The structure of agarose gels,and the accommodation within them of irregularly shaped particles

This paper shows how the number of cross-linkage points (nodes) in a random reticulum, such as an ideal polysaccharide gel, may be calculated in accordance with mathematical principles. The influence of nodal configurations upon the statistical geometry of the reticulum is discussed, and it is shown from experimental evidence that the nodal configurations in agarose gel are nonrandom. A method is given for calculating the accommodation probability of an irregularly shaped particle in a reticulum, which is relevant to the theory of gel chromatography and to the distribution of cells in tissues permeating a network of capillaries or veins.     

Carbamylcholine, TRH, PGF2 alpha and fluoride enhance free intracellular Ca++ and Ca++ translocation in dog thyroid cells

Effects on Ca++ translocation and [Ca++]i were studied in dog thyro?d cell monolayers using both 45Ca++ efflux and the indicator quin-2. Carbamylcholine, a non hydrolysable analog of acetylcholine, through muscarinic receptors, and to a lesser extent TRH and PGF2 alpha increased both these parameters. [Ca++]i increased by 171, 100 and 75% respectively over a basal level of 66 +/- 17 nM (mean +/- SD). The response to carbamylcholine was biphasic. A transient increase in [Ca++]i was followed by a more sustained phase where the [Ca++]i was slightly higher than the basal level. Only the first phase was insensitive to extracellular Ca++ depletion. This phase is probably due to a release of Ca++ from an intracellular store. NaF also induced a sustained rise in [Ca++]i dependent on extracellular Ca++ and affected 45Ca++ efflux. Our data provide direct evidence of an implication of intracellular Ca++ in the response of dog thyro?d cells to all these agents.     

Biosynthesis of porphyrins and corrins. 1. 1H and 13C NMR spectra of (hydroxymethyl)bilane and uroporphyrinogens I and III

High-field NMR spectroscopic methods have been applied to study the reactions catalyzed by porphobilinogen (PBG) deaminase and uroporphyrinogen III (uro'gen III) cosynthase, which are the enzymes responsible for the formation of the porphyrin macrocycle. The action of these enzymes in the conversion of PBG, [2,11-13C]PBG, and [3,5-13C]PBG to uro'gens I and III has been followed by 1H and 13C NMR, and assignments are presented. The principal intermediate that accumulated was the correspondingly labeled (hydroxymethyl)bilane (HMB), the assignments for which are also presented.     

Water stress plating hypersensitivity of yeasts

Saccharomyces cerevisiae, when growing exponentially in batch culture, passed through a phase in which, on average, one cell in 10(4) survived plating onto a low water activity (aw) agar medium. Stationary phase cultures were resistant as were all other species tested, with the exception of Candida krusei. In continuous culture, S. cerevisiae was more resistant at low than at high dilution rates. Plating at low aw was lethal to those cells that were not protected by an adequate content of compatible solute. In naturally resistant yeasts and in S. cerevisiae that had been exposed to an adaptation process, the compatible solute was one or more types of polyhydric alcohol. Resistance in stationary phase was attributable to a different cause.     

Mass characteristics of DNA obtained from chromosomes of a human carcinoma cell line

Sedimentation studies of DNA from chromosomes extracted from human mitotic cells showed that highmolecular-weight DNA can be obtained if cell hypotonic treatments and prolonged metaphase blocks are avoided. Two types of large double-stranded DNA were observed. One of these (M _r = 2.5×10⁸) appeared as a size class with characteristics reminiscent of the chromosomal DNA subunit hypothesis. However, this DNA is the decay product of larger molecules, whose minimum molecular weight is 6×10⁸.     

Constructional morphology: The analysis of constraints in evolution dedicated to A. Seilacher in honour of his 60. birthday

Evolutionary change is opportunistic, but its course is strongly constrained in several fundamental ways. These constraints (historical/phylogenetic, functional/adaptive, constructional/morphogenetic) and their dynamic relationships are discussed here and shown to constitute the conceptual framework of Constructional Morphology. Notwithstanding recent published opinions which claim that the discovery of constraints renders Neodarwinian selection theory obsolete, we regard the insights of Constructional Morphology as being entirely consistent with this theory. As is shown here in the case of the Hyracoidea, formal analysis of the constraints which have framed the evolution of various characters extends our understanding of the evolution of a taxon.     

Catalysis by heteropentalenes of electron transfer to oxygen from ferredoxin-NADP oxidoreductase reduced by NADPH

A range of heteropentalene and bipyridinium compounds have been tested as catalysts of electron transfer to oxygen from spinach ferredoxin-NADP⁺ oxidoreductase reduced by NADPH. For a particular class of compound, the rate of oxygen reduction increased with increasing midpoint potential of the compound under conditions in which reduction of the compound was rate-limiting. Compounds with similar midpoint potentials from different structural classes showed marked differences in rate, attributed to specificity in the interaction with ferredoxin-NADP⁺ oxidoreductase.     

Antagonistic effects of thyrotropin and epidermal growth factor on thyroglobulin mRNA level in cultured thyroid cells

Both thyrotropin (TSH) and epidermal growth factor (EGF) are potent mitogenic agents when added to dog thyroid cells in primary culture [Roger, P. P. and Dumont, J. E. (1984) Mol. Cell. Endocrinol. 36, 79-93]. The concomitant effect of these agents on the differentiation state of the cells was appreciated using cell morphology, iodide trapping, thyroglobulin synthesis and cytoplasmic thyroglobulin mRNA content as markers. Together with previous results [Mol. Cell. Endocrinol. 36, 79-93 (1984)] it is shown that cells cultured in the continuous presence of TSH maintain all the parameters at a near normal level. In the absence of TSH, thyroglobulin mRNA decreased to very low, though still detectable levels. Addition of TSH restored subnormal mRNA levels. Culture of cells in the presence of EGF for 4-6 days affected profoundly their morphology, abolished iodide trapping and decreased thyroglobulin synthesis and cytoplasmic mRNA content to undetectable levels. Addition of TSH to cells previously exposed to EGF reversed the growth factor effect on all four indexes. The redifferentiating effect of TSH was well observed within 3-4 days and was mimicked by the adenylate cyclase activators, forskolin and cholera toxin. When administered simultaneously, TSH and EGF achieved an intermediate situation, EGF antagonizing partially the effect of TSH on the expression of thyroglobulin gene. Another growth factor, fibroblast growth factor, while promoting thyroid cell proliferation also, did not interfere at all with TSH effects on cytoplasmic thyroglobulin mRNA content. Our results make the dog thyroid cell in primary culture an appropriate model to study the mechanisms involved in gene regulation by cyclic AMP and growth factors.     

Clonal variations in keratin : Intermediate filament expression by human somatic cell hybrids

The intermediate filament composition of differentiated vertebrate cells provides a stable phenotype which appears to be specifically regulated in each cell type. In order to analyse the regulation of intermediate filament expression we have constructed human somatic cell hybrids from the fusion of the HeLa-derived cell line HEB7A and a normal human diploid fibroblast, GM2291. These parental cells differ with respect to the presence or absence of keratin intermediate filaments. Isolation of independently arising clones produced two classes of hybrids. One class expresses keratin in a stable manner and the other class lacks keratin altogether. Indirect immunofluorescence of hybrid cells using antikeratin antiserum demonstrates that there are variations in the intensity and organization of cytoskeletal keratin staining. SDS-PAGE comparisons of cell extracts from these hybrids indicates that there are quantitative differences in the relative amounts of individual keratin polypeptides as well. These clonal variations have allowed us to begin assessing the consequences of genetic interactions between cell types that are normally capable of closely regulating different subsets of intermediate filament genes.