Ion Transport in Rat Colon

Abstract

The rat large intestine has been the object of multiple studies of fluid and electrolyte absorption for more than 30 years. Initial experiments performed under in vivo conditions provided an overall description of electrolyte movement, while more recent in vitro studies with both intact tissue and membrane vesicles have provided increasing insight into both the cellular processes responsible for net fluid movement and the mechanisms regulating these transport events. Luminal perfusion experiments that studied the entire colon revealed that sodium (Na) and chloride (Cl) are absorbed, while potassium (K) and bicarbonate (HCO₃) are secreted (Curran and Schwartz 1960; Edmonds 1967b; Powell et al. 1968). Water absorption was considered secondary to the net electrolyte movement. Evaluation of the mechanism(s) responsible for ion movement (based on concentration and potential differences) suggested that Na absorption and HCO₃ secretion are active and energy-dependent, while K secretion is the result of existing electrochemical concentration gradients.