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In Vivo Intestinal Bicarbonate Transport in Infant Rats

Summary: Transport of bicarbonate (HCO3-) was studied in segments of the jejunum and ileum in two-, three-, and eight-wk-old rats using an in vivo, one-pass perfusion technique. From isotonic solutions the net absorption of HCO3- (μmol/hr/g dry wt) in the jejunal segments was about twice as great (P < 0.01) in the two- and three- as in the eight-wk-old rats. In ileal segments, HCO3- transport was quite variable; in the two- and three-wk-old rats, there was net absorption, whereas in the eight-wk-old rats, there was net secretion. In both segments, net absorption was unaffected by addition of glucose (5 mmoles/liter), or acetazolamide (10 mmoles/liter) to the perfusion solution. Perfusion of a hypertonic solution (500 mOsmoles/kg) through the segments of the two-wk-old rats induced metabolic acidosis, which was associated with net secretion of relatively large amounts of bicarbonate into the lumen of both segments. Perfusion of the hypertonic solution in the eight-wk-old rats did not change blood acid-base status. In these rats, net absorption of bicarbonate in the jejunum was decreased, and net secretion in the ileum was enhanced in comparison to values noted during perfusion of the isotonic solution.Speculation: From results of the present study of segments of the jejunum and the ileum, it cannot be assumed that the excessive loss of bicarbonate from the small intestine can explain the metabolic acidosis associated with infantile diarrhea. However, if studies of the colon also show excessive loss of bicarbonate during osmotic diarrhea, they will lend further support to the assumption that the loss of bicarbonate from the small and large intestine is a major cause for the metabolic acidosis noted in infants suffering from osmotic diarrhea.