An analysis of long-distance water transport in the soybean stem using H215O.

The lateral water movement in the intact stem of a transpiring soybean plant was analyzed quantitatively by a real-time measurement system utilizing labeled water, H(2)(15)O and gamma ray detectors. A large volume of water escaping from xylem vessels during its transport was detected. The escape of water was not influenced by evaporation from the stem surface or mass flow in the sieve tubes. It was assumed that the total amount of water transported through xylem vessels was kept almost completely constant along the internode. As a result, most of the escaped water was found to re-enter the xylem vessels, i.e. water exchange occurred. The analysis of radiographs of tritiated water suggested that the self-diffusion effect of water was strong for lateral water movement, although another driving force besides thermal motion was included in the process, and that the process was also affected by the water permeability of the plasma membrane. An analysis based on a mathematical model showed that the net volume of water which escaped from xylem vessels was not dependent on the transpiration rate of the plant.

Visualization of alpha5 subunit of GABAA/benzodiazepine receptor by 11C Ro15-4513 using positron emission tomography.

Although [(11)C]Ro15-4513 and [(11)C]flumazenil both bind to the central benzodiazepine (BZ) receptors, the distributions of the two ligands are not identical in vivo. Moreover, the in vivo pharmacological properties of [(11)C]Ro15-4513 have not been thoroughly examined. In the present study, we examined the pharmacological profile of [(11)C]Ro15-4513 binding in the monkey brain using positron emission tomography (PET). [(11)C]Ro15-4513 showed relatively high accumulation in the anterior cingulate cortex, hippocampus, and insular cortex, with the lowest uptake being observed in the pons. Accumulation in the cerebral cortex was significantly diminished by the BZ antagonist flumazenil (0.1 mg/kg, i.v.), but not that in the pons. Using the pons as a reference region, the specific binding of [(11)C]Ro15-4513 in most of the cerebral cortex including the limbic regions clearly revealed two different affinity sites. On the other hand, specific binding in the occipital cortex and cerebellum showed only a low affinity site. Zolpidem with affinity for alpha1, alpha2, and alpha3 subunits of GABA(A)/BZ receptor fully inhibited [(11)C]Ro15-4513 binding in the occipital cortex and cerebellum, while only about 23% of the binding was blocked in the anterior cingulate cortex. Diazepam with affinity for alpha1, alpha2, alpha3, and alpha5 subunits inhibited the binding in all brain regions. Since Ro15-4513 has relatively high affinity for the alpha5 subunit in vitro, these in vivo bindings of [(11)C]Ro15-4513 can be interpreted as the relatively high accumulation in the fronto-temporal limbic regions representing binding to the GABA(A)/BZ receptor alpha5 subunit.

Synthesis, in vitro and in vivo pharmacology of a C-11 labeled analog of CP-101,606, (+/-)threo-1-(4-hydroxyphenyl)-2-[4-hydroxy-4-(p-[11C]methoxyphenyl)piperidino]-1-propanol, as a PET tracer for NR2B subunit-containing NMDA receptors.

A carbon-11 labeled methoxyl analog of CP-101,606, (+/-)threo-1-(4-hydroxyphenyl)-2-[4-hydroxy-4-(p-[11C]methoxyphenyl)piperidino]-1-propanol [(+/-)[11C]1], was synthesized as a new subtype-selective PET radioligand for NMDA receptors. The in vitro binding studies using rat brain slices demonstrated that (+/-)[11C]1 shows an extremely high-specific binding to the NR2B subunit of NMDA receptors. In contrast to the in vitro binding, the in vivo binding to mouse and monkey brains showed no apparent specific localization of the radioactivity in any of the brain regions. Metabolism and physicochemical properties such as the lipophilicity of (+/-)[11C]1 seemed unlikely to affect the in vivo (+/-)[11C]1 binding. Among the various endogenous ligands acting at the NMDA receptors, polyamines (spermine and spermidine) and divalent cations (Mg(2+,) Zn(2+,) and Ca(2+)) strongly inhibited the in vitro (+/-)[11C]1 binding. Thus, the present studies point to the possibility that the polyamines and cations behave as endogenous inhibitors for (+/-)[11C]1 binding, leading to the loss of the specific binding in vivo.