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Calcium Solubility, Intestinal Sojourn Time and Paracellular Permeability Codetermine Passive Calcium Absorption in Rats^1,2,3,

INSERM U 45 and Ecole Pratique des Hautes Etudes, Hôpital E. Herriot 69437 Lyon CEDEX 03, France ^* The University of Connecticut Health Center, Farmington, CT 06030

To investigate the nonsaturable, paracellular pathway of intestinal Ca absorption, the luminal contents of 12-cm segments of the intestine of 8-wk-old male Sprague-Dawley rats were analyzed for pH, sojourn time and soluble and insoluble Ca over a 24-h period. The rats had been fed one of two high Ca diets for 2 wk: 1.5% Ca (diet group 3a) and 3.1% (diet group 5a). The pH of the small intestine increased from <6.6 to >8.0 from duodenum to ileum; transit time increased from 2.5 min in the duodenum to 58 min in the distal ileum, with the entire ileum accounting on the average for 74% of the transit time of 3 h. The amount of Ca solubilized throughout the intestine was 32 ± 3.3 µmol in diet group 3a and 53 ± 5.3 µmol in diet group 5a, i.e., 27% and 2.0% of the total luminal Ca. Because absorption by diet group 3a was 1.45 ± 0.23 mmol/d and that by diet group 5a was 2.50 ± 0.18 mmol/d, the amounts absorbed were 45.3 and 47.1 times greater than present in the lumen in soluble form at any one time. Thus, over a 24-h period, an average of 3.2% (46.2/1440) of the soluble Ca present in the lumen at any time was absorbed per min. Calculations involving the gradient between luminal and plasma Ca show that the rate of Ca diffusion from lumen to blood is <2% of what it would be if the paracellular path were unrestricted. Thus, intestinal sojourn time, Ca solubility and mucosal permeability to Ca are factors that determine the rate of passive Ca absorption.

KEY WORDS: • transit rate • calcium • rats • pH

¹ Presented in part at the International Conference on Progress in Bone and Mineral Research (Austrian Society for Bone and Mineral Research), October 14–16, 1994, Vienna, Austria.

² C. Duflos has been supported by a joint grant from the Conseil Régional Rhône-Alpes and DIEPAL, Groupe DANONE, Villefranche-sur-Saône, France.

³ The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

⁴ To whom correspondence and reprint requests should be addressed.

Manuscript received 6 October 1994. Revision accepted 30 March 1995.

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