This revised application proposes to extend the observation of these researchers that the precursor mRNA for the peptide hormone uroguanylin is up-regulated by zinc deficiency. Uroguanylin is an intestinal hormone whose receptor, guanylate cyclase-C, plays a significant role in regulating intestinal fluid balance. Literature reports that supplemental zinc can correct secretory diarrhea and thus links uroguanylin with zinc deficiency. The objective of this project is to characterize the true causes of uroguanylin induction during zinc deficiency in rats, and to determine whether this could provide the mechanism by which Zn supplementation reverses the clinical sides of secretory diarrhea. Six specific aims in the revised proposal are listed. 1) The time course of uroguanylin mRNA regulation by zinc during deficiency will be determined. 2) Uroguanylin mRNA regulation by zinc repletion will be characterized. 3) In situ hybridization will be used to determine which cell types express uroguanylin precursor. 4) Using recently prepared antibodies, levels of uroguanylin precursor will be related to levels of uroguanylin mRNA. 5) Using in situ ligated loops, the effect of dietary deficiency on intestinal fluid secretion will be evaluated. 6) The researchers proposed to begin studies with T-84 cells or Caco-2 cells to test the impact of media zinc concentration on uroguanylin precursor mRNA and/or peptide levels.