Magnesium or calcium activated ATPase in mammalian nerve

Purification and characterization of a Ca2+/Mg2+ ecto-ATPase from rat heart sarcolemma

The Ca2+/Mg2+ ATPase of the rat heart sarcolemmal particles was solubilized with Triton X-100 after treating the membranes with trypsin and purified by high speed centrifugation, ammonium sulfate fractionation, hydrophobic chromatography and gel filtration. The purified enzyme was seen as a single protein band in non-denaturing polyacrylamide gel electrophoresis and its molecular weight by gel filtration was found to be about 240,000. The enzyme utilized Ca-ATP or Mg-ATP as a substrate with high affinity sites (Km = 0.12-0.16 mM) and low affinity sites (Km = 1 mM). The enzyme also utilized CTP, GTP, ITP, UTP and ADP as substrates but at a lower rate in comparison to ATP. The enzyme was activated by Ca2+ (Ka = 0.4 mM) and Mg2+ (Ka = 0.2 mM) as well as by other cations in the order Ca2+ greater than Mg2+ greater than Mn2+ greater than Sr2+ greater than Ba2+ greater than Ni2+ greater than Cu2+. The ATPase activity in the presence of Ca2+ was markedly inhibited by Mg2+, Mn2+, Ni2+ and Cu2+ whereas the monovalent cations such as Na+ and K+ were without effect. The enzyme did not exhibit Ca2+ stimulated Mg2+ dependent ATPase activity and was insensitive to calmodulin, ouabain, verapamil, D-600, oligomycin, azide and vanadate. Optimum pH for Ca2+ or Mg2+ ATPase activity was 8.5-9.0. In view of the possible ectoenzyme nature of the ATPase, its role in adenine nucleotide and Ca2+ metabolism in the myocardium is discussed.

Subcellular fractionation of rat sciatic nerve and specific localization of ganglioside LM1 in rat nerve myelin

Subcellular fractionation of rat sciatic nerve was developed to determine the specific localization of gangliosides in the nerve membrane fractions. Myelin, microsomal, and a plasma membrane-like fraction were isolated and purified by sucrose density gradient centrifugation. These subfractions were characterized by electron microscopy, marker enzyme assays, and their protein and lipid profile. In rat sciatic nerve myelin, 90 mol% of the total gangliosides were monosialogangliosides. LM1 (sialosyl-lactoneotetraosylceramide) (61 mol%) and GM3 (21%) were the major gangliosides of the rat nerve myelin. Two other neolacto series of gangliosides, viz., sialosyl-lactoneonorhexaosylceramide and sialosyl-lactoneooctaosylceramide, were also localized mostly in the myelin fraction. GM1 was only a minor (less than 2%) ganglioside in myelin. The ganglioside patterns of the microsomal and plasma membrane-like fractions were similar with minor quantitative differences and were entirely different from that of myelin. Monosialogangliosides were approximately 70-75 mol% of the total in these fractions. The major gangliosides of the microsomal and plasma membrane-like fractions were GM3 (approximately 40%) and GM1 (approximately 20%). LM1 in these fractions was minimal (less than approximately 5%). Significant amounts of GM3 with N-glycolylneuraminic acid (approximately 10%) and GM1b (4-14%) were also identified in the microsomal and plasma membrane-like fractions but not in myelin. These and the higher lactoneo series of gangliosides have not been previously reported to be present in the rat nervous system. Almost exclusive localization of LM1 in myelin in rat peripheral nervous system is consistent with our previous observation that deposition of LM1 in the nerve with age was very similar to that of myelin marker lipids cerebrosides and sulfatides.

Endoneurial ATPase activity in Tangier disease and other peripheral neuropathies

Endoneurial sodium, potassium adenosine triphosphatase (Na+,K+-ATPase) and Mg2+-ATPase activities were determined in routine sural nerve biopsies from patients being evaluated for peripheral neuropathy. A significant reduction of endoneurial Na+,K+-ATPase and Mg2+-ATPase activities was shown in six sural nerve biopsies from patients with Tangier disease complicated by mononeuropathy multiplex or progressive axonal neuropathy. Peripheral nerve ATPase activities did not correlate with myelinated or unmyelinated nerve fiber densities in these biopsies. Other peripheral neuronal disorders with reduced endoneurial Na+,K+-ATPase and Mg2+-ATPase activities included severe vasculitic neuropathy, diabetic neuropathy, tomaculous neuropathy, and motoneuron disease. Such reduced levels of ATPase activity in peripheral nerve may relate to altered endoneurial lipid metabolism and impaired axoplasmic flow.

Calcium requirement for fast axonal transport in frog motoneurons

Calcium is required to sustain fast axonal transport in sensory neurons of frog and cat. We studied the Ca2+ dependence of fast axonal transport in the motoneurons of the lower spinal cord from frog. The accumulation of acetylcholinesterase at a crush on the ventral roots was used to follow axonal transport. Two types of experiments were performed: modification of the medium bathing the ventral roots, alone, and modification of the medium bathing the spinal cord and ventral roots. Incubation (17--18 h) of the ventral roots in Ca2+-free medium markedly inhibited acetylcholinesterase transport, a finding that demonstrates a Ca2+ requirement for fast axonal transport in motoneurons; when 4 mM MgCl2 was added to the Ca2+-free medium, transport was also greatly reduced. During incubation of the ventral roots in normal medium supplemented with 0.18 mM CoCl2 transport proceeded normally; but when the Co2+ concentration was raised to 1.8 mM, transport was diminished as drastically as in the Ca2+-free medium. Incubation of the spinal cord and ventral roots in medium containing 0.18 mM CoCl2 did not reduce the accumulation of acetylcholinesterase at the crush. Similarly, accumulation of acetylcholinesterase at a crush on the dorsal root was not significantly reduced by exposure of the dorsal root ganglion and root to 0.18 mM Co2+. Exposure of sensory cell bodies to 0.18 mM Co2+ thus produces differential effects on transport of acetylcholinesterase and on transport of newly synthesized radiolabeled protein.

MG2+ or Ca2+-activated ATPase in squid giant fiber axoplasm

A divalent cation-activated ATPase in axoplasm from the squid giant axon is described. The enzyme requires Mg2+ or Ca2+, has a K+ optimum of 60 mM, and has a pH optimum of 7.5. Several nucleotide triphosphates other than ATP can serve as substrates. The enzyme is inhibited by excess ATP or Mg2+. The enzyme is enriched in a rapidly sedimenting fraction of the axoplasm, and is eluted in the exclusion volume of a Sepharose 4B column, suggesting that it is associated with a highly aggregated structure. Comparison of the properties of enzyme with those of myosin and Na+-K+-ATPase suggests that differs from both of these enzymes. The enzyme has many similarities to vertebrate nerve ATPases previously described. The demonstration of the presence of this ATPase in squid axoplasm proves the neuronal localization of the enzyme.

Properties of Ca2+- or Mg2+-dependent ATPase in rat heart sarcolemma

Rat heart sarcolemma was shown to hydrolyze ATP in the presence of Ca2+ or Mg2+; Ka values for Ca2+ and Mg2+ were in the range of 0.58-0.67 and 0.72-0.83 mM, whereas Vmax values were 33-38 and 21-28 mumol Pi/mg per hr, respectively. Both Ca2+ ATPase and Mg2+ ATPase showed low- and high-affinity sites for ATP; the Km value for the low-affinity sites for both enzyme activities was 300-325 microM, whereas Km values for high-affinity sites were 75-85 and 100-108 microM, respectively. The pattern of nucleotide hydrolysis in the presence of Ca2+ was found to be different from that with Mg2+. Although both high concentrations of ADP and Pi inhibited the enzyme activities, Mg2+ ATPase was more sensitive to ADP and less sensitive to Pi in comparison to Ca2+ ATPase. Storage of sarcolemma at about 0 degrees C showed a greater increase in ATP hydrolysis with Ca2+ than with Mg2+. The inhibitory effect of Mg2+ on Ca2+ ATPase, unlike that of Ni2+, Co2+, and Cu2+, was more than that on Mg2+ ATPase. Treatment of membranes with sodium dodecylsulfate or deoxycholate produced a greater reduction in Mg2+ ATPase than in Ca2+ ATPase. These results further support the view that Ca2+ ATPase and Mg2+ ATPase may be two separate enzymes in heart sarcolemma. It is suggested that Ca2+-dependent ATPase may be involved in opening calcium channels for the entry of calcium, whereas Mg2+ ATPase may serve as a Mg2+ pump mechanism for the efflux of magnesium from the cardiac cell.

Ca2+- or Mg2+-stimulated ATPase activity in bullfrog spinal nerve: relation to Ca2+ requirements for fast axonal transport

Adenosine triphosphatase (ATPase) activity stimulated by Ca2+ or Mg2+ was characterized in spinal nerve and spinal sensory ganglion of bullfrog. Enzyme activity of homogenates from both sources reached a maximum at a 1-2 mM concentration of either cation, although the level of maximal activity in nerve trunks was approximately twice that in ganglia. Enzyme activation was not observed with 2 mM-Sr2+ or Ba2+. Co2+ or Mn2+, at 2 mM, depressed Ca2+ activation of the enzyme by 50-60% in nerve but had no inhibitory effect on ganglia activity. In intact spinal ganglion/spinal nerve preparations, incubated for 20 h in medium containing 0.2 mM-Co2+, no effect was detected on Ca2+/Mg2+ ATPase activity in ganglia or nerve trunks whereas fast axonal transport was inhibited by 80%. Incubation in medium containing 0.02 mM-Hg2+ depressed enzyme activity in ganglia by 64% and in nerve trunks by 44%, whereas fast transport was again inhibited by 80%. When only nerve trunks were exposed to these ions, Hg2+ but not Co2+ was observed to slow the rate of fast axonal transport. The divalent cation specificity of the Ca2+/Mg2+ ATPase activity is distinct from the ion specificities, determined in previous work, of the Ca2+ requirement during initiation of fast axonal transport in the soma, and of the Ca2+ requirement during translocation in the axon. Thus, previous observations of Ca2+-dependent events in fast axonal transport cannot be taken per se to suggest the involvement of Ca2+/Mg+ ATPase in the transport process.

Microtrabecular structure of the axoplasmic matrix: visualization of cross-linking structures and their distribution

Axoplasmic transport is a dramatic example of cytoplasmic motility. Constituents of axoplasm migrate as far as 400 mm/d or at approximately 5 micron/s. Thin-section studies have identified the major morphological elements within the axoplasm as being microtubules, neurofilaments (100-A filaments), an interconnected and elongated varicose component of smooth endoplasmic reticulum (SER), more dilated and vesicular organelles resembling portions of SER, multivesicular bodies, mitochondria, and, finally, a matrix of ground substance in which the tubules, filaments, and vesicles are suspended. In the ordinary thin-section image, the ground substance is comprised of wispy fragments which, in not being noticeably tied together, do not give the impression of representing more than a condensation of what might be a homogeneous solution of proteins. With the high-voltage microscope on thick (0.5-micron) sections, we have noticed, however, that the so-called wispy fragments are part of a three-dimensional lattice. We contend that this lattice is not an artifact of aldehyde fixation, and our contention is supported by its visability after rapid-freezing and freeze-substitution. This lattice or microtrabecular matrix of axoplasm was found to consist of an organized system of cross-bridges between microtubules, neurofilaments, cisternae of the SER, and the plasma membrane. We propose that formation and deformation of this system are involved in rapid axonal transport. To facilitate electron microscope visualization of the trabecular connections between elements of axoplasm, the following three techniques were used: first, the addition of tannic acid to the primary fixative, OsO4 postfixation, then en bloc staining in uranyl acetate for conventional transmission electron microscope (TEM); second, embedding tissue in polyethylene glycol for thin sectioning, dissolving out the embedding medium from the sections and blocks, critical-point-drying (J. J. Wolosewick, 1980, J. Cell Biol., 86:675-681.), and then observing the matrix-free sections with TEM or the blocks with a scanning electron microscope; and third, rapid freezing of fixed tissue followed by freeze-etching and rotary-shadowing with replicas observed by TEM. All of these procedures yielded images of cross-linking elements between neurofilaments and organelles of the axoplasm. These improvements in visualization should enable us to examine the distribution of trabecular links on motile axonal organelles.

Ca2+- or Mg2+-dependent enzymatic ATP hydrolysis associated with the microsomal fraction of frog sciatic nerves

The microsomal fraction of frog sciatic nerves was found to contain Ca2+- or Mg2+-dependent hydrolytic activity toward different nucleoside di- and triphosphates. In the presence of Ca2+ substrate specificity was in the order CTP > UTP > GTP > ATP. When Mg2+ was used, the triphosphates were approximately equally good substrates. ATP hydrolytic activity was very similar with Ca2+ or Mg2+ as the cofactor, whereas Ca2+ was the more potent activator of hydrolysis of the other triphosphates tested. The preparation showed some activity toward the nucleoside diphosphates but none toward the monophosphates or p-nitrophenylphosphate. The enzymic properties of ATP hydrolysis were more closely studied. The hydrolysis was optimal at 18--24 degrees C in the presence of 1 mM-Ca2+ or 1 mM-Mg2+. Ca2+- and Mg2+-ATP hydrolysis displayed pH maxima around 8.0--8.5 and 7.4--8.0, respectively. Vmax values for Ca2+- and Mg2+-ATP hydrolysis similar: approx. 12 mumol Pi per h per mg protein with a Km value of approx. 0.05 mM. The ATP hydrolysis activity was inhibited by NaF but unaffected by ouabain, vanadate, cytochalasin B, and various drugs known to influence ATPase activity of mitochondria. Zn2+ stimulated the ATP hydrolysis activity at low concentrations (10(-6)-10(-5) M) and inhibited it at higher concentrations. The possibility that these observations account for stimulation and inhibition of axonal transport in frog sciatic nerves exposed to similar concentrations of Zn2+ is discussed.

Axonal transport of the Ca2+-dependent protein modulator of 3':5'-cyclic-AMP phosphodiesterase in the rabbit visual system

Water-soluble proteins were extracted from individual retinas, optic nerves, combined optic tracts and lateral geniculate bodies, and superior colliculi of rabbits at 1, 3, and 18 days after injection of [3H]leucine into the right eye. The Ca2+-dependent protein modulator of 3':5'-cyclic-AMP phosphodiesterase (calmodulin) was isolated from these samples by a two-step polyacrylamide gel electrophoresis procedure. An analysis of the radioactivity incorporated into the total soluble proteins and the calmodulin revealed that most of the calmodulin was axonally transported at a slow rate (2--4 mm/day) and represented about 0.45% of the total transported soluble protein.

Calmodulin in mammalian nerve

Calmodulin, a calcium-dependent regulatory protein has been isolated from mammalian nerve. The protein has similarities to the calcium-binding protein earlier shown to be transported at a fast rate in the nerve fibers. The implication is that calmodulin, which has been shown to be involved in various key cellular processes, may have a relation to axoplasmic transport.

The requirement for calcium ions and the effect of other ions on axoplasmic transport in mammalian nerve

1. Until recently it was believed that axoplasmic transport in vitro was not affected by Ca2+, transport being normal in Ca2+-free medium. This was found due to the presence of the relatively impermeable perineurial sheath around the nerve trunks. Using a desheathed cat peroneal nerve preparation, axoplasmic transport was shown to require an adequate level of Ca2+ in the external medium. In a buffered Ca2+-free medium, transport began to decline within 30 min and a complete block occurred in 2 . 6 hr. A concentration of 5 mM-Ca2+ added to a buffered isotonic sucrose of NaCl solution was able to maintain transport. With lower concentrations of Ca2+ of 1 . 5-3 . 0 mM, those usually present in the extracellular fluid or in a Ringer medium, some impairment of transport was seen but the addition of 4 mM-K+ restored the normal pattern of axoplasmic transport. With Ca2+ concentrations below 0 . 75 mM, however, 4 mM-K+ was unable to sustain transport. 2. Potassium by itself at a concentration of 4 mM when added to a buffered isotonic sucrose of NaCl medium was unable to prolong the time of transport block beyond that seen in buffered isotonic NaCl or sucrose solutions. In concentrations of K+ up to 25 mM, 1 . 5-5 mM-Ca2+ was required for normal transport. With moderately higher concentrations of K+ in the range of 50-100 mM, normal appearing transport was seen with or without Ca2+. This was seen whether or not Na+ was present in the medium. At higher levels of K+, 120-150 mM, decreased transport was seen, with or without the addition of either 15 mM-Na+ or Ca2+ in concentrations of 1 . 5-3 . 0 mM. 3. While Mg2+ could not substitute completely for Ca2+ in maintaining transport, it was able to prolong the time before block occurred. An extra 30-60 min of downflow was seen when 5 mM-Mg2+ was added to a buffered isotonic NaCl medium. Magnesium also acts synergistically with Ca2+. Concentration of Ca2+ as low as 0 . 25 mM was, with the addition of 1 . 5 mM-Mg2+, able to maintain transport. 4. The results are interpreted in the light of studies of the mechanism of Ca2+ regulation known to occur in giant nerve fibres and other clls controlling the level of free Ca2+. The relationship of Ca2+ to the mechanism considered to underlie axoplasmic transport in nerve fibres is also discussed.

Uptake and energy-dependent extrusion of calcium in neural cells in culture

The metabolism of Ca2+ was studied in a neuronal model system, the clonal mouse neuroblastoma x rat glioma hybrid cell line 108CC5. 1. Homogenates of the hybrid cells exhibit a specific activity of Ca2+-ATPase considerably higher than that of homogenates of the parental cells. 2. Uptake and release of 45Ca2+ by the hybrid cells display two and three distinct phases, respectively, and indicate that 40--50% of the cell-associated Ca2+ is located at the cell surface. 3. The influx of 45Ca2+ is insignificantly affected by Mg2+ or Na+, slightly diminished by Ba2+ or Sr2+, strongly inhibited by La3+, Co2+ or prenylamine, and considerably enhanced by high (i.e., depolarizing) concentrations of K+. The efflux of 45Ca2+ is reduced by La3+. 4. The hybrid cells tend to maintain Ca2+ homeostasis with an overall cellular Ca2+ concentration of 0.5--0.7 mM. At 1.8 mM Ca2+ in the medium this implies the necessity of an extrusion pump in the plasma membrane. 5. A reduction in the hybrid cells of the level of ATP is paralleled by a decline in the content of Ca2+. This can only be explained by the existence of energy-dependent intracellular Ca2+ stores that effectively compete for Ca2+ with a Ca2+ pump located in the plasma membrane. The internal stores are not identical with the mitochondria because mitochondrial inhibitors hardly change Ca2+ metabolism. 6. Micromolar concentrations of the ionophore A23187 can switch off the internal Ca2+ stores without affecting considerably the influx of Ca2+ through the plasma membrane. 7. With switched-off Ca2+ stores it is possible to increase the cellular Ca2+ content distinctly and to bring it back again to the control values in an ATP-dependent manner, i.e. to demonstrate the action of a Ca2+-extrusion pump in the plasma membrane. 8. Under some conditions active extrusion of Ca2+ depends not only on ATP but also on the presence of extracellular Na+. 9. Similar results as with hybrid cells are also obtained with rat glioma cells. The methodology of combining energy deprivation with the application of the ionophore A23187 is possibly generally applicable to obtain insight into the Ca2+ metabolism of various cell types.

The identification of two intra-axonally transported polypeptides resembling myosin in some respects in the rabbit visual system

Two polypeptides (M1 and M2) which co-sediment with F-actin in an ATP-reversible way have been detected in extracts of tissue from the rabbit visual system. Both polypeptides resemble skeletal muscle myosin in their ATP-sensitive co-sedimentation with actin, while they resemble the heavy chain of myosin and the lighter polypeptide of erythrocyte spectrin in their electrophoretic mobilities. (The estimated molecular weights are: MI congruent to 195,000; myosin congruent 200,000; M2 and spectrin congruent to 220,000). M1 and M2 were labeled in the cell bodies of the retinal ganglion cells with a radioactive amino acid and subsequently recovered in tissues (optic nerve, optic tract, lateral geniculate nucleus, and superior colliculus) containing segments of the retinal ganglion cell axons. The temporal sequence of labeling M1 and M2 in these tissues indicated that both polypeptides were synthesized in the cell bodies of retinal ganglion cells and subsequently transported down their axons at different maximum velocities. The estimated velocities were: M1, 4-8 mm per day; and M2, 2-4 mm per day.

Fast axonal transport: effect of antimitotic drugs and inhibitors of energy metabolism on the rate and amount of transported protein in frog sciatic nerves

Mitosis inhibitors, drugs affecting the energy metabolism, heavy water, and ouabain were used to partially inhibit fast axonal transport in frog sciatic nerves. Effects on the rate and on the amount of pulse labeled protein could be separated. The pulse of labeled protein, released after a cold-block, rapidly reached a maximum height which indicated that the transport system was saturated in the nerve segment occupied by the pulse. Both the rate and the amount were reduced by the mitosis inhibitors colchicine, vinblastine, and griseofulvin. Colchicine had a differential effect and reduced the rate of material migrating in the advancing front of the pulse less than the rate of that moving in the peak. Preincubation at low temperature potentiated the effects of colchicine. Two inhibitors of energy metabolism, NaCN and IAA, reduced the amount of labeled material in the pulse. The slope of the pulse was markedly reduced and multiple peaks appeared. The distance covered by the migrating pulse was largely unaffected, but some retardation of late components might have occurred. In contrast, 2.4-DNP reduced the rate without any effects on the amount of migrating material. Heavy water uniformly reduced the rate of the migrating pulse, whereas the main effect of ouabain was a diminished amount and multiple peaks as with NaCN and IAA. All drugs were tested for their effects on the electrical activity of sciatic nerves. The compound action potential was not affected by the mitosis inhibitors and heavy water, but was depressed by the inhibitors of energy metabolism and abolished by ouabain. The results indicate that the effects of various transport inhibitory drugs can be differentiated if both the rate and the amount are considered.

Slow axoplasmic transport of mitochondria (MAO) and lactic dehydrogenase in mammalian nerve fibers

The axoplasmic transport of the enzyme monoamine oxidase (MAO) (monoamine:O2 oxidoreductase, (deaminating) EC 1.4.3.4), a marker for mitochondria, and lactic dehydrogenase (LDH) (L-lactate:NAD oxidoreductase, EC 1.1.1927), a soluble component of axoplasm, was studied in cat sciatic nerve. For both these enzymes a linear accumulation was found in the nerve proximal to ligations over a period of at least 20 h. In double-ligation experiments no evidence of a depletion of enzymes within the nerve segment was found over this period of time as would be the case if some portion of the enzymes was carried by fast axoplasmic transport. Both the soluble protein enzyme LDH and the mitochondria, shown by MAO, are thus considered to be moved down the nerve by slow axoplasmic transport. Some differences in the two materials were seen in the greater fall in the level of MAO compared to LDH within the double-ligated segment over the succeeding period from 20 to 48 h. These changes are considered with respect to the transport filament model as modified to take into account slow axoplasmic transport.

Batrachotoxin block of fast axoplasmic transport in mammalian nerve fibers

Batrachotoxin (BTX) irreversibly blocks fast axoplasmic transport in nerve in concentrations as low as 0.2 micromolar. The action of BTX was studied in cat sciatic nerves in vitro by measuring the rate of the crest outflow after injection of the L7 dorsal root ganglion with [3-H]leucine. Tetrodotoxin, which in itself does not affect fast axoplasmic transport, inhibited the blocking action of BTX. Unlike the BTX block of nerve and muscle membrane excitability brought about through increased permeability to sodium ion, the BTX block of fast axoplasmic transport occurs with or without sodium ion in the medium. High concentrations of calcium ion protected against the blocking action of BTX, while magnesium ion did not. An action of BTX on the transport mechanism inside the fibers was indicated by the small reduction of adenosine triphosphate plus creatine phosphate, which in itself did not account for the block of axoplasmic transport.

Low temperature slowing and cold-block of fast axoplasmic transport in mammalian nerves in vitro

1) Fast axoplasmic transport in mammalian nerve in vitro was studied using an isotope labeling technique. The rate of outflow in cat sciatic nerve fibers of 410 mm/day in vitro was reduced at temperatures below 38 degrees C with a Q10 of 2.0 in the range 38-18 degrees C and a Q10 of 2.3 at 38-13 degrees C. 2) At a temperature of 11 degrees C a partial failure of transport occurred. At temperatures below 11 degrees C a complete block of fast axoplasmic transport occurred, a phenomenon termed "cold-block." No transport at all was seen over the temperature range of 10-0 degrees C for times lasting up to 48 hr. 3) Transport was resumed after a period of cold-block lasting up to 22 hr when the nerves were brought back to a temperature of 38 degrees C. Some deleterious effects due to cold-block were seen in the recovery phase as indicated by a reduction in crest amplitude, change in its form, and slowed rate. 4) The approximately P level (combined ATP and creatine phosphate) remained near control level in nerves kept at low or cold-block temperatures for times as long as 64 hr. The reduction in fast axoplasmic transport rate seen at low temperatures for times up to 22 hr was therefore considered due to a decrease in the utilization of ATP, a concept in accord with the "transport filament" model proposed to account for fast axoplasmic transport. 5) The sloping of the front of the crest over the temperature range of 18-13 degrees C suggests an additional factor at the lower temperatures. A disassembly of microtubules is discussed as a possible explanation of the cold-block phenomenon.