Mediated direct measurement

Direct measurements 1 Mediated direct measurements 1 Indirect soil measurements 1 Destructive soil analytical methods J Soil solution Soil solids... 

Explain the difference between direct, mediated direct, and indirect measurement of soil components. 

The Lundh test also includes intestinal intubation and direct measurement of enzyme output in duodenal juice but uses a standardized test meal as a pancreatic stimulus. Because this test requires release of physiological regulatory mediators from the duodenal mucosa, it is less specific than the SC test and may render falsepositive results in intestinal diseases such as celiac sprue. 

Oxalate is excreted primarily by the kidney. Oxalate is freely filtered at the glomerulus, where its concentration is normally 1 5 pM. One of the few physiologic functions of oxalate occurs in the proximal tubule where it plays a role in transcellular reabsorption of chloride (mainly present as sodium chloride). Cl entry across the apical membrane is mediated by Cl /oxalate exchange (oxalate is recycled from the tubular lumen to the cell by oxalate/ sulfate exchange, in parallel with Na /sulfate cotransport) [4]. Early studies of renal oxalate clearance using radio-labeled oxalate showed secretion in almost all subjects studied. More recent studies using direct measurement of serum and urine... 

Finally, it is possible that these techniques can be extended successfully to insect CNS preparations. Methods now exist to prepare functional insect synaptosomes from insect ganglia (1 ), and these preparations have been used to demonstrate veratridine-dependent neurotransmitter release and enhancement of this release by deltamethrin in a superfusion assay (2j)). Further refinement of these methods should allow direct measurement of sodium channel-mediated sodium fluxes in insect CNS preparations, thus allowing the investigation of target site differences not only between mammals and insects but also between susceptible and resistant insect strains. 

A powerful new approach is the direct measurement of released Ca + with fluorescent Ca + indicators such as fura-2 or fluo-3, whereby the permeabilized smooth muscle has to be placed into microcuvettes (lino, 1991). Since force is no longer required as an indicator, Ca + release may be measured in the absence of MgATP, which allowed the study of adenine nucleotides on the IP3-induced Ca + release without the interference by Ca + uptake (lino, 1991 Hirose and lino, 1994). Inclusion of ryanodine allows a separate study of the IP3- and caffeine-sensitive stores (Hirose et al., 1993). Experimental protocols have been designed that allow the use of high concentrations of EGTA, thereby preventing Ca -mediated feedback regulation of the Ca2+ release (Hirose and lino, 1994). Direct measurement of Ca + release with indicators becomes even more powerful in combination with... 

Using mediators in combination with enzyme is a practical alternative for the direct measurement of hydrogen peroxide and for the direa monitoring of enz)nnaric reaaion. Another advantage is that the use of mediator could help to overcome the dependence of the amperometric biosensors on dissolved oxygen since the mediator can provide the essential electrons. ... 

Adenylate cyclase is bound to the inner aspect of the plasma membrane. Its catalytic activity mediates hydrolysis of ATP to form cyclic AMP, and may be measured in fractured membranes in vitro by the conversion of radiolabelled ATP to cyclic AMP °. The activity of the enzyme may also be measured by following changes in intracellular cyclic AMP in whole cells in the absence or presence of a phosphodiesterase inhibitor . A full discussion of the relative merits of the two assay systems lies beyond the scope of the present review, but most information on the detailed biochemistry of receptor-mediated responses has been obtained by direct measurement of the enzymatic activity (i.e. conversion of labelled substrate to labelled product) in washed membrane preparations. 

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