Introduction to Special Issue 2 on the NATMAP Shield Margin Project: The Flin Flon Belt, Trans-Hudson Orogen, Manitoba and Saskatchewan

The Shield Margin Project of the National Geoscience Mapping Program (NATMAP) resulted in a new understanding of the Paleoproterozoic Flin Flon Belt (Manitoba and Saskatchewan) in four dimensions. A multidisciplinary approach was utilized in the NATMAP project, based on partnerships with government, university, and private sector geoscientists, and close cooperation with the Lithoprobe's Trans-Hudson Orogen Transect. Research areas spanned from bedrock and surficial geoscience, to crustal and mantle geophysics, to high precision U-Pb geochronology and tracer isotope studies. This Special Issue contains nine papers covering a wide variety of topics related to the NATMAP Shield Margin Project, including volcanic-hosted massive sulphide deposits in the Flin Flon and Snow Lake camps; structural geology of the Flin Flon townsite and southern flank of the Kisseynew Domain; geochronology and the U-Pb systematics of monazite in metasedimentary rocks; and the geoelectrical and crustal conductivity structure of the Flin Flon Belt.

Transport of glutamine across blood-facing membranes of perfused rat jejunum

Transport of glutamine and other neutral amino acids across the blood-facing membranes of isolated, dually perfused rat jejunum was measured using a paired-tracer isotope-dilution technique. Glutamine, asparagine, histidine, alanine, and leucine showed mutual inhibition of transport. The major component of physiological glutamine transport was saturable (Km = 0.88 +/- 0.15 mM, Vmax = 454 +/- 49 nmol.g-1.min-1; mean +/- SE), stereospecific and Na-independent and appeared to exhibit symmetry of glutamine transport; it most resembled system L. The minor Na-dependent component of glutamine transport resembled system A, i.e., it transported N-methylaminoisobutyric acid (Km approximately equal to 10 microM, Vmax approximately equal to 1.2 nmol.g-1.min-1). At 0.5 mM glutamine transport was insensitive to insulin and glucagon and was unaffected by perfusate pH (7.0-7.8). Glutamine extracted by the jejunum is rapidly utilized; at physiological blood glutamine concentrations the basolateral glutamine-transporter flux may thus not only restrict intestinal glutamine catabolism but also the consequent release of glutamine-derived ammonia (a substrate and stimulant of ureogenesis) into the portal circulation.

Effects of respiratory variables on regional gas transport during high-frequency ventilation

The regional effects of tidal volume (VT), respiratory frequency, and expiratory-to-inspiratory time ratio (TE/TI) during high-frequency ventilation (HFV) were studied in anesthetized and paralyzed dogs. Regional ventilation per unit of lung volume (spVr) was assessed with a positron camera during the washout of the tracer isotope 13NN from the lungs of 12 supine dogs. From the washout data, functional images of the mean residence time (MRT) of 13NN were produced and spVr was estimated as the inverse of the regional MRT. We found that at a constant VT X f product (where f represents frequency), increasing VT resulted in higher overall lung spV through the local enhancement of the basal spVr and with little effect in the apical spVr. In contrast, increasing VT X f at constant VT increased overall ventilation without significantly affecting the distribution of spVr values. TE/TI had no substantial effect in regional spVr distribution. These findings suggest that the dependency of gas transport during HFV of the form VT2 X f is the result of a progressive regional transition in gas transport mechanism. It appears, therefore, that as VT increases, the gas transport mechanism changes from a relative inefficient dispersive mechanism, dependent on VT X f, to the more efficient mechanism of direct fresh gas convection to alveoli with high regional tidal volume-to-dead-space ratio. A mathematical model of gas transport in a nonhomogeneous lung that exhibits such behavior is presented.