The effect of fasting on the potential difference across the brush‐border membrane of enterocytes in rat small intestine.

The effects of starvation on the magnitude of the potential difference across the brush-border membrane of rat small intestine (Vm) have been studied in vitro. The mean values obtained for Vm in jejunal tissue after a fast of either 24 or 72 h duration (-53.5 and -55.0 mV respectively) were significantly greater than the mean value of -45.3 mV recorded using intestine from fed rats. A similar hyperpolarization was observed after a 72 h fast using intestine from a more distal region. The response to fasting seen in the jejunum appears to be caused by a decrease in Na+ permeability of the mucosal membrane. The addition of D-galactose (4 mM) or L-valine (4 mM) to the mucosal fluid caused a depolarization of Vm. Using jejunum from fed animals, Vm in the presence of galactose and valine was found to be -40.3 and -39.2 mV respectively. Following a fast of 72 h duration, the values of Vm for the two substrates were -43.4 and -43.1 mV respectively. Phlorhizin (10(-4) M) abolished the galactose-induced depolarization in both fed and fasted conditions but was without effect on the response to valine. It is concluded that starvation increases the electrical driving force for Na+ during Na+ coupled non-electrolyte transport into the enterocyte. The possible mechanisms involved in this adaptive response are discussed.