The compartmental model of Larsen et al. (2000, 2002) for solute-coupled water transport across toad intestine simulates a leaky epithelium involved in isotonic fluid absorption, allowing the osmolality of the transported fluid to either be fixed at isotonic or to vary depending on parameter selection. When the measured toad intestine water and solute permeabilities and active transport rates are incorporated into the model, a markedly hypertonic absorbate is predicted to exit from the lateral intercellular spaces. Most previous compartmental models of isotonic fluid absorption encountered the hypertonic absorbate problem and solved it either by increasing the water flux into the lateral intercellular spaces or by decreasing the solute diffusion coefficient within those spaces. In the present model, even when the water flux is increased by postulating that 75 % of the water flows across a highly water-permeable tight junction, formation of a hypertonic absorbate is predicted unless solute efflux is greatly retarded. The solution proposed by Larsen et al. (2000, 2002) is that 60-80 % of the solute that has been actively transported out of the cell re-enters it in a reversed flow across the basal membrane. This phenomenon has been denoted 'solute recirculation' and was first suggested by Ussing & Nedergaard (1993) as a way of explaining the non-unity caesium flux ratios in the toad intestine. The concept underwent further refinement and development in their investigations of toad intestine (Nedergaard et al. 1999) and the exocrine glands of frog skin (Ussing et al. 1996).
