Dual-mode stimulation

The concept of dual-mode stimulation can be expanded further. It is possible to devise systems capable of existing in several forms (multistate) that can be intercon-verted by different external stimuli (multifunctional). Such systems can give rise to intricate networks of reactions that, when examined from the viewpoint of molecular-level devices"1311 reveal very interesting properties.1321... 

PTH has a dual effect on bone cells, depending on the temporal mode of administration given intermittently, PTH stimulates osteoblast activity and leads to substantial increases in bone density. In contrast, when given (or secreted) continuously, PTH stimulates osteoclast-mediated bone resorption and suppresses osteoblast activity. Further to its direct effects on bone cells, PTH also enhances renal calcium re-absorption and phosphate clearance, as well as renal synthesis of 1,25-dihydroxy vitamin D. Both PTH and 1,25-dihydroxyvitamin D act synergistically on bone to increase serum calcium levels and are closely involved in the regulation of the calcium/phosphate balance. The anabolic effects of PTH on osteoblasts are probably both direct and indirect via growth factors such as IGF-1 and TGF 3. The multiple signal transduction... 

Fig. 37.11. Use of an NO microsensor for detection of the NO release from cultured endothelial cells. The sensor is a dual probe microsensor. The small sensor is a bare Pt UME used to position the sensor in the feedback mode. Onto the larger Pt electrode a polymer was deposited from an acrylic resin containing Ni(4-lV-tetramethyl) pyridyl porphyrin and served as amperometric NO sensor, (a) Schematic of the sensor, (b) optical microphotograph of the sensor surface, (c) Response of the NO sensor to the stimulation of the cells with bradykinin at different distances of the sensor to the surface of the cells. Reprinted with permission from Ref. [104], Copyright 2004, American Chemical Society.

A mechanistic study has revealed a weak electrophilic character for the competent oxidant, turning to a biphilic nature with respect to sulfoxide oxidation. This behavior is likely to be stimulated by the polynuclear framework, which may foster a dual activation of both the oxidant and the sulfoxide by coordination to proximal Lewis acid sites on the polyoxotungstate surface. The postulated asymmetric binding of the peroxo-ligand, not evolving to a bidentate j -coordination mode, could explain the atypical selectivity behavior, with respect to classical d -peroxometal complexes. 

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