Substantial water fluxes across the small intestine occur during digestion of food, but so far measuring these has required invasive intubation techniques. This paper describes a non-invasive magnetic resonance imaging (MRI) technique for measuring small bowel water content which has been validated using naso-duodenal infusion. Eighteen healthy volunteers were intubated, with the tube position being verified by MRI. After a baseline MRI scan, each volunteer had eight 40 ml boluses of a non-absorbable mannitol and saline solution infused into their proximal small bowel with an MRI scan being acquired after each bolus. The MRI sequence used was an adapted magnetic resonance cholangiopancreatography sequence. The image data were thresholded to allow for intra- and inter-subject signal variations. The MRI measured volumes were then compared to the known infused volumes. This MRI technique gave excellent images of the small bowel, which closely resemble those obtained using conventional radiology with barium contrast. The mean difference between the measured MRI volumes and infused volumes was 2% with a standard deviation of 10%. The maximum 95% limits of agreement between observers were -15% to +17% while measurements by the same operator on separate occasions differed by only 4%. This new technique can now be applied to study alterations in small bowel fluid absorption and secretion due to gastrointestinal disease or drug intervention.
This study was designed to investigate the O2 dependence of K+ influx in sheep red cells. Influx was determined using 86Rb+ as a tracer for K+; glass tonometers coupled to a gas mixing pump were used to equilibrate cell samples to the requisite oxygen tension (PO2). Both volume‐ and H+‐stimulated K+ influxes in low potassium‐containing (LK) sheep red cells were approximately doubled on equilibration with O2 relative to influxes measured in N2. O2‐dependent influxes were abolished when Cl− was replaced with NO3−, consistent with mediation by the KCl cotransporter. At pH 7, PO2 required for half‐maximal stimulation was 56 ± 1 mmHg (mean ± s.e.m., 3 sheep) for the O2‐dependent component of K+ influx: thus PO2 values over the physiological range affected K+ influx. K+ influx in fully deoxygenated sheep red cells showed substantial volume and H+ sensitivity. These residual components in N2 were also Cl− dependent, indicating that the KCl cotransporter of LK sheep red cells was active in the absence of O2. Volume‐sensitive K+ influxes in high potassium‐containing (HK) sheep red cells responded in a similar way to those in cells from LK sheep, although much smaller in magnitude, showing that intracellular [K+] had no significant effect on the O2 dependence of the cotransporter. Intracellular [Mg2+] ([Mg2+]i) was altered by incubating sheep red cells with A23187 (20 μM) and different values of extracellular [Mg2+] ([Mg2+]o). Total [Mg2+]i was determined by atomic absorption spectroscopy and free [Mg2+]i from [Mg2+]o and the Donnan ratio. Total [Mg2+]i was 1.29 ± 0.08 mM (mean ± s.e.m., n= 5), similar to that reported in the literature. Estimates of free [Mg2+]i showed an increase from 0.39 ± 0.05 in oxygenated cells to 0.52 ± 0.04 mM (mean ± s.e.m., n= 5; P < 0.05) in deoxygenated ones. Finally, although K+ influxes were altered by pharmacological loading or depletion of cells with Mg2+, the free [Mg2+]i required to affect influxes significantly was outside the physiological range. Results are difficult to reconcile with PO2 modulating KCl cotransport activity directly via changes in free [Mg2+]i or [Mg2+‐ATP]i.
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