Objective. Quantitative magnetic resonance imaging (MRI) of articular cartilage represents a powerful tool in osteoarthritis (OA) research, but has so far been confined to a field strength of 1.5T. The aim of this study was to evaluate the precision of quantitative MRI assessments of human cartilage morphology at 3.0T and to correlate the measurements at 3.0T with validated measurements at 1.5T.Methods. MR images of the knee of 15 participants with OA and 15 healthy control subjects were acquired using Siemens 1.5T and 3.0T scanners. Double oblique coronal scans were obtained at 1.5T with a 1.5-mm partition thickness, at 3.0T with a 1.5-mm partition thickness, and at 3.0T with a 1.0-mm partition thickness. Cartilage volume, thickness, and surface area of the femorotibial cartilage plates were quantified using proprietary software.Results. For 1.5-mm partition thickness at 1.5T, the precision error was 3.0% and 2.6% for cartilage volume and cartilage thickness, respectively. The error was smaller for a 1.5-mm partition thickness at 3.0T (2.6% and 2.5%) and still smaller for a 1.0-mm partition thickness at 3.0T (2.1% and 2.0%). Correlation coefficients between values obtained at 3.0T and 1.5T were high (r > 0.96), with no significant deviation between the two field strengths.Conclusion. Quantitative MRI measurement of cartilage morphology at 3.0T (partition thickness 1 mm) was found to be accurate and tended to be more reproducible than at 1.5T (partition thickness 1.5 mm). Imaging at 3.0T may therefore provide superior ability to detect changes in cartilage status over time and to determine responses to treatment with structuremodifying drugs.

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Neuronal Ca 2؉ channels are inhibited by a variety of transmitter receptors coupled to G o -type GTPbinding proteins. G o has been postulated to work via a direct interaction between an activated G protein subunit and the Ca 2؉ channel complex. Here we show that the inhibition of sensory neuron N-type Ca 2؉ channels produced by ␥-aminobutyric acid involves a novel, rapidly activating tyrosine kinase signaling pathway that is mediated by G␣ o and a src-like kinase. In contrast to other recently described G protein-coupled tyrosine kinase pathways, the G␣ o -mediated modulation requires neither protein kinase C nor intracellular Ca 2؉ . The results suggest that this pathway mediates rapid receptor-G protein signaling in the nervous system and support the existence of a previously unrecognized form of crosstalk between G protein and tyrosine kinase pathways.Receptor-mediated modulation of ion channels is an important means of regulating intercellular communication in the nervous system. Voltage-dependent Ca 2ϩ channels are effective targets for such modulation by virtue of their involvement in a host of cellular functions. Of the many calcium channel types now recognized, it has been the N-type (or class B) channels that have received the most attention in studies of receptor-mediated regulation. A large number of receptors and a variety of signaling pathways have been identified to target N channels (1, 2).High voltage-activated Ca 2ϩ currents evoked from cultured embryonic chicken sensory neurons are predominantly N-type (as they are blocked Ϸ90% by -conotoxin GVIA) and are inhibited by several neurotransmitters, including ␥-aminobutyric acid (GABA) and norepinephrine (NE). The inhibition is characterized by a slowing in the kinetics of N current activation (kinetic slowing) and an attenuation of the overall current amplitude without changes in current waveform (steady-state inhibition). Previous work has shown that these two components can be separated on the basis of a differential sensitivity to voltage (3).All inhibition by GABA is rapid and mediated through GABA B receptors (4) coupled to G proteins of the G o class (5). The speed of inhibition, coupled with the fact that it is independent of protein kinases A and C, cyclic nucleotides, intracellular Ca 2ϩ , and phosphatases 1 and 2A (6, 7), has led to the suggestion that GABA evokes inhibition through a direct effect of G protein subunits on the Ca 2ϩ channel (1, 2). Results reported below, however, suggest that steady-state inhibition is mediated indirectly through a novel tyrosine kinase pathway that is activated by G␣ o . MATERIALS AND METHODSEmbryonic chicken sensory neurons were grown in culture, and tight-seal whole-cell recording was performed after 1-3 days as in ref. 5. For intracellular application of kinase inhibitors, agents were diluted into intracellular recording solution and delivered to the cell via passive diffusion from the patch pipette. For extracellular application, agents were diluted into standard extracellular saline and ap...

The role of interdomain contact sites in basal GDP release from heterotrimeric G proteins is unknown. G(alpha)(o) and G(alpha)(i1) display a 5-fold difference in the rate of GDP dissociation with half-times of 2.3 +/- 0.2 and 10.4 +/- 1.3 min, respectively. To identify molecular determinants of the GDP release rate, we evaluated the rate of binding of the fluorescent guanine nucleotide 2'(3')-O-(N-methyl-3'-anthraniloyl)guanosine 5'-O-(3-thiotriphosphate) (mGTPgammaS) to chimers of G(alpha)(o) and G(alpha)(i1). Although no one region of the G protein determined the GDP dissociation rate, when the C-terminal 123 amino acids in G(alpha)(i1) were replaced with those of G(alpha)(o), the GDP release rate increased 3.3-fold compared to that of wild-type G(alpha)(i1). Within the C-terminal portion, modification of four amino acids in a coil between beta4 and the alpha3 helix resulted in GDP release kinetics identical to those of wild-type G(alpha)(o). Based on the G(alpha)(i1)-GDP crystal structure of this region, Leu(232) appeared to form a hydrophobic contact with Arg(144) of the helical domain. The role of this interaction was confirmed by G(alpha)(i1) L232Q and G(alpha)(i1) R144A which displayed 2-5-fold faster GDP release rates compared to wild-type G(alpha)(i1) (t(1/2) 4.7 and 1.5 min, respectively), suggesting that interdomain bridging contacts partially determine the basal rate of GDP release from heterotrimeric G proteins.