Receptor-bound thrombin is not internalized through coated pits in mouse embryo cells

The localization of thrombin receptors on mouse embryo (ME) cells was examined using electron microscope (EM) immunocytological techniques. ME cells were fixed with formaldehyde, prior to thrombin binding, and thrombin visualized on cell surfaces using affinity-purified antithrombin rabbit antibody and colloidal gold labeled anti-rabbit IgG. Colloidal gold particles were found in clusters on the surface of cells incubated with thrombin. There were approximately seven particles per cluster observed in thin sections with cluster diameters ranging from 70 to 200 nm. These clusters were not observed on cells incubated without thrombin. The total number of particles present on cells incubated with and without thrombin indicate that the colloidal gold labeling is approximately 98% specific for thrombin. Only four colloidal gold particles out of approximately 1,200 were associated with coated pits. Thus the thrombin receptor clusters do not appear to associate with coated membrane regions. To determine whether receptor-bound thrombin was internalized by receptor-mediated endocytosis, ME cells were incubated with 125I-thrombin and examined using EM autoradiography and the trypsin sensitivity of 125I-thrombin which was associated with the cells. In two types of experiments, where thrombin was incubated with cells at 4 degrees C and the temperature increased to 37 degrees C and where initial incubation was at 37 degrees C, the receptor-directed specific internalization proceeded at approximately the same rate as nonspecific internalization. These studies indicate that thrombin that binds to its receptors on ME cells is not rapidly internalized by receptor-mediated endocytosis.     

Light-controlled inhibition of hypocotyl growth inSinapis alba L. seedlings

Fluence rate-response curves were determined for the inhibition of hypocotyl growth in 54 h old dark-grownSinapis alba L. seedlings by continuous or hourly 5 min red light irradiation (24 h). In both cases a fluence rate-dependence was observed. More than 90% of the continuous light effect could be substituted for by hourly light pulses if the total fluence of the two different light regimes was the same. Measurements of the far red absorbing form of phytochrome ([P _fr]) and [P _fr]/[P tot] (total phytochrome) showed a strong fluence rate-dependence under continuous and pulsed light which partially paralleled the fluence rate-response curves for the inhibition of the hypocotyl growth.Abbreviations R red - HIR high irradiance response - P _rfr phytochrome in its red, far-red absorbing form - [P _tot]=[P _r]+[P _fr] =k ₁/(k ₁+k ₂): photoequilibrium of phytochrome at wavelength , wherebyk _1,2 rate constants ofP _rP _fr,P _frP _r photoconversion - [P _fr]/[P _tot]     

Embryo-lethal mutants of Arabidopsis thaliana: Evidence for gametophytic expression of the mutant genes

Summary Normal and aborted seeds from two recessive embryo-lethal mutants (79A and 124D) of Arabidopsis thaliana were shown to be distributed nonrandomly along the length of heterozygous siliques. Significantly more than half of the aborted seeds in these two mutants were located in the top half of the silique, in the region closest to the stigma surface. Segregation ratios (percent aborted seeds) were unusually low at the base of the silique, and slightly higher than expected at the tip. In contrast, aborted seeds from four other embryo-lethal mutants (87A, 123B, 50B, and 71E) were distributed randomly along the length of the silique. These results suggest that the mutant genes in 79A and 124D are expressed during both the gametophytic (n) and sporophytic (2n) phases of development. These two mutants provide further evidence for the hypothesis that many genes expressed prior to fertilization also perform a critical function during growth and development of the sporophyte. Embryo-lethal mutants of Arabidopsis may therefore be useful in future studies of gametophytic gene expression and the regulation of pollen-tube growth in higher plants.     

EFFECTS OF VARIATIONS IN LIGHT INTENSITY ON THE EFFICIENCY OF GROWTH OF SKELETONEMA COSTATUM (BACILLARIOPHYCEAE) IN A CYCLOSTAT1

The relative importance of respiration and organic carbon release to the efficiency of carbon specific growth of Skeletonema costatum (Grev.) Clave was evaluated over a light range from 1500–15 μE · m^?2· s^?1. Net growth efficiency ranged from 0.45–0.69 with a maximum at 130 μE · m^?2· s^?1. Respiration was 93% or more of the variations in growth efficiency. Organic carbon release ranged from 0–7% of gross production and increased with light intensity. Carbon specific particulate production was a hyperbolic function of incident light intensity and was related exponentially to particulate carbon production per unit chlorophyll a. Full sunlight conditions, 1500 μE · m^?2· s^?1, did not induce photoinhibition of gross production. Variations in the efficiency of growth of S. costatum were minimized over a wide range of light intensities mainly because of variations in cellular pigments which permitted the efficient utilization of available light energy, and a reduction in the losses of carbon which increases the growth rate, possibly as a consequence of the recycling of respired carbon within the cell.     

Nutritional requirements for conidial germination and hyphal growth of Beauveria bassiana

Nutritional requirements for germination and growth of the entomopathogenic fungus Beauveria bassiana are not complex. For germination to occur, a utilizable source of carbon must be present; however, a nitrogen source is needed for continued hyphal growth, otherwise lysis ensues. Compounds that can serve as utilizable carbon-energy sources for germination include glucose, N-acetylglucosamine, glucosamine, chitin, starch, lanolin, hydrocarbons in crude oil, and some longer-chain fatty acids. Both organic and inorganic sources of nitrogen are readily utilized for growth. Conidia undergo active metabolism soon after being placed in a suitable growth medium, indicating that conidia are released from their state of dormancy several hours before emergence of the germ tube can be observed. Because of the nutritional versatility of B. bassiana, this fungus should be able to survive and be infective in several types of natural environments.     

Mutually Independent Cyclic AMP and Sodium Responses to Nerve Growth Factor in Embryonic Chick Dorsal Root Ganglia

Abstract: Chick embryo dorsal root ganglia display a rapid and transient rise in their cyclic AMP content when presented with nerve growth factor. These ganglia also depend on nerve growth factor for control of their intracellular Na⁺ and K⁺ levels. A sequential relationship between the cyclic AMP and Na⁺ responses is not readily apparent. Incubation of chick sensory ganglia in a sodium-free medium does not prevent the cyclic AMP response to nerve growth factor from occurring. When ganglia are first incubated with ouabain for 6 h, presentation of nerve growth factor elicits a cyclic AMP response, but no Na⁺ response. The cyclic AMP response therefore does not depend on the Na⁺ environment. An initial presentation of nerve growth factor to the ganglia for 30 min, followed by its withdrawal and subsequent re-administration at different intervals over several hours failed to result in a second cyclic AMP response. Nevertheless, the expected Na⁺ behaviors were still observed. Dibutyryl cyclic AMP is capable of eliciting a cyclic AMP response in chick sensory ganglia after 6 h of nerve growth factor deprivation. When both agents were presented simultaneously to the ganglia, only a single cyclic AMP response was obtained, corresponding in time to the response elicited by dibutyryl cyclic AMP alone-indicating that this drug acts on the NGF-sensitive cells. At the same time dibutyryl cyclic AMP alone failed to result in a Na⁺ response, leading one to conclude that the cyclic AMP response to nerve growth factor is truly not mediating the Na⁺ response. Additional support for the mutual independence of these two short-latency responses is provided by the apparent inability of nerve growth factor to cause a cyclic AMP response in chick embryo sympathetic ganglia, another traditional target for the factor, which is capable of displaying a Na⁺ response.     

COMPARATIVE KINETIC STUDIES OF NITRATE-LIMITED GROWTH AND NITRATE UPTAKE IN PHYTOPLANKTON IN CONTINUOUS CULTURE1

A comparative study of nitrate-limited growth and nitrate uptake was carried out in chemostat cultures of Ankistrodesmus falcatus (Corda) Ralfs., Asterionella formosa Hass., and Fragilaria crotonensis Kit. In each species growth rate (μ) was related to total cell nitrogen or cell quota (q) by the empirical Droop growth function. Nitrate uptake was a function of both external N concentration and q. The apparent maximum uptake rate (V_m') at a given μ was inversely related to q – q₀, where q₀ is the minimum quota. The apparent half-saturation constant for uptake, (K_m') appears to show a slight inverse trend with μ, although statistical analysis shows that this trend is inconclusive. When q approaches q₀, V_m' is several orders of magnitude greater than μq, the calculated steady-state uptake rate. As q increases, however, the difference between these two variables decreases sharply until q approaches q_m, the cell quota for nitrogen-rich cells. At this point the difference between μq and V_m' disappears. This behavior is explained by the feedback regulation of N uptake. The inverse relationship between V_m' and q – q₀ can be described by an empirical three-parameter equation.