Loss Of Synchrony Research Articles

Effects of changes in excitability and intercellular coupling on synchronization in the rabbit sino-atrial node.

The mechanisms of synchronization between sino-atrial pace-maker cells were studied in biological preparations from rabbit hearts, and in computer simulations of the Hodgkin & Huxley type. For biological experiments, thin strips of sino-atrial node were placed in a three-compartment bath. The electrical properties of the tissue in the middle segment (the 'gap') were manipulated pharmacologically to alter electrical coupling and/or excitability of cells in that segment, and to study the patterns of interaction between two pace-maker centres in the external segments. Superfusion of the gap segment with either verapamil (2 microM) or acetylcholine (10 microM) produced a loss of 1:1 synchrony (entrainment) of spontaneous discharges generated by the external pace-makers but subharmonic (i.e. 3:2; 5:4; 9:8; etc.) entrainment was always maintained. When the gap segment was superfused with heptanol (3.5 mM), which is known to increase intercellular resistance, the pace-maker centres in the external chambers beat independently of one another. Progressive loss of synchrony paralleled reductions in amplitude of electrotonic responses to current pulses applied across the gap. Gap superfusion with hypertonic Tyrode solution (600 mosM) produced a major reduction in the degree of synchronization between the external pace-makers, even though the cells in the central compartment maintained their excitability. Under these conditions, as many as three independent pace-maker centres, one in each chamber, coexisted in a given preparation. Using computerized simulations based on equations of time- and voltage-dependent membrane currents, three 'cells', each capable of maintaining spontaneous activity, were connected in a linear array through ohmic resistances. When selective parameters (e.g. membrane conductances, coupling resistance) were modified appropriately, the mathematical simulations reproduced very closely the interaction patterns observed in the experimental preparations. Our results show that synchronization in the sinus node results from mutual interactions and entrainment between all the cells in this region. These interactions are of the kind expected for a population of coupled, self-sustained oscillators, and are mediated through electrotonic propagation of current across low-resistance junctions.

The cell cycle of symbiotic Chlorella. II. The effect of continuous darkness.

When green hydra were grown in continuous darkness the mean cell size of their symbiotic algae was smaller than when grown in light and numbers of algae per digestive cell were reduced. The former was due to a reduction in size at which the algae divided, and the latter to a loss of synchrony of algal mitosis with that of digestive cells after transfer to darkness. Eventually, algal mitosis regained synchrony with that of digestive cells. Division synchrony was not lost in reinfected hydra (with lower than normal numbers of algae per cell) transferred to darkness; this, and the observation that synchrony in normal animals transferred to darkness was regained when algal numbers per cell had fallen to a new, lower level, suggested that the initial inhibition of algal mitosis was due to competition for a limited supply of heterotrophically required metabolites. When dark-grown hydra were returned to light there was no delay in algal division and a steady increase in the size of dividing cells, suggesting that the smaller division size in darkness was not due simply to the critical size for division being set at a lower value. In light, algal division size varied with frequency of host feeding, but this had less effect on algal division size in darkness. It is suggested that the critical cell size that algae must attain before being able to complete the cell cycle is the same in light and darkness, but in light mitosis is restricted by some exogenous factor so that algae grow beyond the critical size without dividing. In darkness both algal cell growth and division are dependent on exogenously supplied metabolites, and cell growth rather than the division factor is limiting. The precise nature of the restriction on algal division remains unknown.

Conjugation in Tetrahymena thermophila: A temporal analysis of cytological stages

The time course and synchrony of the stages of conjugation in Tetrahymena thermophila as defined by cytologically observable changes in the morphology and position of the nuclei were established. The time required for 50% of the pairs to enter or pass a particular stage, as well as the duration of each stage were determined. The relative synchrony of the pairs as they went through conjugation was followed by correlating the maximum percentage of the population found in a stage with the duration of that stage. The degree of synchrony between the pairs was found to be high under the conditions of this study, with very little decrease in synchrony seen during the initial 9 h of conjugation. Although some variability in the degree of synchrony was seen between different matings, there was little change detected in the duration of each cytological stage. Prolonged starvation of the cells prior to their mating resulted in a gradual loss of synchrony.

Duplication of a spontaneously active neuron in Aplysia: Electrical coupling and effects of a phosphodiesterase inhibitor

An abdominal ganglion from an Aplysia californica is described, in which cell R15 has anomalously duplicated. The two cells exhibited a high degree of electrical coupling, assuring functional synchrony of output in the cells, which are characterized by a complex firing pattern. Exposure of this ganglion to the phosphodiesterase inhibitor IBMX caused a more altered firing rhythm in one of the cells, as well as an enhanced inhibitory component associated with the coupling potentials between cells, resulting in a loss of synchrony between the two cells.

Variation in sister-cell cycle durations and loss of synchrony in cell lineages in root apical meristems

Analyses of identifiable cell lineages of known ages in pea root meristems have allowed estimates to be made of differences in cell cycle duration of sister cells and of the rate of loss of synchrony within lineages as cell proliferation continues. The average difference between sister-cell cycle times is approx. 14%. This results in loss of synchrony within cell lineages, and complete asynchrony is reached after approx. 5 cycles. No evidence for longitudinal gradients being involved in determining differences in cycle times within columns of cells in the proliferating compartment of the meristem was obtained.

Hourly variations in plasma concentrations of growth hormone and insulin and in amino acid uptake and incorporation into protein in diaphragm muscle of the rat.

Groups of young (30-to 32-day-old), freely behaving female rats were decapitated at hourly intervals over a 24-h period, and the concentrations of GH and insulin in the plasma were measured by RIA. Paired hemidiaphragms from these rats were incubated for 40 min in buffer containing 2-[14C]aminoisobutyric acid (AIB) to measure amino acid transport and [3H]-leucine to assess protein synthesis. One member of each pair of hemidiaphragms was also exposed to rat GH (5 μ/ml). Several episodes of GH secretion were evident, as suggested by accumulations of rats having high or low levels of plasma GH at certain times of the day. Out of 192 rats studied, 46 had values above 50 ng/ml (based on a rat GH reference standard having a potency in the immunoassay of 2.7 times NIAMDD-RP-1). Peaks of GH concentration were observed at 0300 h, 0600–0700 h, and 2200 h. No clear peaks were seen at other times, perhaps due to a loss of synchrony in GH secretion among rats during the middle of the day. Mean plasma insulin levels did not vary appreciably over the 24-h period. Amino acid transport and leucine incorporation into protein in hemidiaphragms not exposed to exogenous rat GH varied markedly from hour to hour. During the time when the three peaks of plasma GH were observed, there was a significant correlation between the mean plasma GH concentration and the rate of protein synthesis in diaphragms taken from rats killed 1h later. Adding rat GH to diaphragms in vitro stimulated both AIB transport and leucine incorporation into protein several times during the day, but effects were most evident during the dark period. There was a significant correlation between the effect of exogenous GH on protein synthesis and its effect on AIB uptake. These observations suggest that in prepubertal female rats the plasma concentration of GH reaches levels of 50–150 ng/ml several times during the day and that these episodic peaks in circulating GH may have transient effects on muscle metabolism. (Endocrinology102: 1420, 1978)

La formation des cystes dans l'ovariole de chrysopa perla (L.) (neuroptera: Chrysopidae)

In the polytrophic ovarioles of Chrysopa perla, the germarium is the site of formation of the cyst or functional oocyte-nurse cells complex. Descended from a cystoblast, the cyst shows frequently 12 cystocytes (12–14: exception to the 2 n rule); the latter are connected by intercellular bridges. The relationships between the cystocytes was discovered and a pattern proposed. As early as the second mitotic division, there is loss of synchrony; after 2 other mitoses, the lateral cells develop immediately into nurse cells (early nurse cells); the medial cells divide only 3 times and the complete cyst shows 3 categories of cells: (1) 2 (or 3) cells of type A, one of which grows into the oocyte; (2) 2 (or 3) cells of type B which are smaller; all (except the oocyte) develop into late nurse cells after the beginning of the meiosis; (3) 8 early nurse cells (type C), descended from the lateral cells. The cyst, at first globular, becomes cylindrical as a result of the cell's growth and the presence of a differentiated axial region, the fusome, formed by the alignment of intercellular bridges. Later the fusome loses its cohesion and the differentiated cystocytes take their definitive position in the ovariole: a posterior oocyte preceded by 11 nurse cells.

Determining the base sequence of RNA molecules using continuous degradation kinetics

A kinetic approach to sequence analysis is presented. A polymer chain composed of a small number of monomer types is degraded continuously with degradation proceeding in only one direction of the chain. Ordinarily, the direct determination of the sequence from the amount of degraded monomers is hampered by the rapid loss of reaction synchrony. However, it is shown, that if the monomers can be measured reasonably accurately, one can solve a system of linear equations with the unknowns giving the positions of the monomers in the chain, and the syn chrony loss due to the stochastic nature of the degradation process can be accounted for. Model calculations are presented.

Temporal sharpening within response areas of medullary neurons

It is well known that auditory medullary neurons preserve the temporal characteristics of low‐frequency sounds by discharging at integral multiples of the stimulus period. Monaural recordings from kangaroo rat cochlear nuclei and binaural superior olive neurons show that the variability in synchrony of neural discharges decreases as the frequency of the sound increases. Variability in synchrony is apparent from period histogram displays and may be expressed by vector strength and interquartile range. These results provide a possible neural basis for the finding that the difference limen for pitch and the minimum audible angle, in absolute time, decreases as the frequency of the sound increases. Evidence is also presented that relates the shortest intervals of neural discharge to the loss of synchrony as frequency of sound stimulation increases. [Supported by AFOSR and NINDS.]

CLINICAL CHEMISTRY OF GRAIN-FED CATTLE. II. LIVER FUNCTION

The rate of plasma clearance of bilirubin (BCR), as a test of liver function, was followed in three hay-fed and four grain-fed Hereford steers over the first 77 days of a 106-day feeding period. The grain diet was 90% steam rolled barley, 5% beet pulp pellets and 5% of a protein/mineral/vitamin concentrate given together with a hay supplement of 0.9 kg/head/day. The grain was introduced slowly over a 10-day period. Fluctuations in hematocrit values occurred during the adaptation to the grain diet. Periodic episodes of markedly reduced liver function occurred synchronously in test animals during the introduction of the grain diet. Similar episodes of reduced liver function were observed with decreasing frequency and severity as the feeding progressed. After 25 days of grain feeding, loss of synchrony also occurred, so that high values were observed in only one test animal at any one time. As the feeding period progressed, the time required for the distribution and equilibration of the bilirubin in the blood increased in the grain-fed animals causing interference with the BCR tests. The effects on liver function indicated a reduction in the efficiency of the liver which probably contributes to pathologic conditions in feedlot cattle.

The ultrastructural effects of excess maternal vitamin A on the primitive streak stage rat embryo

ABSTRACT A single injection of 100000 i.u. vitamin A palmitate was administered to pregnant female rats on one of the days 0–14 of pregnancy. The litters were examined for malformations and prenatal death. On the basis of the results of this preliminary investigation, treatment on day 8 was selected for detailed study. Day-8 embryos were examined with the electron microscope from 2 to 24 h after maternal vitamin A administration. Ultrastructural abnormalities were: membrane swelling (cytoplasmic vacuoles, ‘budding’, nuclear membrane irregularities, general swelling leading to cell death); changes in the extra-to intracellular fluid ratio; increase in the number of lipid droplets; autophagic vacuoles; condensed and swollen mitochondria. These changes were not equally distributed among the different cell types. It is suggested that this differential effect leads to a loss of synchrony between the development of structures from the different germ layers, in addition to a general retardation. Loss of contact between mesoderm cells may delay normal migratory movements and induction of neural plate. The general retardation is related to the lower birthweights of treated embryos versus controls, and the differential effects to the subsequent abnormal morphogenesis.

Colcemid action on the division schedule of synchronized Tetrahymena

Synchronized Tetrahymena pyriformis were immersed for varying periods in systematically varied concentrations of Colcemid. With increasingly higher concentrations of Colcemid the cells displayed progressively longer division delays. Cells suspended in inorganic medium were more sensitive to Colcemid than cells in nutrient medium. Possibly Colcemid forms a complex with some constituent(s) of the nutrient medium, thus rendering the agent less effective. A transition from maximum sensitivity to insensitivity was evident when Tetrahymena were exposed to Colcemid at 45–55 min after EH. Moreover, the population became separated into 2 groups during this transition period: an ‘early-maturing group’ dividing with minimal delays; and a ‘late-maturing group’ dividing with maximal delays. On Colcemid reversal, cells divided with no apparent loss in synchrony 75–85 min after Colcemid removal. Biochemical studies revealed that DNA, RNA, and protein synthesis were inhibited approx. 90, 70, and 40%, respectively, in cells continuously exposed to 4 × 10 −4 M Colcemid. Furthermore, the release of cells from a Colcemid block effectively released the block to DNA, RNA, and, to some extent, protein synthesis. Colchicine also inhibited DNA synthesis to a greater degree than RNA or protein synthesis. In addition to affecting the integrity of microtubules, the action of Colcemid and colchicine may be mediated through the inhibition of macromolecular synthesis associated with the cell cycle.

The effects of elevated temperatures on spore swelling and germination in Aspergillus niger.

Exogenous carbon and nitrogen sources were necessary for complete swelling (spherical growth) and germ-tube formation from Aspergillus niger spores. The rate of spherical growth was low at 30°, maximum at 38°, and decreased at higher temperatures until at 47° inhibition was complete. At temperatures from 35° to 44° the final spore size was similar and greater than at 30° although the time taken to reach this size varied. Germ-tube formation occurred in from 97 to 99% of the spores with a good degree of synchrony between 30° and 38°. At temperatures from 38° to 43° the proportion of spores which produced germ tubes gradually decreased with a concurrent loss of synchrony. At 44° germ-tube formation was completely inhibited although spherical growth could occur over a prolonged period to produce large spherical cells. This increase of cell volume was apparently accompanied by a system of wall extension which involved wall synthesis. Under conditions of spore crowding an inhibition became apparent which affected only the initiation of spherical growth and not the spherical-growth process itself. Other effects of spore crowding were observed and were attributed to alterations in the gaseous environment. Increased spherical growth was always accompanied by a greater degree of branching of the germ tube when produced and a hypothesis relating these effects is suggested.

A virtuoso isopod

An extremely elaborate performance, involving endogenous timing with both tidal and lunar frequencies, has been recorded in the locomotor activity of an adult specimen of the intertidal isopod,Excirolana chiltoni. During two months of observation, under constant, non-tidal conditions, this animal showed a persistent tidal rhythm in its swimming activity. Bursts of activity were initially well synchronized with times of tide crest on the shore. The average free-running period of the tidal rhythm was about 24 h 55 min, i.e. about 5 minutes longer than the average period of the tides; thus, the loss of synchrony with the concurrent tides was very gradual. The amount of activity per burst showed a conspicuous pattern of variation, a periodic amplitude modulation which paralleled, in detail, the complex lunar cycle of changes in height of high tide. The free-running period of the bimodal, circa-lunar rhythm of amplitude modulation was one or two days longer than the natural 29-day lunar cycle of tide heights. Each feature of this recording has been qualitatively replicated in activity records from other individuals of this species. Freshly-collectedExcirolana generally show spontaneous bursts of activity at times of tide crest, bursts which are repeated as a persistent tidal rhythm, the period of which commonly departs by only a few minutes from that of the natural tidal cycle. Superimposed on the tidal rhythm is an endogenous monthly pattern of amplitude modulation, which alters the amount of activity per burst. This circa-lunar rhythm has a free-running period between about 26 and 33 days, and generally leads to maximum activity on days of highest of high tides. The net result of the tidal and lunar rhythms is an activity pattern which permits the isopods to recapitulate, in great detail, certain significant ecological aspects of the mixed, semi-diurnal tidal regime of California. The experimental data are not compatible with the hypothesis that uncontrolled environmental factors, such as vibrations from waves, were responsible for the rhythmic behavior of the animals. Neither do the data support the hypothesis that “beats” between the observed tidal rhythm and a hidden daily or circadian rhythm were responsible for the observed circa-lunar rhythm. Furthermore, the pattern of the circa-lunar rhythm cannot be accounted for by a single monthly oscillation in the “excitability” of the animals, such as might be mediated by changes in the level of an excitatory or inhibitory hormone in the circulatory system.

DNA synthesis and mitosis in fused cells. I. HeLa homokaryons.

ABSTRACT When HeLa cells growing in asynchronous culture are fused together by inactivated Sendai virus, synchrony of DNA synthesis and mitosis is rapidly imposed in the resultant multinucleate cells, even though the single cells which fuse to form these multinucleate cells are at different stages of the cell cycle. Some measure of nuclear co-ordination can already be observed within a few hours of cell fusion, and maximal synchrony is achieved within 2 days. By the end of the second day after fusion, asynchronous DNA synthesis or mitosis is rare. In binucleate cells this high order of synchrony is maintained for at least 5 days, but in cells containing greater numbers of nuclei some loss of synchrony begins to appear after the third day. The results indicate that the ability of the multinucleate cell to co-ordinate nuclear events is not impaired by the use of virus to facilitate cell fusion.

Nuclear behavior in multinucleate Schizophyllum commune Germlings.

The ultrastructure of a “fir” morphological mutant containing multinucleate cells is described in Schizophyllum commune. The germlings of basidiospores which arose from mating “fir” with wild-type mycelium were studied in culture by phase contrast microscopy to elucidate behavior of multinucleate cells. Nuclear division appeared synchronous from two nuclei yielding four progeny through six nuclei producing twelve products, beyond which loss of synchrony was indicated. Compensatory nuclear migration into an anucleate cell was presumed during synchronous division of nuclear aggregates in the adjacent cell of an individual germling. The migrant nucleus eventually returned to the cell of origin. However, the return route was not via the central pore of the septum but rather occurred at the juncture of the cross-wall with the germling-periphery. Ultrastructure of a partial septum in “fir” which could accommodate nuclear passage of this sort is described.

Intracellular distribution of sulfur during the synchronous growth of chlorella pyrenoisoda

Intermittent illumination was used to synchronize cells of Chlorella pyrenoidosa. The degree of synchrony of the cultures was evaluated during three consecutive generations of growth in continuous light following the synchronization procedure. The loss in synchrony, as measured by the coefficient of variation, was only 8.53% for the three generations.The uptake and intracellular distribution of [35S]sulfate was followed during synchronous growth. The total cellular-35S (percentage of cellular dry wt.) remained essentially constant throughout cellular development. However, the percentage 35S (percentage of total cellular-35S) in the total cellular protein, cold trichloroacetic acid extract, and sulfolipid fractions of the cells showed dennite periodism.Between the third and fifth hours of cellular development, during a 14-h synchronous growth cycle, the cold trichloroacetic acid-soluble fraction showed a marked increase in percent 35S while the protein fraction showed a decrease of equal magnitude. The decrease in protein-35S was traced to a decrease in the percentage of protein cystine-cysteine-35S. The percentage of protein methionine-35S increased during this period but not enough to compensate for the decrease in protein cystine-cysteine-35S. Thus, the percentage of protein-35S decreased. Between the fifth and seventh hours of cellular maturation, the percentage of 35S in the protein and trichloroacetic acid-soluble fractions returned to their original levels. The protein-sulfur-amino acid composition likewise returned to its original value. The change in the 35S content of the cold trichloroacetic acid-soluble fraction was attributed to a single spot on paper chromatograms which absorbed ultraviolet light and was ninhydrin negative. Acid hydrolysis of the compound(s) in this spot released six or seven ninhydrin-positive spots and two ultraviolet-absorbing spots. After acid hydrolysis, most of the radioactivity was associated with a ninhydrin-positive spot which had the characteristics of cysteic acid.Immediately after the 35S content of the protein and trichloroacetic acid-soluble fractions returned to their original values, nuclear division was initiated. A close relationship between these shifts in sulfur metabolism and the initiation of nuclear division seems possible.