Intestinal transport: studies with isolated epithelial cells

Isolated intestinal epithelial cells have been extremely useful for characterizing the nature of intestinal absorption processes and for providing insight into the energetics of Na(+)-dependent transport systems. This report describes a number of experimental approaches which have been used for investigating the specific epithelial transport systems involved in sugar absorption, but provides information which ultimately should prove useful for characterizing a number of different intestinal transport events. Similar experiments should also prove useful for exploring the effect of environmental agents on the function of intestinal tissue. In the case of sugars, net absorption is accomplished via a mucosal, Na(+)-dependent concentrative transport system acting in sequence with a passive serosal system which does not require Na(+). The serosal system limits the full gradient-forming capability of the muscosal system. Agents such as phloretin or cytochalasin B which inhibit serosal transport allow the cells to establish sugar gradients as high as 70 fold in contrast to 10-15 fold gradients observed for control cells. Seventy-fold sugar gradients cannot be explained in terms of the energy available in the electrochemical potential for Na(+) if the Na(+):sugar coupling stoichiometry is 1:1 as commonly assumed. New information indicates that the true Na(+):sugar stoichiometry is in fact 2:1. Flow of two Na(+) ions per sugar molecule down the transmembrane electrochemical potential for Na(+) provides more than sufficient energy to account for observed 70 fold sugar gradients. If flow of sugar by other routes could be completely inhibited, theoretical sugar gradients as high as 400 could be achieved assuming that the cells maintain a membrane potential of -36 mV as measured for intact tissue.