Increasing returns activity analysis

The second modification to the increasing returns activity analysis model is the possibility of substitution among inputs along a given process ray. For example, it may be possible to use less feedstock and more fuel to produce the same output from fixed capital equipment. The extent of this substitution is limited by physical and chemical laws, as well as by legal restrictions and by equipment limits in the short run. 

Although innovation is an extremely complex activity, some simplification can be achieved for purposes of empirical study by delineating categories of innovation in the context of a model of production. Production is assumed to take place within a finite set of well-defined processes, rather than on a smooth neoclassical production function. In this respect, the model is a version of the "activity analysis" model. In contrast to the usual activity analysis model, however, some of the inputs are characterized by increasing returns to scale. Specifically, engineers often assume that capital and labor (and perhaps other inputs) may be increased at a slower rate than output, while raw materials, energy, and other inputs are increased at the same or nearly the same rate as output. The economics literature on production functions discusses these points (12.13,14). 

IL-6 expression can be activated in neutrophils upon exposure to GM-CSF and TNF, whilst IL-3, G-CSF, y-interferon and lymphotoxin do not induce expression of this cytokine. Expression of IL-6 has been identified by analysis of mRNA levels and by protein analysis. Following GM-CSF exposure, expression is detectable by 2 h, maximal by 6 h and then returns to base-line levels. LPS, PMA (but not fMet-Leu-Phe) and cycloheximide also induce IL-6 transcripts. The finding that this protein-synthesis inhibitor increases mRNA levels suggests either that transcription is regulated by a short-lived repressor, or else that decay of its mRNA may be regulated by a short-lived RNase. 

Fig. 3. Changes in Akt and GSK-3/3 phosphorylation induced by transient retinal ischemia and effect ofintravitreal application of MK801 in rat. Animals were subjected to retinal ischemia for 50 min in the right eye (R) and reperfusion was allowed for 1, 6, or 24 h. The left eye (L) was used as control. (A) Phosphorylation of Akt on Ser473 is significantly diminished after retinal ischemia and is accompanied by a transient dephosphorylation (activation) of GSK-3/3. During the reperfusion phase, Akt activation is increased within 1 h whereas GSK-3(3 phosphorylation status returns to basal level. (B) Intravitreal treatment with MK801 enhances the phosphorylation of Akt reported after I h reperfusion. Histograms show the results of the densitometric analysis of immunoreactive bands. P < 0.05 versus vehicle.

Similarly to T. cruzi, inositol phospholipids have been identified in T. brucei brucei (44). Moreover, two distinct PI synthase activities (CDP-diglyceride-dependent and -independent) are present in membrane fractions. In the absence of added lipid precursor, the incorporation of free inositol into PI is strongly stimulated by the addition of Mn " and less by Mg . Addition of exogenous CDP-diglyceride to the system increases the uptake of inositol by a factor of 1.5 to 3.5. These inositol phosphates may mediate part of the T. brucei brucei response to external signals. Previous work have shown that transformation of bloodstream forms to procyclic cells can be induced by cis-aconitate or citrate. Analysis of inositol phosphates in [ H]inositol-labeled cells exposed to ds-aconitate revealed that levels of InsP3 increased rapidly (maximum levels at one hour) and subsequently returned to basal level. The levels of InsP and InsP2 were not or little affected by cis-aconitate. 

Recently [3] we described a mutant (actl) which was almost completely deficient in activity of the chloroplast glycerol-3-phosphate acyltransferase the first enzyme of the prokaryotic pathway. Analysis of the fatty acid composition of leaf lipids and C-labelling experiments indicated that PG was essentially the only lipid produced by the prokaryotic pathway in this mutant. However, this blockage in the prokaryotic pathway is compensated for by increased flux through the eukaryotic pathway and furthermore the proportions of the different chloroplast lipids made by the eukaryotic pathway are altered so that there is only a small alteration in lipid composition in the actl mutant compared with wild type [3]. Thus in the actl mutant (Fig. lb) 950 of every 1000 fatty acids enter the eukaryotic pathway and 650 of these are subsequently returned to the chloroplast. The mutant is essentially an 18 3-plant and clearly demonstrates the existence of controls which regulate reactions in the chloroplast and endoplasmic reticulum so as to provide the complement of lipids required for correct membrane synthesis in leaf cells. 

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