Measurement
of intestinal
water
The following is the abstract the subsequent letter:
absorption
of the article
discussed
in
Gisolfi, C. V., R. W. Summers, H. P. Schedl, T. L. Bleiler, and R. A. Oppliger. Human intestinal water absorption: direct vs. indirect measurements. Am. J. PhysioZ. 258 (Gastrointest. Liver Physiol 21): G377-G378, 1990.-Distilled water, a carbohydrate-electrolyte (CE; 4% sucrose, 2% glucose, 17.2 meq/l NaCl, and 2.8 meq/l KCl) solution, or a 10% glucose solution, all containing the nonabsorbed indicator polyethylene glycol (PEG) and deuterium oxide (D20, 30 ppm), were infused (15 ml/min) into the duodenojejunum of seven men by using the triple lumen technique. Net water absorption was determined directly from the change in PEG concentration and was calculated from plasma D,O derived from D20 in the perfusion solutions. The protocol included a 45min equilibration period followed by a 9O-min test period. Intestinal samples were drawn at lo-min intervals from 15 to 45 min and at 15-min intervals thereafter. Blood was drawn at 45, 50, 55, 60, 75, 90, 105, 120, and 135 min. Intestinal samples were analyzed for DzO, Na+, K’, osmolality, PEG, and glucose; blood was analyzed for D,O. Results (ME; positive values secretion, negative values absorption) showed net fluid absorption from distilled water (-9.40 t 1.28 ml-h-l-cm-l) and the CE (-13.30 t 1.22 ml*h-lcm-l) solution, but net secretion (4.40 t 1.25 ml. h-l cm-‘) from the 10% glucose solution. All values were significantly (P c 0.05) different from each other. Perfusing the CE solution caused net Na’ and K+ absorption, whereas perfusing the 10% dextrose solution caused net electrolyte secretion. Rates of D20 accumulation in the plasma were independent of the solutions perfused. We conclude that 1) the rate of D20 accumulation in the plasma is not a valid measure of net fluid movement from the duodenojejunum into the blood, and 2) the CE solution studied in this investigation promoted greater fluid absorption than distilled water. l
To the Editor: In the paper by Gisolfi et al., the authors conclude that D,O absorption into plasma does not reflect net water absorption from the lumen of the intestine. However, DZO was being absorbed from the entire small bowel and perhaps the colon, whereas net absorption from the lumen was being measured from a 30-cm segment of proximal jejunum. So, the comparison may not be entirely relevant. If the entire intestine is considered, fluid probably was absorbed during perfusion of the 10% dextrose solution, even though fluid was secreted by the 30-cm segment of proximal jejunum. I wonder why the authors did not report D,O absorption from the 30-cm intestinal segment in the same manner that they measured glucose and electrolyte absorption. It seems t/o me that they could easily do this from the data on D,O concentration in intestinal samples (measurement mentioned on p. G217). If the authors analyze their data as suggested, I believe they will find that D,O will be absorbed by the 30-cm segment regardless of the direction of net water movement. I would guessthat D20 absorption would be somewhat less with 10% dextrose than from the other solu-
tions because of solvent drag effects. I believe that this would be a valuable piece of information that would shed additional light on the differences between isotopic and net absorption and on the serious limitations in the interpretation of isotopic absorption by the intestine. John S. Fordtran Department of Internal Medicine Baylor University Medical Center Dallas, Texas 75246 REPLY
To the Editor: We are writing in response to the letter of Dr. John S. Fordtran regarding our paper. First, we would like to thank Fordtran for his interest and careful reading of our manuscript. This response to his letter provides further evidence in support of the conclusion drawn in our manuscript that D,O accumulation in the plasma does not reflect net water flux across the intestinal mucosa. Dr. Fordtran’s first point is that our comparison of D,O absorption from the 40-cm test segment may not be relevant because the D20 was being absorbed from the test segment. However, disappearance of D,O from the lumen was so rapid that little remained for absorption beyond the sampling site. We infused D20 at 30,000 parts per million (ppm) and collected it at 1,000 ppm at the distal sampling site. This information, as well as D,O absorption data given below, were not available in the paper. In addition, the paper contained a misstatement in METHODS: the distal occlusive balloon was not deflated. (We deflated the balloon in subsequent studies conducted while writing the manuscript, but in this initial study we purposely did not deflate the balloon.) Thus the D,O concentration of any fluid escaping past the distal inflated balloon was so low as to have a negligible effect on plasma D,O compared with D,O absorbed proximally. In response to Dr. Fordtran’s second point, we have done what he suggested and present the D,O absorption data in Table 1 together with the net water flux from Fig. 3 of the paper. Despite the fact that the net water flux data revealed net absorption of water from the 6% carbohydrate-electrolyte solution and net secretion of water from the 10% dextrose solution, the D,O results in Table 1 indicate net absorption of water for all solutions. Dr. Fordtran was Table 1. DzO absorption data and net Hz0 flux Solution
Water 6% CE 10% dextrose
D,O
Absorption*
-18.71t0.8 -21.12t0.37'f -13.80+2.2"f
Net
H,O
Flux*
-9.4Ok1.28 -13.30t1.22 4.40t1.25
Values are means * SE. CE, carbohydrate-electrolyte. * Negative values designate net loss from lumen, positive values net secretion; values in ml mh-’ . cm? t Significantly different from water.
Downloaded from www.physiology.org/journal/ajpgi at Tulane University (129.081.226.078) on February 13, 2019.
G378
LETTERS
TO
correct in predicting this result and in predicting that D20 absorption would be less from the 10% dextrose solution than from the other solutions. We believe the reason for this is that D20 measures only unidirectional flux out of the lumen. The D20 content of the body before the experiment is essentially zero. Luminal D20 exchanges with mucosal and luminal Hz0 and is carried into the body Hz0 pool. Hence, D20 will not reenter the lumen from the mucosa. HZ0 will reenter the lumen, but D20 is so diluted by the large body HZ0 pool, there will be virtually no D20 reentry. Thus, D20 will always be absorbed. D20 absorption was lower for the 10% dextrose solution because of opposing solvent drag effects, i.e., net movement of water into the lumen. Because D20 absorption represents unidirectional flux of HZ0 out of the lumen, it differs from net HZ0 absorption, which is the
THE
EDITOR
difference between flux into and flux out of the lumen. In summary, our paper shows that D20 absorption into the plasma does not reflect net water absorption from the lumen of the intestine. Evidence in support of this conclusion is that plasma accumulation of D20 was the same for all solutions; however, the 10% dextrose solution produced net water secretion. Thus DzO can only be absorbed, regardless of net water secretion. These results, as suggested by Dr. Fordtran, reveal the limitations in interpretation of isotopic absorption by the intestine. Carl V. Gisolfi, Harold P. Schedl, and Robert D. Summers Department of Exercise Science University of Iowa Iowa City, Iowa 52242
Downloaded from www.physiology.org/journal/ajpgi at Tulane University (129.081.226.078) on February 13, 2019.
of intestinal
water
The following is the abstract the subsequent letter:
absorption
of the article
discussed
in
Gisolfi, C. V., R. W. Summers, H. P. Schedl, T. L. Bleiler, and R. A. Oppliger. Human intestinal water absorption: direct vs. indirect measurements. Am. J. PhysioZ. 258 (Gastrointest. Liver Physiol 21): G377-G378, 1990.-Distilled water, a carbohydrate-electrolyte (CE; 4% sucrose, 2% glucose, 17.2 meq/l NaCl, and 2.8 meq/l KCl) solution, or a 10% glucose solution, all containing the nonabsorbed indicator polyethylene glycol (PEG) and deuterium oxide (D20, 30 ppm), were infused (15 ml/min) into the duodenojejunum of seven men by using the triple lumen technique. Net water absorption was determined directly from the change in PEG concentration and was calculated from plasma D,O derived from D20 in the perfusion solutions. The protocol included a 45min equilibration period followed by a 9O-min test period. Intestinal samples were drawn at lo-min intervals from 15 to 45 min and at 15-min intervals thereafter. Blood was drawn at 45, 50, 55, 60, 75, 90, 105, 120, and 135 min. Intestinal samples were analyzed for DzO, Na+, K’, osmolality, PEG, and glucose; blood was analyzed for D,O. Results (ME; positive values secretion, negative values absorption) showed net fluid absorption from distilled water (-9.40 t 1.28 ml-h-l-cm-l) and the CE (-13.30 t 1.22 ml*h-lcm-l) solution, but net secretion (4.40 t 1.25 ml. h-l cm-‘) from the 10% glucose solution. All values were significantly (P c 0.05) different from each other. Perfusing the CE solution caused net Na’ and K+ absorption, whereas perfusing the 10% dextrose solution caused net electrolyte secretion. Rates of D20 accumulation in the plasma were independent of the solutions perfused. We conclude that 1) the rate of D20 accumulation in the plasma is not a valid measure of net fluid movement from the duodenojejunum into the blood, and 2) the CE solution studied in this investigation promoted greater fluid absorption than distilled water. l
To the Editor: In the paper by Gisolfi et al., the authors conclude that D,O absorption into plasma does not reflect net water absorption from the lumen of the intestine. However, DZO was being absorbed from the entire small bowel and perhaps the colon, whereas net absorption from the lumen was being measured from a 30-cm segment of proximal jejunum. So, the comparison may not be entirely relevant. If the entire intestine is considered, fluid probably was absorbed during perfusion of the 10% dextrose solution, even though fluid was secreted by the 30-cm segment of proximal jejunum. I wonder why the authors did not report D,O absorption from the 30-cm intestinal segment in the same manner that they measured glucose and electrolyte absorption. It seems t/o me that they could easily do this from the data on D,O concentration in intestinal samples (measurement mentioned on p. G217). If the authors analyze their data as suggested, I believe they will find that D,O will be absorbed by the 30-cm segment regardless of the direction of net water movement. I would guessthat D20 absorption would be somewhat less with 10% dextrose than from the other solu-
tions because of solvent drag effects. I believe that this would be a valuable piece of information that would shed additional light on the differences between isotopic and net absorption and on the serious limitations in the interpretation of isotopic absorption by the intestine. John S. Fordtran Department of Internal Medicine Baylor University Medical Center Dallas, Texas 75246 REPLY
To the Editor: We are writing in response to the letter of Dr. John S. Fordtran regarding our paper. First, we would like to thank Fordtran for his interest and careful reading of our manuscript. This response to his letter provides further evidence in support of the conclusion drawn in our manuscript that D,O accumulation in the plasma does not reflect net water flux across the intestinal mucosa. Dr. Fordtran’s first point is that our comparison of D,O absorption from the 40-cm test segment may not be relevant because the D20 was being absorbed from the test segment. However, disappearance of D,O from the lumen was so rapid that little remained for absorption beyond the sampling site. We infused D20 at 30,000 parts per million (ppm) and collected it at 1,000 ppm at the distal sampling site. This information, as well as D,O absorption data given below, were not available in the paper. In addition, the paper contained a misstatement in METHODS: the distal occlusive balloon was not deflated. (We deflated the balloon in subsequent studies conducted while writing the manuscript, but in this initial study we purposely did not deflate the balloon.) Thus the D,O concentration of any fluid escaping past the distal inflated balloon was so low as to have a negligible effect on plasma D,O compared with D,O absorbed proximally. In response to Dr. Fordtran’s second point, we have done what he suggested and present the D,O absorption data in Table 1 together with the net water flux from Fig. 3 of the paper. Despite the fact that the net water flux data revealed net absorption of water from the 6% carbohydrate-electrolyte solution and net secretion of water from the 10% dextrose solution, the D,O results in Table 1 indicate net absorption of water for all solutions. Dr. Fordtran was Table 1. DzO absorption data and net Hz0 flux Solution
Water 6% CE 10% dextrose
D,O
Absorption*
-18.71t0.8 -21.12t0.37'f -13.80+2.2"f
Net
H,O
Flux*
-9.4Ok1.28 -13.30t1.22 4.40t1.25
Values are means * SE. CE, carbohydrate-electrolyte. * Negative values designate net loss from lumen, positive values net secretion; values in ml mh-’ . cm? t Significantly different from water.
Downloaded from www.physiology.org/journal/ajpgi at Tulane University (129.081.226.078) on February 13, 2019.
G378
LETTERS
TO
correct in predicting this result and in predicting that D20 absorption would be less from the 10% dextrose solution than from the other solutions. We believe the reason for this is that D20 measures only unidirectional flux out of the lumen. The D20 content of the body before the experiment is essentially zero. Luminal D20 exchanges with mucosal and luminal Hz0 and is carried into the body Hz0 pool. Hence, D20 will not reenter the lumen from the mucosa. HZ0 will reenter the lumen, but D20 is so diluted by the large body HZ0 pool, there will be virtually no D20 reentry. Thus, D20 will always be absorbed. D20 absorption was lower for the 10% dextrose solution because of opposing solvent drag effects, i.e., net movement of water into the lumen. Because D20 absorption represents unidirectional flux of HZ0 out of the lumen, it differs from net HZ0 absorption, which is the
THE
EDITOR
difference between flux into and flux out of the lumen. In summary, our paper shows that D20 absorption into the plasma does not reflect net water absorption from the lumen of the intestine. Evidence in support of this conclusion is that plasma accumulation of D20 was the same for all solutions; however, the 10% dextrose solution produced net water secretion. Thus DzO can only be absorbed, regardless of net water secretion. These results, as suggested by Dr. Fordtran, reveal the limitations in interpretation of isotopic absorption by the intestine. Carl V. Gisolfi, Harold P. Schedl, and Robert D. Summers Department of Exercise Science University of Iowa Iowa City, Iowa 52242
Downloaded from www.physiology.org/journal/ajpgi at Tulane University (129.081.226.078) on February 13, 2019.
