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Department of Biomedical Science, University of Sheffield, Western Bank.

1. The effects of starvation and undernutrition were assessed on rat colonic electrogenic Na+ absorption in fed controls, 72 h starved and acute undernourished (fed one-third of the control group's daily food intake for up to 9 days). The basal short-circuit currents (Isc) of three segments of rat colon (proximal, mid- and distal), stripped of their external muscle layers were monitored before and during addition of 0.1 mM-mucosal amiloride. The decrease in Isc was used as the measure of the electrogenic Na+ absorption. 2. Acute undernutrition and to a lesser extent 72 h starvation elevated the basal Isc only in the distal colon. The increase was inhibited by amiloride (0.1 mM, mucosal) indicating that it was due to electrogenic Na+ transport. 3. Allowing the 9 days acute undernourished rats to drink 0.9% NaCl failed to prevent the increase in the basal Isc in the distal colon but it was reduced by administration of spironolactone. 4. Adrenalectomy completely abolished the increased basal Isc in the distal colon induced by the 9 day undernutrition. However, the plasma aldosterone levels in the fed and 9 day undernutrition groups were not significantly different. 5. Injection of aldosterone into adrenalectomized rats drinking 0.9% NaCl and which were undernourished for 9 days induced a large increase in their distal colonic Isc which was inhibited by mucosal amiloride. Similar treatment of sham-operated rats on 0.9% NaCl or adrenalectomized control fed rats on 0.9% NaCl had no effect on the distal colonic Isc. 6. The results indicate that acute undernutrition for 9 days makes the distal colonic epithelium more sensitive to the prevailing plasma aldosterone level allowing an enhanced electrogenic Na+ absorptive capacity to be induced.