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

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Abstract
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 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 2 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. 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.