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The opiate receptor antagonist naltrexone is also a non-competitive blocker of the nicotinic acetylcholine receptor ion channel complex. This blockade is usually thought to be due to physical occlusion of the channel pore. However, the most recent structural data for the nicotinic receptor show that the pore is generally 20-25 A in diameter, with a much narrower section in the plane of the membrane which is presumed to be the gating region. We have completed a single-crystal X-ray structure determination of naltrexone hydrochloride, (C20H24NO4)+Cl-.4H2O, at room temperature. Crystals are orthorhombic, P212121, a 18.099(5), b 15.926(6), c 7.768(11) A, Z 4; the structure was refined to a residual of 0.056 for 1988 'observed' reflections. The maximal dimension of this molecule (c. 11 - 6 A) suggests that physical occlusion of the channel pore can only occur if binding is directly in the gating region.

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A new flow-through slurry test developed by the Bureau of Mines significantly reduces abrasive particle degradation and contamination of wear debris in the slurry during the test. Fresh slurry is continuously fed into the test container as used slurry is discharged. Sixteen stationary specimens form the walls of the container in which the slurry is circulated by an impeller. Slurries of silica sand in water can range from 1.3 to 34 weight % solids as the sand flow ranges from 90 to 1600 g/min. An American Foundrymen's Society (AFS) 50/70 mesh test sand was used. Results of tests conducted on a variety of materials showed that the wear rate was constant with time for each material. In contrast, when no fresh slurry was added, as in a conventional slurry pot test, the wear rate decreased significantly as the test proceeded. This decrease is attributed to micropolishing of the surfaces of the abrasive particles. The relative effects of cutting wear and deformation wear during slurry erosion are described.