House sparrows (Passer domesticus) had plasma aldosterone concentrations of about 180 pg ml-1 while maintained on a low-sodium diet (LS, 0.1 mequiv Na+ ingested per day), 135 pg ml-1 on a sodium intake of 0.9 mequiv day-1 (high-salt diet, HS) and 45 pg ml-1 on a Na+ intake of 3.8 mequiv day-1 (high-salt diet with saline drinking water, HSS). The plasma concentration of aldosterone changed to the LS or the HS level within 1 day of switching from the HS or the LS diet, respectively. Neither dehydration (22 h, 14.5 % loss in body mass) nor brief periods of stress (1–5 min of handling) caused a change in circulating levels of aldosterone. The electrical properties of the lower intestine acclimated to the different sodium intakes with a time course similar to that of the changes in aldosterone levels. On the LS diet, the lower intestine generated an electrical potential difference (PD) of 5 mV (lumen negative) and a short-circuit current (Isc) of about 50 muA cm-2; these were consistently inhibited by amiloride (resulting in a lumen-positive PD) and were stimulated by glucose or amino acids (leucine and lysine) in about half of the tissues. In HS birds, the PD and Isc were abolished and the effects of glucose and amino acids were reduced, but amiloride still caused a significant change in transmural PD (to a mucosa-positive value). These properties resemble those of the chicken coprodeum more than they do those of chicken colon, although the tissues tested were from the mid-region of the large intestine and their histology resembled that of colon. Sparrows tested immediately upon capture from the wild had plasma aldosterone levels not significantly different from those of birds on the LS diet, which is consistent with the known diet of this species. However, Isc was higher and tissue resistance was lower in wild birds compared with low-salt birds in the laboratory, perhaps indicating the influence of other hormones in addition to aldosterone.

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