Sreenivas Devidas

Molecular Characterization of a Broad Selectivity Neutral Solute Channel

In all living cells, coordination of solute and water movement across cell membranes is of critical importance for osmotic balance. The current concept is that these processes are of distinct biophysical nature. Here we report the expression cloning of a liver cDNA encoding a unique promiscuous solute channel (AQP9) that confers high permeability for both solutes and water. AQP9 mediates passage of a wide variety of non-charged solutes including carbamides, polyols, purines, and pyrimidines in a phloretin- and mercury-sensitive manner, whereas amino acids, cyclic sugars, Na+, K+, Cl−, and deprotonated monocarboxylates are excluded. The properties of AQP9 define a new evolutionary branch of the major intrinsic protein family of aquaporin proteins and describe a previously unknown mechanism by which a large variety of solutes and water can pass through a single pore, enabling rapid cellular uptake or exit of metabolites with minimal osmotic perturbation.

The cystic fibrosis transmembrane conductance regulator and ATP.

A controversy in the field of cystic fibrosis (CF) research has arisen concerning the role of the cystic fibrosis transmembrane conductance regulator (CFTR) in the transport of ATP. Does the CFTR actually conduct ATP or does it regulate the conductance of ATP? Recent findings either support or reject the hypothesis that the CFTR can transport ATP. In addition, recent research from several laboratories has suggested that ATP mediates its effects after traversing the plasma membrane and reaching the extracellular surface. The current model suggests that the released ATP exerts its various influences via a purinergic receptor to regulate outwardly rectifying chloride channels and epithelial sodium channels.

CFTR: domains, structure, and function.

Mutations in the gene encoding the cystic fibrosis transmembrane conductance regulator (CFTR) cause cystic fibrosis (CF) (Collins, 1992). Over 500 naturally occurring mutations have been identified in CF gene which are located in all of the domains of the protein (Kerem et al., 1990; Mercier et al., 1993; Ghanem et al., 1994; Fanen et al., 1992; Ferec et al., 1992; Cutting et al., 1990). Early studies by several investigators characterized CFTR as a chloride channel (Anderson et al.; 1991b,c; Bear et al., 1991). The complex secondary structure of the protein suggested that CFTR might possess other functions in addition to being a chloride channel. Studies have established that the CFTR functions not only as a chloride channel but is indeed a regulator of sodium channels (Stutts et al., 1995), outwardly rectifying chloride channels (ORCC) (Gray et al., 1989; Garber et al., 1992; Egan et al., 1992; Hwang et al., 1989; Schwiebert et al., 1995) and also the transport of ATP (Schwiebert et al., 1995; Reisin et al., 1994). This mini-review deals with the studies which elucidate the functions of the various domains of CFTR, namely the transmembrane domains, TMD1 and TMD2, the two cytoplasmic nucleotide binding domains, NBD1 and NBD2, and the regulatory, R, domain.

Engineering Aspects of a Kinestatic Charge Detector

The engineering aspects of a nine-channel digital radiographic system developed for bioimaging research, based on high gas pressure ionography and kinestatic principles, are presented. The research imaging system uses a pulsed x-ray beam which allows one to study simultaneously the ionic signal characteristics at 10 different ionization sites along the drift axis. This research imaging detector system allows one to investigate methods to improve the detection and image quality parameters as part of the development of a large scale prototype medical imaging system.

The Second Half of the Cystic Fibrosis Transmembrane Conductance Regulator Forms a Functional Chloride Channel

The cystic fibrosis transmembrane conductance regulator (CFTR) consists of two transmembrane domains (TMDs), TMD1 and TMD2, two cytoplasmic nucleotide binding domains (NBDs), NBD1 and NBD2, and a regulatory domain. To elucidate the complex function of the CFTR, deletion constructs encompassing the second half of the CFTR distal to the first transmembrane domain were expressed in Xenopusoocytes and IB3 cells (a cystic fibrosis cell line). Constructs containing the regulatory domain, the second transmembrane domain, and the second nucleotide binding domain formed constitutively active channels, which were further stimulated upon the addition of cAMP. On the other hand, a construct encompassing the second transmembrane domain and the second nucleotide binding domain was stimulated to a small but noticeable extent upon the addition of cAMP. The selectivity of the second-half construct was the same for iodide and chloride, in contrast to the selectivity of wild-type CFTR, which is Cl− > I−. However, both constructs displayed single-channel conductances that were significantly smaller than those displayed by the first half of the CFTR. We conclude that regions of the second transmembrane domain may contribute to the overall channel of the pore, although the first half of the CFTR may confer its selectivity.

Enhanced X-Ray Detectors Using Polar Dopants for KCD Digital Radiography.

The goal of this study is to develop high resolution imaging detectors with applications in digital radiography and computed tomography. A physical treatment aimed at a better understanding of the line-spread function response of kinestatic charge detector (KCD) gas media, using dopants with permanent electric dipoles, is presented. Experimental results were obtained by operating a KCD krypton-filled detector at pressures up to 60 atm and constant electric field-to-gas density ratio doped with small amounts of polar or nonpolar polyatomic molecules with low or high ionization potential. The results clearly indicate that the addition of dopants having both low ionization potential and high dipole moment significantly enhance the imaging signal quality. An analysis of the experimental results aimed at providing a plausible interpretation of the reported observations is offered.

Initial clinical performance of a large-field KCD digital radiography system

The initial clinical performance of a research prototype digital radiographic system based on a large-field (2016-channel) kinestatic charge detector and data acquisition system is discussed. The first clinical images from the large-field system are compared with images of the same patients taken with commercial systems. Future directions are discussed.

Imaging performance of a large-field kinestatic charge detector for digital radiography

The initial performance of a digital radiographic system incorporating a large-field (2016- channel) kinestatic charge detector and data acquisition electronics is discussed. The measured modulation transfer function of the system is 20% at 4 cy/mm. The measured detective quantum efficiency is 40 - 60%. These results are comparable with or better than those of current clinical (rare-earth film-screen and storage phosphor) systems. First images from the large-field system are shown and compared with those from commercial systems. Future system improvements in process or in planning are discussed.

Quantum mechanical and electrodynamical effects on charge carriers in KCD imaging detectors

A theoretical and an experimental study in the area of electronic medical imaging devices operating through Kinestatic Charge Detection principles is in progress, aimed at a better understanding of both the macroscopic electric potentials and the charge transfer (exchange) reactions. It will be demonstrated that long range attractive dipolar moment forces associated with low ionization potential polar molecules with a high dipole moment in an inert gas detective medium offer superior imaging performance over the non-polar counterparts with zero electric dipole. The dopant works by introducing a drastic reduction of the ion mobility dispersion. The goal of this study is to implement and develop high resolution imaging detectors with applications in medical detector technology.

Progress in gas detector technology for medical imaging research

The x-ray capture, conversion into charge carriers, ion transport mechanisms and image formation mechanisms within a high-gas pressure digital radiographic system, operating up to 60 atm., are presented and analyzed. In detail, the physics of the high-pressure KCD imaging detectors is exposed, analyzed and related to the detector and image quality parameters. Specifically, this study indicates that ion diffusion cannot account for all the experimental observations. It advances the hypothesis that, at sufficiently high pressures, formation of molecular clusters with narrowed mobility distribution take place, through energy exchange mechanism, with local potential forces such that they compensate the space charge distortion of the applied field strength.