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A method for the measurement of 51CrEDTA absorption from the ventral sac of the rumen with an error of the order of +/-10% is described. When a solution present in the rumen was hypotonic or isotonic, the absorption rate of 51CrEDTA expressed as a clearance was about 0.2 ml/min. This gave rise to negligible errors when 51CrEDTA was used as an unabsorbed marker to calculate net water movements. When the osmotic pressure in the rumen exceeded that of plasma by 30-40 mos-mol/kg, the absorption rate of 51CrEDTA appeared to be related to the degree of hypertonicity. Absorption rates as high as 8 ml/min were observed within a range of osmotic pressures normally encountered postprandially in the rumen. Under hypertonic conditions, a correction for the absorption of this large anion was necessary if passage of water into the lumen were not to be systematically overestimated.
Dobson et al. (Mon,) studied this question.