium influx signalled by arsenazo III accompanies broadening action potentials.

Effect of measured calcium chloride injections on the membrane potential and internal pH of snail neurones

1. Ion-sensitive micro-electrodes were used to measure changes in intracellular pH (pHi) and internal chloride which resulted from the pressure injection of calcium chloride into identified Helix aspersa neurones. The internal chloride measurement allowed the quantity of calcium chloride injected to be estimated. 2. Application of the metabolic inhibitor carbonyl cyanide m-chlorophenyl hydrazone (CCmP) to a calcium-loaded cell caused an increase in the membrane potential comparable to the effect of injecting calcium itself. The effect was not observed in normal cells. 3. When injected in the presence of CCmp, calcium caused a much larger and longer-lasting effect on the membrane potential than that observed in untreated cells. 4. The injection of ruthenium red can increase and/or prolong the hyperpolarization caused by a given quantity of injected calcium. The pHi changes following calcium injection were biphasic and slower than normal. 5. In seven experiments, both hydrogen chloride and calcium chloride were injected into the same cell. The relative changes in pHi corresponded to the production of one hydrogen ion for each calcium ion injected. 6. The relationship between the quantity of calcium injected and the size of the induced hyperpolarization suggested that at least three calcium ions acting cooperatively are required to activate a potassium 'channel'. 7. Injection of barium chloride hyperpolarized the membrane after a delay of about 1 min. After several injections of barium the cell lost this response while retaining its normal response to calcium injection. Injection of barium also caused a slowly developing (biphasic) fall in pHi. 8. We conclude that injected calcium is normally rapidly taken up by mitochondria in exchange for hydrogen ions. If this uptake process is blocked by ruthenium red or CCmP, the calcium is taken up by a second, slower, process which also releases hydrogen ions. When pre-loaded, but not otherwise, the mitochondria will release calcium ions on treatment with CCmP. Injection of barium does not directly affect the membrane conductance, but causes the release of calcium from intracellular binding sites.

Cyclic variation of K+ conductance in pancreatic beta-cells: Ca2+ and voltage dependence

Pulses of hyperpolarizing current were injected through the microelectrode recording the electrical activity of beta-cells in order to measure input resistance. Increase in resistance during depolarization of the slow oscillation ("burst") indicates inactivation of an outward current, probably K+. Decrease in resistance as the plateau commences suggests that the previous depolarization causes activation of an inward current, probably calcium. The postburst hyperpolarization, caused by a late activation of potassium permeability (PK), would result from the increase of intracellular free calcium. An intracellular buffering system may control this intracellular free calcium level. By restoring the silent phases, in the presence of ouabain or high potassium, injection of hyperpolarizing current shows also a voltage dependency of the PK involved in the postburst hyperpolarization. Glucose, by stimulating intracellular binding of calcium, would cause membrane depolarization at glucose levels below threshold and elongation of the plateau phase at higher concentrations.

Calcium movement in nerve fibres

Given the existence of a difference in electrical potential between the interior of a nerve cell and the media surrounding it, where the cytoplasm is some 70 mV negative (Hodgkin, 1958), it must be expected that any positively charged ion to which the cell membrane is permeable is more concentrated in the cell interior. For monovalent cations such as Na and divalent cations such as Ca and Mg this is not the case in the majority of the cells such as the squid giant axon. In other words, nerve cells maintain a lower intracellular concentration of these ions, as compared with their concentration in the extracellular fluid. For Mg, Ca and Na ions, this lower internal concentration must, in the steady state, be effected by some membrane based mechanism which consumes energy.